Chapter 9 Muscle Tissue Lab exam next Thurs. 2/12 CPR practice, essay entry complete this morning, remainder due next Tuesday Knee dissection Tuesday2/5

Chapter 9 Muscle Tissue

Lab exam next Thurs. 2/12

CPR practice, essay entry complete this morning, remainder due next Tuesday

Knee dissection Tuesday2/5

Human Anatomy Spring 2012 2

Objectives

Discuss organization and functions of muscle as an organ including all tissue types

Describe the structural modifications of muscle cells and their functional significance

Describe the neuromuscular junction and events Discuss the sliding filament theory Describe a motor unit and neural control of muscle Explain the link between anatomy & physiology exemplified in

the length/tension curve Compare and contrast the three types of skeletal muscle cells

and relate these to muscular performance Compare and contrast the three types of muscle tissue Discuss developmental changes of muscle Apply knowledge of levers to human skeletal muscles(if time)


Muscle Function

1. Movement – including moving substances within body

• Contract against resistance Skeletal move against bone Cardiac move against fluid – blood Smooth move against other contents

2. Muscles also maintain posture & stabilize joints

3. Regulate organ volume

4. Generate heat


Functional Characteristics of Muscle Tissue

Contractility – the ability to shorten forcibly is the unique feature of muscle

Excitability, or irritability – the ability to receive and respond to stimuli, have action potentials like neurons

Extensibility – the ability to be stretched or extended

Elasticity – the ability to recoil and resume the original resting length

EpimysiumMuscle fascicle

EndomysiumPerimysium

Nerve

Muscle fibers

Blood vessels

SKELETAL MUSCLE(organ)

MUSCLE FASCICLE(bundle of cells)

Perimysium

Muscle fiber

Endomysium

Epimysium

Blood vesselsand nerves

Endomysium

Perimysium

Tendon

MUSCLE FIBER(cell)

Mitochondria

Sarcolemma

Myofibril

AxonSarcoplasm

CapillaryEndomysium

Myosatellitecell

Nucleus


Human Anatomy 6


Skeletal Muscle Organs Organs include muscle tissue,

blood vessels, nerve fibers, and connective tissue

The three connective tissue wrappings are: Epimysium – an overcoat of

dense regular CT that surrounds the entire muscle

Perimysium – fibrous CT that surrounds groups of muscle fibers called fascicles

Endomysium – fine sheath of CT composed of reticular fibers surrounding each muscle fiber


Skeletal Muscle: Nerve and Blood Supply

Each muscle is served by at least one nerve, artery, and vein Each skeletal muscle fiber is supplied with a

nerve ending that controls contraction – a neuromuscular junction

Contracting fibers require continuous delivery of oxygen and nutrients via arteries

Wastes must be removed via veins


Skeletal Muscle: Attachments

Muscles span joints and are attached in at least two places

When muscles contract the movable bone, the muscle’s insertion moves toward the immovable bone – the muscle’s origin (i.e., origin is stationary— flawed concept, but customary)

Muscles attach: Directly – epimysium of the muscle is fused to the

periosteum of a bone Indirectly (more common) – CT wrappings extend

beyond the muscle as ropelike tendon or sheetlike aponeurosis


Microscopic Anatomy of a Skeletal Muscle Fiber

Each fiber is a long, cylindrical cell with multiple nuclei just beneath the sarcolemma

Fibers are 10 to 100 m in diameter, and up to hundreds of centimeters long

Each cell is a syncytium produced by fusion of myoblasts (embryonic cells)

Sarcoplasm has a unique oxygen-binding protein called myoglobin

Fibers contain the usual organelles plus myofibrils, sarcoplasmic reticulum, and T tubules


Muscle Cell Formation

Myoblasts

Syncytium

Note satellite cells


Sarcoplasmic Reticulum (SR)

SR is an elaborate smooth endoplasmic reticulum that mostly runs longitudinally and surrounds each myofibril

Paired terminal cisternae form perpendicular cross channels Functions in the regulation of intracellular calcium levels Elongated tubes called T tubules penetrate into the cell’s interior at

each A band–I band junction T tubules associate with the paired terminal cisternae to form triads

Internal organization of a muscle fiber.Note the relationships among myofibrils,sarcoplasmic reticulum, mitochondria,triads, and thick and thin filaments.

Mitochondria

Sarcolemma

Myofibril

Thin filament

Thick filament

Triad T tubulesSarcoplasmicreticulum

Terminal cisterna

Sarcolemma

Sarcoplasm

Myofibrils


T Tubules

T tubules are continuous with the sarcolemma They conduct impulses to the deepest regions

of the muscle These impulses signal for the release of Ca2+

from adjacent terminal cisternae

Internal organization of a muscle fiber.Note the relationships among myofibrils,sarcoplasmic reticulum, mitochondria,triads, and thick and thin filaments.

Mitochondria

Sarcolemma

Myofibril

Thin filament

Thick filament

Triad T tubulesSarcoplasmicreticulum

Terminal cisterna

Sarcolemma

Sarcoplasm

Myofibrils


Myofibrils

Myofibrils are densely packed, rodlike contractile elements

They make up most of the muscle volume The arrangement of myofibrils within a fiber is

such that a perfectly aligned repeating series of dark A bands and light I bands is evident

Figure 9.2b


Sarcomeres

The smallest contractile unit of a muscle The region of a myofibril between two successive

Z discs Composed of myofilaments made up of contractile

proteins Myofilaments are of two major types – thick and

thin


Myofilaments: Banding Pattern

Thick filaments – extend the entire length of an A band

Thin filaments – extend across the I band and partway into the A band

Z-disc – coin-shaped sheet of proteins (connectins) that anchors the thin filaments and connects myofibrils to one another


Myofilaments: Banding Pattern

Thin filaments do not overlap thick filaments in the lighter H zone

M lines appear darker due to the presence of the protein desmin

Elastic filaments of protein titin


Ultrastructure of Myofilaments:Thick Filaments

Each myosin molecule has a rodlike tail and two globular heads Tails – two

interwoven, heavy polypeptide chains

Heads – two smaller, light polypeptide chains called cross bridges

Figure 9.3a, b

Myofibril

Z line M line

H bandSarcomere

A single myosin molecule detailing the structure andmovement of the myosin head after cross-bridgebinding occurs

The structure ofthick filaments

Titin

M line

Myosin tail

Myosin head

Hinge

19

Ultrastructure of Myofilaments:Thick Filaments

Thick filaments are composed of the protein myosin

Myosin heads contain: 2 smaller, light polypeptide chains that

act as cross bridges during contraction Binding sites for actin of thin filaments Binding sites for ATP ATPase enzymes

Human Anatomy Spring 2012

The attachmentof thin filamentsto the Z line

The detailed structure of a thin filament showingthe organization of G actin, troponin, andtropomyosin

Myofibril

Z line M line

H band

Sarcomere

Actinin Z line Titin

Troponin Nebulin TropomyosinActive

siteG actin

molecules

F actinstrand


Ultrastructure of Myofilaments: Thin Filaments

Thin filaments are chiefly composed of protein actin Each actin molecule is a helical polymer of globular

subunits called G actin The subunits contain the active sites to which myosin

heads attach during contraction Tropomyosin (filamentous protein) and troponin are

regulatory subunits bound to actin


Arrangement of Filaments in a Sarcomere

Longitudinal section within one sarcomere

Figure 9.3d

22

Sliding Filament Mechanism of Contraction Thin filaments slide past the thick ones so that the

actin and myosin filaments overlap to a greater degree

In the relaxed state, thin and thick filaments overlap only slightly

Upon stimulation, myosin heads bind to actin and sliding begins (interactive physiology page 17)

Each myosin head binds and detaches several times during contraction, acting like a ratchet to generate tension and propel the thin filaments to the center of the sarcomere

As this event occurs throughout the sarcomeres, the muscle shortensHuman Anatomy Spring 2012



Figure 9.7 Changes in a Sarcomere

The Sliding Filament Theory



Figure 9.4b Sarcomere Structure

Sarcomere Structure


Regulation of Contraction

In order to contract, a skeletal muscle must: Be stimulated by a nerve ending (NMJ) Propagate an electrical current, or action potential,

along its sarcolemma (T-tubules) Have a rise in intracellular Ca2+ levels, the final

trigger for contraction (SR)


Nerve Stimulus of Skeletal Muscle

Skeletal muscles are stimulated by motor neurons of the somatic nervous system

Axons of these neurons travel in nerves to muscle cells

Axons of motor neurons branch profusely as they enter muscles

Each axonal branch forms a neuromuscular junction with a single muscle fiber

Motorneuron

Axon

Muscle fiber

Path of actionpotential

Neuromuscularsynapse

Motorend plate

Myofibril


Neuromuscular Junction

The neuromuscular junction is: Axonal endings, which have small

membranous sacs (synaptic vesicles) that contain the neurotransmitter acetylcholine (ACh)

The motor end plate of a muscle, which is a specific part of the sarcolemma that contains ACh receptors that helps form the neuromuscular junction

Though exceedingly close, axonal ends and muscle fibers are always separated by a space called the synaptic cleft

30Figure 9.8a, b





When a nerve impulse reaches the end of an axon at the neuromuscular junction: Voltage-regulated calcium channels open and

allow Ca2+ to enter the axon Ca2+ inside the axon terminal causes axonal

vesicles to fuse with the axonal membrane This fusion releases ACh into the synaptic cleft via

exocytosis ACh diffuses across the synaptic cleft to ACh

receptors on the sarcolemma Binding of ACh to its receptors initiates an action

potential in the muscle



Figure 9.8c


Summary



Excitation-Contraction Coupling

Spinal cord

Motor neuroncell body

Muscle

Nerve

Motorunit 1

Motorunit 2

Musclefibers

Motorneuronaxon

Axon terminals atneuromuscular junctions

36

Motor Unit: The Nerve-Muscle Functional Unit

A motor unit is a motor neuron and all the muscle fibers it supplies

The number of muscle fibers per motor unit can vary from four to several hundred

Muscles that control fine movements (fingers, eyes) have small motor units


37

Motor Unit: The Nerve-Muscle Functional Unit

Large weight-bearing muscles (thighs, hips) have large motor units

Muscle fibers from a motor unit are spread throughout the muscle; therefore, contraction of a single motor unit causes weak contraction of the entire muscle

Spinal cord

Motor neuroncell body

Muscle

Nerve

Motorunit 1

Motorunit 2

Musclefibers

Motorneuronaxon

Axon terminals atneuromuscular junctions





Muscle Tone

Muscle tone: The constant, slightly contracted state of all

muscles, which does not produce active movements

Keeps the muscles firm, healthy, and ready to respond to stimulus

Spinal reflexes account for muscle tone by: Activating one motor unit and then another Responding to activation of stretch receptors in

muscles and tendons


Force of Contraction The force of contraction is affected by:

The number of muscle fibers contracting – the more motor fibers in a muscle, the stronger the contraction

The relative size of the muscle – the bulkier the muscle, the greater its strength

Figure 9.19a


Force of Contraction

Frequency of stimulation Length-tension

relationships Series-elastic elements –

the noncontractile structures in a muscle

Degree of muscle stretch – muscles contract strongest when muscle fibers are 80-120% of their normal resting length

Figure 9.19a

42

Length/Tension Relationship

Maximum force generated between 80-120% of resting length, interactive physiology page 16

Think of cross-bridge mechanism for explanation

Sarcomeresgreatly

shortened

Sarcomeres atresting length

Sarcomeres excessivelystretched

170%

Optimal sarcomereoperating length(80%–120% ofresting length)

100%75%


43Human Anatomy Spring 2012


Muscle Fiber Type: Functional Characteristics

Speed of contraction – determined by speed in which ATPases split ATP The three types of fibers are slow and fast and

intermediate ATP-forming pathways

Oxidative fibers – use aerobic pathways Glycolytic fibers – use anaerobic glycolysis

These two criteria define three categories – slow oxidative fibers, fast oxidative fibers, and fast glycolytic fibers




Training

Slow, oxidative respond to endurance training. Diameter changes little.

Fast, oxidative respond to strength and power training. Diameter increases.

Intermediate can take on characteristics of fast or slow, depending on type of training.

In what birds do you expect to find FT? And ST?

Exercise causes: An increase in the number of mitochondria An increase in the activity of muscle spindles An increase in the concentration of glycolytic enzymes An increase in the glycogen reserves An increase in the number of myofibrils The net effect is an enlargement of the muscle

(hypertrophy) Disuse causes atrophy:

A decrease in muscle size A decrease in muscle tone

Muscle Hypertrophy


Developmental Aspects

Muscle tissue develops from embryonic mesoderm called myoblasts

Multinucleated skeletal muscles form by fusion of myoblasts forming a syncytium

The growth factor agrin stimulates the clustering of ACh receptors at newly forming motor end plates

As muscles are brought under the control of the somatic nervous system, the numbers of fast and slow fibers are also determined


Developmental Aspects

Cardiac myoblasts do not fuse but develop gap junctions at an early embryonic stage

Most smooth muscle follows the same pattern of gap junctions rather than fusion


Developmental Aspects: After Birth

Muscular development reflects neuromuscular coordination

Development occurs head-to-toe, and proximal-to-distal

Peak natural neural control of muscles is achieved by midadolescence

Athletics and training can improve neuromuscular control


Developmental Aspects: Male and Female

There is a biological basis for greater strength in men than in women

Women’s skeletal muscle makes up 36% of their body mass

Men’s skeletal muscle makes up 42% of their body mass

These differences are due primarily to the male sex hormone testosterone

With more muscle mass, men are generally stronger than women

Body strength per unit muscle mass, however, is the same


Homeostatic Imbalance: Age Related

With age, connective tissue increases and myofibrils, glycogen and myoglobin decrease

Muscles become stringier and more sinewy By age 80, 50% of muscle mass is lost

(sarcopenia), and myosatellite cells decrease Regular exercise reverses sarcopenia Aging of the cardiovascular system affects every

organ in the body Atherosclerosis may block distal arteries, leading to

intermittent claudication and causing severe pain in leg muscles


Developmental Aspects: Regeneration

Cardiac and skeletal muscle become amitotic, but can lengthen and thicken (hypertrophy)

Myoblastlike satellite cells of skeletal muscle show very limited regenerative ability (Cardiac tissue lacks satellite cells)

Smooth muscle has good regenerative ability (hyperplasia)

Levers

F1L1 = F2L2

Mass=Force L=length

There are several ways to increase the force efficiency of a lever: increasing the length of the in-lever arm decreasing the length of the out-lever or doing both of the above



See-saw

WheelbarrowLess distance

Hotdog tongsmost common, least mechanical advantage, more force, more speed/distance

Figure 12-21b

The Arm is a Lever and Fulcrum System

57

Figure 12-22

The Lever-Fulcrum System Amplifies the Load Distance Traveled and the Speed of Movement

58


Smooth Muscle

Composed of spindle-shaped fibers diameter of 2-10 m and lengths of several hundred m

Lack the coarse CT sheaths of skeletal muscle, but have fine endomysium

Figure 9.23


Smooth Muscle

Are generally organized into two layers (longitudinal and circular) of closely apposed fibers

Found in walls of hollow organs (except the heart)

Figure 9.23


Innervation of Smooth Muscle Most smooth muscle lacks neuromuscular

junctions Innervating nerves have bulbous swellings

called varicosities Varicosities release neurotransmitters into

wide synaptic clefts called diffuse junctions


Microscopic Anatomy of Smooth Muscle

SR is less developed than in skeletal muscle and lacks a specific pattern (no cisterns)

T tubules are absent Plasma membranes have pouchlike infoldings

called caveoli Ca2+ is sequestered in the extracellular space

near the caveoli, allowing rapid influx when channels are opened

There are no visible striations and no sarcomeres Thin and thick filaments are present


Ratio of thick to thin filaments (1:2) is much lower than in skeletal (1:6) or cardiac (1:4)

Thick filaments have heads along their entire length

There is no troponin complex

Figure 9.25

Proportion and Organization of Myofilaments in Smooth Muscle


Thick and thin filaments are arranged diagonally, causing smooth muscle to contract in a corkscrew manner

Figure 9.25



Noncontractile intermediate filament bundles attach to dense bodies (analogous to Z discs) at regular intervals

Figure 9.25



Contraction of Smooth Muscle

Whole sheets of smooth muscle exhibit slow, synchronized contraction

They contract in unison, reflecting their electrical coupling with gap junctions

Action potentials are transmitted from cell to cell Some smooth muscle cells:

Act as pacemakers and set the contractile pace for whole sheets of muscle

Are self-excitatory and depolarize without external stimuli


Contractile Mechanism

Actin and myosin interact according to the sliding filament mechanism

The final trigger for contractions is a rise in intracellular Ca2+

Ca2+ is released from the SR and from the extracellular space

Ca2+ interacts with calmodulin and myosin light chain kinase to activate myosin


Special Features of Smooth Muscle Contraction Unique characteristics of smooth muscle

include: Smooth muscle tone Slow, prolonged contractile activity Low energy requirements Response to stretch


Response to Stretch

Smooth muscles exhibits a phenomenon called stress-relaxation response in which: Smooth muscle responds to stretch only briefly,

and then adapts to its new length The new length, however, retains its ability to

contract This enables organs such as the stomach and

bladder to temporarily store contents


Types of Smooth Muscle: Single Unit

The cells of single unit smooth muscle, commonly called visceral muscle: Contract rhythmically as a unit Are electrically coupled to one another via gap

junctions Often exhibit spontaneous action potentials Are arranged in opposing sheets and exhibit

stress-relaxation response


Types of Smooth Muscle: Multiunit

Multiunit smooth muscles are found: In large airways to the lungs In large arteries In arrector pili muscles In the internal eye muscles

Characteristics include: Rare gap junctions Infrequent spontaneous depolarizations Structurally independent muscle fibers A rich nerve supply, which, with a number of muscle

fibers, forms motor units Graded contractions in response to neural stimuli

Muscle Comparison Summary

Table 12-3

Documents

Chapter 9 Muscle Tissue Lab exam next Thurs. 2/12 CPR practice, essay entry complete this morning, remainder due next Tuesday Knee dissection Tuesday2/5