Anatomy & Physiology
Chapter 12:
Physiology of the Muscular System
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Introduction


Muscular system is responsible for
moving the framework of the body
In addition to movement, muscle
tissue performs various other
functions
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General Functions

Movement of the body as a whole
or movement of its parts

Heat production

Posture
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Function of Skeletal Muscle Tissue
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Characteristics of skeletal muscle
cells
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Excitability (irritability)—ability to be
stimulated
Contractility—ability to contract, or
shorten, and produce body
movement
Extensibility—ability to extend, or
stretch, thereby allowing muscles to
return to their resting length
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Function of Skeletal Muscle Tissue

Overview of the muscle cell (Figures 12-1 and 122)

Muscle cells are called fibers because of their
threadlike shape
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Sarcolemma—plasma membrane of muscle fibers

Sarcoplasmic reticulum (SR)


T tubules—network of tubules and sacs found within
muscle fibers
Membrane of the SR continually pumps calcium ions from
the sarcoplasm and stores the ions within its sacs for later
release (Figure 12-3)
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Function of Skeletal Muscle Tissue

Overview of the muscle cell (cont)
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Muscle fibers contain many mitochondria
and several nuclei
Myofibrils—numerous fine fibers packed
close together in sarcoplasm
Sarcomere



Segment of myofibril between two successive Z
disks
Each myofibril consists of many sarcomeres
Contractile unit of muscle fibers
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Function of Skeletal Muscle Tissue
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Overview of the muscle cell (cont)
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T tubules

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
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Transverse tubules extend across the
sarcoplasm at right angles to the long axis of
the muscle fiber
Formed by inward extensions of the
sarcolemma
Membrane has ion pumps that continually
transport Ca++ ions inward from the sarcoplasm
Allow electrical impulses traveling along the
sarcolemma to move deeper into the cell
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Function of Skeletal Muscle Tissue
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Overview of the muscle cell (cont)

Striated muscle (Figure 12-4)



Dark stripes called A bands; light H band runs
across the midsection of each dark A band
Light stripes called I bands; dark Z disk extends
across the center of each light I band
Triad
Triplet of tubules; a T tubule sandwiched
between two sacs of sarcoplasmic reticulum
 Allows an electrical impulse traveling along a T
tubule to stimulate the membranes of adjacent
sacs of the sarcoplasmic reticulum

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Function of Skeletal Muscle Tissue
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Myofilaments (Figures 12-5 and 12-6)


Each myofibril contains thousands of thick
and thin myofilaments
Four different kinds of protein molecules
make up myofilaments

Myosin
 Makes up almost all the thick filament
 Myosin “heads” are chemically attracted
to actin molecules
 Myosin “heads” are known as cross
bridges when attached to actin
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Function of Skeletal Muscle Tissue
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Four different kinds of protein
molecules make up myofilaments
(cont)
Actin—globular protein that forms two
fibrous strands twisted around each
other to form the bulk of the thin filament
 Tropomyosin—protein that blocks the
active sites on actin molecules
 Troponin—protein that holds
tropomyosin molecules in place

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Function of Skeletal Muscle Tissue

Mechanism of contraction

Excitation and contraction (Figures 12-7
through 12-13; Table 12-1)
 A skeletal muscle fiber remains at rest
until stimulated by a motor neuron
 Neuromuscular junction—motor
neurons connect to the sarcolemma at
the motor endplate (Figure 12-7)
 Neuromuscular junction is a synapse
where neurotransmitter molecules
transmit signals
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Function of Skeletal Muscle Tissue

Excitation and contraction (cont)
Acetylcholine—the neurotransmitter released
into the synaptic cleft that diffuses across the
gap, stimulates the receptors, and initiates an
impulse in the sarcolemma
 Nerve impulse travels over the sarcolemma
and inward along the T tubules, which
triggers the release of calcium ions
 Calcium binds to troponin, which causes
tropomyosin to shift and expose active sites
on actin

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Function of Skeletal Muscle Tissue

Excitation and contraction (cont)
 Sliding filament model (Figures 12-12
and 12-13)
 When active sites on actin are
exposed, myosin heads bind to them
 Myosin heads bend and pull the thin
filaments past them
 Each head releases, binds to the
next active site, and pulls again
 The entire myofibril shortens
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Function of Skeletal Muscle Tissue
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Mechanism of contraction (cont)

Relaxation
 Immediately after the Ca++ ions are
released, the sarcoplasmic reticulum
begins actively pumping them back
into the sacs (Figure 12-3)
 Ca++ ions are removed from the
troponin molecules, thereby shutting
down the contraction
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Function of Skeletal Muscle Tissue
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Mechanism of contraction (cont)

Energy sources for muscle contraction (Figure 1214)


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Hydrolysis of ATP yields the energy required for
muscular contraction
ATP binds to the myosin head and then transfers its
energy to the myosin head to perform the work of
pulling the thin filament during contraction
Muscle fibers continually resynthesize ATP from the
breakdown of creatine phosphate (CP)
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Function of Skeletal Muscle Tissue

Energy sources for muscle contraction
(cont)
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Catabolic pathways
 Aerobic pathway


Occurs when adequate O2 is available
from blood (Figure 12-15)
Slower than anaerobic pathway, thus
supplies energy for the long term
rather than the short term
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Function of Skeletal Muscle Tissue
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Energy sources for muscle contraction (cont)
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Catabolic pathways (cont)
 Anaerobic pathway (Figure 12-16)
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

Very rapid, providing energy during first minutes
of maximal exercise (Figure 12-16)
May occur when low levels of O2 are available
Results in the formation of lactic acid, which
requires oxygen to convert back to glucose, the
producing of an “oxygen debt” or excess
postexercise oxygen consumption (EPOC)
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Function of Skeletal Muscle Tissue

Energy sources for muscle contraction
(cont)

Skeletal muscle contraction produces
waste heat that can be used to help
maintain the set point body temperature
(Figure 12-17)
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Function of Skeletal Muscle Organs
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
Muscles are composed of bundles of muscle
fibers held together by fibrous connective tissue
Motor unit (Figure 12-18)
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

Motor unit—motor neuron plus the muscle fibers to which it
attaches
Some motor units consist of only a few muscle fibers,
whereas others consist of numerous fibers
Generally, the smaller the number of fibers in a motor unit,
the more precise are the available movements; the larger
the number of fibers in a motor unit, the more powerful the
contraction available
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Function of Skeletal Muscle Organs


Myography—method of graphing the
changing tension of a muscle as it
contracts (Figure 12-19)
Twitch contraction (Figure 12-20)

A quick jerk of a muscle that is produced
as a result of a single, brief threshold
stimulus (generally occurs only in
experimental situations)
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Function of Skeletal Muscle Organs

Twitch contraction (cont)

The twitch contraction has three phases
 Latent phase—nerve impulse travels to
the sarcoplasmic reticulum to trigger
release of Ca++
 Contraction phase—Ca++ binds to
troponin and sliding of filaments occurs
 Relaxation phase—sliding of filaments
ceases
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Function of Skeletal Muscle Organs

Treppe—the staircase phenomenon
(Figure 12-21, B)
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

Gradual, steplike increase in the strength
of contraction that is seen in a series of
twitch contractions that occur 1 second
apart
Eventually, the muscle responds with less
forceful contractions, and the relaxation
phase becomes shorter
If the relaxation phase disappears
completely, a contracture occurs
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Function of Skeletal Muscle Organs

Tetanus—smooth, sustained
contractions
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
Multiple wave summation—multiple
twitch waves are added together to
sustain muscle tension for a longer
time
Incomplete tetanus—very short
periods of relaxation occur between
peaks of tension (Figure 12-21, C)
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Function of Skeletal Muscle Organs

Tetanus—smooth, sustained
contractions (cont)


Complete tetanus—the stimulation is
such that twitch waves fuse into a
single, sustained peak (Figure 12-21, D)
The availability of calcium determines
whether a muscle will contract; if the
calcium is continuously available, then
contraction will be sustained (Figure 1222)
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Function of Skeletal Muscle Organs
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Muscle tone


Tonic contraction—continual, partial contraction of
a muscle
At any one time, a small number of muscle fibers
within a muscle contract and produce a tightness or
muscle tone

Muscles with less tone than normal are flaccid

Muscles with more tone than normal are spastic

Muscle tone is maintained by negative feedback
mechanisms
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Graded Strength Principle
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Graded strength principle—skeletal muscles
contract with varying degrees of strength at
different times
Factors that contribute to the phenomenon of
graded strength (Figure 12-26)


Metabolic condition of individual fibers
Number of muscle fibers contracting simultaneously; the
greater the number of fibers contracting, the stronger the
contraction

Number of motor units recruited

Intensity and frequency of stimulation (Figure 12-23)
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Graded Strength Principle
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Graded strength (cont)

Length-tension relationship (Figure 12-24)
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Maximal strength that a muscle can develop bears a direct
relationship to the initial length of its fibers
A shortened muscle’s sarcomeres are compressed;
therefore, the muscle cannot develop much tension
An overstretched muscle cannot develop much tension
because the thick myofilaments are too far from the thin
myofilaments
Strongest maximal contraction is possible only when the
skeletal muscle has been stretched to its optimal length
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Graded Strength Principle
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Graded strength (cont)

Stretch reflex (Figure 12-25)
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

The load imposed on a muscle influences
the strength of a skeletal contraction
Stretch reflex—the body tries to maintain
constancy of muscle length in response to
increased load
Maintains a relatively constant length as load
is increased up to a maximum sustainable
level
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Graded Strength Principle

Isotonic and isometric contractions
(Figure 12-27)

Isotonic contraction
 Contraction in which the tone or
tension within a muscle remains the
same as the length of the muscle
changes
Concentric—muscle shortens as it
contracts
 Eccentric—muscle lengthens while
contracting

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Graded Strength Principle

Isotonic contraction (cont)
Isotonic—literally means “same
tension”
 All of the energy of contraction is
used to pull on thin myofilaments
and thereby change the length of a
fiber’s sarcomeres

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Graded Strength Principle

Isotonic and isometric contractions
(cont)


Isometric contraction
 Contraction in which muscle length
remains the same while muscle
tension increases
 Isometric—literally means “same
length”
Most body movements occur as a result
of both types of contractions
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Function of Cardiac and Smooth
Muscle Tissue (Table 12-1)

Cardiac muscle (Figure 12-28)



Found only in the heart; forms the bulk
of the wall of each chamber
Also known as striated involuntary
muscle
Contracts rhythmically and continuously
to provide the pumping action needed to
maintain constant blood flow
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Function of Cardiac and Smooth
Muscle Tissue

Cardiac muscle (cont)

Cardiac muscle resembles skeletal muscle but
has unique features related to its role in
continuously pumping blood



Each cardiac muscle contains parallel myofibrils
(Figure 12-28)
Cardiac muscle fibers form strong, electrically
coupled junctions (intercalated disks) with other
fibers; individual cells also exhibit branching
Syncytium—continuous, electrically coupled mass
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Function of Cardiac and Smooth
Muscle Tissue

Cardiac muscle (cont)





Cardiac muscle fibers form a continuous, contractile band
around the heart chambers that conducts a single impulse
across a virtually continuous sarcolemma
T tubules are larger and form diads with a rather sparse
sarcoplasmic reticulum
Cardiac muscle sustains each impulse longer than in skeletal
muscle; therefore, impulses cannot come rapidly enough to
produce tetanus (Figure 12-29)
Cardiac muscle does not run low on ATP and does not
experience fatigue
Cardiac muscle is self-stimulating
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Function of Cardiac and Smooth
Muscle Tissue

Smooth muscle




Smooth muscle is composed of small, tapered
cells with single nuclei (Figure 12-30)
No T tubules are present, and only a loosely
organized sarcoplasmic reticulum is present
Ca++ comes from outside the cell and binds to
calmodulin instead of troponin to trigger a
contraction
No striations because thick and thin myofilaments
are arranged differently than in skeletal or cardiac
muscle fibers; myofilaments are not organized into
sarcomeres
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Function of Cardiac and Smooth
Muscle Tissue

Smooth muscle (cont)

Two types of smooth muscle tissue (Figure
12-31)
 Single-unit (visceral) smooth muscle
Gap junctions join smooth muscle fibers
into large, continuous sheets
 Most common type; forms a muscular layer
in the walls of hollow structures such as the
digestive, urinary, and reproductive tracts
 Exhibits autorhythmicity and produces
peristalsis

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The Big Picture: Muscle Tissue and
The Whole Body




Function of all three major types of muscle
is integral to function of the entire body
All three types of muscle tissue provide the
movement necessary for survival
Relative constancy of the body’s internal
temperature is maintained by “waste” heat
generated by muscle tissue
Maintains the body in a relatively stable
position
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