Ch11

Chapter 11

Muscular Tissue

ANATOMY & PHYSIOLOGY

The Unity of Form and Function

NINTH EDITION

KENNETH S. SALADIN

No reproduction or further distribution permitted without the prior written consent of McGraw Hill.© McGraw Hill 2

Introduction

Movement is a fundamental characteristic of all living

organisms. Collagen played a key role in the evolution of

movement. What happened to those organisms that lacked

collagen? What allowed animals to move through the

evolutionary bottleneck that required movement?

Three types of muscular tissue: skeletal, cardiac, and smooth

muscle

Important to understand muscle at the molecular, cellular,

and tissue levels of organization.

striations, nuclei, intercalated discs, muscle shape,

muscle function.© McGraw Hill 3

11.1 Types and Characteristics of Muscular Tissue

Expected Learning Outcomes:

• Describe the physiological properties that all muscle types have in common.

• List the defining characteristics of skeletal muscle.

• Discuss the elastic functions of the connective tissue components of a muscle.© McGraw Hill 4

Universal Characteristics of Muscle

• Excitability (responsiveness)—to

chemical/electrical signals,

stretch, and electrical changes

across the plasma membrane

• Conductivity—local electrical

excitation sets off a wave of

excitation that travels along the

muscle fiber

• Contractility—shortens when

stimulated

• Extensibility—capable of being

stretched between contractions

• Elasticity—returns to its original

rest length after being stretched© McGraw Hill 5

Skeletal Muscle

Skeletal muscle—voluntary, striated muscle usually attached

to bones

• Striations—alternating light and dark transverse bands

• Results from arrangement of internal contractile proteins

• Voluntary—usually subject to conscious control

Muscle cell is a muscle fiber (myofiber)—as long as 30 cm© McGraw Hill 6© McGraw Hill 7

Skeletal Muscle Fibers

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Figure 11.1

Skeletal Muscle 2

Connective tissue wrappings

• Endomysium: connective tissue around muscle cell

• Perimysium: connective tissue around muscle fascicle

• Epimysium: connective tissue surrounding entire muscle

Tendons are attachments between muscle and bone matrix

Collagen is somewhat extensible and elastic

• Stretches slightly under tension and recoils when released

• Resists excessive stretching and protects muscle from injury

• Returns muscle to its resting length

• Contributes to power output and muscle efficiency© McGraw Hill 9

11.2 Skeletal Muscle Cells

Expected Learning Outcomes:

• Describe the structural components of a muscle fiber.

• Relate the striations of a muscle fiber to the overlapping arrangement of its

protein filaments.

• Name the major proteins of a muscle fiber and state the function of each.© McGraw Hill 10

The Muscle Fiber

Sarcolemma—plasma

membrane of a muscle fiber

Sarcoplasm—cytoplasm of a

muscle fiber

• Myofibrils: long protein

cords occupying most of

sarcoplasm

• Glycogen: carbohydrate

stored to provide energy for

exercise

• Myoglobin: red pigment;

provides some oxygen

needed for muscle activity© McGraw Hill 11

The Muscle Fiber 2

Multiple nuclei—flattened

nuclei pressed against the

inside of the sarcolemma

• Myoblasts: stem cells that fused

to form each muscle fiber early in

development

• Satellite cells: unspecialized

myoblasts remaining between the

muscle fiber and endomysium

• Play a role in regeneration of

damaged skeletal muscle tissue

Mitochondria—packed into

spaces between myofibrils© McGraw Hill 12

The Muscle Fiber 3

Sarcoplasmic reticulum (SR)—smooth ER that forms a

network around each myofibril:

• Terminal cisterns—dilated end-sacs of SR which

cross the muscle fiber from one side to the other

• Acts as a calcium reservoir; it releases calcium

through channels to activate contraction

T tubules—tubular infoldings of the sarcolemma which

penetrate through the cell and emerge on the other side

Triad—a T tubule and two terminal cisterns associated

with it© McGraw Hill 13

Structure of a Skeletal Muscle Fiber

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Figure 11.2© McGraw Hill 14

Myofilaments

Thick filaments—made of several hundred myosin molecules

• Each molecule shaped like a golf club

• Two chains intertwined to form a shaft-like tail

• Double globular head

• Heads directed outward in a helical array around the

bundle

• Heads on one half of the thick filament angle to the left, while heads

on other half angle to the right

• Bare zone with no heads in the middle© McGraw Hill 15

Molecular Structure of Thick Filaments

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Figure 11.3 a, b, d

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Myofilaments2

Thin filaments

• Fibrous (F) actin: two intertwined strands

• String of globular (G) actin subunits each with an active site that can bind to

head of myosin molecule

• Tropomyosin molecules

• Each blocking six or seven active sites on G actin subunits

• Troponin molecule: small, calcium-binding protein on each

tropomyosin molecule© McGraw Hill 17

Molecular Structure of Thin Filaments

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Figure 11.3 c© McGraw Hill 18

Myofilaments3

Elastic filaments

• Titin: huge, springy protein

• Run through core of thin filament and anchor it to Z disc

and M line

• Help stabilize and position the thick filament

• Prevent overstretching and provide recoil© McGraw Hill 19

Muscle Striations and Their Molecular Basis

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Figure 11.5 b© McGraw Hill 20

Myofilaments4

Contractile proteins—myosin and actin do the work of

contraction

Regulatory proteins—tropomyosin and troponin

• Act like a switch that determines when fiber can (and

cannot) contract

• Contraction activated by release of calcium into

sarcoplasm and its binding to troponin

• Troponin changes shape and moves tropomyosin off the

active sites on actin© McGraw Hill 21

Myofilaments5

Several other proteins associate with myofilaments to

anchor, align, and regulate them

Dystrophin—clinically important protein

• Links actin in outermost myofilaments to membrane

proteins that link to endomysium

• Transfers forces of muscle contraction to connective tissue

ultimately leading to tendon

• Genetic defects in dystrophin produce disabling disease

muscular dystrophy© McGraw Hill 22

Dystrophin

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Figure 11.4© McGraw Hill 23

Striations

Striations result from the precise organization of myosin and actin

in cardiac and skeletal muscle cells

Striations are alternating A-bands (dark) and I-bands (light)

• A band: dark; “A” stands for anisotropic

• Darkest part is where thick filaments overlap a hexagonal array of

thin filaments

• H band: not as dark; middle of A band; thick filaments only

• M line: middle of H band

• I band: light; “I” stands for isotropic

• The way the bands reflect polarized light

• Z disc: provides anchorage for thin filaments and elastic filaments© McGraw Hill 24

Muscle Striations and Their Molecular Basis2

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Figure 11.5 b© McGraw Hill 25

Muscle Striations and Their Molecular Basis3

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Figure 11.5 a

Striations 2

Sarcomere—segment from Z disc to Z disc

• Functional contractile unit of muscle fiber

Muscle cells shorten because their individual sarcomeres shorten

• Z disc (Z lines) are pulled closer together as thick and thin

filaments slide past each other

Neither thick nor thin filaments change length during shortening

• Only the amount of overlap changes

During shortening, dystrophin and linking proteins also pull on

extracellular proteins

• Transfers pull to extracellular tissue© McGraw Hill 27

Structural Hierarchy of Skeletal Muscle

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TABLE 11.1 The Structural Hierarchy of a Skeletal Muscle

Structural Level Description

Muscle A contractile organ, usually attached to bones by way of

tendons. Composed of bundles (fascicles) of tightly packed,

long, parallel cells (muscle fibers). Supplied with nerves and

blood vessels and enclosed in a fibrous epimysium that

separates it from neighboring muscles.

Fascicle A bundle of muscle fibers within a muscle. Supplied by nerves

and blood vessels and enclosed in a fibrous perimysium that

separates it from neighboring fascicles.

Muscle Fiber A single muscle cell. Slender, elongated, threadlike, enclosed in

a specialized plasma membrane (sarcolemma). Contains

densely packed bundles (myofibrils) of contractile protein

myofilaments, multiple nuclei immediately beneath the

sarcolemma, and an extensive network of specialized smooth

endoplasmic reticulum (sarcoplasmic reticulum). Enclosed in a

thin fibrous sleeve called endomysium.© McGraw Hill 28

Structural Hierarchy of Skeletal Muscle2

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TABLE 11.1 The Structural Hierarchy of a Skeletal Muscle

Structural Level Description

Myofibril A bundle of protein myofilaments within a muscle fiber;

myofibrils collectively fill most of the cytoplasm. Each

surrounded by sarcoplasmic reticulum and mitochondria.

Has a banded (striated) appearance due to orderly overlap

of protein myofilaments.

Sarcomere A segment of myofibril from one Z disc to the next in the

fiber’s striation pattern. Hundreds of sarcomeres end to end

compose a myofibril. The functional, contractile unit of the

muscle fiber.

Myofilaments Fibrous protein strands that carry out the contraction

process. Two types: thick myofilaments composed mainly of

myosin, and thin myofilaments composed mainly of actin.

Thick and thin myofilaments slide over each other to

shorten each sarcomere. Shortening of end-to-end

sarcomeres shortens the entire muscle.© McGraw Hill 29

11.3 The Nerve—Muscle Relationship

Expected Learning Outcomes:

• Explain what a motor unit is and how it relates to muscle contraction.

• Describe the structure of the junction where a nerve fiber meets a

muscle fiber.

• Explain why a cell has an electrical charge difference across its

plasma membrane and, in general terms, how this relates to muscle

contraction.© McGraw Hill 30

The Nerve—Muscle Relationship

Skeletal muscle cannot contract unless stimulated by a nerve

If nerve connections are severed or poisoned, a muscle is

paralyzed

• Denervation atrophy: shrinkage of paralyzed muscle when

nerve remains disconnected© McGraw Hill 31

Motor Neurons and Motor Units

Somatic motor neurons

• Nerve cells whose cell bodies are in the brainstem and

spinal cord that serve skeletal muscles

• Somatic motor fibers—their axons that lead to the skeletal

muscle

• Each nerve fiber branches out to a number of muscle

fibers

• Each muscle fiber is supplied by only one motor neuron© McGraw Hill 32

Motor Neurons and Motor Units2

Motor unit—one nerve fiber and all the muscle fibers

innervated by it

Muscle fibers of one motor unit

• Dispersed throughout muscle

• Contract in unison

• Produce weak contraction over wide area

• Provide ability to sustain long-term contraction as motor

units take turns contracting

• Effective contraction usually requires contraction of

several motor units at once© McGraw Hill 33

Motor Units

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Figure 11.6

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Motor Neurons and Motor Units3

Average motor unit contains 200 muscle fibers

Small motor units—fine degree of control

• Three to six muscle fibers per neuron

• Eye and hand muscles

Large motor units—more strength than control

• Powerful contractions supplied by large motor units with

hundreds of fibers

• Quadriceps femoris and gastrocnemius have 1,000

muscle fibers per neuron© McGraw Hill 35

The Neuromuscular Junction

Synapse—point where a nerve fiber meets its target cell

Neuromuscular junction (NMJ)—when target cell is a muscle

fiber

Each terminal branch of the nerve fiber within the NMJ forms

a separate synapse with the muscle fiber consisting of:

• Axon terminal—swollen end of nerve fiber

• Contains synaptic vesicles with acetylcholine (ACh)

• Synaptic cleft—gap between axon terminal and

sarcolemma

• Schwann cell envelops and isolates NMJ© McGraw Hill 36

Innervation of Skeletal Muscle

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Figure 11.7 b© McGraw Hill 37

The Neuromuscular Junction2

Nerve impulse causes synaptic vesicles to undergo exocytosis

releasing ACh into synaptic cleft

Muscle cell has millions of ACh receptors—proteins incorporated

into its membrane

• Junctional folds of sarcolemma beneath axon terminal increase

surface area holding ACh receptors

• Lack of receptors causes weakness in myasthenia gravis

Basal lamina—thin layer of collagen and glycoprotein separating

Schwann cell and muscle cell from surrounding tissues

• Contains acetylcholinesterase (AChE) that breaks down ACh,

allowing for relaxation© McGraw Hill 38

Electrically Excitable Cells

Muscle fibers and neurons are electrically excitable

• Their membranes exhibit voltage changes in response to

stimulation

Electrophysiology—the study of the electrical activity of cells

Voltage (electrical potential)—a difference in electrical charge

from one point to another

Resting membrane potential—about −90 mV in skeletal

muscle cells

• Maintained by sodium–potassium pump© McGraw Hill 39

Electrically Excitable Cells2

In an unstimulated (resting) cell:

• There are more anions (negatively charged particles) on the

inside of the membrane than on the outside

• These anions make the inside of the plasma membrane

negatively charged by comparison to its outer surface

• The plasma membrane is electrically polarized (charged) with a

negative resting membrane potential (RMP)

• There are excess sodium ions (Na+) in the extracellular fluid

(ECF)

• There are excess potassium ions (K+) in the intracellular fluid

(ICF)© McGraw Hill 40

Electrically Excitable Cells3

In a stimulated (active) muscle fiber or nerve cell:

• Na+ ion gates open in the plasma membrane

• Na+ flows into cell down its electrochemical gradient

• These cations override the negative charges in the ICF

• Depolarization: inside of plasma membrane becomes positive

• Immediately, Na+ gates close and K+ gates open

• K+ rushes out of cell partly repelled by positive sodium charge and

partly because of its concentration gradient

• Loss of positive potassium ions turns the membrane negative again

(repolarization)

• This quick up-and-down voltage shift (depolarization and

repolarization) is called an action potential© McGraw Hill 41

Electrically Excitable Cells4

A resting membrane potential (RMP) is seen in a waiting

excitable cell, whereas an action potential is a quick event

seen in a stimulated excitable cell

An action potential perpetuates itself down the length of a

cell’s membrane

• An action potential at one point causes another one to

happen immediately in front of it, which triggers another

one a little farther along and so forth

• This wave of excitation is called an impulse© McGraw Hill 42

Neuromuscular Toxins and Paralysis

Toxins interfering with synaptic function can paralyze muscles

Some pesticides contain cholinesterase inhibitors

• Bind to acetylcholinesterase and prevent it from degrading ACh

• Spastic paralysis: a state of continual contraction of the

muscles; possible suffocation

Tetanus (lockjaw) is a form of spastic paralysis caused by toxin

Clostridium tetani

• Glycine in the spinal cord normally stops motor neurons from

producing unwanted muscle contractions

• Tetanus toxin blocks glycine release in the spinal cord and

causes overstimulation and spastic paralysis of the muscles© McGraw Hill 43

Neuromuscular Toxins and Paralysis 2

Flaccid paralysis—a state in

which the muscles are limp and

cannot contract

• Curare: competes with ACh for receptor

sites, but does not stimulate the

muscles

• Plant poison used by South American

natives to poison blowgun darts

Botulism—type of food poisoning

caused by a neuromuscular toxin

secreted by the bacterium

Clostridium botulinum

• Blocks release of ACh causing flaccid

paralysis

• Botox cosmetic injections used for

wrinkle removal© McGraw Hill 44

Botulism toxin.© McGraw Hill 45© McGraw Hill 46

11.4 Behavior of Skeletal Muscle Fibers

Expected Learning Outcomes:

• Explain how a nerve fiber stimulates a skeletal muscle

fiber.

• Explain how stimulation of a muscle fiber activates its

contractile mechanism.

• Explain the mechanism of muscle contraction.

• Explain how a muscle fiber relaxes.

• Explain why the force of a muscle contraction depends on

the muscle’s length prior to stimulation.© McGraw Hill 47

Excitation of a Muscle Fiber

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Figure 11.8 (1, 2)

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Excitation of a Muscle Fiber 2

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Figure 11.8 (3, 4)

Access the text alternative for slide images.© McGraw Hill 49

Excitation of a Muscle Fiber 3

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Figure 11.8 (5)© McGraw Hill 50

Excitation–Contraction Coupling

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Figure 11.9 (6, 7)

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Excitation–Contraction Coupling2

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Figure 11.9 (8, 9)© McGraw Hill 52

Contraction

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Figure 11.10 (10, 11)

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Contraction 2

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Figure 11.10 (12, 13)© McGraw Hill 54

Relaxation

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Figure 11.11 (14, 15)

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Relaxation 2

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Figure 11.11 (16)© McGraw Hill 56

Relaxation 3

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Figure 11.11 (17, 18)© McGraw Hill 57

The Length–Tension Relationship and Muscle Tone

Length–tension relationship

Define

• Tension

• Muscle tone© McGraw Hill 58

Length–Tension Relationship

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Figure 11.12© McGraw Hill 59

Rigor Mortis

Rigor mortis

Recall the sliding filament hypothesis

What is the role of Ca? ATP?

Why does RM go away?© McGraw Hill 60

Threshold, Latent Period, and Twitch

Myogram

Threshold

Twitch© McGraw Hill 61

A Muscle Twitch

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Figure 11.13© McGraw Hill 62

Contraction Strength of Twitches2

Muscle contraction varies depending on the tasks.

Nerve signal strength usually matches the tasks within our

capabilities.

• Higher voltages excite more nerve fibers which stimulate

more motor units to contract

• Recruitment or summation helps us use our muscles

effectively.

• multiple motor unit (MMU) summation—the process of

bringing more motor units into play with stronger stimuli© McGraw Hill 63

The Relationship Between Stimulus Intensity (Voltage)

and Muscle Tension

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Figure 11.14© McGraw Hill 64

Contraction Strength of Twitches3

Low frequency stimuli produce identical twitches

Higher frequency stimuli (e.g., 20 stimuli/s) produce temporal

(wave) summation

• Each new twitch rides on the previous one generating

higher tension

• Only partial relaxation between stimuli resulting in

fluttering, incomplete tetanus

Unnaturally high stimulus frequencies (in lab experiments)

cause a steady, contraction called complete (fused) tetanus© McGraw Hill 65

The Relationship Between Stimulus

Frequency and Muscle Tension

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Figure 11.15

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Isometric and Isotonic Contraction

Isometric muscle contraction

• Muscle produces internal tension but external resistance

causes it to stay the same length

• Can be a prelude to movement when tension is absorbed by

elastic component of muscle

• Important in postural muscle function and antagonistic muscle

joint stabilization

Isotonic muscle contraction

• Muscle changes in length with no change in tension

• Concentric contraction: muscle shortens as it maintains tension

(example: lifting weight)

• Eccentric contraction: muscle lengthens as it maintains tension

(example: slowly lowering weight)© McGraw Hill 67

Isometric and Isotonic Contraction 2

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Figure 11.16

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Isometric and Isotonic Contraction working together

At the beginning of contraction—isometric phase

• Muscle tension rises but muscle does not shorten

When tension overcomes resistance of the load

• Tension levels off

Muscle begins to shorten and move the load—isotonic phase© McGraw Hill 69

Isometric and Isotonic Phases of Contraction

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Figure 11.17© McGraw Hill 70

11.6 Muscle Metabolism

Expected Learning Outcomes:

• Explain how skeletal muscle meets its energy demands during rest

and exercise.

• Explain the basis of muscle fatigue and soreness.

• Discuss why extra oxygen is needed even after an exercise has

ended.

• Distinguish between two physiological types of muscle fibers, and

explain their functional roles.

• Discuss the factors that affect muscular strength.

• Discuss the effects of resistance and endurance exercises on

muscles.© McGraw Hill 71

ATP Sources

All muscle contraction depends on ATP for?

ATP supply depends on availability of:

• Oxygen and organic energy sources

(e.g., glucose and fatty acids)

Two main pathways of ATP synthesis

• Anaerobic fermentation

• no oxygen used in glycolysis.

• yields little ATP and lactate, which

needs to be disposed of by the liver

• Aerobic respiration—produces far more

ATP; does not generate lactate; requires

a continual supply of oxygen© McGraw Hill 72© McGraw Hill 73

This physiological effect is called excess post-exercise

oxygen consumption, or EPOC. Also known as oxygen debt,

EPOC is the amount of oxygen required to restore your body

to its normal, resting level of metabolic function (called

homeostasis). It also explains how your body can continue

to burn calories long after you’ve finished your workout.© McGraw Hill 74© McGraw Hill 75© McGraw Hill 76

Modes of ATP Synthesis During Exercise

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Figure 11.18© McGraw Hill 77© McGraw Hill 78© McGraw Hill 79

Short-Term Energy

• Muscles obtain glucose from blood and their own stored

glycogen

• In the absence of oxygen, glycolysis can generate a net gain of 2

ATP for every glucose molecule consumed

• Converts glucose to lactate

As the phosphagen system is exhausted, muscles

shift to anaerobic fermentation

Anaerobic threshold (lactate threshold)—point at

which lactate becomes detectable in the blood

Glycogen–lactate system—the pathway from

glycogen to lactate

Produces enough ATP for 30–40 s of maximum

activity© McGraw Hill 80

Long-Term Energy

Aerobic respiration produces more ATP per glucose

than glycolysis does (another 30 ATP per glucose)

Efficient means of meeting

the ATP demands of

prolonged exercise

After 3–4 min, the rate of

oxygen consumption levels

off to a steady state where

aerobic ATP production

keeps pace with demand

For 30 min energy comes

equally from glucose and

fatty acids

Beyond 30 min, depletion of

glucose causes fatty acids

to become the more

significant fuel

After about 40 s, the respiratory and cardiovascular

systems start to deliver oxygen fast enough for aerobic

Fatigue and Endurance

Muscle fatigue—progressive weakness from prolonged use of

muscles

Fatigue in high-intensity exercise is thought to result from:

• Potassium accumulation in the T tubules reduces

excitability

• Excess ADP and P i slow cross-bridge movements, inhibit

calcium release and decrease force production in

myofibrils

Fatigue in low-intensity (long duration) exercise is thought to

result from:

• Fuel depletion as glycogen and glucose levels decline

• Electrolyte loss through sweat can decrease muscle

excitability

• Central fatigue when less motor signals are issued from

brain

• Brain cells inhibited by exercising muscles’ release

of ammonia

• Psychological will to persevere—not well

Fatigue and Endurance 2

Maximum oxygen uptake (VO2 max) is

major determinant of one’s ability to

maintain high-intensity exercise for more

than 4–5 min

• VO2 max: the point at which the rate

of oxygen consumption plateaus and

does not increase further with added

workload

• Proportional to body size

• Peaks at around age 20

• Usually greater in males than

females

• Can be twice as great in trained

endurance athlete as in untrained

Excess Postexercise Oxygen Consumption (EPOC)

EPOC meets a metabolic demand also known as oxygen debt

It is the difference between the elevated rate of oxygen

consumption following exercise and the usual resting rate

Needed for the following purposes:

• To aerobically replenish ATP (some of which helps regenerate

CP stores)

• To replace oxygen reserves on myoglobin

• To provide oxygen to liver that is busy disposing of lactate

• To provide oxygen to many cells that have elevated metabolic

rates after exercise

Physiological Classes of Muscle Fibers

Fast versus slow-twitch fibers can predominate in certain muscle groups

• Muscles of the back contract relatively quickly (100 ms to peak

tension) whereas muscles that move the eyes contract quickly (8 ms

to peak tension)

Slow-twitch, slow oxidative (SO), red or type I fibers

• Well adapted for endurance; resist fatigue by oxidative (aerobic) ATP

production

• Important for muscles that maintain posture (e.g., erector spinae of the

back, soleus of calf)

• Thin cells with abundant mitochondria, capillaries, myoglobin (deep

red color) and contain a form of myosin with slow ATPase, and a SR

that releases calcium slowly

• Grouped in small motor units controlled by small, easily excited motor

Physiological Classes of Muscle Fibers 2

Fast-twitch, fast glycolytic (FG), white, or type II

fibers

• Fibers are well adapted for quick responses

• Important for quick and powerful muscles: eye

and hand muscles, gastrocnemius of calf and

biceps brachii

• Contain a form of myosin with fast ATPase and

a large SR that releases calcium quickly

• Utilize glycolysis and anaerobic fermentation for

energy

• Abundant glycogen and creatine phosphate

• Lack of myoglobin gives them pale (white)

appearance

• Fibers are thick and strong

• Grouped in large motor units controlled by

larger, less excitable neurons allowing for

Physiological Classes of Muscle Fibers3

Fast-twitch, intermediate, or type IIA fibers

• Fast twitch but fatigue resistant

• Known in other animals but rare in humans

Every muscle contains a mix of fiber types, but one type predominates

depending on muscle function

Fiber type within a muscle differs across individuals

• Some individuals seem genetically predisposed to be sprinters, while

Muscular Strength and Conditioning

Muscles can generate more tension

than the bones and tendons can

withstand

Muscular strength depends on:

• Primarily muscle size—thicker

muscle forms more cross-bridges; a

muscle can exert a tension of 3 or 4

kg/cm2 of cross-sectional area

• Fascicle arrangement—pennate are

stronger than parallel, and parallel

stronger than circular

• Size of active motor units—the

larger the motor unit, the stronger

the contraction

• Multiple motor unit summation—

simultaneous activation of more

Muscular Strength and Conditioning 2

Muscular strength depends on:

• Temporal summation

• The greater the frequency of

stimulation, the more strongly

a muscle contracts

• Length–tension relationship

• A muscle resting at optimal

length is prepared to contract

more forcefully than a muscle

that is excessively contracted

or stretched

• Fatigue

• Fatigued muscles contract

more weakly than rested

Muscular Strength and Conditioning 3

Resistance training (example: weightlifting)

• Contraction of a muscle against a load that resists movement

• A few minutes of resistance exercise a few times a week is enough to

stimulate muscle growth

• Growth is from cellular enlargement

• Muscle fibers synthesize more myofilaments and myofibrils and grow

Benefits of Muscular Strength and Conditioning 4

Endurance training (aerobic exercise)

• Improves fatigue-resistant muscles

• Slow twitch fibers produce more

mitochondria, glycogen, and acquire

a greater density of blood capillaries

• Improves skeletal strength

• Increases the red blood cell count

and oxygen transport capacity of the

blood

• Enhances the function of the

cardiovascular, respiratory, and

Muscular Dystrophy

Muscular dystrophy―group of hereditary

diseases in which skeletal muscles

degenerate and weaken, and are replaced

with fat and fibrous scar tissue

Duchenne muscular dystrophy is caused by a

sex-linked recessive trait (1 of 3,500 live-born

boys)

• Most common form; a disease of males;

diagnosed between 2 and 10 years of

age

• Mutation in gene for muscle protein

dystrophin

• Actin not linked to sarcolemma and

cell membranes damaged during

contraction; necrosis and scar tissue

result

• Rarely live past 20 years of age due to

effects on respiratory and cardiac muscle;

Muscular Dystrophy 2

Facioscapulohumeral

MD―autosomal dominant trait

affecting both sexes equally

• Facial and shoulder muscles

more than pelvic muscles

Limb-girdle dystrophy

• Combination of several

diseases of intermediate

severity

• Affects shoulder, arm, and

Myasthenia Gravis

Autoimmune disease in which

antibodies attack neuromuscular

junctions and bind ACh receptors

together in clusters

• Usually occurs in women

between 20 and 40

• Muscle fibers then remove

the clusters of receptors from

the sarcolemma by

endocytosis

• Fiber becomes less and less

sensitive to ACh

• Effects usually first appear in

facial muscles

• Drooping eyelids and

double vision, difficulty

swallowing, and

weakness of the limbs

• Strabismus: inability to fixate

on the same point with both

Myasthenia Gravis 2

Treatments for Myasthenia Gravis

• Cholinesterase inhibitors retard

breakdown of ACh allowing it to

stimulate the muscle longer

• Immunosuppressive agents

suppress the production of

antibodies that destroy ACh

receptors

• Thymus removal (thymectomy)

helps to dampen the overactive

immune response that causes

myasthenia gravis

• Plasmapheresis: technique to

remove harmful antibodies from

Test of Myasthenia Gravis

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Figure 11.25