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Biotogy

Nerve & Muscle

Types Of Tissues

Types Of TissuesATP (which was generated in glycoiysis, the Krebs cvcle, and oxidative phosphorylation) become converted into the mechanical movement of, say, muscle cells? How is it that the chemical energy of ATP is converted into an electrical signal that al1ows various nerves to communicate with those muscles?Before we can discuss the cellular mechanisms of muscles and nerves, we first need to consider some of the general characteristics of cells, tissues, and organs. The general body plan of an animal is fairly simple and can be divided into a number of systems that represent a variety of organs working in concert with one another. For example, one body system you are probably quite familiar with is the skeletal system. Another is the muscular system. Others are the circulatory, integumentary (skin), endocrine, nervous, and digestive systems, to name but a Let's consider a series of discussions on cellular physiology. For example, we will consider how muscle and nerve cells function. How does the chemical energy of

few. The digestive system is formed by an alimentary canal (gastrointestinal "tube") that begins at the mouth and ends at the anus. This system is suspended within a body cavity re-ferred to as the coelom. The coelom is separated into a thoracic cavity (upper) and an abdominal cavity (lower). These two cavities are separated by the dome-shaped mass of skeletal muscle called the diaphragm. Within the thoracic cavity, one finds the lungs and the heart. The abdominal cavity contains the liver, stomach, and iatestines.

As we examine

l"hese

various systems, we will find different levels of

stomach. some of the simple epithelial cells within the siomach secrete hydrochloric acid (pH = 1) to aid in the digestion of food. Other epithelial cells of the stomach secrete mucus to help prevent that acid from digesting the lining of the stomach. Still other epithelial cells secrete enzymes. These epithelial cells are just one type of tissue that is involved in forming the stomach. The stomach is also composed of other types of tissue. For example, nervous tissue helps to innervate the stomach, connective tissue helps to hold the stomach in its proper position, and muscle tissue helps to propel food through the stomach. Thus, these four groups of primary tissue (epitheliai, connective, muscle, and nerve) have the abiiity to form the various organs of the body. An organ is n structure that is composed of two 0r ffLlre tissues that act in such a way as to perform a specific.function.

organization. There are individual cells, and then there are cells of a particular type which coalesce to form tissue. One example of a tissue is the layer of epithelial cells that line one of the principal organs of the alimentary canal, the

Epithelial TissuesLet's examine the epithelialce11s in a little more detail. The epithelial tissue that constitutes the various organs of the body can be either simple epithelium (consisting of a single layer of cells) or stratified epithelium (consisting of two or more layers of cells). These epithelial celtrs come in a variety of shapes and sizes. For example, there are squamous (flat), cuboidal, and columnar epithelial cells (refer to Figure 1-1).

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Biology

Nerve & Muscle

Types Of Tissues

On the lumenal side of the simple epithelial cells are projections cailed microvilli (singular, microviilus--see Figure 1-1 and Figure 1-2). Theseprojections increase the total absorptive area of the cell (sometimes by as much as 25%). Sometimes you find specialized structures cailed cilia (singular, cilium) projecting outward on the apical surface of these cells. For example, in the respiratory tract these hair-like appendages move in a coordinated unidirectional wave to move foreign particles out of the mucous lining of the lungs and bronchial tubes.

Simple squamous epithelial cel1s

Simple columnar epithelial cells

\-JCuboidal and columnar epithelial cells

Basal .--------\lamina

Stratified squamous epithelial cells (non-keratinized)

Figure l - lTypes of epithelial cells.

Thebe cells are !_l-undea by a number of specialized junctions. For example, tight junctionS aet-fs a permeability barrier (see Figure 1-2). Not oniy do they prevent

the transport of protein molecules from the lumenal side of the cell towards the basolaterai side of the cell, but they also act to hold neighboring cells together.

Epithelial cells are also held together by structures called desmosomes (see Figure 1-2) One type of desmosome joins the epithelial cell to a structure on the basal side of the ceil called the basal lamina (or basement membrane). The basal iamina is in close contact with connective tissue that helps to anchor the cells inplace.

Gap junctions provide a means for water-soluble molecules to pass from the cytoplasm of one cel1 to the cytoplasm of another cell (see Figure 1-2). These lunclions allow for equilibration within the connected epithelial celis and therefore allow those celis to function as a unit. For example, the beating cilia appear to be coordinated by waves of calcium, which flow in the plane of thejuxtaposed epithelial cells.

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BiologyLUMENApical surface

Nerve & Muscle

Types Of Tissues

Tight junctionsGap

(=l

Microvillus Tightjunctions

.lunctlonDesmosome Desmosomes Basal lateral surface

C) f

BLOODFigure 1.2Different components of an epithelial cell.

Basal iamina (basement membrane)

As we have mentioned, epithelial cells can secrete substances into a lumenal space. For example, hydrochloric acid can be secreted into the lumen of the stomach. If a cell secretes a substance into the lumen by way of a duct, it is referred to as an exocrine gland. Endocrine glands secrete substances into the blood. For example, insulin is a protein hormone secreted into the blood byclusters of specialized epithelial cells in the pancreas.Dead skin cells

&r \t-

(keratinizedl

II noio..-ur f cellsBasement membrane

)t)ce1ls.

Collagen fibers

] conn..,,u. tissue J

Figure 1.3Stratified squamous epithelial

Stratified squamous epithelium usualiy has a protective function. Your skin is composed of many layers of stratified squamous epithelial cells. The outer cells of your skin are dead, and they contain a large amount of the fibrous protein keratin (Figure 1-3). These cells are constantly being lost and replaced, as cells begin to move toward the surface from beiow.

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Biology

Nerve Er Muscle

Types Of fissues

Consider a segment of skin. This organ comprises about 15o,i, of r-our total body weight. The epidermal region contains stratified epithelial cells that act to protect the deeper layers of the skin. Belou'the epidermis is the dermis. Within the dermis are a variety of structures. Surrounding the hair follicles are erector muscles, which act to straighten the hair shaft. This causes the skrn close to the hair follicle to become depressed and gives the characteristic appearance of "goose pimples." Those erector muscles are innervated biz nerves which cause them to contract at specific times (e.g., when it is cold outside). The skin is also a highiy vascularized organ. When it is hot outside, the blood is shunted towards the surface of the skin where it can dissipate some of its heat to the outside environment. Below the dermis is the subcutaneous tissue. This is where one

finds adipose deposits.

Connective fissuesThis type of tissue helps to anchor and support the various structures of the body. There are a variety of types of connective tissues, a few of which are structural, blood cells, mast cells, adipose cells, and melanocytes. Many of the proteins that make up structural connective tissue are secreted by cells cailed fibroblasts. Collagen, reticulirl and elastin are structural proteins which are secreted by these cells. Collagen is a triple-stranded, insoluble, fibrous protein (see Figure 1-3) that is highly ooss-linked, a feature that makes these fibers quite strong and rather flexible. Besides having a very high tensile strength, collagen is also the most abundant protein found in mammals. Reticulin is a thin fiber found in the spleen and lymph nodes. It is not as highly coiled as collagen. Elastin is also a highly cross-linked protein found associated with organs that require some degree of elasticity (like the lungs, skin, and bloodvesseis).

Another type of structural connective tissue, cartilage, is secreted by a specialized fibrobiast cell called a chondrocyte. There are different types of cartilage, but in general it is found in places where there is a certain amount ofstress placed on the body. For example, cartilage can be found in the nose, on the articulating surfaces of bones (including the intervertebral discs of the vertebrai column), and in the external ear.

Bone is also a structural connective tissue. About one-third of the weight of bone comes from organic materiai such as collagen, while the remaining two-thirds is inorganic material such as calcium phosphate and calcium carbonate. The collagen found in bone matrix is secreted by specialized fibroblast cells called osteoblasts. Collagen lends flexibility to bone, while the inorganic crystals lend rigidity. Within the centrai cavity of bone, we find a spongy marrow where red blood cells and white blood cells are formed. Towards the surface of bone the

ceilular arrangement is more compact. [As a comparison, the main structural component of chitin (found in the exoskeleton of insects) consists of specially modified glucose residues linked to one another to form long polymers. Associated with these polymers is calcium carbonate (CaCO3). This combrnation adds rigidity to the exoskeleton, but offers little in the way of flexibility.l

will

We mentioned that blood cells and mast cells are kinds of connective tissue. We discuss blood cells in a separate lecture. Mast cells can be found in the respiratory tract, as well as in the gastrointestinal tract. Mast cells can release

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Biologyregion.

Nerve 8r Muscle

TYpes Of Tissues

histamines in response to an allergic reaction, an infection, or even an injury. Histamine causes an increase in blood flow to the blood vessels of the affectedOther types of connective tissue involve adipose cells and melanocytes. Adipose cells are simply ceils that store fat whereas melanocytes are cells which store pigments.

Muscle fissuesWe

will be discussing various types of muscie in future lectures. For example, r,vhen we examine skeletal muscle we will find that it is voluntary muscle. Thatis, we can generally control its action. Cardiac muscle and smooth muscle are examples of involuntary muscles.

Nervous fissuesThe nervous systems allow one to adapt rather quickly to external stimuli. For example, consider a simple reflex arc. If someone were to tap on your knee with a

rubber hammer, your lower leg would extend outward. As the hammer impinged upon the patellar tendon in your knee, an eiectrical impuise was generated and traveled via a sensory nerve to your spinal cord. That sensory neuron svnapsed with a motor neuron, which returned the impulse to the muscle that was initialiy stimulated and caused it to contract. We will come back io this exampie and examine it in a bit more detail later. First, let's consider someterrninology.

Dendrites

Neurotransmittersare released from sYnaPtic Uulbs

--i

A Typical NeuronCeil body

1r:=

Figure I-4:nrjor components of a neuron. -s

\en-e cellsand

associated supporting cells make up the newous system. Nerve are also called neurons, and ihey are the basic structural unit that make up --:r: r.ervous system. The major anatomical features of a neuron are the cell body -:.-,-oh'ed in integration of information), the dendrites (involved in receiving and ::a:.smitting information towards the cel1 body), and the axon (involved in :,:.ducting information away from the cell body). When a neuron becomes :\.-rid and receives electrical information in the form of a stimulus, the celi body ::it.esses that information and transmits it down the axon in the form of a nerve rnpuise called an action potential. When that action potential reaches the end*o{ -j-.. a\on (referred to as the synaptic bulb or bouton terminal), it causes the

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Biology

Nerve & Muscle

Types OfTissues

release of a chemical substance called a neurotransmitter (see Figure 1-4). The neurotransmitter diffuses across the synaptic cleft and induces an identical action potential in an adjoining neuron, muscle cell, or gland cell. The junction between two such cells is called a synapse.

(

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Biology

Nerve & Muscle

Membrane Potentials

The generation of electrical signals in the nervous system is concerned with the diffusion of ions from a high to a low concentration (see Figure 1-5). in other words, charged ions diffuse down their concentration gradient. In the extracellular space of vertebrates the concentration of Nao is about 150 mM, while that of Ko is about 5 mM. The concentration of Cle is about 130 mM, while ihat of HCO3e is about 25 mM. Note that the concentrations of the cations (Nae and KCI) and the anions (Cl and HCO3O) balance one another. In other words, we find 155 mM of the cations and 155 mM of the anions. This represents

electroneutrality.Inside Cell Outside Cell

K* =

120 mM

to

140

mM

K+=5mM

Na* = 10 mM to 15 mM

]..,,"",

Na* = 150 mMCl- = 130 mM HCO3- = 25 mM

Cl-=5mMto40mMHCO3- = 12 mM to 25 mMProteins-

l^","",

Figure I -5 I. :.::r ceiiular"

concentrations of the common ions.

Ke (about 120 mM to 140 mM) =,:. a lorv concentration of Nao (about 10 mM to 15 mM). We also find a lower : -:.::nfation of Cle (about 5 mM to 40 mN! and usually a lower concentrationr--r'n the cel1, we find a high concentration of

: :-rCO3 3 (about 12 mM to 25 mM). There are many negativeiy charged proteins ,' :--:ir. the cell. Electroneutrality can also be found on the inside of the cell as

:

-.

the distribution of ions across the cell's membrane, you will find it - r: :.::.-mrnetri.cal. Let's consider a resting nerve (i.e., a nerve that is waiting to :: .=:;-:: an action potential) that has a permeabiiity to potassium which is much .r:-:i:r :han its permeabiiity to sodium. In other words, PKe >>> PNao (where F :=:::s to permeability). Because the concentration of Ke is higher inside the ,: --r.;r: outside the cell, potassium will diffuse down its concentration gradient .: : -.:'" e 'Jee ceil (see Figure 1-6). As the positive Ko cation leaves the cell, there : -::::>:cndrngiy less positive charge remaining inside the cell. In other words, -- = ,: s:ie oi the cell is now more negatively charged with respect to the outside . ---- ::--. This sets up a voltage that is positive on the side of the celi to which ::::::-'':. is trving to diffuse to (i.e., the outside of the celi). As that positive :-,:i- ::gins to build up, it tends to push potassium back into the cell :;::::,:.:. irke charges repel each other).

:

. - .. -ook at

I


Biology

Nerve & Fluscle

Membrane Potentials

Those tn'o forces (chemical and electrical) do not exactly match each other. It lums out that the force of diffusion is a little larger than the electrical force. This results in a little bit of leakage of Ko out of the cell, as well as a little bit of Ieakage of Nao into the cell. The Ke that leaked out of the cell has to be pumped back into the cell, and the Nao that leaked into the cell has to be pumped out of the cell. The pumping action of these two ions is provided for by the Nao/K@ ATPase pump. This pump is responsible for the generation of the asymmetrical concentration gradient of Nao and Ke across the cell's membrane.Cellmembrane

Inside cell

Diffusion

Outside cell

-87mVNa*

Na++ 87

Where P* ))) P"u

mV

and the anions are

impermeable

Figure l-6Cellular gradients where PK >>> PNa.

We can calculate the membrane voltage (the potential difference) across the cell's membrane using the Nernst equation as shown in (L-1). In this equation, V is the voltage in miilivolts (1 mV = 10-3 volts), i refers to inside, o refers to outside, R is the gas constant, T is the temperature in Kelvin, Z is the ion's valance, and F is the Faraday constant. If we let the cell's membrane be permeabie to just Ko, we find that the voitage is '87 mV inside the cell with respect to outside the cell. Remember, this is if potassium is the only permeable ion. It is ihe gradient of potassium alone across the cell's membrane that is able to generate this potential.

V'^ = 2.3 RL ,on [K*]o ZF " [K*]rVio = 60 log [5 mM]" [140 mM]r

(t-1)

(1-2)(1-3)

Vio=-87mV

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BiologyIf we stimulate

Nerve & Muscle

Membrane Potentials

a nerve, it leaves its resting state and enters an active state in r,r'hich the cell's membrane is more permeable to sodium (Nuo) than it is to potassiurn. In other words, PNa >> PK. Since the concentration of sodium is higher outside the ce1l than inside the cel1, sodium will tend to diffuse down its concentration gradient and into the cell.Once again, we can establish a separation of charge. As the Nao ions enter the ce11 ivith their positive charges, there is that much iess positive charge outside the .e11. The outside of the cell becomes more negativeiy charged than it was before \a3 started to diffuse into the cell. Similarly, as the Nao ions diffuse into the -e11, the inside of the celi begir-rs to accumulate more positive charge. The inside :- rhe cell becomes more positively charged than it was before the Nao ions :t.:ied to diffuse in (see Figure 1-7). As the Nao ions diffuse into the cell down --:-:ir concentration gradient, a chemical and an electrical equilibrium is being

=r:a:rished.

Insidecell

Where P*, )) F

Outside cell

" Na+Voltage

:x,qLu'e l. / : . - -r :-:- :1::. 'ihere P\a >> PK

: -:-- -:-,:-:ie:he magnitude of the potential across the ceil's membrane by -- - - : I\-:llL,8ffi=%QuyosinH-zone

Z-line

H-zone

Z-line

Sarcomere

Sarcomere

Figure

I - 16

Sarcomere details

Myosin fiiaments are arranged toward the center of the sarcomere. They give rise to the characteristic dark bands one sees when examining muscle tissue. Actin filaments are attached to the Z-lines. The actin and myosin filaments interact with each other through projections stemming from the myosin fiiaments. Those projections are sometimes referred to as myosin heads. The myosin thick filament does not have any of these head groups in its central region but rather concentrates those head groups towards its terminal regions.The actin thin filament is composed of a protein subunit called G actin ("G for giobular), which is roughly spherical in shape. The actin filaments can grow by the addition of G actin to the ends of the already existing filament. Each actin

fiiament is composed of two rows of G actin monomers wound around each other to form a helix.

Actin and MyosinLet's consider how myosin and actin interact with each other at the molecular level. When a muscle is in its relaxed state, ATP is bound to the myosin head groups. The myosin head is not bound to the actin filament, because ATP reduces myosin's affinity for actin. \,Vhen ATP is bound to the myosin head, the myosin head itself is at about a 45' angle with respect to the actin filament. Since actin is not bound to myosin, the ATP on the myosin heads is hydrolyzed to ADP and Pi (inorganic phosphate). These hydrolysis products remain on the

myosin head. More importantiy, the myosin head now undergoes a conformational change, so that it is situated at a 90' angle with respect to theactin filament. This high energy, stable, myosin'ADP-P1 head complex binds to the actin filament. As we will see, this step is dependent on Ca2e being present.Copyright O by The Berkeley Review

22


Biology

Nerve & Muscle

The Sarcomere

The interaction between the actin filaments and myosin head groups causes the reiease of Pi and then ADP from the myosin heads. This process causes a conformational change iir the myosin heads, so that they shift by about 45' in a direction that is away from the Z-lines. This step, in which the actin filaments move relative to the myosin filaments, is cailed the power stroke, and the product of this step is referred to as the rigor complex or rigor state. See Figure1-17.

ATP binds and causes myosin head

Mvosin

This is the rigor complex

ADP

r

f-ilament

:ioVes

Actin and myosin bind

rxgure l.I/-'a

:

-l::

-:Lrn Cf'Cle.

'-"

:-i.:: --:.. :TL\-osirl heads have swiveled and have pulled the actin filament past : : '- : :-rr, tilament, the myosin heads remain bound to the actin filaments. This:=:..:-:alied rigor state. In order for the myosin heads to dissociate from theheads. Remember, ATP

:-:

'.:,::s:r-.-osin's affinity for actin. Without ATP, muscles remain in a state of .".-:-: -:: : gir-en period of time. This is what happens to muscles in your body .-.-- -.----. :ie (i.e., rigormortis). The myosin head groups can no longer bind

i::*- 1ir1ents, ATP needs to bind to those myosin

:lT :=-;use the metabolic ",i = -=-..i to function.

pathways that generate this energy-rich nucleotide

lfi'u'oponin, Tropomyosin, Et Calcium r" r: : ': .'ra::Llnd actin, we saw that it was composed of two long rows of

*::- , ;::.:rica1 protein subunits, which polymerized together and then wound : -r :;:: :--.Ler rn a helical fashion. If we look closely at the helical structure of r--:-i .:.:ii:e tn-o grooves. Running the length of these grooves is a coiled : *, ::*- :.--=j tropomyosin, composed of two helical polymers wound about --: =: :ee Figure 1-18). When tropomyosin resides in the actin groove, it -; - : :u:ding sites for the myosin heads and prevents those head groups-: -:':.1:o the actin filament.

fhe Berkeley Review


Biology

Nerve & Muscle

The Sarcomere

In the case of striated (skeletal) muscle, this is often referred to as the actin-based regulation of muscie contraction. Cardiac muscle and smooth muscle are both controlled by myosin-based regulation.Troponin Protien Complex

ca2+ Binding Tropom yosinSite

o/tRemoval

RelaxationAddition otCalcium

"t ll f,l Calcium ll \1/

li

ContractionMyosin Head Binding Sites

Figure

l-lB

Sarcomere detail.

Troponin, a multi-subunit binding protein, interacts with tropomyosin, actin, and Ca2o. \A/hen Ca2o is bound to a particular subunit of the troponin complex, it causes tropomyosin to shift its position and expose the myosin head binding sites. The myosin heads then bind to the actin filaments (see Figure '1'-17) and muscle contraction follows. [In Figure L-17, Ca2@ would bind at Step 3.] lf Ca2o is not in the medium, there witl be none to bind to the troponin complex and tropomyosin wili remain in the actin gloves and cover the myosin head bindingsites. This is the relaxed state.

Surrounding each myofibrii is a membranous structure,

a

modified version of the

endoplasmic reticulum called the sarcoplasmic reticulum. Calcium is sequestered within this smooth membranous structure. Also surrounding eachmvofibril is an invaeination of the sarcoleuuna (i.e., the plasma membrane) called the transverse tubule (abbreviated as T-tubules). These T-tubules follow lhe Zlines of each myofibril. Selected anatomical features of the structures associated with the sarcoplasmic reticulum are shown in Figure 1-19. After an action potential crosses the iast synaptic junction on its way to a muscle cell, it passes down each T-tubule and somehow stimulates the release of Ca2e from the sarcoplasmic reticulum.' [At the present time, it is thought that the lumen of the T-tubules and the lumen of the sarcoplasmic reticulum are not

Copyright O by The Berkeley Review

24


Biology

Nerve

8e

Muscle

The Sarcomere

continuous.] As the Ca2s flows from a region of high concentration (thesarcoplasmic reticulum) to a region of low concentration (the cytosol), it binds to

its binding site on the troponin complex. Each sarcomere

contracts simuitaneously because of the way in which the nerve impuise is carried along the sarcolemma and into the T-tubules.

+

Sarcolemma

Myofibrils

ransverse tubule

Figure l-19-

=,.:,'oplasmic reticulum.

:cntraction has taken place anC the nerve impulse has ceased, the Ca2e in --= :-.:osol is pumped back into the sarcoplasmic reticulum by a Ca2o-ATPase --::-: In the sarcoplasmic reticulum Ca2o associates with a specific binding ::,:: :.. \Vhen the next action potential passes down the T-tubuies, the cycle will -:.larn and another muscie contraction will take place.:.:e a number of ways ir-r which the strength of muscle contraction can be The strength of contraction can be varied by (1) the size of the motor unit :efined in a moment), (2) the number of available motor units, and (3) the ': of acth and myosin contained with each cell.

- ,- :-

tor unit is simply a motor neuron and the muscle fibers that it innervates. .. e already examined the muscle fibers. What is a motor neuron? Motor . :-s are nerve cel1s whose ceII bodies are located in the centrai nervous :- e.g., the spinal cord or brain stem), and whose myelinated axons :,e skeletal muscle. Reca11 that myelinated axons allow action potentials to ::-,-::-,itted rapidly to the desired effector organ (in this case, a muscle),, anted to precisely move a muscle (e.g., the muscles associated with your - :ngers), then you wouid need motor units which were rather small in -:r smalier the size of the motor unit, the smaller the strength of . : ; ::- rn, In contraSt the posturai muscles of the back or the legs require large - . : .-::.:ts to be effective. Not only can strength be controlled but precrsion can

. : .--led as wel1.::. :


25


Biology

Nerve 6t Muscle

The Sarcomere

The number of motor units also controls the strength of contraction. For example,

if you wanted to pick up a light object (e.9., u pencil), you would need to employ only a few motor units. However, if you wanted to pick up heavier objects (e.g., gym weights), you would need to utilize more motor units. When you lift weights regularly, the size of each muscle cell increases, because the amount of actin and myosin in each muscle cell has increased. The more actin and myosin ina muscle cell, the greater the

strength o{ contraction that can be generated.

Even though the strength of muscle contraction can vary with the size of the motor units, the number of motor units, and the amount of actin and myosin in each muscie cell, the ultimate determinant of muscie contraction is the concentration of ATP in your muscle cells. The energy for contraction comes from the hydrolysis of ATP to ADP and Pi (inorganic phosphate). See equation(1-7\:

ATP

----a

ADP + P; + Energy

(r-7)

For muscie contraction to continue, ATP must be constantly regenerated. Recail

from basic biology that under aerobic conditions (when 02 is not limited) glucose will be oxidized to CO2 and H2O. Energy can be extracted (in the form of 36 molecules of ATP--if we use the glycerol-phosphate shuttle) during this catabolic process from glycolysis, the Krebs cycle, and oxidative phosphorylation/electron transport. See equation (1-8):Aerobic (Slow)

Glucose

CO2

+ H2O +

36 ATP

(1-8)

During anaerobic conditions (when 02 is limited), glucose is metabolized tolactate (the anion of lactic acid). Only 2 ATP molecules are extracted for the use of energy in this process. See equation (1-9). [This equation is not balanced.]Anaerobic

Glucose

(Fast)

Lactate

+ 2ATP

(1-e)

Even though the efficiency of ATP production is greater for aerobic metabolism than for anaerobic metabolism (36 ATPs compared to 2 ATPs), ATP can be produced much more quickly through anaerobic metabolism than through aerobic metabolism, owing to enzyme regulation in the glycolytic pathway.

During anaerobic metabolism, the concentration of lactic acid begins to increase. This means that the pH of the medium will decrease (i.e., become more acidic). One of the key regulatory enzymes in the glycolytic pathway cannot function weli below a certain pH value. This enzyme has some optimum pH range at whictr it functions and if the pH falls below (or above) that range, the enzyme is inJribitedQlycolysis comes to a halt and the ATP yield becomes rnsufficient to /iatry out ndlmal metabolic processes. In other words, fatigue sets in.


26


BiologyN. ti$,#$,ft

Nerve Ef Musclei,,,.'r,,".,,,,' ,..

Nervous System Components

t,

..,.,.

The nervous system in its simplest form can be found among the cnidarians. All the neurons in these creatures are linked together in a nerve net (much like a rr.eb). stimulation of the nerve net causes muscle contraction. This simple procedure is referred to as a reflex arc. In the case of the cnidarians there is no

associative activity, meaning that the transmission of the action potential is notlinfluenced by other neuronal activity.

-{ssociative neurons are {ound in higher animals (Platyhelminthes and above). Here, different neurons can interact to produce a given response. when a rrrlnber of these associative neurons are grouped together, the nervous system erpands. A grouping of nerve cells is called a ganglion (or a nucleus or nuclei). Groupings of neurons in higher animals also lead to more elaborate sense organs, :jtterentiation into a central region of cells (the central nervous system, ot -Ns; =ld a peripheral region of celis (the peripheral nervous system, or pNS), various :rssociative areas, and a brain.The three basic anatomicai divisions of the vertebrate brain can be seen in Figure 1-?0- Those divisions are: (1) the forebrain (prosencephalon), (2) the midbrain :resorcephaion), and (3) the hindbr ain (rhombencephalon).

Rhombencephalon

rfignr l-2o'MlmcaiCirisions of the vertebrate brain.

erufurlcr icl rmeaning "below") the brain is the spinal cord. Recall that the brain ffiie spinaf cord together are the central neraous system (CNS). From the spinal ''rlrmfi r@!d* rrrmr-es extend into the limbs and extremities (the periphery) of the body. ffime mru1:es represent lhe peripheral neroous system (PNS). If a neuron carries

into the spinal cord and brain, that neuron is said to be an afferent i{lm*.'*yt nenron- U a neuron carries the information away from the brain ormnffimnnnmrnn#.ncne

qryilltumf rmmd-

that neuron is said to be an efferent (motor) neuron.

ilhihr

l[ffie Sunbrain has several anatomical subdivisions, including the cerebrum, md hrpothalamus (see Figure 1-21'.a). The cerebrum is divided into @{ilfril mr,{ lc{t cerebral hemispheres, joined by the corpus callosum. The cerebral lfirnlil'ilr1iql'ruttmsres are dirtded into the frontal lobes (associated with movement and Wmrmnlilittmy ri" parietal lobes (associated with touch and stretch sensation),niltntldmdtll

rfti

ldes

(associated

nirilmtwr,,os

shonrr in Figure

'1,-21b.

with vision), and temporal lobes (associated with

i{0fu@fh,@

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Tbs Berkeley Review

27


Biology(a)

Nerve Er Muscle

Nervous System Components (b)

Thalamus Cerebral

Central Sulcus

CortexCorpus

Callosum

)/s /a

\Parjetal

."o

o .)

Pons

The Cerebrum and its Four Lobes

\MedullaSpinal Cord Cerebe llum

Figure l-2 IAnatomical divisions of the vertebrate brain.

The outermost layer of the cerebrum is called the cerebral cortex. It consists of gray matter, which is simply nerve cel1 bodies and their dendrites. Beneath the gray matter is the white matter, or myelinated axons of the nerve cells. In the spinal cord, the situation is reversed: The gray matter is more centralized, while the white matter is more peripheral.The cerebral cortex has many important landmarks, one being the central sulcus, a prominent groove that separates the frontal lobes and the parietal lobes.

Anterior to this sulcus is the motor cortex, which controls the movement of individual muscles. Posterior to this sulcus is the (somatic) sensory cortex, whichdetects sensations in various parts of the body.

Somatic receptors send their information into the spinal cord via afferent nerve fibers, which either cross over to the opposite side of the spinal cord and then ascend to the sensory cortex in the brain, or ascend in the spinal cord and then cross over in the brainstem before reaching the sensory cortex.

flomunculusWilder Penfield, a Canadian neurosurgeon, was able to rnap the sensory andmotor areas of the brain by electrically stimuiating certain regions of the brains of

his patients during sulgery and observing their actions. Throughout thisprocedure his patients were conscious but were unable to feel any pain because there are no sensory receptors for pain il the cerebrum. He was able to show that the largest number of cortical neurons found in the sensory cortex register sensation in the fingers, hands, lips, and tongue. This is represented as the sensory homunculus (a schematic model of a human being mapped out on the sensory cortex) shown in Figure 1-22. Penfield was also able to show that the largest number of cortical neurons found

in the motor cortex control individual movement of the fingers, hands,

and speech. Groups of muscles are controlled by an area just anterior to the motor cortex called the premotor cortex. This is an association area. Stemming from

2a


Biotogy

Nerve & Muscle

Nervous System Components

this association area are neuronal connections with the thalamus and cerebellum. Together these structures plan specific movements that the motor cortex then executes. It is thought that cognitive functioning like speech, writing, and reading are localized in the left hemisphere of the brain, whiie intuitive functions are localized in the right hemisphere of the brain. This is not proven, only theoretical.

-lSensory receptors on the right side of the body project to the somatosensory cortex on the left hemisphere of the brain and vice versa

-J

Tongue

Hemisphere J,

Left

i

L,

RightHemisphere

l:.:

Figure l-22:.c,munculus.

thalamus is a relay station for much of the visual and auditory information -,\-e receive from our environment. The hypothalamus is concerned with the activities of the body. The pituitary gland is the master endocrine gland -':t:a1 ,,' -:.: body. It receives information from the hypothalamus and sends out ::::rahon to regulate different parts of the body. -*-::

.:.

'' = brainstem contains such anatomical feafures as the midbrain, cerebellum, : ::ls medulla, and the reticular formation. These areas coordinate motor and -.:=:;-- activities. The midbrain detects movement and can direct the head and:iir-drds that movement. The midbrain can also sense pleasure and pain. is responsible for the bulk of regulation and coordination of -: ..-.:-^iar activity. The pons and medulla coordinate visceral activities. The -. * ----:-:: formation, which is the core of the brainstem, is essentially an activating . :::::.;esigned to alert the brain. The reticular formation also inhibits motor i.- - ::.*ior:\- impulses and can induce sleep. Below the medulla is the spinal cord.--- = :e:ebel1um

: :!

br The Berkeley Review


Biology

Nerve & Muscle

Control Of Body Activity

G;dffitfdlrri drdyrfffifiiffiiRecall that when we first mentioned the nervous system, we iooked at a simple reflex arc. If someone were to tap you on your knee with a rubber-headed hammer, your lower leg would extend outward. The mechanism behind this is quite simple. As the hammer impinges on the patellar tendon, sensory neurons located in the tendon of the quadriceps muscle are excited. These impuises travel along an axon that enters the spinal cord through the dorsal root gangiion and synapses with two neurons (Figure 1-23).This area, even though it is supposed to be gray matter, is shown as being clear so you can see the synapses. Biceps ('hamstring") muscie is a flexor

\*u.

Ganglion

I

Dorsal

Root

Grav Matter

Spinal Cord

Motor Nerve TibiaPolysynaptic Reflex ArcInterneuron

Figure l-23The reflex arc.

One of the synapses is to a motor neuron that immediateiy leaves the spinal cord and returns to the quadriceps muscle, causing a contraction. As this muscle contracts, the leg straightens (extends) at the knee joint. Because it elicits this kind of action, the quadriceps muscle is termed an extensor muscle. The type of synapse just described (making just one synaptic connection), is referred to as a

monosynaptic reflex arc.The other synapse is made to an interneuron which, in this case, is inhibitory. This interneuron in turn synapses with a motor neuron that innervates the bicep ("hamstring") muscle in the back of the leg. When the bicep muscle contracts, the lower portion of the leg bends or flexes at the knee joint. We call this kind of muscle a flexor muscle, and this type of synapse (because it makes at least two synaptic connections) is referred to as a polysynaptic reflex arc.

If contraction of the biceps muscle is inhibited

as the quadriceps muscle

contracts, then one observes a smooth and coordinated movement at the knee joint, as the lower portion of the leg extends outward after stimulation by the tapping of the hammer on the patellar tendon.


50

The Berkeley Kevibw Specializing in MCAT Preparation

BiologyNeurovisceral Control

Nerve & Muscle


The autonomic nervous system, which is part of the efferent division of the peripheral nervous system, can be subdivided into the sympathetic and the parasympathetic systems. Nerve fibers from the autonomic nervous system leave the spinal cord to irrrervate various glands, smooth muscle, and cardiac muscle. Let's examine these two subdivisions.

Paras;rrmpathetic DivisionThe parasympathetic division of the autonomic nervous system has nerve fibers which leave from the sacral portion of the spinal cord and from the midbrain (mesencephalon) and medulla (part of the rhombencephalon), as shown in Figure L-24. Parasympathetic nerve impulses tend to increase the rate of digestion and lower the heart rate. The blood pressure is also iowered, and the pupils constrict (contract). In general, the parasympathetic division conserves energy and helps in the restoration of various bodily functions (".9., by aiding in

the digestion of food for later use in metabolic processes).GanglionEve

Neurotransmitter is acetylcholine

Ganglion

1Lacrimal glands

Postganglionicnerve fiber

o

\ur, Itr \ l.a

Heart

lr

o (,)

Small intestines

Lungs and Bronchi

I

Q.i (t

\ \Large intestinesI

(

1

Sexuai organs

Figure l-24Parasympathetic nerves.

The parasympathetic division has both preganglionic and postganglionic nerve fbers. The cell bodies of the preganglionic neurons are found in the sacral regionCopyrightby The Berkeley Review

@

3l


Biology

Nerve & Muscle


of the spinai cord and in the brainstem. The ganglia of the parasympathetic division lie near or in the organs that are to be innervated. Therefore, thepreganglionic nerve fibers are rather long, while the postganglionic nerve fibers are rather short. Both the preganglionic and the postganglionic nerve fibers in the parasympathetic division release acetylcholine (A C h ) as the neurotransmitter are called cholinergic nerve fibers.] The most prominent nerve in the parasympathetic division is the vagus nerve [also called the (tenth) X cranial nerve]. Roughly three-quarters of all the neurons in the parasympathetic division can be found in the vagus nerve. The reason for this should become obvious, if you look at Figure 1-24 The vagus nerve innervates the heart, lungs, stomach, liver (not shown), small intestine, large intestine, and kidneys (not shown), among other organs.

neurotransmitter. INerve fibers that release acetylcholine as their

Sympathetic DivisionThe sympathetic division of the autonomic nervous system has nerve fibers branching off from the thoracic and lumbar regions of the spinal cord, as shown in Figure 1-25. Sympathetic nerve fibers tend to condition the body for a "fightor-flight" response (a term first proposed by the Harvard physiologist Walter Cannon in the early 1900s). The heart rate increases, blood pressure elevates, pupils dilate (open wider), and the digestive functions decrease, all as a result of sympathetic nervous innervation.The sympathetic nerves that leave the thoracic and lumbar regions of the spinal cord first enter chains of ganglia connected by nerve fibers on either side of the spinal column. These chains of ganglia are collectively called the sympathetic trunk. As these spinal nerves, called preganglionic nerve fibers, enter the sympathetic trunk, they can either (a) pass through this collection of ganglia to synapse with other ganglia outside the sympathetic trunk, (b) pass into the sympathetic trunk and ascend or descend to synapse with ganglia at other levels,

or (c) pass into the sympathetic trunk and directlyganglion.

synapse

with a given

The nerve fibers leaving a synapse in a given ganglion are referred to as postganglionic nerve fibers. In the case of the sympathetic division the preganglionic nerve fibers tend to be short, while the postganglionic nerve fibers tend to be longer. The preganglionic fibers release acetylcholine, while the postganglionic fibers release norepinephrine as their neurottansmitters. [Nerve fibers that release norepinephrine (or epinephrine (adrenaline)) are called adrenergic nerve fibers.lOne set of spinal nerves that originates in the lower thoracic region of the spinal

cord send long preganglionic nerve fibers to the adrenal medull4 a region within the adrenal glands (located on the superior surface of the kidneys). These nerve fibers synapse directly on the ganglia in the adrenal medulla. There are no postganglionic nerve fibers. When the adrenal medulla is stimulated, both norepinephrine and epinephrine are released directly into the bloodstream. Because these substances are released into the bloodstream, we can refer to themas hormones. Therefore, the adrenal medulla can be considered as an endocrine

gland. These hormones, distributed throughout the body by the circulatorysystem, can increase the heart rate and cause the pupils to dilate.


32


Biology

Nerve & MusclePostganglionicnerve fiber Eye


Neurotransmitters are acetylcholine andnorepinephrine

Preganglionicnerve flber

t( .& LJ /^

{Vi,

Hearr

Thoracic Lungs and Bronchi

bo

StomachOd)

Pancreas

o,

Small(n

Kidney

intestine

'o kO

Large intestine

a a

Bladder

Figure I -25 . -:.:::irC nefves.

Somatic vs Autonomic Nervous System:

-: , : :evierv the basic differences between the somatic nervous svstem and

the

-: - ,-:r-ic nervous system. In the somatic nervous system, we find that (a) once . :.=:r,'e fibers ieave the central nervous system, they do not make a synapse :-- -:,ev have reached their effector organ. When the synapse is made at the -:--: organ, (b) the neurotransmiiter that is released is acetylcholine. The:--:-:

nervous system (c) innervates skeletal muscle. Innervation of that nuscle (d) Ieads to excitation of the muscle itseif. =:-'autonomic nervous system, we find that (a) once the nerve fibers leave the

:::- :,en-ous system, they synapse with a ganglion before they make the final =:-= rvith their effector organ. The preganglionic fibers in both the ::':-rathetic and sympathetic divisions release (b) acetylcholine as the -:,-:=.smitter. The postganglionic fibers in the parasympathetic division =:'= a:eivicholine; in the sympathetic division, norepinephrine is released. ::nomic nervous system (c) innervates glands, and smooth and cardiac

l:'e

ce1ls innervated

by the autonomic nervous system can (d) be either

or inhibitory

The Berkelev Review


Biology

Nerve & Muscle


The adrenal medulla, a specialized ganglion in the sympathetic division of the autonomic nervous system, is directly stimulated by a preganglionic fiber. This nerve fiber releases acetylcholine, which causes the ceils of the adrenal meduila to release two types of hormones into the biood. The major hormone released is epinephrine. The minor hormone released into the blood is norepinephrine. These differences are illustrated in Figure 1-26.

CNSSomaticNervous System Skeletal muscle

Acetvlcholine Ganglion Postganglionic nerve fiber

Giands, cardiac and smooth muscle

Acetylcholine

lIENorepinephrineOrgan

AutonomicNervous System

.*_|or-g".eAdrenal Medulla Preganglionic nerve fiber

(Epineprhine)

F+

Biood

r-)

)l6'Glands, cardiac and smooth musclela t

>s t4 l5 lo l-

t'ts

Ganglion

t:IrJ ID J5@

o

Figure l-26CNS and PNS review.


34


Biology

Nerve & Muscle

Receptors and Sensory Input

KeGeptof5,:,and,ri .Sn5,6ff

,.,Iffit

There are many different types of receptors receiving sensory information from

the environment and passing that information to the nervous system. For erample, there are mechanoreceptors, which are concerned with pressure,hearlng, balance and blood pressure. Nociceptors sense pain. Mechanoreceptors :espond to a change in their configuration. Some can respond to a light pressure, -,r-hile others respond to a deeper pressure. There are specialized regions on the =:des of fish called lateral line organs that respond to a change in the pressure of :re rvater. There are pressure receptors in the walls of the aorta that are able to ::nse an increase in blood pressure.

Thermoreceptors detect cold and warmth, while chemoreceptors can detect -:--te, smell, oxygen, carbon dioxide, hydrogen ions, and blood giucose ievels. -:-.e taste receptors in the tongue can distinguish between food which is bitter, .:'.r, salty, or sweet. Olfactory cells rn the nasal cavity can distinguish hundreds - ilfferent odors. Photoreceptors in the retina of the eye can respond to the ::.sence of a single photon of light. There are also receptors concerned with :,ectricity and magnetism. For example, catfish have electrical receptors that ..=.r them detect prey, and birds have magnetic receptors that help them to---:' 1gate.- -.= sensory information that a receptor receives is specific for that particular :=:E:tor. The stimulus that is received by that receptor changes (converts or ::ansduces) the potentiai (V*) across the receptor's membrane. This change in =::'.biane potential is called a receptor potential. If this receptor potential were ', ='.:eed a specific threshold, an action potential would be generated. We also :: -,rat if the pressure on this receptor is great and the change in membrane : :=:.iial is large, then the receptor potential will exceed the threshoid level. The :::*-: is an increase in the frequency of the action potentials being generated. . . = ,:nplitude of the action potentials r,vi11 not be change, only the frequency.

Receptors and Transducers.: . :onsider the receptor potential for a special type of mechanoreceptor ca11ed :acinian corpuscle. A pacinian corpuscle has an unmyelinated nerve ending . -::. is encapsulated in layers of connective tissue. However, the afferent nerve-- :, . -=ar-es

this encapsulated ending is myelinated.SaltatoryPressure

_/\:*v++

\----l--\------r'

'','gUre 1.27

',, - .--. -'.rrpuscle.

.ii

0 by The Berkeley Review

cc


Biology\zVhen the nerve

Nerve Et Muscle


ending is depressed, a local deformation causes sodium channels to open and Nao ions to rush into the nerve ending. An action potential is not generated in the receptor region. Instead, the establishment of this receptor potential causes a localized flow of current to be propagated along the nerve fiber. If the threshold has been reached, then this local current flow will jnitiate an action potential at the first node of Ranvier, located at the outer edge of the pacinian corpuscle. The action poiential can spread down the afferent nerve and towards the central nervous system by saltatory conduction, as shown in Figure7-27.

Suppose we were to depress the pacinian corpuscle just a smail amount. If the threshold is not reached, an action potentiai will not be generated. This means one would not feel the pressure that is being applied. If we apply a second stimulus, we might depolarize the membrane even more. If we were to apply a third stimulus that was even stronger, we could exceed threshold and an action potentiai would be generated. If we maintain this pressure such that we are just

above threshold by a certain amount, we will continue to generate action potentials at the rate of, say,2 action potentials every 10 milliseconds.2 action potentials per 10 miliiseconds 3 action potentials per 10 milliseconds

Threshold

olstWeak

[fr

Time (ms)

3rd

tl4rh

Strength

of

stimulus

Strong

Figure l-28Acti0n potential frequencies.

What wouid happen if we apply a fourth stimulus, much stronger than the previous stimuli? The membrane would become more depolarized, and we would move further above the threshoid level. Once this happens, the frequency of action potentials generated would increase, say, to 3 action potentials every 10 milliseconds. This is shown in Figure 1-28. Note that in each case the amplitude of the action potentials remains the same. \Alhat changes is the frequency of the action potentials propagating along the axon" It is the frequency of action potentials that signals to the central nervous system the magnitude of the stimulus being received. All receptors function on this basic principle.


36


BiologyAdaptation

Nerve 6r Muscle

Receptors and Sensory tnPut

wheriyou wake up in the morning and take a cool shower, you {eel the coolness of the water on your skin. When you get dressed, you feel the clothes touching your skin. In each case, though, after a brief period of time you get used to the water or the clothes touching your body. These are examples of sensory adaptation. One type of receptor that undergoes sensory adaptation is the pr"ir.r." receptor. if we plot the frequency of action potentiais (as they travel Lack to the central ,r*tno.tt system aiong a given axon) as a function of the time of a sustained stimulus, we would find that pressule receptors adapt rapidly' F{owever, pain receptors do not adapt raprdly, as is shown-in Figure 1-29' If the pressure ,u."ptott did not adapt to the touch of the clothes that we wear, it rvould plove to be rather inconvenient. [On the other hand, the body does not adapt ai readily to the sensation of pain--a good thing, evolutionarily speaking, silce pain is a warning of potentiai damage to the body's tissue and not something to be responded tb as a matter of convenience, but as a matter of sunival.]

)\= 'r O

Time of sustained stimulus (sec)

Figure l-29-.:.::aiion to pressureand to pain.

Tactile Discrimination

-"-=:J that the end of a neuron can be divided into many branches. Each of these ::::rches in turn can end at a receptor (such as a pacinian corpuscle). These 1.,--ra-l branches and their corresponding receptors constitute a receptive field. there can be many -.=:ending on which area of the body one is describing, :=,::iiit'e f-ieids, some of which overlap, or there can be a few receptive fields, :::-: o j tvhich do not overlap. Let's consider a set of receptive fields that overlap : :,rier io d.etermine how iwo points in space can be distinguished from oneci- :

same touch. =er bv the

:-::l=e \\e have a set of receptive fields like the ones shown in Figure 1-30a. ,--.., ."" generally finds is thit a greater frequency of action potentiais will be:

r.:=:.:ed fass,tming threshold has been -ore-,,.

l0 3

if the central region rather than seem *: = :e:rphery of a glven receptive field is stimulated. In other words, there in itsperiphery. ,, , ," receptors in the central region of a receptive field thanreached)

e

n.e

outd happen if receptive fields overlapped, as shown in Figure 1-30? If ri: -.r-ere to sti.mulate the receptive field represented by (b) in Figure 1-30 ,:r:t:-i.*- enough, then because of the field overlap, we would also stimulate l::-= :::e::ive fields in (a) and (c). It would feel to a subject as if three different .r::. _: e body were being stimulated when, in fact, just the receptive field in from b -. :eing stimulated. HJw can the information being received (b) these is being only field .:5s.-r-. a-ierent neurons be fine-tuned to let us know that

r,,:::

ra'

- :' :::::

3

b1' The BerkeleY

Review

5/

tn

The BerkeleY Keview Specializing in MCAT PreParation

Biology

Nerve Ef Muscle


stimulated, and not field (a) or field (c)? This is accomplished by a process called lateral inhibition, mediated through interneurons within the spinal cord. In Figure 1-30, the axon that leads away from field (b) has lateral connections to interneurons that inhibit the impulses being sent down the axons from fields (a) and (c). In other words, because of lateral inhibition the impulses generated in receptive fields (a) and (c) are suppressed, allowing the impulses from the receptive field in (b) reach the proper spinal tracts that ascend to the brain.To Brain

(a)

+

6 a

(b)

D+Lateral Inhibition (interneuron)

(c)

Figure l-3OReceptive fields.

Somatic Sensory PathwaysIn other words, the sensory input from the right side of the body witlOnce the sensory afferent nerve fibers enter the spinal cord they cross to the side opposite from the side they entered, either in the spinal cord or in the brainstem.be

represented on the somatosensory cortex of the left cerebral hemisphere, and sensory input from the left side of the body will be represented on the somatosensory cortex of the right cerebral hemisphere. [In Figure 1-39 which side of the body are the receptive fields located, the right side or the left side? Will the sensory input go to the somatosensory cortex of the right or left cerebral hemisphere? How do you know?]

Where do these sensory neurons synapse as they ascend to the brain? As a general rule, there are three neurons involved in sensory pathways. In the case of pressure, we find that the first-order neurons carrying information from the receptive field(s) enters the spinal cord and synapse with second-order neurons that ascend on the opposite side of the spinal cord to the thalamus. Here, another synapse is made with third-order neurons that continue to ascend, until they reach a specific region of the somatosensory cortex of the cerebral hemisphere opposite to the side of the body in which the sensation was perceived. The cerebral cortex itself contains ceils that are organized into 6 horizontal layers. The sensations of pressure would be registered in cells arranged in columns that cross these different lavers"Copyright @ by The Berkeley Review

3a


Nerrre & Musc[e To Go15 Passages

98 Questions

Time for All Passages Taken Together as a Practice Exam125 Minutes

I. il. IIII. IV. V. VI. VIII. WII. IX. X. XL XIII. XIIII. XIV. XV.

Passage Titles Types ofTransportAutonomic Nervons SYstem Action Potentials Local Anesthetic The Lens, the lris, & Associated Muscles Resting Membrane Potential Nicotine Replacement RetinalProjection Axonctl Transport Huntington's Disease Photoreceptors Sound Transmission in the Ear T-r,-ptophan & Serotonin Experiment Frog Muscle Experiment Skeletal Muscle Groups

Questions

1-89-1.3

2t-26 a l/./ - JJ

14-20

4r-46

34-40

47 -52

53-58 59-64 65-71 12-19 80-85 86-92 93-98

,TftE

EIEru{EMY

L,g.p-y.1.fl-1ry'

Specializing in MCAT Preparation

SuggestionsThe passages that follow are designed to get you to think irr a conceptual manner about the processes of physiology at the organismal ievel. If you have a solid foundation in physiology, *u1y of these answers will be straightforward. If you have not had a pleasant experience with the topic, some of these answers might appear to come from the void past the Oort field of the solar system. Pick a few passage topics at random. For these initial few passages, do not worry about the time. Just focus on what is expected of you. First, read the passage. Second, look at any diagrams, charts, or graphs. Third, read each question and the accompanying answers carefully. Fourth, answer the questions the best you can. Check the soiutions and see how you did" \A4rether you got the answers right or wrong/ it is important to read the explanations and see if you understand (and agree with) whal is berng explained. Keep a record of your results.

passages and1

approaches to a passage. For example, you might feei well versed enough to read the questions first and then try to answer some of them, without ever having read the passage. Maybe yor.un answer some of the questions by just looking at the diagrams, charts, or graphs that are pr"r"rrtud in a particular passage. Remember, we are not clones of one another. You need to begin to develop a format that works best for you. Keeping a record of your results may be helpful.

group. If you feel comfortable with the outline presented above, fine. If not, then try different

minute and 15 seconds to complete a question. Be a little more creative in how you approach this next

After you feel comfortable with the format of those initial few passages, pick another block of try them. Be aware that time is going to become important. Generally, you will have about

The last block of passages might contain topics that are unfamiiiar to you. Find a place where the level of distraction is at a minimum. Get out your watch and time yontr!ff on these parruges, either individually or as a group. It is important to have a feei for time, and how much is passrng aslou try to answer each question. Never let a question get you flustered. If you carurot figure ont *nui the answer is from information given to you in the passage, or from your own knowledge-Lase, dump it and move on to the next question. As vou do this, make a note of that pesky question ind come back to it at the end, when you have more time. When you are finished, check your answers and make sure you understand the solutions. Be inquisitive. If you do not know the answer to something, look it up. The solution tends to stay with you longer. (For example, what is the Oort field?)

The estimated score conversions for 100 questions are shown below. At best, these are rough approximations and should be used only to give one a feel for which ballpark they are sitting in.

Section I Estimated Score ConversionsScaled Score

Raw Score86

>12

- 100

l07 6 5

11

19 -85

8-9

EB WA. The Fleart

Fteart t uEegs

&

l. 2. 3. 4.5.

CardiorrascnlarAnatomy Cardiac Output Poiseuille's Law DiffusionLJSMOSIS

6. 7.

Lvmphatics Blood ClottingCases Gas Exchange

The Lungs

t. 2.

lF====

-,-''/

,T{te

ffiflffiffifltumYSpecializing in MCAT Preparation

AiK

Biology

Ileart & Lungs

Cardiovascular Anatomy

Within iiving creatures there are two types of circulatory systems. In an open circulatory system the circulating fluid within the body is mixed with the body fluid itself. In other words, the blood in these creatures does not circulate entirely within the confines of vessels like arteries and veins. Most mollusks andarthropods have an open circulatory system. Conversely, in a closed circulatory

system the blood that flows throughout the body is confined to vessels like arteries and veins. It does not freely mix with the fluid of the body. Annelids and mammals like ourselves have a closed circulatory system. The function of the circulatory system is to bring nutrients and oxygen to the tissues of the body while simultaneously removing waste producis from those very tissues. Because the circulatory system is continually flowing it helps to maintain body temperature. Also, the circulatory system can act as a means to transport hormones to various locations within the body. Ultimately, though, as the blood enters into the smallest vessels, the capillaries, there will be diffusion between those capillaries and the cells in the immediate environment.

General Anatomical FeaturesLet's examine the general anatomical features of the mammalian cardiovascular system. The foundation of the circulatory system is the heart. The heart can be thought of as being divided into two halves. These two haives are often referred to as the right heart and the left heart. \44ren we look at a picture of a heart we must remember that we are viewing that heart in its anatomical position. In the anatomical position the body is standing upright, the arms to the sides, hands

with plams up, and the face positioned forward. Thus, when we look at adiagram of a heart on paper, the right side of that paper really represents the left side of the heart and vice versa. The right heart pumps blood to the lungs and back to the left heart. The left heart pumps that blood to the remaining tissues of the body and back to the right heart. The blood that is pumped from the right heart to the lungs and back to the left heart is called the pulmonary circulation while the blood that is pumped from the left heart to the rest of the tissues and back to the right heart is called the systemic circulation. Both the pulmonary and systemic circulations lie in series

with

one another.

We can pick a starting point in the circulatory system and follow a red blood cell as it rnigrates through the pulmonary and systemic circulations. Let's start with a red blood cell in the right heart. The right heart is composed of the right atrium and right ventricle. As deoxygenatedblood passes from the right atrium into the right ventricle it is pumped rnto the pulmonary artery and to the lungs where it is oxygenated. The oxygenated blood returns to the left heart by way of the (left and right) pulmonary veins. The blood enters the left atrium and passes into the left ventricle where it is pumped out the aorta and to the branching arteries, arterioles, and capillaries. It is at the level of the capiilaries that the blood exchanges nutrients and oxygen for waste products created by metabolism. Deoxygenated blood passes from the capillaries to venules and then to larger veins and eventually to the superior and inferior vena cava which enter the rightCopyright@


a5

The BerkeleY Keview Specializing in MCAT Preparation

Biotogy

fleart & Lungs


atrium. The circulation of our red blood cell can now start again. As you are studying this circuit it is important to note that arteries carry iiood away from the heart while veins carry blood from the tissues and lungs towards the heart. This can be seen in the diagram shown in Figure 2-1.Capillaries of Head and Arms

Superior VenaCava

Pulmonary

Artery

J.cbo

O\3 t

q !

o

o o(D

a)

-

+O

l0a

Right

Atrium

lnferior VenaCava

7

tr

-'""*,

Pulmonary

Vein Left Atrium LeftVentricle

Capillaries of Abdomen and Legs

Figure 2- IBlood flow through the cardiovascular system.

Aorta and ArteriesAs shown in Figure 2-L, blood ieaves the left ventricle of the heart and exits through the aorta and passes to both the superior and inferior portions of the body. The total cross-sectional area of the aorta is about s cm2. under resting conditions the velocity of blood flow in the aorta is about 30 cm/sec. The blooJ pressure fluctuates betrveen 120 and 80 mmHg with an average being about 100 mmHg. The volume of blood found in the aorta and arteries ui u.ty given time isabout'l.6oh to 20'/..

As the left ventricle contracts (systole) it propeis blood out the aorta and into the arteries with a pressure of about 120 mmHg. At sea level this force is enough to raise a column of mercury 120 mm above the ground. After contraction takesCopyright O by The Berkeley Review

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Biotogyn the arteries

fleart & Lungsfill r /ith blood, the pressurea


irlace and the ventricles begin to relax (diastole) and is about 80 mmHg.

'-riessure cuff is placed around your upper arm and a stethoscope is piaced over

\Ieasurement of Blood Pressure: When you have your blood pressure taken

-re antecubital artery (the artery at the bend between your upper and lower :.:rnl. The cuff is inflated above arterial systoiic pressure. This causes the artery in , .ur arm to collapse thus stopping the flow of blood. The pressure in the cuff is :e:reased and the pressure needle monitored. As soon as the pressure in the cuff -' relou' the systolic pressure, blood will jet through the small opening in the '::er\'. The flow through the artery is turbulent because of the great pressure :ushing the blood through such a narrow opening. The "tapping" sounds that are ,-.=a:d in the stethoscope approximately correspond to the systolic pressure in the ,-.=:.rt, The cuff is continualiy deflated and the artery slowly regains its original :j-.:le. The blood has an easier time passing through the vessel and the fiow :=i:rr.s to change from turbulent to laminar (smooth). When the turbulent sounds -rr,rfopear it means that you are recording the diastolic pressure of the heart. ---..d pressure readings are given as systolic/diastolic (e.g., 720 mmHg/8O ,:::'i{g). These are the approximate readings that the baroreceptors are sensing -:: --.re\- monitor vour blood pressure.- -.- rlood that moves through the arteries is under a lot of pressure. Therefore, ,: -.e arteries have to be rather durabie. They have thick walls and are composed : smooth muscle and connective tissue that contain both collagenous and "1,::tic fibers. The elasticity of the arteries prevents the blood pressure from - :: -::ing too high when it is ejected out of the heart and it also maintains a high : r.=r- jrl pressure between the systolic and diastolic phases of the heart. This : - ,.': blood to flow to the rest of the circulatory system without a sudden loss of

- = -::nen of all the blood vessels in the body are lined with epithetial cells and -: i>e these ceils are inside the cardiovascular system they are referred to as .rl,rihelial cells. Damage to these endothelial celis b,v the pulsating arterial::-

--::s.,:ie or even by abrasive substances in the blood can lead to the disease --.,':-: as atherosclerosis. Once these ceils are damaged choiesterol can build up :-- = site of the lesion and a plaque will form. During the later stages of the - ,:: is- the arteries become "hardened" from layer upon iayer of deposit. This is -: -:-:= j to as "hardening of the arteries" or arteriosclerosis.

:

--r :::asvmpathetic divisions of the autonomic nervous system.

- :'.^-::ron

of the circulatory system is controlled (in part) by the sympatheticThe

:-::.:.eti.c division is the more important of the two. Besides nervous control of : :- :-,.ir- there is also humoral control from the action of ions or hormones and -.- ::rtrol at the level of the individual tissues from various metabolites.

:re metabolites important? If you were to start exercising a particular ".:--, "-ou would notice that the blood flow to that muscle would increase.

:'

--,rrat

u.hen muscles contract ATP is hydrolyzed io ADP and Pi, lactic acid

.,

::',=i, and otirer such metabolites circulate in the tissue. Whenr

these

-:;es diffuse from the tissue out to the arterioles the smooth muscle dilates -- -: :::ases the flow of blood to that particular area. The blood brings not only :-::.:s and oxygen to the working tissue, but it carries away the waste . r-::i of metaboiism as well.

.;:

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Biology

fleart & LungsArterioles


branch to form arterioles. The arterioles are important because they represent the major area of resistance in the cardiovascular system. Arterioles have strong muscular wails and are regulated as mentioned above.

After the aorta branches to form a variety of arteries, the arteries themselves

CapillariesDiffusion takes place at the level of the capillaries. The total cross sectional area of the capiliaries is about 3000 cm2. The velocity of blood flow has been red.uced to about 2 cm/sec. The pressure is roughly 25 mmHg and the total volume of blood is about 5o/o to7%. The wall of the capillaries are"composed of a unicellular iayer of endothelial cells. surounding tirese cells is a basement membrane. Flowever, there is no connective tissue or smooth muscle. The capillary itself is just large enough for a red blood cell to squeeze through. At the entrance to the capillary bed is a precapillary sphinct"r .o*pos*d Lf smooth muscle which helps to regulates the flow of blood to the area.

Veinsonce the blood flows through the capillaries and reaches the veins the blood pressure has been reduced to a value between 0 mmHg and 25 mmHg. The velocity of blood flow through the veins is about 20 cm/Jec and the total cross sectional area is roughly T.cm2. Roughly s0'/. ofthe total blood volume is present in the veins at any given time.

Figure 2-2A valve ina vein showing directionality.

since there is not much pressure in the veins the amount of smooth muscle and elastic tissue surrounding the veins is reduced. However, they are under control of the sympathetic nerves. Note that the rate of blood flow inihe veins is similar to that in the arteries. This is due to specialized valves that allow the blood to flow in only one direction (see Figure 2-2). As the muscles that surround the veins in your body contract they squeeze the blood back towards the heart. Since these valves are "one-way valves" blood is prevented from flowing backward. trf the valves become damaged and blood is aliowed to flow backwarls, pressure in the veins can increase. This has a tendency to cause varicose veins, which are protrusions of the dilated veins beneath the skin.

Blood FIow in the tleartbegins to relax blood from the right atrium is pumped into the rig:ht ventricle. The right ventricle contracts and forces blood out the pulmonary irtery and to the lungs where it is oxygenated. Blood returning to the heart enters the left atrium via the right and left pulmonary veins. As the left ventricle begins to relax Copyight O by The Berkeley ReviewWe have mentioned that deoxygenated blood returns from the tissues and enters the right atrium via the superior and inferior vena cava. As the right ventricle

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Biology

Ileart & Lungs


blood from the left atrium is pumped into the left ventricle. Contraction forces the blood in the left ventricle out the aorta and to the tissues of the body. See Figure 2-3.

Pulmonary

Artery

Pulmonary Artery

Pulmonary Veins

SuperiorVena Cava

SA Aortic Valves(semilunar) L. Atrioventricular Valves (mitral)

Pulmonary Valves

(semilunar)

AV

Kight AtriumR. Atrioventricular

\ alves (tricuspid) Inferior

VentricleBundle of flis gives rise to L. E( R. Bundle Branches (frrrther divides into Purkinje Fibers)

Left

Vena Cava

Chordae tendineae

Kight Ventricle

::1. Descending

.:->

Aortari.lre 2.5 - -.. -:ndmarks of the heart.

PapillaryMuscle

: r l-- i atriorrentricular

::.e heart there are valves between the right atrium and right ventricle valve), between the right ventricle and the pulmonary .:.. rulmonary valve, or tricuspid), between the left atrium and the left -:. :,= 'ie left atrioventricular valve, or mitral), and between the left ventricle -- .: r ird (the aortic valve). Once the ventricles are filied with blood and they - = :ontract the valves between the atria and the ventricles close. This :, : i: :r.\- backflow of blood into the atria and ensures that the blood will be : :-- rhe pulmonary and systemic systems of the body. The closing of the :.ricular valves between the atria and the ventricles as the ventricies are .- : ::1\'es the characteristic "lub" sound when listening to the heart. As ,: :-:rrs out of the pulmonary artery and the aorta, the pulmonary valve - :lrlic valve close, giving the characteristic "dub" sound. The closing of -. as blood is pumped either from the atria into the ventricles or from :-=s lo the puimonary or systemic tissues prevents backflow into either

-

-:

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Biology

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the atria or the ventricles, respectivery. The valves between the atria and the ventricles themselves do not invert because of tendinous cords, called chordae tendineae, which hoid them in place.Located near the junction of the superior vena cava and the right atrium is a specialized region of myocardium called the sinoatrial node (sA node) or the pacemaker of the heart (Figure 2-4).

Electrical Activity of the lleart

SA

AVNode

Bundle of His

Figure 2.4The SA and AV nodes of rhe heart.

The SA node is the point of origin for the electrical impulse that propagates through the rest of the heart. Th[ electrical impulse spreads out over the atria causing them to contract and filt the ventricles. Located in the lower portion of the right atrium and near the right ventricle is the atrioventricular node (AV node). Impulses from the sA node also spread to the AV node and then from the AV node through a coliection of fibers called the bundle o{ His. Branches of the bundle of His surround the ventricles, and when this bundle receives an impuises it causes the ventricles to contract and eject blood to the pulmonary and systemic systems. Anatomicaily speaking, ventricular contraction is from the apex of the heart towards the base of the heart. It is interesting to note that if the sA node is damaged, the AV node takes over and slows the heart down to about 40 beats per minute.

Cardiovascul.rr ControlThe average blood pressure leaving the aorta is about 100 mmHg. This blood pressure is monitored by baroreceptors and chemoreceptors located in the carotid arteries and in the aortic arch. The baroreceptors (pressure receptors) are continually monitoring how much the aortic and carotid arteries are eipandingCopyright@


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Biology

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and contracting. Suppose we were to decrease the arterial pressure. There would be less stretch on the aortic and carotid arteries. The baroieceptors would sense this and send impulses to the medulla in the brain stem. The medulla responds by activating the sympathetic nerves of the autonomic nervous system. Impulses are sent via the sympathetic nerves and norepinephrine is released at the sA node to increase the rate of the heart. This action helps to increase the contraction of the heart, which in turn wiil elevate the blood pressrrre. Sympathetic nerve fibers will also stimulate the adrenal medulla to releise epinephrine (adrenaline) into the blood. This hormone acts to increase the rate of the heart, therefore increasing the contraction of the heart. Both of these actions act in a negative feedback manner to "negate" the initial loss of pressure due toi say, hemorrhaging.The sympathetic nerves will also cause constriction of the blood vessels that lead to the gastrointestinal system and the kidneys. During hemorrhaging the brain and heart receive first priority in terms of blood. The rest of the organ systems of the body see a decrease in the flow of blood until the probiem is corrected.

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Biology

Ileart & Lungs tffiffiffi

Cardiac Output:iiiiiijr;iilliijiiiti::

Every time your heart beats, a certain volume of blood is pushed out into the circulatory system. The cardiac output is that amount of blood which is pumped per minute by each of the two individual ventricles of the heart. We can define the cardiac output (for either ventricle in liters/minute) as being equal to the heart rate (in beats/minute) times the stroke volume (in liters/beat). This is shown in equation (2-1). The stroke volume is simply the amount of blood ejected by each ventricle during one beat of the heart.

If the average heart rate is T2beats per minute and the average stroke volume is 70 milliliters (or 0.07liters) per beat, then the cardiac ouput would be about 5 liters per minute. This value is for the average resting adult male. As we will seelater, the volume of the cardiac output can be influenced by the diameter of the blood vessels in the periphery, the amount of blood returned to the heart by the superior and inferior vena cava (i.e., return of the venous blood), and the heart rate and force of ventricular contraction.

Cardiac Output = (Stroke Volume) (Heart Rate)

(2-L)

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BiologyPoiseuille's Law

Ileart & Lungs

Poiseuille's Law

Not all of the arteries and veins in the circulatory system have the same diameter. flow of biood in blood vessels, established a relationship between the radius and length of a tube, the change in pressure between the two ends of the tube, the viscosity, and the flow rate of a fluid in that tube. This relationship, known as Poiseuille's law, is given in equation (2-2).Jean Poiseuille, a French physician studying the

Flow = AP ftRr = (P, - P, 'SnL ) 7IR*

SnL

(2-2)

(2-2), AP is the pressure drop between the two ends of the tube (i.e., P1 - Pz), R is the radius of the tube, eta (r1) is the coefficient of viscosity, L is the length of the tube, and n/8 is a proportionality constant adjusting for the cross-sectional area of the tube. What can we say about this equation?

in this equation

(a)

Notice that the fiow rate is proportional to R4. This tells us that the rate of blood flow is extremely dependent on the radius of the vessel. If the radius of the vessel were reduced by a factor of 2, then the rate of blood fiow would be reduced by a factor of 16. similarly, if the radius of the vessel were increased by a factor of '1,.5, then the flow rate would increase by a factor of5.1.

(b)

The

flow rate is also inversely proportional to the length of the vessel. In other words, the longer the vessel, the slower the rate of flow. The shorter the vessel, the faster the rate of flow.

(c)

The flow rate is also inversely proportional to the viscosity of the solution. This tells us that a high viscosity gives a 1ow flow rate.The value of AP is provided for by the strength of the heart's contraction. In

(d)

other words, the difference in the pressure is what drives ihe blood in the cardiovascular system.

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Biology

Ileart

6e

Lungs

Diffusion

molecules tend to move faster than iarger molecuies. Similarly, a higher temperature provides more energy to a system and therefore imparts more thermal motion to the various moiecules in that system. you might think that because smaller molecules move relatively fast they would have no problem traversing their environment. However, don't forget that there are millions and millions of molecules within a given system and that each of those molecules, even thought they are moving at a given velocity, collide with their neighbors. These collisions tend to alter the path o{ the molecules, thus confining them to a random walk through their medium.Consider an imaginary sphere of water with a given radius. suppose we place some dye outside this sphere of water and ask how long it will take to reach the center. The answer clearly depends on the radius of that sphere. If the radius were 7 microns, it would take the dye about 5.4 seconds to reach the center. Flowever, if the radius were L centimeter, it would take about 11,000 seconds or a little more than 3 hours to reach the center. [As a reference a red blood cell is about 7 microns in length. There are about 1,400 red blood cells end to end in 1 centimeter.l what this is telling us is that simple diffusion is a rather poor way for a molecule to trek across long distances.The Law that governs diffusion is given in equation (2-3) where J is the net flux or net rate of diffusion (in moles per unit time, usually in seconds; that is, mol/sec), D is the proportionality constant called the diffusion coefficient, A is the area of the plane of interest (in cm2), and AC/Ax is the concentration gradient across that plane (in mol/cm4, because concentration is in units of mol/cri3 and distance is in units of cm). Since the net flux always proceeds down a concentration gradient, from a high concentration to a low concentration, we need to add a minus sign in front of this equation. [The minus sign indicates the direction of the flux or diffusion.] This equation is sometimes referred to as Fick's law (after the German physiologist who postulated it in the 19th century). In equation (2-3), what are the units of the diffusion coefficient, D?

Diffusion is simply the process by which molecules randomly move from one place to the next. The molecular weight of a moiecule and the temperature of the medium have a lot to do with the velocity at which a molecule moves. Smaller

r=-(DX^)f

(DXAXCour - Cin) Ax

(2-3)

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BiologyOsmosis

fleart

8r Lungs

Osmosis

In order to understand hydrostatic pressure we must first review osmosis. Recall that osmosis is simply the net movement of water from a region of high concentration to a region of 1ow concentration. General chemistry tells us that the concentration of pure water is 55.5 moles/liter. If we had a beaker of pure water and u,e added a solute to that beaker (say glucose), then we would decrease the concentration of the water in the beaker. Remember, when we add a solute nolecule (or molecules) we are occupying a volume of space that was once occupied by a water molecule. The more solute molecules we add to our beaker ,.f pure water, the more water molecules we will displace and the lower r.t'ill be re concentration of pure water. Jlucose, when added to a solution of pure water, does not ionize. It stays as :1ucose. Flowever, if we add a molecule of sodium chioride to a solution of pure .,-ater, it u'ill ionize into a Nae ion and a Cle ion. Because we now have added a .-.,lium chloride molecule which has dissociated into two ions in solution, we :.ar-e displaced (iowered the concentration of) the water molecules twice as much :-s rre would if we had added a glucose mo1ecu1e. \\hat this is telling us is that --:.e concentration of water in a given solution depends on the number of solute :a:ticles (e.g., glucose, Nae, or Cle) in that solution. We can define the total .-ir-rte concentration in our solution as the osmolarity, where one osmol is :-::prlv one mole of a molecule that does not ionize. If we had a 1M solution of :-r.ose, it would have a concentration of 1 osmol per liter. If we have a 1M .: -'.ihon of sodium chloride, we r,r,ould find that it wouid have a concentration of I osmols per liter (one from the Nae ion and one from the Cle ion). If we have a 1i0mM concentration of NaCl, then after ionization we would have 150 mM of :: rons and 150 mM of Cle ions or a total of 300 milliosmols (mosmols) per ,-:=:. Therefore, the osmolarity refers to the concentration of solute particles that : :.ave in our solution.

Io

Ah

Hzo- >. ooPermeable Membrane

L

to toaa

a Oaa

Semipermeable Membrane

o

lj

'a )ao .o

F,gure 2-5-

.: ::.ovement across a permeablerve have a U-tube

and semipermeable membrane.

:

I fure 2-5a. Water is free to pass back and forth across this membrane and| : - irS of this the height of the water in each of the columns of the U-tube will r : --.: Sdrrl. However, suppose we now replace the permeable membrane with a ""

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