Chapter 7
The Muscular System
The Human Body: Concepts of Anatomy and Physiology, 3rd ed.
Bruce D. Wingerd
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Chapter Outline
• The Big Picture
• Muscle Structure
Connective Tissues of
Muscle
Microscopic Structure of
Muscle
Nerve Supply
• Physiology of Muscle
Contraction
The Muscle Fiber at Rest
Role of the Stimulus
Muscle Contraction
Return to Rest
Energy for Contraction
Metabolism and Fitness
Comparing Muscle Tissues
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Chapter Outline (cont.)
• Muscle Mechanics
All-or-None Response
Measuring Muscle
Contractions
Sustained Muscle Contraction
Isotonic and Isometric
Contractions
• Production of Movement
• Major Muscles of the Body
Muscles of the Head
and Neck
Muscles Moving the
Pectoral Girdle and Trunk
Muscles of the Upper
Limbs
Muscles of the Lower
Limbs
Origin and Insertion
Group Actions
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Learning Objectives
1. Indicate the primary function of
muscles.
2. Describe the connective tissues
associated with muscle.
3. Identify and describe the
microscopic components of
skeletal muscle tissue.
4. Identify the parts of the
neuromuscular junction.
5. Explain the sliding filament
mechanism of muscle
contraction.
6. Describe in their proper order
of occurrence the events
leading to muscle contraction.
7. Indicate the roles of ATP in
muscle contraction and how
this energy is supplied.
8. Describe the oxygen debt and
muscle fatigue.
9. Define threshold stimulus, and
relate it to the concept of the
all-or-none response.
10.Compare twitch, tetanic,
isotonic, and isometric
contractions.
11.Define origin and insertion, and
describe the role of group
actions in producing movement
12.Identify the primary muscles
on the basis of their locations,
origin, insertions, and actions.
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The Big Picture
• The muscular system is composed of about 600 muscles, making
up about 60% of total body weight.
• Each muscle is an organ composed mainly of skeletal muscle tissue.
• Muscles are specialized to contract. Functions include:
–
Movement
–
Support
–
Heat production
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Muscle Structure
• A muscle extends from one bone to another, and includes skeletal
muscle tissue, connective tissue, nerves, and blood vessels.
• Connective Tissues of Muscle
–
The primary connective tissue of muscle is fascia.
• Fascia includes the superficial fascia beneath the skin (the
hypodermis), and muscle wrappings known as deep fascia.
• Deep fascia includes 3 layers, carrying blood vessels and
nerves:
• Epimysium: the tough outermost layer, composed of
dense connective tissue
• Perimysium: the middle layer, surrounding fascicles.
• Endomysium: the inner layer, a thin loose connective
tissue layer surrounding individual muscle cells.
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Figure 7.1. Muscle structure. A. A muscle is supported by several layers of connective tissue, which
are made visible in this cutaway view. The muscle itself is composed of smaller units, called muscle
bundles (or fasciculi), and each muscle bundle is composed of numerous skeletal muscle cells or
muscle fibers. Each muscle fiber is composed of cylindrical subunits called myofibrils. B. In this closer
view, you can see that each myofibril is composed of thick and thin filaments arranged in repeating
units.
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Muscle Structure (cont.)
–
All three fascia layers converge to form the connecting band
between a muscle and bone, called a tendon.
• Most muscles attach to bone by a single tendon, although
some muscles have more than one tendon and several are
attached by a broad tendinous sheet called an aponeurosis.
• Tendons are very strong due to the regular dense connective
tissue, composed of collagen fibers, allowing them to accept
stresses as they are pulled and stretched.
–
Other forms of connective tissue in muscle include loose
connective tissue and adipose tissue.
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Figure 7.2. Tendon repair surgery. The white band-like structure is the tendon of the pronator teres
muscle of the forearm, which had severed and is being reattached to the radius.
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Muscle Structure (cont.)
• Microscopic Structure of Muscle
–
Skeletal muscle cells are unlike other cells of the body.
• They are extremely long and filamentous, with many nuclei,
little cytoplasm, and crammed with protein.
• Terms specific to skeletal muscle cells:
• Muscle fiber: the skeletal muscle cell.
• Sarcolemma: the cell membrane.
• Sarcoplasm: the cytoplasm.
• Sarcoplasmic reticulum (SR): cytoplasmic sacs similar
to endoplasmic reticulum, but contains stored calcium ions.
• Transverse tubule (TT): a tube that extends
perpendicular to SR.
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Figure 7.3. The filaments of the myofibril. Thick
filaments are composed of myosin proteins braided
together, and thin filaments are composed of actin,
troponin, and tropomyosin intertwined together.
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Muscle Structure (cont.)
• Microscopic Structure of Muscle (cont.)
• The cytoplasm is packed with cylindrical protein subunits
called myofibrils.
• Each myofibril extends the distance of the muscle fiber.
• Each myofibril consists of two types of protein filaments:
• Thin filaments are composed of actin, tropomyosin,
and troponin proteins intertwined to form a narrow
thread.
• Thick filaments are composed of the large protein,
myosin. Myosin includes a long tail and a bulbous
head (cross bridge).
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Muscle Structure (cont.)
• Microscopic Structure of Muscle (cont.)
• Each myofibril consists of a linear arrangement of
subunits known as sarcomeres. Each sarcomere
contains the following, visible under an electron
microscope:
• A band: region where thick and thin filaments
overlap.
• I band: region of only thin filaments.
• H zone: region in the center of only thick filaments.
• Z lines: zig-zag lines in center of I bands, and
indicate the lateral borders of sarcomere.
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Figure 7.4. The sarcomere. A photograph from an electron microscope is compared with a
corresponding illustration. Note that one sarcomere extends from Z line to Z line. A myofibril is a linear
arrangement of many sarcomeres.
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Muscle Structure (cont.)
• Nerve Supply
–
Muscle fibers require an external stimulus to contract.
–
The stimulus begins at the brain and travels along a motor
neuron to arrive at the muscle fiber.
• The distal end of the motor neuron divides into numerous
branches.
• Each branch terminates at a muscle fiber, at a swelling
called a synaptic knob.
• A single motor neuron, its terminal branches, and the
numerous muscle fibers it stimulates is called a motor unit.
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Figure 7.5. The motor unit. A. A motor unit consists of a single motor neuron and its connections to
numerous skeletal muscle fibers. B. A photomicrograph of a motor unit.
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Muscle Structure (cont.)
• Nerve Supply (cont.)
–
The junction between the motor neuron and a muscle fiber is
the neuromuscular junction (NMJ). It includes:
• Motor end plate: a highly folded region of the sarcolemma
where the motor neuron is received.
• Synaptic cleft: the space, filled with interstitial fluid,
between the motor neuron and the muscle fiber.
• Synaptic vesicles: cytoplasmic sacs in the synaptic knob
filled with chemicals called neurotransmitters.
• Acetylcholine: the neurotransmitter at the NMJ.
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Figure 7.6. The neuromuscular junction. The neuromuscular junction consists of a terminal end of a
motor neuron containing synaptic vesicles with neurotransmitter, a narrow gap filled with interstitial fluid
called the synaptic cleft, and the modified sarcolemma of the muscle fiber called the motor end plate.
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Physiology of Muscle Contraction
• An individual fiber contracts when it undergoes a shift of thin filaments
in all of its sarcomeres, in which they move toward the sarcomere
center. This causes the muscle fiber to shorten in length--the sliding
filament mechanism.
• The muscle fiber at rest:
–
Calcium ions are in storage in the SR.
–
ATP is chemically bound to thick filaments.
–
Thin filaments are intact with all three proteins bound tightly
together (actin, troponin, and tropomyosin).
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Physiology of Muscle Contraction (cont.)
• Role of the Stimulus
–
The external stimulus triggers a series of events resulting in
muscle fiber contraction. The steps include:
• The nerve impulse travels along the motor neuron, arriving
at the NMJ.
• Synaptic vesicles migrate and fuse with the neuron cell
membrane, releasing ACh into the synaptic cleft.
• ACh binds with receptors on the motor end plate, triggering
a nerve impulse in the muscle fiber.
• The impulse travels along the sarcolemma, through the
transverse tubules, and to the SR.
• Calcium ions are released from the SR into the sarcoplasm.
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Physiology of Muscle Contraction (cont.)
• Muscle Contraction
–
Muscle contraction is triggered by the presence of calcium ions
in the sarcoplasm.
• Calcium ions bind to troponin in thin filaments.
• Binding causes actin and troponin to change in shape,
exposing actin binding sites.
• Myosin heads bind to actin at their exposed sites forming
coupling between thin and thick filaments.
• Calcium ions activate the breakdown of ATP that is bound to
the thick filaments, releasing energy.
• The energy pivots the myosin head. This is the power stroke,
which shifts the attached thin filament toward the center.
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Physiology of Muscle Contraction (cont.)
• Muscle Contraction (cont.)
• As soon as the pivot action is complete, another ATP binds to
the myosin head and is broken to release energy used to
break the coupling between the myosin head and the actin
binding site.
• Now uncoupled, the myosin head returns to its cocked
position and couples once again to a different actin binding
site.
• If ATP is available, it will power another stroke, shifting the
thin filament yet closer to the center of the sarcomere.
• The cycle of coupling, power stroke, uncoupling repeats over
and over, resulting in the shift of thin filaments to the
sarcomere center, and contraction.
• In death, rigor mortis occurs as ATP becomes unavailable.
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Physiology of Muscle Contraction (cont.)
• Return to Rest
–
Rest returns to the fiber when the nerve impulse passing down
the motor neuron stops.
• An enzyme in the motor end plate, acetylcholinesterase,
inactivates ACh remaining in receptors.
• Calcium ions are then actively transported back to the SR by
enzymes, which requires energy.
• The absence of calcium ions in the sarcoplasm causes the
thin filaments to return to their original (resting) shape,
covering up the actin binding sites.
• Covering of the binding sites prevents coupling.
• ATP is regenerated to prepare for the next stimulus.
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Figure 7.7. The sliding filament mechanism of contraction. A. At rest. B. Calcium ion availability
leads to binding with troponin, causing a change in structure that exposes the actin binding site. C. As
a result, the myosin head attaches to actin. D. The splitting of ATP on the myosin head provides the
energy to shift the cross bridges, which slide the thin filament toward the center of the sarcomere
(arrow). E. A second ATP molecule provides the energy to detach the thin and thick filaments, and if
calcium and ATP remain available, the process will cycle again and again.
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Physiology of Muscle Contraction (cont.)
• Energy for Contraction
–
Energy is required for muscle contraction in three ways:
• The power stroke
• Uncoupling
• Return of calcium ions to the SR
–
ATP is obtained from the metabolism of glucose and can be
broken to release energy or re-used:
ADP + PO42- + Energy  ATP
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Physiology of Muscle Contraction (cont.)
• Energy for Contraction (cont.)
–
ATP is the most immediate form of energy, but is used up in
seconds of contracting.
–
Creatine phosphate in muscle tissue is then used to
regenerate ATP quickly.
–
Activity lasting more than 15 seconds relies on glucose
metabolism in muscle fibers and other cells.
–
Once free glucose is utilized, glycogen is metabolized to
release more glucose.
–
Lipids may be utilized during long-term strenuous exercise.
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Physiology of Muscle Contraction (cont.)
• Metabolism and Fitness
–
Aerobic respiration involves cellular respiration when oxygen
is available.
• Aerobic respiration is the most efficient form of catabolism,
generating the maximum number of ATP molecules from
glucose.
–
Anaerobic respiration occurs when oxygen is not available.
• Anaerobic respiration provides a minimal number of ATP
molecules and the waste product, lactic acid. It is incapable
of producing enough ATP to achieve sustained contraction.
–
Thus, muscle contraction requires oxygen. During exercise,
myoglobin in muscle tissue improves the delivery of oxygen.
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Physiology of Muscle Contraction (cont.)
• Metabolism and Fitness (cont.)
–
In the event oxygen becomes unavailable during activity, the
muscle tissue will enter oxygen debt.
• Oxygen debt is “repaid” when the muscle rests, deep
breathing restores oxygen levels, and ATP is regenerated.
• “Fit” individuals have an increased oxygen availability.
–
Muscle fatigue occurs when muscle is unable to contract
normally.
• Usually due to reduced availability of ATP during strenuous
exercise.
• A cramp occurs when a muscle reacts to reduced ATP
availability by spasmodic contractions and failure to rest.
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Physiology of Muscle Contraction (cont.)
• Comparing Muscle Tissues
–
Skeletal muscle tissue (striated, voluntary)
• Highly organized sarcomeres and many fibers provide great
strength of contraction, but fatigues fairly quickly due to a
limited oxygen availability.
–
Cardiac muscle tissue (striated, involuntary)
• Also organized into sarcomeres, but fibers are not as linearly
arranged, providing less strength.
• Large volume of myoglobin and blood supply provide a superior
oxygen availability, and therefore no oxygen debt or fatigue.
• Contraction is autorhythmic, and thereby does not require an
external stimulus.
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Physiology of Muscle Contraction (cont.)
• Comparing Muscle Tissues (cont.)
–
Smooth muscle tissue (nonstriated, involuntary)
• Cells are small and spindle shaped without a regular
arrangement of sarcomeres, so contraction is slower and with
less force than that of skeletal and cardiac muscle tissue.
• Does not fatigue readily or develop an oxygen debt, enabling
it to sustain a contraction.
• Requires an external stimulus to contract (by nerve or
hormone stimulation).
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Muscle Mechanics
• All-or-None Response
–
The weakest stimulus resulting in muscle contraction is
called the threshold stimulus.
–
Upon receiving a threshold stimulus a muscle fiber will
contract completely. If the stimulus is subthreshold it will
not contract at all. This is the all-or-none response.
–
How can you vary muscle strength for different tasks?
• Muscle strength can be adjusted by the brain by
controlling the number of motor units stimulated. The
more motor units stimulated, the greater the force of
contraction.
• Adding motor units to increase the force of contraction is
known as recruitment.
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Muscle Mechanics (cont.)
• Measuring Muscle Contraction
–
Measurement of muscle contraction can be performed in a
laboratory by electrically stimulating a dissected muscle,
measuring its contraction strength, and plotting it on a graph.
The graph is a myogram.
–
Twitch: occurs when a single muscle fiber is stimulated at
threshold, resulting in its contraction. It includes:
• Latent period: the time required for calcium ions to be
released and coupling to occur.
• Period of contraction: the upward tracing on the
myogram, it is the increase of tension as sarcomeres
shorten.
• Period of relaxation: the downward tracing, it is the
return of the fiber to its original length.
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Figure 7.8. A myogram. The single muscle twitch is plotted in this myogram.
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Muscle Mechanics (cont.)
• Sustained Muscle Contraction
–
When a muscle fiber receives a series of threshold stimuli, the
myogram will reveal increasing tension.
–
When the stimuli are spaced closely, preventing complete
return to rest, the myogram reveals fused peaks called
summation.
–
When the stimuli are increased in frequency further, the
myogram reveals a fusion of peaks at maximal force called
tetanic contraction. A forced, sustained maximal contraction
is known as complete tetanus.
–
Muscle tone is the complete tetanus at periodic intervals, and
is a normal function of muscle to keep it in a ready state.
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Figure 7.9. Types of muscle contractions. When the frequency of stimuli increases, the nature of
muscle contraction changes. A. Twitch. If a muscle is allowed to relax between stimuli, the contractions
will be simple twitches. B. Wave summation. If a muscle is not allowed to relax between stimuli,
contractions increase considerably in strength. C. Complete tetanus. Should the frequency of stimuli
increase yet further, no relaxation will occur between stimuli and contractions will fuse completely at a
maximum force.
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Muscle Mechanics (cont.)
• Isotonic and Isometric Contractions
–
Tension is the force exerted by a muscle contraction.
• Two terms used to describe the effects of tension on muscles
are isotonic and isometric.
–
Isotonic contraction: muscle contraction that pulls on
attached bones to produce movement.
• Isotonic contractions are the usual method of body
movement.
–
Isometric contraction: muscle contraction that produces
tension but does not cause body movement.
• Isometric contractions occur when pushing against an
immovable object. They utilize energy also and can be used
to lose weight and strengthen joints without risk of joint
injury.
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Production of Movement
• Muscles produce movement by pulling on their attachments to
bone.
• Origin and Insertion
–
Most muscles extend from bone to bone, crossing over a joint.
–
In most cases, the attachment to a bone is by way of one or
more tendons.
–
As a muscle contracts, one bone moves while the other bone is
kept stationary.
–
The point of attachment of a tendon to the stationary bone is
called the origin.
–
The point of attachment of a tendon to the moving bone is the
insertion.
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Figure 7.10. Origin and insertion of a muscle. The muscle shown is the biceps brachii, located in
the upper arm. Notice that its origins are proximal and its insertions are distal, with a joint (the elbow)
located between them.
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Production of Movement (cont.)
• Group Actions
–
A group action is a coordinated response of a group of
muscles to produce a body movement. The roles played by
muscles in a group action include:
• Agonists: the “prime movers” that cause the desired
action.
• Antagonists: relax during the action, yielding to the
agonists.
• Synergists: assist the agonists in performing the action,
usually by steadying the movement.
• Fixators: stabilize the origin of the prime mover.
–
Example: flexing the arm at the elbow. The agonist is the
biceps brachii, the antagonist is the triceps brachii, the
brachialis is a synergist, and the deltoid and trapezius are
fixators.
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Major Muscles of the Body
• Naming the Muscles
–
Students often learn the muscle names, locations, origins,
insertions, and actions.
–
Muscle names are based on Latin word parts that relate to a fact
about the muscle, such as:
• Appearance
• Location
• Action
• Relationship to other body parts.
–
Examples: pectoralis major, trapezius, biceps brachii
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Figure 7.11A. Major muscles of the body. A. Anterior view.
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Figure 7.11B. Major muscles of the body. B. Posterior view.
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Figure Credits
•
Figure 7.2. Reprinted with permission from: Strickland JW, Graham TJ. Master Techniques in Orthopaedic Surgery: The Hand,
2nd ed. Philadelphia, PA: Lippincott Williams & Wilkins, 2005.
•
Figure 7.4A. Reprinted with permission from: Mills SE, Histology for Pathologists, 3rd ed. Philadelphia, PA: Lippincott Williams
& Wilkins, 2007.
•
Figure 7.5B. Reprinted with permission from Cui, D. Atlas of Histology with Functional and Clinical Correlations. Baltimore:
MD: Lippincott Williams & Wilkins, 2011.
•
Figure 7.6. Reprinted with permission from: Premkumar K. Anatomy & Physiology: The Massage Connection. Baltimore, MD:
Lippincott Williams & Wilkins, 2012.
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