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Chapter 11

Lecture Outline

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Introduction

• Movement is a fundamental characteristic of

all living organisms

• Three types of muscular tissue—skeletal,

cardiac, and smooth muscle

• Important to understand muscle at the

molecular, cellular, and tissue levels of

organization

11-2

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 possible elastic functions of the

connective tissue components of a muscle.

11-3

11-4

Universal Characteristics of Muscle

• Excitability (responsiveness)

– To chemical signals, stretch, and electrical changes across the

plasma membrane

• Conductivity

– Local electrical change triggers 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

11-5

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

Figure 11.1

11-6

Skeletal Muscle

• 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

– Continuous with collagen fibers of tendons

– In turn, with connective tissue of 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

Microscopic Anatomy of

Skeletal Muscle

• 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.

11-7

11-8

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

• 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

11-9

The Muscle Fiber

11-10

• Sarcoplasmic reticulum (SR)—smooth ER

that forms a network around each myofibril:

– Terminal cisternae—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 cisternae

associated with it

The Muscle Fiber

Sarcoplasm

Sarcolemma

Openings into

transverse tubules

Sarcoplasmic

reticulum

Mitochondria

Myofibrils

Myofilaments

A band

I band

Z disc

Nucleus

Triad:

Terminal cisternae

Transverse tubule

Muscle

fiber

11-11

Figure 11.2

The Muscle Fiber

11-12

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

Figure 11.3a, b, d

(a) Myosin molecule

Head

Tail

(b) Thick filament

Myosin head

11-13

Myofilaments

• 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

Troponin complex G actinTropomyosin

Figure 11.3c

11-14

Myofilaments

• 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

Figure 11.5

11-15

Myofilaments

• 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

(b) Thick filament

Myosin head

Troponin complex G actinTropomyosin

Figure 11.3b,c

11-16

Myofilaments

(d) Portion of a sarcomere showing the overlap

of thick and thin filaments

Bare zone

Thin filament

Thick filament

Figure 11.3d

11-17

Myofilaments

• 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

11-18

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)

Figure 11.5b

11-19

Striations

– 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

• Bisects I band

Figure 11.5b

Striations

11-20

Figure 11.5a

11-21

Striations

• 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

11-22

Structural Hierarchy of Skeletal Muscle

11-23

Structural Hierarchy of Skeletal Muscle

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.

11-24

11-25

The Nerve—Muscle Relationship

• Skeletal muscle never contracts 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

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

11-26

11-27

Motor Neurons and Motor Units

• 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

Figure 11.6

Spinal cord

Neuromuscular

junction

Skeletal

muscle

fibers

Motor

neuron 1

Motor

neuron 2

Motor Neurons and Motor Units

• 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

– Gastrocnemius of calf has 1,000 muscle fibers per

neuron

11-28

11-29

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 separate synapse with the

muscle fiber

• One nerve fiber stimulates the muscle fiber at

several points within the NMJ

11-30

The Neuromuscular Junction

• Synaptic knob—swollen end of nerve fiber

– Contains synaptic vesicles with acetylcholine (ACh)

• Synaptic cleft—gap between synaptic knob and sarcolemma

• Schwann cell envelops and isolates NMJ

Figure 11.7b

11-31

The Neuromuscular Junction

• 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 synaptic knob

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

11-32

The Neuromuscular Junction

Figure 11.7a

11-33

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

Electrically Excitable Cells

• 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)

11-34

11-35

Electrically Excitable Cells

• 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

11-36

Electrically Excitable Cells

• Resting membrane potential (RMP) is seen in a

waiting excitable cell, whereas 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

11-37

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

11-38

Neuromuscular Toxins and Paralysis

• 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

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 sarcomere length prior to stimulation.

11-39

11-40

Behavior of Skeletal Muscle Fibers

• Four major phases of contraction and relaxation

– Excitation

• Process in which nerve action potentials lead to muscle action

potentials

– Excitation–contraction coupling

• Events that link the action potentials on the sarcolemma to

activation of the myofilaments, thereby preparing them to contract

– Contraction

• Step in which the muscle fiber develops tension and may shorten

– Relaxation

• When stimulation ends, a muscle fiber relaxes and returns to its

resting length

11-41

Excitation

• Nerve signal opens voltage-gated calcium channels in synaptic knob

• Calcium enters knob and stimulates release of ACh from synaptic

vesicles into synaptic cleft

• Ach diffuses across cleft

Figure 11.8 (1, 2)

Arrival of nerve signal

Acetylcholine (ACh) release2

Motor

nerve

fiber

Sarcolemma

Synaptic

knob

Synaptic

vesicles

ACh

receptors

Nerve signal

ACh

enters

synaptic knob

Synaptic

cleft

11-42

Excitation

• Two ACh molecules bind to each receptor and open its channel

• Na

enters; shifting membrane potential from −90 mV to +75 mV

• Then K

exits and potential returns to −90 mV

• The quick voltage shift is called an end-plate potential (EPP)

Figure 11.8 (3, 4)

Binding of ACh to receptor

Sarcolemma

ACh receptor

AChACh

Opening of ligand-regulated ion gate;

creation of end-plate potential

11-43

Excitation

• Voltage change in end-plate region (EPP) opens nearby voltage-

gated channels producing an action potential that spreads over

muscle surface

Opening of voltage-regulated ion gates;

creation of action potentials

Sarcolemma

Plasma

membrane

of synaptic

knob

Voltage-regulated

ion gates

Figure 11.8 (5)

11-44

Excitation–Contraction Coupling

• Action potential spreads down T tubules

• Opens voltage-gated ion channels in T tubules and Ca

channels in SR

• Ca

leaves SR and enters cytosol

Action potentials propagated

down T tubules

Calcium released from

terminal cisternae

T tubule

Terminal

cisterna

of SR

Sarcoplasmic

reticulum

Figure 11.9 (6, 7)

11-45

• Calcium binds to troponin in thin filaments

• Troponin–tropomyosin complex changes shape and exposes active sites

on actin

Figure 11.9 (8, 9)

Shifting of tropomyosin;

exposure of active sites

on actin

Active sites

Myosin

Troponin

Tropomyosin Actin Thin filament

Binding of calcium

to troponin

Excitation–Contraction Coupling

11-46

Contraction

• ATPase in myosin

head hydrolyzes an

ATP molecule

• Activates the head

“cocking” it in an

extended position

– ADP + P

remain

attached

• Head binds to actin

active site forming

a myosin–actin

cross-bridge

Figure 11.10 (10, 11)

11-47

Contraction

• Myosin releases

ADP and P

, and flexes pulling

thin filament with it—power

stroke

• Upon binding more ATP,

myosin releases actin

and process can be repeated

– Recovery stroke recocks

head

• Each head performs five power

strokes per second

– Each stroke utilizes one

molecule of ATP

Power stroke; sliding of thin

filament over thick filament

Binding of new ATP;

breaking of cross-bridge

ATP

ADP

Figure 11.10 (12, 13)

11-48

Relaxation

• Nerve stimulation and ACh release stop

• AChE breaks down ACh and fragments are reabsorbed into knob

• Stimulation by ACh stops

AChE

Cessation of nervous stimulation

and ACh release

ACh breakdown by

acetylcholinesterase (AChE)

ACh

Figure 11.11 (14, 15)

11-49

Relaxation

• Ca

pumped back into SR by active transport

• Ca

binds to calsequestrin while in storage in SR

Figure 11.11 (16)

Reabsorption of calcium ions by

sarcoplasmic reticulum

Terminal cisterna

of SR

11-50

Relaxation

• Ca

removed from troponin

is pumped back into SR

• Tropomyosin reblocks the

active sites of actin

• Muscle fiber ceases to

produce or maintain tension

• Muscle fiber returns to its

resting length

– Due to recoil of elastic

components and contraction

of antagonistic muscles

Figure 11.11 (17, 18)

Loss of calcium ions from troponin

ADP

Return of tropomyosin to position

blocking active sites of actin

Tropomyosin

ATP

11-51

The Length–Tension Relationship

and Muscle Tone

• Length–tension relationship—the amount of tension

generated by a muscle depends on how stretched or

shortened it was before it was stimulated

– If overly shortened before stimulated, a weak contraction

results, as thick filaments just butt against Z discs

– If too stretched before stimulated, a weak contraction results,

as minimal overlap between thick and thin filaments results in

minimal cross-bridge formation

– Optimum resting length produces greatest force when

muscle contracts

• The nervous system maintains muscle tone (partial contraction) to

ensure that resting muscles are near this length

11-52

Length–Tension Relationship

Figure 11.12

11-53

Rigor Mortis

• Rigor mortis—hardening of muscles and stiffening of

body beginning 3 to 4 hours after death

– Deteriorating sarcoplasmic reticulum releases Ca

– Deteriorating sarcolemma allows Ca

to enter cytosol

– Ca

activates myosin-actin cross-bridging

– Muscle contracts, but cannot relax

• Muscle relaxation requires ATP, and ATP production

is no longer produced after death

– Fibers remain contracted until myofilaments begin to decay

• Rigor mortis peaks about 12 hours after death, then

diminishes over the next 48 to 60 hours

Behavior of Whole Muscles

• Expected Learning Outcomes

– Describe the stages of a muscle twitch.

– Explain how successive muscle twitches can add up

to produce stronger muscle contractions.

– Distinguish between isometric and isotonic

contraction.

– Distinguish between concentric and eccentric

contraction.

11-54

11-55

Threshold, Latent Period, and Twitch

• Myogram—a chart of the

timing and strength of a

muscle’s contraction

• Threshold—minimum voltage

necessary to generate an

action potential in the muscle

fiber and produce a contraction

• Twitch—a quick cycle of

contraction and relaxation

when stimulus is at threshold

or higher

Figure 11.13

display.

Contraction

phase

Relaxation

phase

Time

Latent

period

Time of

stimulation

Muscle tension

11-56

Threshold, Latent Period, and Twitch

Figure 11.13

display.

Contraction

phase

Relaxation

phase

Time

Latent

period

Time of

stimulation

Muscle tension

• Latent period—very brief delay

between stimulus and contraction

– Time required for excitation,

excitation–contraction coupling, and

tensing of elastic components of

muscle (generating internal tension)

• Contraction phase—time when

muscle generates external tension

– Force generated can overcome the

load and cause movement

• Relaxation phase—time when

tension declines to baseline

– SR reabsorbs Ca

, myosin releases

actin and tension decreases

– Takes longer than contraction

• Entire twitch duration varies

between 7 and 100 ms

11-57

Contraction Strength of Twitches

• With subthreshold stimuli—no contraction at all

• At threshold intensity and above—twitch produced

• Even if the same voltage is delivered, different

stimuli cause twitches varying in strength,

because:

– The muscle’s starting length influences tension generation

– Muscles fatigue after continual use

– Warmer muscles’ enzymes work more quickly

– Muscle cell’s hydration level influences cross-bridge

formation

– Increasing the frequency of stimulus delivery increases

tension output

11-58

Contraction Strength of Twitches

• Muscles must contract with variable

strength for different tasks

• Stimulating the nerve with higher

voltages produces stronger

contractions

– Higher voltages excite more nerve fibers

which stimulate more motor units to

contract

– Recruitment or multiple motor unit

(MMU) summation—the process of

bringing more motor units into play with

stronger stimuli

– Occurs according to the size principle:

weak stimuli (low voltage) recruit small

units, while strong stimuli recruit small

and large units for powerful movements

1 2 3 4 5 6 7 8 9

Threshold

Stimulus voltage

Stimuli to nerve

Tension

Proportion of nerve fibers excited

Responses of muscle

Maximum contraction

Figure 11.14

11-59

Contraction Strength of Twitches

• Low frequency stimuli produce identical twitches

• Higher frequency stimuli (eg., 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

11-60

• 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

Muscle develops

tension but does

not shorten

No movement

(a) Isometric contraction

Isometric and Isotonic Contraction

Figure 11.16a

Isometric and Isotonic Contraction

• 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)

11-61

Figure 11.16b, c

11-62

Isometric and Isotonic Contraction

• 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

Muscle

tension

Muscle

length

Isometric

phase

Isotonic

phase

Time

Length or Tension

Figure 11.17

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.

– Define oxygen debt and explain 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.

11-63

11-64

ATP Sources

• All muscle contraction depends on ATP

• 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

• Enables cells to produce ATP in the absence of oxygen

• Yields little ATP and lactic acid, possibly contributing to muscle

fatigue

– Aerobic respiration

• Produces far more ATP

• Does not generate lactic acid

• Requires a continual supply of oxygen

11-65

ATP Sources

Figure 11.18

Aerobic respiration

using oxygen from

myoglobin

Glycogen–

lactic acid

system

(anaerobic

fermentation)

Phosphagen

system

Duration of exercise

0 10 seconds 40 seconds

Aerobic

respiration

supported by

cardiopulmonary

function

Repayment of

oxygen debt

Mode of ATP synthesis

11-66

Immediate Energy

• Short, intense exercise (100 m dash)

– Oxygen is briefly supplied by myoglobin but is rapidly depleted

– Muscles meet most ATP demand by borrowing phosphate

groups (P

) from other molecules and transferring them to ADP

– Two enzyme systems control these phosphate transfers

• Myokinase: transfers P

from one ADP to another, converting the

latter to ATP

• Creatine kinase: obtains P

from a phosphate-storage molecule

creatine phosphate (CP) and gives it to ADP

– Phosphagen system—the combination of ATP and CP which

provides nearly all energy for short bursts of activity

• Enough energy for 6 seconds of sprinting

11-67

Creatine

phosphate

Creatine

kinase

Myokinase

ATP

ADP ADP

ADP

AMP

Figure 11.19

Immediate Energy

11-68

Short-Term Energy

• As the phosphagen system is exhausted, muscles

shift to anaerobic fermentation

– 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 lactic acid

• Glycogen–lactic acid system—the pathway from

glycogen to lactic acid

• Produces enough ATP for 30 to 40 seconds of

maximum activity

11-69

Long-Term Energy

• After about 40 seconds, the respiratory and

cardiovascular systems start to deliver oxygen fast

enough for aerobic respiration to meet most of muscle’s

ATP demand

• 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 minutes, the rate of oxygen consumption levels off to a

steady state where aerobic ATP production keeps pace with demand

– For 30 minutes energy comes equally from glucose and fatty acids

– Beyond 30 minutes, depletion of glucose causes fatty acids to

become the more significant fuel

11-70

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

slow cross-bridge movements, inhibit calcium

release and decrease force production in myofibrils

– (Possibly a drop in pH, but lactic acid does not seem to accumulate

in the muscle cell)

• 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 understood

11-71

Fatigue and Endurance

• Maximum oxygen uptake (VO

max) is major

determinant of one’s ability to maintain high-intensity

exercise for more than 4 to 5 minutes

– VO

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 person

11-72

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 lactic acid

– To provide oxygen to many cells that have elevated

metabolic rates after exercise

• EPOC can be six times basal consumption and last an

hour

11-73

Physiological Classes of

Muscle Fibers

11-74

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)

– Abundant mitochondria, capillaries, myoglobin: deep red color

– Contain a form of myosin with slow ATPase, and a SR that

releases calcium slowly

– Relatively thin fibers

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

motor neurons allowing for precise movements

11-75

Physiological Classes of

Muscle Fibers

• Fast-twitch, fast glycolytic (FG), white, or type IIb 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 powerful movements

11-76

Physiological Classes of

Muscle Fibers

• 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 others more suited for endurance

Physiological Classes of

Muscle Fibers

• Staining for enzymes

(oxidative versus

glycolytic) reveals the

distribution of fiber

types within one

muscle

11-77

Figure 11.20

11-78

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/cm

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 units increases tension

11-79

Muscular Strength and Conditioning

(continued)

• 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

muscles

11-80

Muscular Strength and Conditioning

• 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 thicker

11-81

Muscular Strength and Conditioning

• 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 nervous systems

Cardiac and Smooth Muscle

• Expected Learning Outcomes

– Describe the structural and physiological differences

between cardiac muscle and skeletal muscle.

– Explain why these differences are important to

cardiac function.

– Describe the structural and physiological differences

between smooth muscle and skeletal muscle.

– Relate the unique properties of smooth muscle to its

locations and functions.

11-82

Cardiac and Smooth Muscle

• Cardiac and smooth muscle share certain

properties

– Their cells are myocytes—not as long and

fibrous as skeletal muscles; they have one

nucleus

– They are involuntary

– They receive innervation from the autonomic

nervous system (not from somatic motor neurons)

11-83

11-84

Cardiac Muscle

• Properties of cardiac muscle

– Contracts with regular rhythm

– Works in sleep or wakefulness, without fail, and

without conscious attention

– Highly resistant to fatigue

– Muscle cells of a given chamber must contract in

unison

– Contractions must last long enough to expel blood

11-85

Cardiac Muscle

• Characteristics of cardiac muscle cells

– Striated like skeletal muscle, but myocytes (cardiocytes)

are shorter and thicker

• Sarcoplasmic reticulum less developed, but T tubules are larger

and admit Ca

from the extracellular fluid

– Myocyte is joined at its ends to other myocytes by

intercalated discs

• Appear as thick, dark lines in stained tissue sections

• Electrical gap junctions allow each myocyte to directly stimulate

its neighbors

• Mechanical junctions that keep the myocytes from pulling apart

– Damaged cardiac muscle cells repair by fibrosis

• Unfortunately, after a heart attack, functional muscle is not

regenerated

11-86

Cardiac Muscle

• Can contract without need for nervous

stimulation

– Contains a built-in pacemaker that rhythmically sets

off a wave of electrical excitation

– Wave travels through the muscle and triggers

contraction of heart chambers

– Autorhythmic: able to contract rhythmically and

independently

11-87

Cardiac Muscle

• Autonomic nervous system can increase or decrease

heart rate and contraction strength

• Very slow twitches; does not exhibit quick twitches

like skeletal muscle

- Maintains tension for about 200 to 250 ms

- Gives the heart time to expel blood

• Uses aerobic respiration almost exclusively

- Rich in myoglobin and glycogen

- Has especially large mitochondria

• 25% of volume of cardiac muscle cell

• 2% of skeletal muscle cell with smaller mitochondria

- Highly resistant to fatigue

11-88

Smooth Muscle

• Smooth muscle is named for its lack of

striations

• Some smooth muscles lack nerve supply; others

receive input from autonomic fibers with many

varicosities containing synaptic vesicles

• Capable of mitosis and hyperplasia

• Injured smooth muscle regenerates well

• Smooth muscle is slower than skeletal and

cardiac muscle

– Takes longer to contract but can remain contracted for

a long time without fatigue

11-89

Smooth Muscle

• Smooth muscle forms layers within walls of

hollow organs

– It can propel contents of an organ (e.g., food in GI tract)

– It can modify pressure and flow of blood in the

circulatory system and air in the respiratory system

• Can provide fine control in some locations

– Smooth muscle of iris controls pupil size

– Piloerector muscles raise hairs in skin

11-90

Smooth Muscle Myocyte Structure

• Myocytes have a fusiform shape

– There is one nucleus, located near the middle of the cell

– Thick and thin filaments are present, but not aligned with

each other (“smooth” not striated)

• Sarcoplasmic reticulum is scanty and there are no T

tubules

• Ca

needed for muscle contraction comes from

ECF by way of Ca

channels in sarcolemma

• Z discs are absent and replaced by dense bodies

– Well-ordered array of protein masses that form protein

plaques on the inner face of the plasma membrane and

scattered throughout sarcoplasm

– Dense bodies of neighboring cells are linked together

11-91

Types of Smooth Muscle

• Multiunit smooth muscle

– Occurs in some of the largest

arteries and air passages, in

piloerector muscles, and in

iris of the eye

– Autonomic innervation forms

motor units

• Terminal branches of a nerve

fiber synapse with individual

myocytes

• Each motor unit contracts

independently of the others

Synapses

Autonomic

nerve fibers

(a) Multiunit

smooth muscle

Figure 11.23a

11-92

Types of Smooth Muscle

• Single-unit smooth muscle

– More common

– Occurs in most blood

vessels, in digestive,

respiratory, urinary, and

reproductive tracts

– Also called visceral muscle

• Often in two layers: inner

circular and outer

longitudinal

– Myocytes of this cell type are

electrically coupled to each

other by gap junctions

– They directly stimulate each

other and a large number of

cells contract as one

Figure 11.23b

Varicosities

Gap junctions

Autonomic

nerve fibers

(b) Single-unit

smooth muscle

11-93

Types of Smooth Muscle

Figure 11.21

Synaptic

vesicle

Mitochondrion

Autonomic

nerve fiber

Varicosities

Single-unit

smooth muscle

11-94

Types of Smooth Muscle

Epithelium

Mucosa:

Muscularis externa:

Lamina propria

Muscularis

mucosae

Circular layer

Longitudinal

layer

Figure 11.22

11-95

Excitation of Smooth Muscle

• Smooth muscle is involuntary and can contract (or

relax) in response to a variety of stimuli

– Autonomic activity

• Parasympathetic nerves secrete acetylcholine stimulating GI tract

smooth muscle

• Sympathetic nerves secrete norepinephrine relaxing smooth

muscle in bronchioles (dilating them)

– Hormones, carbon dioxide, oxygen, and pH

• Hormone oxytocin stimulates uterine contractions

– Temperature

• Cold excites piloerector muscles

• Warmth relaxes muscle in skin blood vessels

– Stretch

• Stomach contracts when stretched by food

– Autorhythmicity

• Some single-unit smooth muscle cells in GI tract are

pacemakers

11-96

Contraction and Relaxation

• Contraction is always triggered by Ca

energized by ATP, and achieved by sliding

filaments

• Smooth muscle gets most Ca

from ECF

– Gated Ca

channels open to allow Ca

to enter cell

– Calcium channels are concentrated in caveolae—

pockets on sarcolemma

– Different channels gated by different stimuli

• Can respond to mechanical stretch, voltage, or chemical ligands

11-97

Contraction and Relaxation

• Calcium binds to calmodulin on thick filaments

– Activates myosin light-chain kinase; adds phosphate to

regulatory protein on myosin head

• Myosin ATPase, hydrolyzes ATP

– Enables myosin similar power and recovery strokes like

skeletal muscle

• Thick filaments pull on thin ones, thin ones pull on

dense bodies and membrane plaques

• Force is transferred to plasma membrane and entire

cell shortens

– Puckers and twists like someone wringing out a wet towel

11-98

Smooth Muscle Contraction

Dense body

Intermediate filaments

of cytoskeleton

Actin filaments

(b) Contracted smooth

muscle cells

(a) Relaxed smooth muscle cells

Plaque

Myosin

Figure 11.24a,b

11-99

• Contraction and relaxation very slow in

comparison to skeletal muscle

– Latent period in smooth muscle 50 to 100 ms (versus

2 ms in skeletal muscle)

– Tension peaks at about 500 ms (0.5 sec)

– Declines over a period of 1 to 2 seconds

– Slow myosin ATPase enzyme and pumps that

remove Ca

– Ca

binds to calmodulin instead of troponin

• Activates kinases and ATPases that hydrolyze ATP

Contraction and Relaxation

11-100

• Latch-bridge mechanism is resistant to fatigue

– Heads of myosin molecules do not detach from actin

immediately

– Do not consume any more ATP

– Maintains tetanus tonic contraction (smooth muscle

tone)

• Arteries—vasomotor tone; intestinal tone

– Makes most of its ATP aerobically

Contraction and Relaxation

11-101

Response to Stretch

• Stretch can open mechanically gated calcium

channels in the sarcolemma causing contraction

– Peristalsis: waves of contraction brought about by food

distending the esophagus or feces distending the colon

• Propels contents along the organ

• Stress–relaxation response (receptive relaxation)—

helps hollow organs gradually fill (urinary bladder)

– When stretched, tissue briefly contracts then relaxes;

helps prevent emptying while filling

11-102

• Skeletal muscle cannot contract forcefully if

overstretched

• Smooth muscle contracts forcefully even

when greatly stretched

– Allows hollow organs such as the stomach and

bladder to fill and then expel their contents efficiently

• Plasticity—the ability to adjust its tension to the

degree of stretch

– A hollow organ such as the bladder can be greatly

stretched yet not become flabby when empty

Response to Stretch

11-103

• Smooth muscle can be anywhere from half to

twice its resting length and still contract

powerfully

• Three reasons

– There are no Z discs, so thick filaments cannot butt

against them and stop contraction

– Since the thick and thin filaments are not arranged in

orderly sarcomeres, stretching does not cause a

situation where there is too little overlap for cross-

bridges to form

– The thick filaments of smooth muscle have myosin

heads along their entire length, so cross-bridges can

form anywhere

Response to Stretch

11-104

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

– 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; incurable

11-105

Muscular Dystrophy

• 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 pelvic muscles

11-106

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 eyes

11-107

Myasthenia Gravis

• 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 blood plasma

11-108

Myasthenia Gravis

• Drooping eyelids and weakness of muscles of eye

Figure 11.25