Muscle
281
SECTION A SUMMARY
I. There are three types of muscle—skeletal, smooth, and cardiac.
Skeletal muscle is attached to bones and moves and supports
the skeleton. Smooth muscle surrounds hollow cavities and
tubes. Cardiac muscle is the muscle of the heart.
Structure
I. Skeletal muscles, composed of cylindrical muscle fi
bers (cells),
are linked to bones by tendons at each end of the muscle.
II. Skeletal muscle fi bers have a repeating, striated pattern of light
and dark bands due to the arrangement of the thick and thin
fi laments within the myofi brils.
III. Actin-containing thin fi laments are anchored to the Z lines at
each end of a sarcomere. Their free ends partially overlap the
myosin-containing thick fi laments in the A band at the center
of the sarcomere.
Molecular Mechanisms of Skeletal Muscle
Contraction
I. When a skeletal muscle fi ber actively shortens, the thin
fi laments are propelled toward the center of their sarcomere by
movements of the myosin cross-bridges that bind to actin.
a. The two globular heads of each cross-bridge contain a
binding site for actin and an enzymatic site that splits ATP.
b. The four steps occurring during each cross-bridge cycle
are summarized in Figure 9–8. The cross-bridges undergo
repeated cycles during a contraction, each cycle producing
only a small increment of movement.
c. The three functions of ATP in muscle contraction are
summarized in Table 9–1.
II. In a resting muscle, tropomyosin molecules that are in
contact with the actin subunits of the thin fi laments block the
attachment of cross-bridges to actin.
III. Contraction is initiated by an increase in cytosolic calcium
concentration. The calcium ions bind to troponin, producing
a change in its shape that is transmitted via tropomyosin to
uncover the binding sites on actin, allowing the cross-bridges
to bind to the thin fi laments.
a. The rise in cytosolic calcium concentration is triggered by
an action potential in the plasma membrane. The action
potential is propagated into the interior of the fi ber along
the transverse tubules to the region of the sarcoplasmic
reticulum, where dihydropyridine receptors sense the
voltage change and pull open ryanodine receptors, releasing
calcium ions from the reticulum.
b. Relaxation of a contracting muscle fi ber occurs as a result of
the active transport of cytosolic calcium ions back into the
sarcoplasmic reticulum.
IV. Branches of a motor neuron axon form neuromuscular
junctions with the muscle fi bers in its motor unit. Each muscle
fi ber is innervated by a branch from only one motor neuron.
a. Acetylcholine released by an action potential in a motor
neuron binds to receptors on the motor end plate of the
muscle membrane, opening ion channels that allow the
passage of sodium and potassium ions, which depolarize the
end-plate membrane.
b. A single action potential in a motor neuron is suffi cient to
produce an action potential in a skeletal muscle fi
ber.
c. Figure 9–15 summarizes events at the neuromuscular
junction.
V. Table 9–2 summarizes the events leading to the contraction of
a skeletal muscle fi ber.
Mechanics of Single-Fiber Contraction
I. Contraction refers to the turning on of the cross-bridge cycle.
Whether there is an accompanying change in muscle length
depends upon the external forces acting on the muscle.
II. Three types of contractions can occur following activation of a
muscle fi ber: (1) an isometric contraction in which the muscle
generates tension but does not change length; (2) an isotonic
contraction in which the muscle shortens (concentric), moving
a load; and (3) a lengthening (eccentric) contraction in which
the external load on the muscle causes the muscle to lengthen
during the period of contractile activity.
III. Increasing the frequency of action potentials in a muscle fi
ber
increases the mechanical response (tension or shortening) up
to the level of maximal tetanic tension.
12
3
4
5
Figure 9–31
Boy with Duchenne muscular dystrophy. Muscles of the hip girdle and trunk are the fi rst to weaken, requiring patients to use their arms to
“climb up” the legs in order to go from lying to standing.
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