Contraction of Skeletal Muscle pdf - Membrane Physiology and Nerve and Muscle

From this disc, these filaments extend in both directions to

Z disc.

Figure 6–1

cross-bridges.

These are

filaments in Figure 6–1

polarized light. Note also the small projections from the sides of the myosin

overlap the myosin, and are called

contain myosin filaments, as well as the ends of the actin filaments where they

to polarized light. The dark bands

illustrated in Figure 6–2. The light bands contain only actin filaments and are

tate and thus cause the myofibrils to have alternate light and dark bands, as

Note in Figure 6–1

The thick filaments in the diagrams are

6–1, parts

tron micrograph of Figure 6–2 and are represented diagrammatically in Figure

the actual muscle contraction. These can be seen in longitudinal view in the elec-

aments,

Each myofibril (Figure 6–1

dots in the cross-sectional view of Figure 6–1

myofibrils,

into bundles to form the muscle tendons that then insert into the bones.

of the sarcolemma fuses with a tendon fiber, and the tendon fibers in turn collect

numerous thin collagen fibrils. At each end of the muscle fiber, this surface layer

plasma membrane,

colemma consists of a true cell membrane, called the

The sarcolemma is the cell membrane of the muscle fiber. The sar-

one nerve ending, located near the middle of the fiber.

Except for about 2 per cent of the fibers, each fiber is usually innervated by only

In most skeletal muscles, each fiber extends the entire length of the muscle.

subunits, also shown in Figure 6–1 and described in subsequent paragraphs.

micrometers in diameter. Each of these fibers is made up of successively smaller

Figure 6–1 shows the organization of skeletal muscle, demonstrating that all

smooth muscle are discussed in Chapter 8, and

is considered mainly; the specialized functions of

muscle. In this chapter, function of skeletal muscle

cardiac muscle. Some of the same basic principles of

About 40 per cent of the body is skeletal muscle,

Contraction of

Skeletal Muscle

and perhaps another 10 per cent is smooth and

contraction apply to all these different types of

cardiac muscle is discussed in Chapter 9.

Physiologic Anatomy of Skeletal Muscle

Skeletal Muscle Fiber

skeletal muscles are composed of numerous fibers ranging from 10 to 80

Sarcolemma.

and an

outer coat made up of a thin layer of polysaccharide material that contains

Myofibrils; Actin and Myosin Filaments.

Each muscle fiber contains several hundred

to several thousand

which are demonstrated by the many small open

and E) is composed of about 1500 adjacent myosin filaments and 3000 actin fil-

which are large polymerized protein molecules that are responsible for

E through L.

myosin, and the

thin filaments are actin.

E that the myosin and actin filaments partially interdigi-

called I bands because they are isotropic

A bands because they are anisotropic to

E and L.

It is the interaction

between these cross-bridges and the actin filaments that causes contraction.

E also shows that the ends of the actin filaments are attached to a

so-called

comere will work. There is reason to believe that the

These springy titin molecules act as a

very springy.

ecules in the body. Also, because it is filamentous, it is

million, which makes it one of the largest protein mol-

ficult to maintain. This is achieved by a large number

The side-by-side rela-

Titin Filamentous Molecules.

What Keeps the Myosin and Actin Filaments in Place?

that, at this length, the muscle is capable of generating

beginning to overlap one another. We will see later

filaments, and the tips of the actin filaments are just

the sarcomere is about 2 micrometers. At this length,

shown at the bottom of Figure 6–4, the length of

When the muscle fiber is contracted, as

sarcomere.

The portion of the myofibril (or of the whole muscle

cardiac muscle their striated appearance.

the individual myofibrils.These bands give skeletal and

the entire muscle fiber has light and dark bands, as do

another all the way across the muscle fiber. Therefore,

myofibril to myofibril, attaching the myofibrils to one

ferent from the actin and myosin filaments, passes

interdigitate with the myosin filaments. The Z disc,

Chapter 6

Contraction of Skeletal Muscle

which itself is composed of filamentous proteins dif-

crosswise across the myofibril and also crosswise from

fiber) that lies between two successive Z discs is called
a

the actin filaments completely overlap the myosin

its greatest force of contraction.

tionship between the myosin and actin filaments is dif-

of filamentous molecules of a protein called titin. Each
titin molecule has a molecular weight of about 3

framework that holds the myosin and actin filaments
in place so that the contractile machinery of the sar-

Sarcomere Z

SKELETAL MUSCLE

G-Actin molecules

F-Actin filament

Myofilaments

Myosin filament

Myosin molecule

Light

meromyosin

Heavy

meromyosin

Muscle fasciculus

Muscle

Muscle fiber

Myofibril

band

disc

band

book of Histology. Philadelphia:

DW: Bloom and Fawcett: A Text-

Keene. Modified from Fawcett

(Drawing by Sylvia Colard

are cross

F, G, H,

from the gross to the molecular

Figure 6–1

Organization of skeletal muscle,

level.

and I

sections at the levels indicated.

WB Saunders, 1986.)

another. Conversely, in the contracted state, these actin

state, the ends of the actin filaments extending from

(top) and the contracted state (bottom). In the relaxed

traction. It shows the relaxed state of a sarcomere

Figure

muscle contractile process.

We now describe the molecular machinery of the

myofibrils causes the muscle contraction to cease.

comes along; this removal of calcium ions from the

membrane pump, and they remain stored in

8. After a fraction of a second, the calcium ions are

process.

slide alongside each other, which is the contractile

the actin and myosin filaments, causing them to

7. The calcium ions initiate attractive forces between

fiber. Here it causes the sarcoplasmic reticulum to

membrane, and much of the action potential

6. The action potential depolarizes the muscle

travel along nerve fiber membranes.

5. The action potential travels along the muscle fiber

an action potential at the membrane.

interior of the muscle fiber membrane. This initiates

4. Opening of the acetylcholine-gated channels allows

in the membrane.

gated” channels through protein molecules floating

fiber membrane to open multiple “acetylcholine-

3. The acetylcholine acts on a local area of the muscle

acetylcholine.

2. At each ending, the nerve secretes a small amount

its endings on muscle fibers.

1. An action potential travels along a motor nerve to

occur in the following sequential steps.

The initiation and execution of muscle contraction

contraction, as discussed in Chapter 7.The very rapidly

This reticulum has a special organiza-

extensive reticulum (Figure 6–3), called the

triphosphate (ATP) formed by the mitochondria.

myofibrils. These supply the contracting myofibrils

mitochondria

multiple protein enzymes. Also present are tremen-

ties of potassium, magnesium, and phosphate, plus

lular fluid called sarcoplasm, containing large quanti-

are suspended side by side in the muscle fiber. The

The many myofibrils of each muscle fiber

sarcomere, especially the myosin filaments.

Membrane Physiology, Nerve, and Muscle

Unit II

titin molecule itself acts as template for initial forma-
tion of portions of the contractile filaments of the

Sarcoplasm.

spaces between the myofibrils are filled with intracel-

dous numbers of

that lie parallel to the

with large amounts of energy in the form of adenosine

Sarcoplasmic Reticulum.

Also in the sarcoplasm sur-

rounding the myofibrils of each muscle fiber is an

sarcoplas-

mic reticulum.
tion that is extremely important in controlling muscle

contracting types of muscle fibers have especially
extensive sarcoplasmic reticula.

General Mechanism of Muscle
Contraction

of the neurotransmitter substance

large quantities of sodium ions to diffuse to the

membrane in the same way that action potentials

electricity flows through the center of the muscle

release large quantities of calcium ions that have
been stored within this reticulum.

pumped back into the sarcoplasmic reticulum by a
Ca

the reticulum until a new muscle action potential

Molecular Mechanism
of Muscle Contraction

Sliding Filament Mechanism of Muscle Contraction.

6–4 demonstrates the basic mechanism of muscle con-

two successive Z discs barely begin to overlap one

filaments have been pulled inward among the myosin

lying between the myofibrils. (From Fawcett DW: The Cell.

Electron micrograph of muscle myofibrils showing the detailed

Figure 6–2

organization of actin and myosin filaments. Note the mitochondria

Philadelphia: WB Saunders, 1981.)

(From Fawcett DW: The Cell. Philadelphia: WB Saunders, 1981.)

ducting the electrical signal into the center of the muscle fiber.

to the exterior of the fiber membrane and are important for con-

(arrows)

brils. Also shown in cross section are T tubules

Sarcoplasmic reticulum in the extracellular spaces between the

Figure 6–3

myofibrils, showing a longitudinal system paralleling the myofi-

that lead

from the center.

almost exactly 1.6 micrometers. Note, however, that

The total length of each myosin filament is uniform,

ing sections.

actual contraction process, as discussed in the follow-

the body. The hinged heads in turn participate in the

the head attaches to the arm. The hinged arms allow

the body of the myosin filament, and the other where

cross-bridges.

the figure. The protruding arms and heads together are

extends the head outward from the body, as shown in

side along with the head, thus providing an

molecules hang outward to the sides of the body. Also,

of the filament, while many heads of the

these filaments is shown in Figure 6–5

vidual myosin molecules. The central portion of one of

The

to each head. These light chains help control the func-

four light chains are also part of the myosin head, two

at one end of the double-helix myosin molecule. The

Thus, there are two free heads

myosin molecule. One end of each of these chains is

to form a double helix, which is called the

The two heavy chains wrap spirally around each other

chains

a molecular weight of about 200,000, and four

heavy chains,

(see Figure 6–5

The

of two actin filaments.

of many molecules to form a myosin filament, as well

vidual molecule; Figure 6–5

weight of about 480,000. Figure 6–5

multiple myosin molecules, each having a molecular

The myosin filament is composed of

Contractile Filaments

Molecular Characteristics of the

energy. In the next few sections, we describe what is

energy bonds in the ATP molecule, which is degraded

process to proceed. This energy comes from high-

tion begins. But energy is needed for the contractile

between the myosin and actin filaments, and contrac-

myofibrils. The calcium ions in turn activate the forces

an action potential travels along the muscle fiber,

resting conditions, these forces are inactive, but when

the myosin filaments with the actin filaments. Under

among the myosin filaments? This is caused by forces

sliding filament mechanism.

myosin filaments. Thus, muscle contraction occurs by a

their maximum extent. Also, the Z discs have been

filaments, so that their ends overlap one another to

Chapter 6

Contraction of Skeletal Muscle

pulled by the actin filaments up to the ends of the

But what causes the actin filaments to slide inward

generated by interaction of the cross-bridges from

this causes the sarcoplasmic reticulum to release large
quantities of calcium ions that rapidly surround the

to adenosine diphosphate (ADP) to liberate the

known about the details of these molecular processes
of contraction.

Myosin Filament.

A shows an indi-

B shows the organization

as interaction of this filament on one side with the ends

myosin molecule

A) is composed

of six polypeptide chains—two

each with

light

with molecular weights of about 20,000 each.

tail of the

folded bilaterally into a globular polypeptide structure
called a myosin head.

tion of the head during muscle contraction.

myosin filament is made up of 200 or more indi-

B, displaying the

tails of the myosin molecules bundled together to form
the body

part of the body of each myosin molecule hangs to the

arm that

called

Each cross-bridge is flexible at

two points called hinges—one where the arm leaves

the heads either to be extended far outward from the
body of the myosin filament or to be brought close to

there are no cross-bridge heads in the very center of
the myosin filament for a distance of about 0.2
micrometer because the hinged arms extend away

Relaxed

Contracted

each other.

pulling of the Z membranes toward

(red),

Figure 6–4

Relaxed and contracted states of a myofibril showing (top) sliding
of the actin filaments (pink) into the spaces between the myosin
filaments

and (bottom)

Two heavy chains

Tail

Head

Light chains

Actin filaments

Hinges

Myosin filament

Body

Cross-bridges

f the cross-

cross-bridges

to form a myosin filament. Also shown are thousands of myosin

Figure 6–5

A, Myosin molecule. B, Combination of many myosin molecules

and interaction between the heads o

bridges with adjacent actin filaments.

causes contraction to occur. Although the precise

active sites of the actin filament, and this, in some way,

by the calcium ions, the heads of the cross-bridges

Myosin Cross-Bridges—The “Walk-Along” Theory of Contrac-

is altered by calcium ions, producing a new condition

mechanism, it does emphasize that the normal relation

traction to proceed. Although this is a hypothetical

ers” the active sites of the actin, thus allowing these to

the groove between the two actin strands. This “uncov-

calcium ions, the troponin complex supposedly under-

ing: When calcium ions combine with troponin C, each

of this is not known, but one suggestion is the follow-

the actin filaments is itself inhibited. The mechanism

the presence of large amounts of calcium ions, the

This brings us to the role of the calcium ions. In

contraction. Before contraction can take place, the

tropomyosin complex. Consequently, the sites cannot

place. Therefore, it is believed that the active sites on

of the myosin molecules. Then, if the troponin-

ATP) binds instantly and strongly with the heads

Inhibition of the Actin Filament by the Troponin-Tropomyosin

Two Actin Filaments, and Calcium Ions

contraction process, as explained in the next section.

the tropomyosin to the actin. The strong affinity of the

for calcium ions. This complex is believed to attach

(troponin T) for tropomyosin, and a third (troponin C)

(troponin I) has a strong affinity for actin, another

controlling muscle contraction. One of the subunits

protein subunits, each of which plays a specific role in

These are actually complexes of three loosely bound

Troponin and Its Role in Muscle Contraction.

the active sites of the actin strands, so that attraction

resting state, the tropomyosin molecules lie on top of

spirally around the sides of the F-actin helix. In the

length of 40 nanometers. These molecules are wrapped

The actin filament also con-

Tropomyosin Molecules.

molecules, as shown in Figure 6–4.

the Z discs; the ends of the filaments protrude in both

Each actin filament is about 1 micrometer long. The

nanometers.

strands of the double helix are staggered, giving one

muscle contraction. The active sites on the two F-actin

ADP. It is believed that these ADP molecules are the

molecular weight of about 42,000. Attached to each

, each having a

G-actin molecules

myosin molecule.

two lighter-colored strands in Figure 6–6. The two

, represented by the

The backbone of the actin filament is a double-

The actin filament is also complex.

tion process.

ATP and to use the energy derived from the ATP’s

explained later, this property allows the head to cleave

ATPase enzyme.

ATPase Activity of the Myosin Head.

120 degrees. This ensures that the cross-bridges extend

Now, to complete the picture, the myosin filament

Membrane Physiology, Nerve, and Muscle

Unit II

itself is twisted so that each successive pair of cross-
bridges is axially displaced from the previous pair by

in all directions around the filament.

Another feature

of the myosin head that is essential for muscle con-
traction is that it functions as an

high-energy phosphate bond to energize the contrac-

Actin Filament.

It is composed of three protein components: actin,
tropomyosin, and troponin.

stranded F-actin protein molecule

strands are wound in a helix in the same manner as the

Each strand of the double F-actin helix is composed

of polymerized

one of the G-actin molecules is one molecule of

active sites on the actin filaments with which the cross-
bridges of the myosin filaments interact to cause

active site on the overall actin filament about every 2.7

bases of the actin filaments are inserted strongly into

directions to lie in the spaces between the myosin

tains another protein, tropomyosin. Each molecule of
tropomyosin has a molecular weight of 70,000 and a

cannot occur between the actin and myosin filaments
to cause contraction.

Attached

intermittently along the sides of the tropomyosin mol-
ecules are still other protein molecules called troponin.

troponin for calcium ions is believed to initiate the

Interaction of One Myosin Filament,

to Cause Contraction

Complex; Activation by Calcium Ions.

A pure actin filament

without the presence of the troponin-tropomyosin
complex (but in the presence of magnesium ions and

tropomyosin complex is added to the actin filament,
the binding between myosin and actin does not take

the normal actin filament of the relaxed muscle are
inhibited or physically covered by the troponin-

attach to the heads of the myosin filaments to cause

inhibitory effect of the troponin-tropomyosin complex
must itself be inhibited.

inhibitory effect of the troponin-tropomyosin on

molecule of which can bind strongly with up to four

goes a conformational change that in some way tugs
on the tropomyosin molecule and moves it deeper into

attract the myosin cross-bridge heads and cause con-

between the troponin-tropomyosin complex and actin

that leads to contraction.

Interaction Between the “Activated” Actin Filament and the

tion.

As soon as the actin filament becomes activated

from the myosin filaments become attracted to the

Troponin complex

F-actin

Tropomyosin

Active sites

troponin

tropomyosin molecule is a

grooves between the actin strands. Attached to one end of each

molecules that fit in the

tropomyosin

Figure 6–6

Actin filament, composed of two helical strands of F-actin mole-
cules and two strands of

complex that initiates

contraction.

yet reached the center of the myosin filament. With

point, the actin filament has already overlapped all

length decreases to about 2.2 micrometers. At this

filament begins to overlap the myosin filament, the

is zero. Then, as the sarcomere shortens and the actin

point, the tension developed by the activated muscle

myosin filament, with no actin-myosin overlap. At this

mere lengths. At point D on the diagram, the actin

right, shown in black, are different degrees of overlap

tension developed by a contracting muscle fiber. To the

Figure 6–8 shows the effect of sarcomere length and

Developed by the Contracting Muscle

Filament Overlap on Tension

Effect of Amount of Actin and Myosin

occur.

Thus, the process proceeds again and again until the

stroke.

active site on the actin filament, it becomes

derived from the cleaved ATP) binds with a new

6. When the cocked head (with its stored energy

power stroke cycle.

perpendicular condition, ready to begin the new

the energy again “cocks” the head back to its

cycle, leading to a new power stroke. That is,

new molecule of ATP is cleaved to begin the next

5. After the head has detached from the actin, the

This binding of new ATP causes detachment of

release of the ADP, a new molecule of ATP binds.

previously attached to the head. At the site of

release of the ADP and phosphate ion that were

4. Once the head of the cross-bridge tilts, this allows

was cleaved earlier.

that occurred in the head when the ATP molecule

“cocked” spring, by the conformational change

power stroke is the energy already stored, like a

actin filament. The energy that activates the

power stroke

This provides the

head to tilt toward the arm of the cross-bridge.

conformational change in the head, prompting the

3. The bond between the head of the cross-bridge

with these, as shown in Figure 6–7.

are uncovered, and the myosin heads then bind

with calcium ions, active sites on the actin filament

2. When the troponin-tropomyosin complex binds

ion, bound to the head. In this state, the

leaves the cleavage products, ADP plus phosphate

myosin head immediately cleaves the ATP but

bridges bind with ATP. The ATPase activity of the

1. Before contraction begins, the heads of the cross-

The

Fenn effect.

ATP that is cleaved, which is called the

performed by the muscle, the greater the amount of

contraction process; the greater the amount of work

amounts of ATP are cleaved to form ADP during the

tracts, work is performed and energy is required. Large

When a muscle con-

ATP as the Source of Energy for Contraction—Chemical Events

filament at any given time, the greater, theoretically,

in a continuous repeated cycle. Therefore, the greater

independently of all others, each attaching and pulling

along the actin filament, pulling the ends of two suc-

moves another step. Thus, the heads of the cross-

to cause a new power stroke, and the actin filament

down along the actin filament; then the head tilts again

position, it combines with a new active site farther

the head returns to its extended direction. In this

automatically breaks away from the active site. Next,

Then, immediately after tilting, the head

power stroke.

ment along with it. This tilt of the head is called the

cross-bridge. The new alignment of forces causes the

when a head attaches to an active site, this attach-

active sites of an actin filament. It is postulated that

mechanism for contraction. The figure shows the heads

Figure 6–7 demonstrates this postulated walk-along

“ratchet”

dence exists is the “walk-along” theory (or

theoretical, one hypothesis for which considerable evi-

Chapter 6

Contraction of Skeletal Muscle

manner by which this interaction between the cross-
bridges and the actin causes contraction is still partly

theory) of contraction.

of two cross-bridges attaching to and disengaging from

ment simultaneously causes profound changes in the
intramolecular forces between the head and arm of its

head to tilt toward the arm and to drag the actin fila-

bridges bend back and forth and step by step walk

cessive actin filaments toward the center of the myosin
filament.

Each one of the cross-bridges is believed to operate

the number of cross-bridges in contact with the actin

the force of contraction.

in the Motion of the Myosin Heads.

following sequence of events is believed to be the
means by which this occurs:

conformation of the head is such that it extends
perpendicularly toward the actin filament but is
not yet attached to the actin.

and the active site of the actin filament causes a

for pulling the

the head from the actin.

uncocked and once again provides a new power

actin filaments pull the Z membrane up against
the ends of the myosin filaments or until the load on
the muscle becomes too great for further pulling to

amount of myosin-actin filament overlap on the active

of the myosin and actin filaments at different sarco-

filament has pulled all the way out to the end of the

tension increases progressively until the sarcomere

the cross-bridges of the myosin filament but has not

Actin filament

Active sites

Myosin filament

Hinges

Power
stroke

Movement

“Walk-along” mechanism for contraction of the muscle.

Figure 6–7

In mathematical terms, work is defined by the

This means that

When a muscle contracts against a load, it performs

Work Output During Muscle

Energetics of Muscle

caused by muscle contraction. Therefore, the net force

This decreasing velocity of contraction with load is

results, despite activation of the muscle fiber.

the maximum force that the muscle can exert, the veloc-

6–10. That is, when the load has been increased to equal

gressively less as the load increases, as shown in Figure

are applied, the velocity of contraction becomes pro-

in about 0.1 second for the average muscle. When loads

Relation of Velocity of Contraction

eters. This is demonstrated by the decreased length of

muscle is stretched beyond its normal length—that is,

contraction, called

However, the

of about 2 micrometers, it contracts upon activation

length, which is at a sarcomere length

Note in Figure 6–9 that when the muscle is at its

as noted in Figure 6–9.

individual muscle fiber, but it exhibits the same general

the same amount. Therefore, the curve has somewhat

amount of connective tissue in it; also, the sarcomeres

single muscle fiber. The whole muscle has a large

tension of the intact, whole muscle rather than of a

that in Figure 6–8, but the curve in Figure 6–9 depicts

The top curve of Figure 6–9 is similar to

of contraction approaches zero, but the entire muscle

are crumpled and, as shown in the figure, the strength

sarcomere lengths, the ends of the myosin filaments

aments. Then, as contraction proceeds to still shorter

contraction decreases rapidly. At this point, the two Z

about 1.65 micrometers, at point A, the strength of

addition to overlapping the myosin filaments. As the

of about 2 micrometers. At this point, the ends of the

tension until point B is reached, at a sarcomere length

further shortening, the sarcomere maintains full

Membrane Physiology, Nerve, and Muscle

Unit II

two actin filaments begin to overlap each other in

sarcomere length falls from 2 micrometers down to

discs of the sarcomere abut the ends of the myosin fil-

has now contracted to its shortest length.

Effect of Muscle Length on Force of Contraction in the Whole
Intact Muscle.

in different parts of the muscle do not always contract

different dimensions from those shown for the

form for the slope in the normal range of contraction,

normal resting

with the approximate maximum force of contraction.

increase in tension that occurs during

active tension, decreases as the

to a sarcomere length greater than about 2.2 microm-

the arrow in the figure at greater than normal muscle
length.

to Load

A skeletal muscle contracts extremely rapidly when it
contracts against no load—to a state of full contraction

ity of contraction becomes zero and no contraction

caused by the fact that a load on a contracting muscle
is a reverse force that opposes the contractile force

that is available to cause velocity of shortening is
correspondingly reduced.

Contraction

work.

energy is transferred from the

muscle to the external load to lift an object to a greater
height or to overcome resistance to movement.

following equation:

100

Length of sarcomere (micrometers)

B C

Tension developed

(per cent)

vertebrate striated muscle fibers. J Physiol 171:28P, 1964.)

AM, Huxley AF, Julian FJ: The length-tension diagram of single

comere lengths from point A to point D. (Modified from Gordon

tive positions of the actin and myosin filaments at different sar-

2.0 to 2.2 micrometers in length. At the upper right are the rela-

showing maximum strength of contraction when the sarcomere is

Length-tension diagram for a single fully contracted sarcomere,

Figure 6–8

Length

Tension of muscle

1/2

normal

2 x

normal

Normal

Increase in tension

during contraction

Tension during

contraction

Normal range of contraction

Tension

before contraction

Relation of muscle length to tension in the muscle both before and

Figure 6–9

during muscle contraction.

ATP itself can later be converted into work.

even then, only 40 to 45 per cent of the energy in the

in foodstuffs is lost during the formation of ATP, and

this low efficiency is that about one half of the energy

cent, with the remainder becoming heat. The reason for

work, even under the best conditions, is less than 25 per

The percentage of the input energy to muscle (the

The efficiency of an

physiology.

In addition, the importance of the different mecha-

The detailed mechanisms of these energetic

energy can come from stored carbohydrates.

periods of 2 to 4 hours, as much as one half of the

greatest proportion of energy comes from fats, but for

and protein. For extremely long-term maximal muscle

foodstuffs that are consumed are carbohydrates, fats,

term contraction is derived from this source. The

of all energy used by the muscles for sustained, long-

lular foodstuffs to liberate ATP. More than 95 per cent

This means combining oxygen with the

oxidative

The third and final source of energy is

minute.

lar foodstuffs reacting with oxygen. However, so

times as rapid as ATP formation in response to cellu-

mation of ATP by the glycolytic process is about 2.5

from the blood is not available. Second, the rate of for-

to more than a minute, even when oxygen delivery

in the absence of oxygen, so that muscle contraction

twofold. First, the glycolytic reactions can occur even

The importance of this glycolysis mechanism is

phosphocreatine.

that is used to convert ADP to ATP; the ATP can

muscle cells. Rapid enzymatic breakdown of the glyco-

is “glycolysis” of

used to reconstitute both ATP and phosphocreatine,

The second important source of energy, which is

seconds.

the combined energy of both the stored ATP and

only about five times as great as the ATP. Therefore,

reconstitute the ATP. However, the total amount of

causes bonding of a new phosphate ion to ADP to

creatine is instantly cleaved, and its released energy

more fully in Chapters 67 and 72. Therefore, phospho-

energy than that of each ATP bond, as is discussed

bonds of ATP. The high-energy phosphate bond of

phosphocreatine,

tute the ATP is the substance

The first source of energy that is used to reconsti-

traction. There are several sources of the energy for

second, which allows the muscle to continue its con-

lated to form new ATP within another fraction of a

as described in Chapter 2, the ADP is rephosphory-

to the contracting machinery of the muscle fiber. Then,

ADP, which transfers energy from the ATP molecule

for only 1 to 2 seconds at most.The ATP is split to form

4 millimolar, is sufficient to maintain full contraction

The concentration of ATP in the muscle fiber, about

for propagation of muscle fiber action potentials.

ulum after the contraction is over, and (2) pumping

which the cross-bridges pull the actin filaments, but

on energy supplied by ATP. Most of this energy is

We have already seen that muscle contraction depends

tion, as described in the following sections.

the distance of movement against the load. The energy

in which W is the work output, L is the load, and D is

Chapter 6

Contraction of Skeletal Muscle

= L ¥ D

required to perform the work is derived from the
chemical reactions in the muscle cells during contrac-

Sources of Energy for Muscle
Contraction

required to actuate the walk-along mechanism by

small amounts are required for (1) pumping calcium
ions from the sarcoplasm into the sarcoplasmic retic-

sodium and potassium ions through the muscle fiber
membrane to maintain appropriate ionic environment

this rephosphorylation.

which

carries a high-energy phosphate bond similar to the

phosphocreatine has a slightly higher amount of free

phosphocreatine in the muscle fiber is also very little—

the phosphocreatine in the muscle is capable of
causing maximal muscle contraction for only 5 to 8

glycogen previously stored in the

gen to pyruvic acid and lactic acid liberates energy

then be used directly to energize additional muscle
contraction and also to re-form the stores of

can be sustained for many seconds and sometimes up

many end products of glycolysis accumulate in the
muscle cells that glycolysis also loses its capability to
sustain maximum muscle contraction after about 1

metabolism.
end products of glycolysis and with various other cel-

activity—over a period of many hours—by far the

processes are discussed in Chapters 67 through 72.

nisms of energy release during performance of
different sports is discussed in Chapter 84 on sports

Efficiency of Muscle Contraction.

engine or a motor is calculated as the percentage of
energy input that is converted into work instead of heat.

chemical energy in nutrients) that can be converted into

Velocity of contraction (cm

/sec)

Load-opposing contraction (kg)

with a cross section of 1 square centimeter and a length of 8

Figure 6–10

Relation of load to velocity of contraction in a skeletal muscle

centimeters.

Membrane Physiology, Nerve, and Muscle

Unit II

120

160

200

Milliseconds

Force of contraction

Duration of

depolarization

Ocular
muscle

Gastrocnemius

Soleus

energy by the glycolytic process. (4) Less extensive

release of calcium ions to initiate contraction. (3) Large

traction. (2) Extensive sarcoplasmic reticulum for rapid

Fast Fibers.

between these two types of fibers are as follows.

composed mainly of “slow” fibers. The differences

numbers of the slow variety. Conversely, the muscles

composed mainly of “fast” fibers with only small

these two extremes. The muscles that react rapidly are

muscle fibers, with still other fibers gradated between

slow

Chapter 84 on sports physiology, every muscle of the

Fast Versus Slow Muscle Fibers.

long-term support of the body against gravity.

running and jumping, and the soleus muscle is con-

specific objects to provide accuracy of vision. The gas-

respective muscles. Ocular movements must be

tion of about 1/3 second. It is interesting that these dura-

soleus muscle, which has a duration of contrac-

a duration of contraction of about 1/15 second; and the

than 1/40 second; the gastrocnemius muscle, which has

of three types of skeletal muscle: an ocular muscle,

Figure 6–12 shows records of isometric contractions

contraction differ among muscles.

siderably from one muscle to another. Therefore, it is no

Finally, the energetics of muscle contraction vary con-

micrometers in diameter or as large as 80 micrometers.

stapedius. Further, the fibers may be as small as 10

quadriceps muscle, a half million times as large as the

a millimeter or so in diameter, up to the very large

middle ear, measuring only a few millimeters long and

The human body has many sizes of skeletal

Characteristics of Isometric Twitches Recorded from Different

functional characteristics of different muscle types.

in force of muscle contraction itself. Therefore, the iso-

However, the isometric system records strictly changes

the muscle contracts, as well as the inertia of the load.

muscle lifting a pan of weights. The characteristics of

this is illustrated on the left in the figure, showing a

tonic system, the muscle shortens against a fixed load;

length, as shown on the right in Figure 6–11. In the iso-

In the isometric system, the muscle contracts against

muscle contraction are shown in Figure 6–11.

the contraction. Systems for recording the two types of

Isometric Versus Isotonic Contraction.

stimulus through the muscle itself, giving rise to a single,

This can be

muscle twitches.

ciency is developed when the velocity of contraction is

efficiency of contraction. Ordinarily, maximum effi-

friction within the muscle itself, and this, too, reduces the

as zero. Conversely, if contraction is too rapid, large pro-

thereby decreasing the conversion efficiency to as little

traction, even though little or no work is performed,

contracts slowly or without any movement, small

muscle contracts at a moderate velocity. If the muscle

Maximum efficiency can be realized only when the

Duration of isometric contractions for different types of mammalian

Figure 6–12

skeletal muscles, showing a latent period between the action
potential (depolarization) and muscle contraction.

amounts of maintenance heat are released during con-

portions of the energy are used to overcome viscous

about 30 per cent of maximum.

Characteristics of Whole
Muscle Contraction

Many features of muscle contraction can be demon-
strated by eliciting single
accomplished by instantaneous electrical excitation of
the nerve to a muscle or by passing a short electrical

sudden contraction lasting for a fraction of a second.

Muscle contraction is

said to be isometric when the muscle does not shorten
during contraction and isotonic when it does shorten but
the tension on the muscle remains constant throughout

a force transducer without decreasing the muscle

isotonic contraction depend on the load against which

metric system is most often used when comparing the

Muscles.

muscles—from the very small stapedius muscle in the

wonder that the mechanical characteristics of muscle

which has a duration of isometric contraction of less

tions of contraction are adapted to the functions of the

extremely rapid to maintain fixation of the eyes on

trocnemius muscle must contract moderately rapidly
to provide sufficient velocity of limb movement for

cerned principally with slow contraction for continual,

As we discuss more fully in

body is composed of a mixture of so-called fast and

that respond slowly but with prolonged contraction are

(1) Large fibers for great strength of con-

amounts of glycolytic enzymes for rapid release of

blood supply because oxidative metabolism is of

To electronic

ISOTONIC SYSTEM

ISOMETRIC SYSTEM

Weights

Kymograph

Muscle

Stimulating
electrodes

Electronic force

transducer

recorder

Isotonic and isometric systems for recording muscle contractions.

Figure 6–11

tract after a long period of rest, its initial strength of

When a muscle begins to con-

Staircase Effect (Treppe).

bone.

of tension may be applied to the patellar tendon. Thus,

16 square inches of muscle belly, as much as 800 pounds

inch. Because a quadriceps muscle can have as much as

square centimeter of muscle, or 50 pounds per square

The maximum strength

any relaxation between the action potentials.

muscle sarcoplasm, even between action potentials, so

further effect in increasing contractile force. This occurs

tional increase in frequency beyond that point has no

of contraction reaches its maximum, so that any addi-

At a slightly higher frequency, the strength

and continuous, as shown in the figure. This is called

become so rapid that they fuse together, and the whole

critical level, the successive contractions eventually

increasing frequency. When the frequency reaches a

contraction is added partially to the first, so that the

before the preceding one is over. As a result, the second

quency of stimulation. Then, as the frequency increases,

tetanization. To the left are displayed individual twitch

shows the principles of frequency summation and

Figure 6–13

Frequency Summation and Tetanization.

of nerve signals.

alternates among motor units one after the other, thus

chronously by the spinal cord, so that contraction

the larger ones, so they naturally are excited first.

driven by small motor nerve fibers, and the small

when large amounts of force are required. The cause of

steps, whereas the steps become progressively greater

It is important, because it allows the gradations of

ple.

size princi-

force of the smallest units. This is called the

units begin to be excited as well, with the largest motor

strength of the signal increases, larger and larger motor

preference to the larger motor units. Then, as the

system sends a weak signal to contract a muscle, the

When the central nervous

and (2) by increasing the frequency of contraction,

simultaneously, which is called

muscle contraction. Summation occurs in two ways: (1)

ual segments.

units in microbundles of 3 to 15 fibers. This interdigita-

The muscle fibers in each motor unit are not all

figure for all the muscles of the body is questionable, but

hundred muscle fibers in a motor unit. An average

control, such as the soleus muscle, may have several

Conversely, large muscles that do not require fine

muscle fibers (for instance, as few as two or three muscle

general, small muscles that react rapidly and whose

ing on the type of muscle. All the muscle fibers inner-

innervates multiple muscle fibers, the number depend-

white muscle.

red muscle,

to the mitochondria. The myoglobin gives the slow

until needed; this also greatly speeds oxygen transport

cells. Myoglobin combines with oxygen and stores it

Fibers contain large amounts of myoglobin, an iron-

also to support high levels of oxidative metabolism. (5)

oxygen. (4) Greatly increased numbers of mitochondria,

smaller nerve fibers. (3) More extensive blood vessel

(1) Smaller fibers. (2) Also innervated by

because oxidative metabolism is secondary.

secondary importance. (5) Fewer mitochondria, also

Chapter 6

Contraction of Skeletal Muscle

Slow Fibers.

system and capillaries to supply extra amounts of

containing protein similar to hemoglobin in red blood

muscle a reddish appearance and the name
whereas a deficit of red myoglobin in fast muscle gives
it the name

Mechanics of Skeletal Muscle
Contraction

Motor Unit.

Each motoneuron that leaves the spinal cord

vated by a single nerve fiber are called a motor unit. In

control must be exact have more nerve fibers for fewer

fibers per motor unit in some of the laryngeal muscles).

a good guess would be about 80 to 100 muscle fibers to
a motor unit.

bunched together in the muscle but overlap other motor

tion allows the separate motor units to contract in
support of one another rather than entirely as individ-

Muscle Contractions of Different Force—Force Summation.

Summation means the adding together of individual
twitch contractions to increase the intensity of overall

by increasing the number of motor units contracting

multiple fiber summa-

tion,
which is called frequency summation and can lead to
tetanization.

Multiple Fiber Summation.

smaller motor units of the muscle may be stimulated in

units often having as much as 50 times the contractile

muscle force during weak contraction to occur in small

this size principle is that the smaller motor units are

motoneurons in the spinal cord are more excitable than

Another important feature of multiple fiber summa-

tion is that the different motor units are driven asyn-

providing smooth contraction even at low frequencies

contractions occurring one after another at low fre-

there comes a point where each new contraction occurs

total strength of contraction rises progressively with

muscle contraction appears to be completely smooth

tetanization.

because enough calcium ions are maintained in the

that full contractile state is sustained without allowing

Maximum Strength of Contraction.

of tetanic contraction of a muscle operating at a normal
muscle length averages between 3 and 4 kilograms per

one can readily understand how it is possible for
muscles to pull their tendons out of their insertions in

Changes in Muscle Strength at the Onset of Contraction—The

10 15 20 25 30 35 40 45 50 55

Rate of stimulation (times per second)

Strength of muscle contraction

Tetanization

Frequency summation and tetanization.

Figure 6–13

Remodeling of Muscle

positioning process.

We learn in Chapter 54 that the motor nervous

system directs the positioning of the arm or leg.

of the agonist and antagonist muscles, the nervous

Thus, by varying the ratios of the degree of activation

other. At this point, movement of the arm or leg stops.

muscle decreases, whereas the strength of the shorter

toward its midposition, the strength of the longer

muscle on the opposite side. As an arm or leg moves

Therefore, the elongated muscle on one side of a joint

demonstrated in Figure 6–9, showing maximum strength

with more force than a shortened muscle, which was

equally. Remember that an elongated muscle contracts

this, agonist and antagonist muscles are excited about

is to be placed in a midrange position. To achieve

muscles. For instance, let us assume that an arm or a leg

as an arm or a leg, is determined by the relative degrees

The position of each separate part of the body, such

muscles, and it is controlled by the motor control centers

antagonist muscles on opposite sides of joints. This

Virtually all body movements are

physioanatomy.

systems, and their movements is called

tances. The study of different types of muscles, lever

are long and contract a long distance, and some are

reason, there are many different types of muscle; some

of which need large distances of movement. For this

body, some of which need great strength and others

lever. Many types of movement are required in the

the length of the lever arm, and (4) the position of the

tion, (2) its distance from the fulcrum of the lever, (3)

In short, an analysis of the lever systems of the body

also much less than 43 pounds.

much less than 2 inches anterior to the fulcrum, and the

arm is fully extended, the attachment of the biceps is

pounds of muscle force, or about 43 pounds. When the

inches. Therefore, the amount of lifting power of the

is about 2 inches anterior to the fulcrum at the elbow,

with the upper arm, the tendon attachment of the biceps

about 300 pounds. When the forearm is at right angles

inches, the maximum force of contraction would be

muscle to lift the forearm. If we assume that a large

bones in turn form various types of lever systems. Figure

tension to their points of insertion into bones, and the

Lever Systems of the Body.

supply, especially loss of oxygen.

contraction. Interruption of blood flow through a con-

longed muscle activity, thus further diminishing muscle

7, can diminish at least a small amount after intense pro-

neuromuscular junction, which is discussed in Chapter

same work output. However, experiments have also

depletion of muscle glycogen. Therefore, fatigue results

muscle leads to the well-known state of muscle fatigue.

itself. Both of these are discussed in relation to muscle

spinal cord. These, in turn, are controlled partly by

stimulate the fibers, skeletal muscle tone results entirely

muscle tone.

certain amount of tautness usually remains. This is

Even when muscles are at rest, a

Skeletal Muscle Tone.

immediately.

increasing calcium ions in the cytosol because of the

are not known, it is believed to be caused primarily by

treppe.

traction increases to a plateau, a phenomenon called the

to 50 muscle twitches later. That is, the strength of con-

Membrane Physiology, Nerve, and Muscle

Unit II

contraction may be as little as one half its strength 10

staircase effect, or

Although all the possible causes of the staircase effect

release of more and more ions from the sarcoplasmic
reticulum with each successive muscle action potential
and failure of the sarcoplasm to recapture the ions

called

Because normal skeletal muscle

fibers do not contract without an action potential to

from a low rate of nerve impulses coming from the

signals transmitted from the brain to the appropriate
spinal cord anterior motoneurons and partly by signals
that originate in muscle spindles located in the muscle

spindle and spinal cord function in Chapter 54.

Muscle Fatigue.

Prolonged and strong contraction of a

Studies in athletes have shown that muscle fatigue
increases in almost direct proportion to the rate of

mainly from inability of the contractile and metabolic
processes of the muscle fibers to continue supplying the

shown that transmission of the nerve signal through the

tracting muscle leads to almost complete muscle fatigue
within 1 or 2 minutes because of the loss of nutrient

Muscles operate by applying

6–14 shows the lever system activated by the biceps

biceps muscle has a cross-sectional area of 6 square

and the total length of the forearm lever is about 14

biceps at the hand would be only one seventh of the 300

force with which the hand can be brought forward is

depends on knowledge of (1) the point of muscle inser-

short but have large cross-sectional areas and can
provide extreme strength of contraction over short dis-

kinesiology

and is an important scientific component of human

“Positioning” of a Body Part by Contraction of Agonist and Antag-
onist Muscles on Opposite Sides of a Joint—“Coactivation”
of Antagonist Muscles.

caused by simultaneous contraction of agonist and

is called coactivation of the agonist and antagonist

of the brain and spinal cord.

of contraction of the agonist and antagonist sets of

of contraction at full functional muscle length and
almost no strength of contraction at half normal length.

can contract with far greater force than the shorter

muscle increases until the two strengths equal each

system has additional important mechanisms to com-
pensate for different muscle loads when directing this

to Match Function

All the muscles of the body are continually being
remodeled to match the functions that are required of

Figure 6–14

Lever system activated by the biceps muscle.

traction in striated muscle. Physiol Rev 80:853, 2000.

Gordon AM, Homsher E, Regnier M: Regulation of con-

hypertrophy and atrophy. Nat Cell Biol 5:87, 2003.

Glass DJ: Signalling pathways that mediate skeletal muscle

Trends Mol Med 8:344, 2003.

Glass DJ: Molecular mechanisms modulating muscle mass.

tractility. Physiol Rev 83:1269, 2003.

Clausen T: Na

iology. Adv Physiol Educ 27:171, 2003.

Brooks SV: Current topics for teaching skeletal muscle phys-

plasticity, and disease. Physiol Rev 80:1215, 2000.

in skeletal muscle: its crucial role for muscle function,

Berchtold MW, Brinkmeier H, Muntener M: Calcium ion

rapidly at higher temperatures.

released from lysosomes. All these events occur more

proteins deteriorate about 15 to 25 hours later, which

process. The muscles remain in rigor until the muscle

ATP, which is required to cause separation of the cross-

action potentials. This rigidity results from loss of all the

the muscles contract and become rigid, even without

called “rigor mortis”; that is,

Several hours after death, all the muscles of the body go

for each motoneuron coming from the spinal cord. This

macromotor units,

of the paralyzed muscle fibers. This causes large motor

occurs in poliomyelitis, the remaining nerve fibers

nerve fibers to a muscle are destroyed, as commonly

When some but not all

stretched during the atrophying process.

contractures. This is achieved by daily stretching of the

Therefore, one of the most important problems

tracture.

tinue shortening for many months, which is called

during denervation atrophy also has a tendency to con-

The fibrous tissue that replaces the muscle fibers

regrow.

fatty tissue. The fibers that do remain are composed of

In the final stage of denervation atrophy, most of the

after 1 to 2 years.

becomes less and less, with no further return of function

that time onward, the capability of functional return

of function can occur in as little as 3 months, but from

supply to the muscle grows back rapidly, full return

appear in the muscle fibers themselves. If the nerve

about 2 months, degenerative changes also begin to

Therefore, atrophy begins almost immediately. After

that are required to maintain normal muscle size.

nerve supply, it no longer receives the contractile signals

When a muscle loses its

Effects of Muscle Denervation.

anism is linear splitting of previously enlarged fibers.

When it does occur, the mech-

hypertrophy process. This increase in fiber number is

by a few percentage points), in addition to the fiber

extreme muscle force generation, the actual number of

ends of the muscle fibers can actually disappear. It is by

ened to less than its normal length, sarcomeres at the

Conversely, when a muscle continually remains short-

hypertrophy.

muscle, illustrating the rapidity of this type of

to the tendons. In fact, new sarcomeres can be added

added at the ends of the muscle fibers, where they attach

than normal length. This causes new sarcomeres to be

occurs.

than the rate of replacement. Therefore, muscle atrophy

When a muscle remains unused for many weeks, the

is especially true of the enzymes for glycolysis, allowing

enzyme systems that provide energy also increase. This

Along with the increasing size of myofibrils, the

muscle to form new myofibrils, but how important this

50 per cent. In turn, some of the myofibrils themselves

filaments in the myofibrils, often increasing as much as

greater when hypertrophy is developing, leading also to

hypertrophy is not known. It is known, however, that the

The manner in which forceful contraction leads to

significant hypertrophy within 6 to 10 weeks.

muscle is loaded during the contractile process. Only a

fiber hypertrophy.

ual muscle fibers; this is called simply

each muscle fiber, causing enlargement of the individ-

Virtually all muscle hypertrophy results from an

atrophy.

When it decreases, the process is called

trophy.

mass of a muscle increases, this is called

When the total

Muscle Hypertrophy and Muscle Atrophy.

little as 2 weeks.

some smaller, more active muscles can be replaced in as

quite rapid, within a few weeks. Indeed, experiments in

altered at least slightly. This remodeling process is often

plies are altered, and even the types of muscle fibers are

altered, their strengths are altered, their vascular sup-

them. Their diameters are altered, their lengths are

Chapter 6

Contraction of Skeletal Muscle

animals have shown that muscle contractile proteins in

muscle hyper-

muscle

increase in the number of actin and myosin filaments in

Hypertrophy occurs to a much greater extent when the

few strong contractions each day are required to cause

rate of synthesis of muscle contractile proteins is far

progressively greater numbers of both actin and myosin

have been observed to split within hypertrophying

is in usual muscle hypertrophy is still unknown.

rapid supply of energy during short-term forceful
muscle contraction.

rate of decay of the contractile proteins is more rapid

Adjustment of Muscle Length.

Another type of hyper-

trophy occurs when muscles are stretched to greater

as rapidly as several per minute in newly developing

these processes that muscles are continually remodeled
to have the appropriate length for proper muscle
contraction.

Hyperplasia of Muscle Fibers.

Under rare conditions of

muscle fibers has been observed to increase (but only

called fiber hyperplasia.

muscle fibers are destroyed and replaced by fibrous and

a long cell membrane with a lineup of muscle cell nuclei
but with few or no contractile properties and little or no
capability of regenerating myofibrils if a nerve does

con-

in the practice of physical therapy is to keep atrophying
muscles from developing debilitating and disfiguring

muscles or use of appliances that keep the muscles

Recovery of Muscle Contraction in Poliomyelitis: Devel-
opment of Macromotor Units.

branch off to form new axons that then innervate many

units called

which can contain as

many as five times the normal number of muscle fibers

decreases the fineness of control one has over the
muscles but does allow the muscles to regain varying
degrees of strength.

Rigor Mortis

into a state of contracture

bridges from the actin filaments during the relaxation

presumably results from autolysis caused by enzymes

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Unit II

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