Chapter 48 Nervous System. 1. Nervous systems perform the three overlapping functions of sensory input, integration, and motor output

Chapter 48

Nervous System

1. Nervous systems perform the three overlapping functions of sensory input,

integration, and motor output

• Neuron Structure and Synapses.– The neuron is the structural and functional unit of the

nervous system.

• Nerve impulses are conducted along a neuron.– Dentrite cell body axon hillock axon– Some axons are insulated by a myelin sheath.

Networks of neurons either intricate connections form nervous systems

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• Axon endings are called synaptic terminals.

– They contain neurotransmitters which conduct a signal across a synapse.• A synapse is the junction between a presynaptic

and postsynaptic neuron.

• Neurons differ in terms of both function and shape.

Fig. 48.4

• Types of Nerve Circuits.– Single presynaptic neuron several postsynaptic

neurons.– Several presynaptic neurons single postsynaptic

neuron.– Circular paths.


Supporting Cells (Glia)

• Glia are supporting cells– That are essential for the structural integrity of

the nervous system and for the normal functioning of neurons

• In the CNS, astrocytes– Provide structural support for neurons and

regulate the extracellular concentrations of ions and neurotransmitters

Figure 48.7 50 µ

m

• Oligodendrocytes (in the CNS) and Schwann cells (in the PNS)– Are glia that form the myelin sheaths around

the axons of many vertebrate neurons

Myelin sheathNodes of Ranvier

Schwanncell Schwann

cellNucleus of Schwann cell

Axon

Layers of myelin

Node of Ranvier

0.1 µm

Axon

Figure 48.8

• A Simple Nerve Circuit – the Reflex Arc.– A reflex is

an autonomic response.

ANIMATION

http://www.sumanasinc.com/webcontent/animations/content/reflexarcs.html

• A ganglion is a cluster of nerve cell bodies within the PNS.

• A nucleus is a cluster of nerve cell bodies within the CNS.


• The membrane potential of a cell can be measured

Figure 48.9

APPLICATIONElectrophysiologists use intracellular recording to measure the

membrane potential of neurons and other cells.

TECHNIQUE A microelectrode is made from a glass capillary tube filled with an electrically conductive salt solution. One end of the tube tapers to an extremely fine tip (diameter < 1 µm). While looking through a microscope, the experimenter uses a micropositioner to insert the tip of the microelectrode into a cell. A voltage recorder (usually an oscilloscope or a computer-based system) measures the voltage between the microelectrode tip inside the cell and a reference electrode placed in the solution outside the cell.

Microelectrode

Referenceelectrode

Voltage recorder

–70 mV

• How a Cell Maintains a Membrane Potential.– Cations.

• Na+ is the principal extracellular cation.

• K+ the principal intracellular cation.

– Anions.• Cl– is principal

extracellular anion.• Proteins, amino acids,

sulfate, and phosphate are the principal intracellular anions.

-70 mV

The Resting Potential

• The resting potential– Is the membrane potential of a neuron that is

not transmitting signals

• In all neurons, the resting potential– Depends on the ionic gradients that exist across

the plasma membrane

CYTOSOL EXTRACELLULARFLUID

[Na+]15 mM

[K+]150 mM

[Cl–]10 mM

[A–]100 mM

[Na+]150 mM

[K+]5 mM

[Cl–]120 mM

–

–

–

–

–

+

+

+

+

+

Plasmamembrane

Figure 48.10

• The concentration of Na+ is higher in the extracellular fluid than in the cytosol– While the opposite is true for K+

• A neuron that is not transmitting signals– Contains many open K+ channels and fewer open

Na+ channels in its plasma membrane

• The diffusion of K+ and Na+ through these channels– Leads to a separation of charges across the

membrane, producing the resting potential

• Ungated ion channels allow ions to diffuse across the plasma membrane.– These channels are always open.

• This diffusion does not achieve an equilibrium since sodium-potassium pump transports these ions against their concentration gradients.


Fig. 48.7

• Excitable cells have the ability to generate large changes in their membrane potentials.– Gated ion channels open or close in response to

stimuli.• The subsequent diffusion of ions leads to a change in the

membrane potential.

Changes in the membrane potential of a neuron give rise to nerve impulses


• Hyperpolarization.– Gated K+ channels open

K+ diffuses out of the cell the membrane potential becomes more negative.


Fig. 48.8a

• Depolarization.– Gated Na+ channels open

Na+ diffuses into the cell the membrane potential becomes less negative.


Fig. 48.8b

• The Action Potential: All or Nothing Depolarization.– If graded potentials sum

to -55mV a threshold potential is achieved.• This triggers an action

potential.– Axons only.


Fig. 48.8c

• In the resting state closed voltage-gated K+ channels open slowly in response to depolarization.

• Voltage-gated Na+ channels have two gates.– Closed activation gates open rapidly in response to

depolarization.– Open inactivation gates close slowly in response to

depolarization.


• Step 1: Resting State.


Fig. 48.9

• Step 2: Threshold.


Fig. 48.9

• Step 3: Depolarization phase of the action potential.


Fig. 48.9

• Step 4: Repolarizing phase of the action potential.


Fig. 48.9

Animation

http://highered.mcgraw-hill.com/olc/dl/120107/anim0013.swf

Animation 2


• Schwann cells are found within the PNS.– Form a myelin sheath by insulating axons.


Fig. 48.5

• Saltatory conduction.– In myelinated neurons only unmyelinated regions of

the axon depolarize.• Thus, the impulse moves faster than in unmyelinated

neurons.


Fig. 48.11

• Electrical Synapses.– Action potentials travels directly from the presynaptic

to the postsynaptic cells via gap junctions.

Chemical or electrical communication between cells occurs at synapses


• Chemical Synapses.– More common than electrical synapses.– Postsynaptic chemically-gated channels exist for ions

such as Na+, K+, and Cl-.• Depending on which gates open the postsynaptic neuron

can depolarize or hyperpolarize.


• In a chemical synapse, a presynaptic neuron – Releases chemical neurotransmitters, which are

stored in the synaptic terminal

Figure 48.16

Postsynapticneuron

Synapticterminalof presynapticneurons

5 µ

m

Animation

Animation 2



• Excitatory postsynaptic potentials (EPSP) depolarize the postsynaptic neuron.– The binding of neurotransmitter to postsynaptic

receptors open gated channels that allow Na+ to diffuse into and K+ to diffuse out of the cell.

5. Neural integration occurs at the cellular level


• Inhibitory postsynaptic potential (IPSP) hyperpolarize the postsynaptic neuron.– The binding of neurotransmitter to postsynaptic

receptors open gated channels that allow K+ to diffuse out of the cell and/or Cl- to diffuse into the cell.


• Summation: graded potentials (EPSPs and IPSPs) are summed to either depolarize or hyperpolarize a postsynaptic neuron.

Fig. 48.14

Inhibits pain

Organization of Nervous Systems• The simplest animals with nervous systems,

the cnidarians– Have neurons arranged in nerve nets

Figure 48.2a

Nerve net

(a) Hydra (cnidarian)

• Sea stars have a nerve net in each arm– Connected by radial nerves to a central nerve

ring

Figure 48.2b

Nervering

Radialnerve

(b) Sea star (echinoderm)

• In relatively simple cephalized animals, such as flatworms– A central nervous system (CNS) is evident

Figure 48.2c

Eyespot

Brain

Nerve cord

Transversenerve

(c) Planarian (flatworm)

• Annelids and arthropods– Have segmentally arranged clusters of neurons called

ganglia

• These ganglia connect to the CNS– And make up a peripheral nervous system (PNS)

Brain

Ventral nervecord

Segmentalganglion

Brain

Ventralnerve cord

Segmentalganglia

Figure 48.2d, e (d) Leech (annelid) (e) Insect (arthropod)

Anteriornerve ring

Longitudinalnerve cords

Ganglia

Brain

Ganglia

Figure 48.2f, g (f) Chiton (mollusc) (g) Squid (mollusc)

• Nervous systems in molluscs– Correlate with the animals’ lifestyles

• Sessile molluscs have simple systems– While more complex molluscs have more

sophisticated systems

• In vertebrates– The central nervous system consists of a brain

and dorsal spinal cord– The PNS connects to the CNS

Figure 48.2h

Brain

Spinalcord(dorsalnervecord)

Sensoryganglion

(h) Salamander (chordate)

Evolutionary Trends• Nervous systems become centralized - formation

of longitudinal cords• Conduction along pathway becomes one way -

afferent and efferent fibers• pathways within CNS become more complex

(interneurons) - more flexible behavior• More segregation and specialization• Formation of a Brain - Cephalization• More and complex organs

1. Vertebrate nervous systems have central and peripheral components


• Central nervous system (CNS).– Brain and spinal cord.

• Both contain fluid-filled spaces which contain cerebrospinal fluid (CSF).

– The central canal of the spinal cord is continuous with the ventricles of the brain.

– White matter is composed of bundles of myelinated axons

– Gray matter consists of unmyelinated axons, nuclei, and dendrites.

• Peripheral nervous system.– Everything outside the CNS.

• The brain provides the integrative power– That underlies the complex behavior of vertebrates

• The spinal cord integrates simple responses to certain kinds of stimuli– And conveys information to and from the brain

• The central canal of the spinal cord and the four ventricles of the brain– Are hollow, since they are derived from the

dorsal embryonic nerve cord

Gray matter

Whitematter

Ventricles

Figure 48.20

• Structural composition of the PNS.

– Paired cranial nerves that originate in the brain and innervate the head and upper body.

– Paired spinal nerves that originate in the spinal cord and innervate the entire body.

– Ganglia associated with the cranial and spinal nerves.

The divisions of the peripheral nervous system interact in maintaining

homeostasis


• The cranial nerves originate in the brain– And terminate mostly in organs of the head and

upper body

• The spinal nerves originate in the spinal cord– And extend to parts of the body below the head

• The PNS can be divided into two functional components– The somatic nervous system and the autonomic

nervous systemPeripheral

nervous system

Somaticnervoussystem

Autonomicnervoussystem

Sympatheticdivision

Parasympatheticdivision

Entericdivision

Figure 48.21

• A closer look at the (often antagonistic) divisions of the autonomic nervous system (ANS).

Fig. 48.18

heart experiment.mov

Embryonic development of the vertebrate brain reflects its evolution from three

anterior bulges of the neural tube


Fig. 48.19

• As a human brain develops further– The most profound change occurs in the

forebrain, which gives rise to the cerebrum

Figure 48.23c

Brain structures present in adult

Cerebrum (cerebral hemispheres; includes cerebralcortex, white matter, basal nuclei)

Diencephalon (thalamus, hypothalamus, epithalamus)

Midbrain (part of brainstem)

Pons (part of brainstem), cerebellum

Medulla oblongata (part of brainstem)

(c) Adult

Cerebral hemisphere

Diencephalon:

Hypothalamus

Thalamus

Pineal gland(part of epithalamus)

Brainstem:

Midbrain

Pons

Medullaoblongata

Cerebellum

Central canal

Spinal cord

Pituitarygland

• The Brainstem.– The “lower brain.”

– Consists of the medulla oblongata, pons, and midbrain.

– Derived from the embryonic hindbrain and midbrain.

– Functions in homeostasis, coordination of movement, conduction of impulses to higher brain centers.

4. Evolutionary older structures of the vertebrate brain regulate essential

autonomic and integrative functions


• The Medulla and Pons.– Medulla oblongata.

• Contains nuclei that control visceral (autonomic homeostatic) functions.– Breathing.

– Heart and blood vessel activity.

– Swallowing.

– Vomiting.

– Digestion.

• Relays information to and from higher brain centers.

• Sleep


• Pons.

– Contains nuclei involved in the regulation of visceral activities such as breathing.

– Relays information to and from higher brain centers.


• The Midbrain.

– Contains nuclei involved in the integration of sensory information.• Superior colliculi are involved in the regulation of

visual reflexes.• Inferior colliculi are involved in the regulation of

auditory reflexes.

– Relays information to and from higher brain centers.


• The Reticular System, Arousal, and Sleep.

– The reticular activating system (RAS) of the reticular formation.• Regulates sleep

and arousal.• Acts as a

sensory filter.


Fig. 48.21

– Sleep and wakefulness produces patterns of electrical activity in the brain that can be recorded as an electroencephalogram (EEG).• Most dreaming

occurs during REM (rapid eye movement) sleep.


Fig. 48.22b-d

The Cerebellum• The cerebellum

– Is important for coordination and error checking during motor, perceptual, and cognitive functions

– The Cerebellum.• Develops from part of the metencephalon.• Functions to error-check and coordinate motor

activities, and perceptual and cognitive factors.• Relays sensory information about joints, muscles,

sight, and sound to the cerebrum.• Coordinates motor commands issued by the

cerebrum.


The Diencephalon• The embryonic diencephalon develops into

three adult brain regions– The epithalamus, thalamus, and hypothalamus

– Epithalamus.• Includes a choroid plexus and the pineal gland.


– Thalamus.• Relays all sensory information to the cerebrum.

– Contains one nucleus for each type of sensory information.

• Relays motor information from the cerebrum.• Receives input from the cerebrum.• Receives input from brain centers involved in the

regulation of emotion and arousal.


– Hypothalamus.• Regulates autonomic activity.

– Contains nuclei involved in thermoregulation, hunger, thirst, sexual and mating behavior, etc.

– Regulates the pituitary gland.


– The Hypothalamus and Circadian Rhythms.• The biological clock is the internal timekeeper.

– The clock’s rhythm usually does not exactly match environmental events.

– Experiments in which humans have been deprived of external cues have shown that biological clock has a period of about 25 hours.

• In mammals, the hypothalamic suprachiasmatic nuclei (SCN) function as a biological clock.

– Produce proteins in response to light/dark cycles.

• This, and other biological clocks, may be responsive to hormonal release, hunger, and various external stimuli.


• Biological clocks usually require external cues– To remain synchronized with environmental cycles

Figure 48.25

In the northern flying squirrel (Glaucomys sabrinus), activity normally begins with the onset of darkness and endsat dawn, which suggests that light is an important external cue for the squirrel. To test this idea, researchers monitored the activity of captivesquirrels for 23 days under two sets of conditions: (a) a regular cycle of 12 hours of light and 12 hours of darkness and (b) constant darkness.The squirrels were given free access to an exercise wheel and a rest cage. A recorder automatically noted when the wheel was rotating andwhen it was still.

EXPERIMENT

Light Dark Light

20

15

10

5

1

(a) 12 hr light-12 hr dark cycle (b) Constant darkness

12 16 20 24 4 8 12 12 16 20 24 4 8 12

Time of day (hr) Time of day (hr)

When the squirrelswere exposed to a regular light/darkcycle, their wheel-turning activity (indicated by the dark bars) occurredat roughly the same time every day.However, when they were kept inconstant darkness, their activity phasebegan about 21 minutes later each day.

RESULTS

The northern flying squirrel’s internal clock can run in constant darkness, but it does so onits own cycle, which lasts about 24 hours and 21 minutes. External (light) cues keep the clock running on a 24-hour cycle.

CONCLUSION

Dark

Day

s of

exp

erim

ent

• The cerebrum is derived from the embryonic telencephalon.

The cerebrum is the most highly evolved structure of the mammalian

brain


Fig. 48.24a

• The cerebrum is divided into left and right cerebrum hemispheres.– The corpus callosum is the major connection

between the two hemispheres.– The left hemisphere is primarily responsible for the

right side of the body.– The right hemisphere is primarily responsible for the

left side of the body.• Cerebral cortex: outer covering of gray matter.

– Neocortex: region unique to mammals.• The more convoluted the surface of the neocortex the more

surface area the more neurons.

• Basal nuclei: internal clusters of nuclei.


• The cerebrum is divided into frontal, temporal, occipital, and parietal lobes.

6. Regions of the cerebrum are specialized for different functions


Fig. 48.24b

• Frontal lobe.– Contains the primary motor cortex.

• Parietal lobe.– Contains the primary somatosensory cortex.


Information Processing in the Cerebral Cortex

• Specific types of sensory input– Enter the primary sensory areas

• Adjacent association areas– Process particular features in the sensory input and

integrate information from different sensory areas

Controls visceral functions: breathing, heart, blood vessel, swallowing, vomiting, digestion

Controls visceral functions: breathing, heart, blood vessel, swallowing, vomiting, digestion

Coordination of movement and balance

Coordination of movement and balance

Major input center for sensory info going to cerebrum also main output center for motor info leaving cerebrum

Major input center for sensory info going to cerebrum also main output center for motor info leaving cerebrum

Homeostasis regulation: thermostat, hunger, thirst, sex, fight or flight

Superchiasmatic nuclei acts as a biological clock

Homeostasis regulation: thermostat, hunger, thirst, sex, fight or flight

Superchiasmatic nuclei acts as a biological clock

Connects right and left hemispheres

Connects right and left hemispheres

Link to Probe da Brain

http://www.pbs.org/wgbh/aso/tryit/brain/

• Integrative Function of the Association Areas.– Much of the cerebrum is given over to

association areas.• Areas where sensory information is integrated and

assessed and motor responses are planned.


• The brain exhibits plasticity of function.– For example, infants with intractable epilepsy

may have an entire cerebral hemisphere removed.• The remaining hemisphere can provide the function

normally provided by both hemispheres.

• Lateralization of Brain Function.– The left hemisphere.

• Specializes in language, math, logic operations, and the processing of serial sequences of information, and visual and auditory details.

• Specializes in detailed activities required for motor control.

– The right hemisphere.• Specializes in pattern recognition, spatial relationships,

nonverbal ideation, emotional processing, and the parallel processing of information.

• Language and Speech.– Broca’s area.

• Usually located in the left hemisphere’s frontal lobe• Responsible for speech production.

– Wernicke’s area.• Usually located in the right hemisphere’s temporal lobe• Responsible for the comprehension of speech.

– Other speech areas are involved generating verbs to match nouns, grouping together related words, etc.

Written words translated into sounds

Written words translated into sounds

Linguistic meaning determined on left side - comprehension

Higher frequency sounds sent to right area of brain for emotional overtones

Linguistic meaning determined on left side - comprehension

Higher frequency sounds sent to right area of brain for emotional overtones

Grammatical refinement of words – speech production

Grammatical refinement of words – speech production

• Emotions.– In mammals, the limbic system is composed of

the hippocampus, olfactory cortex, inner portions of the cortex’s lobes, and parts of the thalamus and hypothalamus.• Mediates basic emotions (fear, anger), involved in

emotional bonding, establishes emotional memory– For example,

the amygdala is involved in recognizing the emotional content of facial expression.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 48.27

Connects higher brain

involved in complex learning, reasoning and personality and emotion

Connects higher brain

involved in complex learning, reasoning and personality and emotion

Center of convergence for sensory data and a major organizer of emotional information-may act as a memory filter - tying info to an event or emotion

Center of convergence for sensory data and a major organizer of emotional information-may act as a memory filter - tying info to an event or emotion

• Memory and Learning.– Short-term memory stored in the frontal

lobes.– The establishment of long-term memory

involves the hippocampus.• The transfer of information from short-term to long-

term memory.– Is enhanced by repetition (remember that when you are

preparing for an exam).– Influenced by emotional states mediated by the amygdala.– Influenced by association with previously stored information.


– Different types of long-term memories are stored in different regions of the brain.

– Memorization-type memory can be rapid.• Primarily involves changes in the strength of

existing nerve connections.

– Learning of skills and procedures is slower.• Appears to involves cellular mechanisms similar to

those involved in brain growth and development.


Cellular Mechanisms of Learning• Experiments on invertebrates

– Have revealed the cellular basis of some types of learning

Figure 48.31a, b

(a) Touching the siphon triggers a reflex thatcauses the gill to withdraw. If the tail isshocked just before the siphon is touched,the withdrawal reflex is stronger. Thisstrengthening of the reflex is a simple formof learning called sensitization.

(b) Sensitization involves interneurons thatmake synapses on the synaptic terminals ofthe siphon sensory neurons. When the tailis shocked, the interneurons releaseserotonin, which activates a signaltransduction pathway that closes K+

channels in the synaptic terminals ofthe siphon sensory neurons. As a result,action potentials in the siphon sensoryneurons produce a prolongeddepolarization of the terminals. That allowsmore Ca2+ to diffuse into the terminals, which causes the terminals to release more of their excitatory neurotransmitter onto the gill motor neurons. In response, the motor neuronsgenerate action potentials at a higher frequency,producing a more forceful gill withdrawal.

Siphon

Mantle

Gill

Tail

Head

Gill withdrawal pathway

Touchingthe siphon

Shockingthe tail Tail sensory

neuron

Interneuron

Sensitization pathway

Siphon sensoryneuron

Gill motorneuron

Gill

EPSPs

• In the vertebrate brain, a form of learning called long-term potentiation (LTP)– Involves an increase in the strength of synaptic

transmission

Figure 48.32

PRESYNAPTIC NEURON

NO

Glutamate

NMDAreceptor

Signal transduction pathways

NO

Ca2+

AMPA receptor

POSTSYNAPTIC NEURON

Ca2+ initiates the phos-phorylation of AMPA receptors,making them more responsive.Ca2+ also causes more AMPAreceptors to appear in thepostsynaptic membrane.

5

Ca2+ stimulates thepostsynaptic neuron toproduce nitric oxide (NO).

6

The presynapticneuron releases glutamate.

1

Glutamate binds to AMPAreceptors, opening the AMPA-receptor channel and depolarizingthe postsynaptic membrane.

2

Glutamate also binds to NMDAreceptors. If the postsynapticmembrane is simultaneouslydepolarized, the NMDA-receptorchannel opens.

3

Ca2+ diffuses into thepostsynaptic neuron.

4

NO diffuses into thepresynaptic neuron, causing it to release more glutamate.

7

P

• Functional changes in synapses in synapses of the hippocampus and amygdala are related to memory storage and emotional conditioning.– Long-term depression (LTD) occurs when a

postsynaptic neuron displays decreased responsiveness to action potentials.• Induced by repeated, weak stimulation.

– Long-term potentiation (LTP) occurs when a postsynaptic neuron displays increased responsiveness to stimuli.• Induced by brief, repeated action potentials that strongly

depolarize the postsynaptic membrane.• May be associated with memory storage and learning.


• Human Consciousness.– Brain imaging can show neural activity

associated with:• Conscious perceptual choice• Unconscious processing• Memory retrieval• Working memory.

– Consciousness appears to be a whole-brain phenomenon.


• The mammalian PNS has the ability to repair itself, the CNS does not.

– Research on nerve cell development and neural stem cells may be the future of treatment for damage to the CNS.

7. Research on neuron development and neural stem cells may lead to new approaches for treating CNS

injuries and diseases


Nerve Cell Development

• Signal molecules direct an axon’s growth – By binding to receptors on the plasma

membrane of the growth cone

• Nerve Cell Development.

Fig. 48.28

• Neural Stem Cells.– The adult human brain does produce new

nerve cells.• New nerve cells have been found in the

hippocampus.• Since mature human brain cells cannot undergo cell

division the new cells must have arisen from stem cells.


Neural Stem Cells• The adult human brain

– Contains stem cells that can differentiate into mature neurons

Figure 48.34

10

m

• The induction of stem cell differentiation and the transplantation of cultured stem cells– Are potential methods for replacing neurons

lost to trauma or disease

Diseases and Disorders of the Nervous System

• Mental illnesses and neurological disorders– Take an enormous toll on society, in both the

patient’s loss of a productive life and the high cost of long-term health care

Schizophrenia

• About 1% of the world’s population– Suffers from schizophrenia

• Schizophrenia is characterized by– Hallucinations, delusions, blunted emotions, and

many other symptoms

• Available treatments have focused on– Brain pathways that use dopamine as a

neurotransmitter

Depression

• Two broad forms of depressive illness are known– Bipolar disorder and major depression

• Bipolar disorder is characterized by– Manic (high-mood) and depressive (low-mood)

phases

• In major depression– Patients have a persistent low mood

• Treatments for these types of depression include– A variety of drugs such as Prozac and lithium

Alzheimer’s Disease

• Alzheimer’s disease (AD)– Is a mental deterioration characterized by

confusion, memory loss, and other symptoms

• AD is caused by the formation of– Neurofibrillary tangles and senile plaques in the

brain

Figure 48.35

Senile plaque Neurofibrillary tangle20 m

• A successful treatment for AD in humans– May hinge on early detection of senile plaques

Parkinson’s Disease• Parkinson’s disease is a motor disorder

– Caused by the death of dopamine-secreting neurons in the substantia nigra

– Characterized by difficulty in initiating movements, slowness of movement, and rigidity

• There is no cure for Parkinson’s disease– Although various approaches are used to

manage the symptoms

Documents

Chapter 48 Nervous System. 1. Nervous systems perform the three overlapping functions of sensory input, integration, and motor output