Heckman, Ph.D. OT
pages 308 ‑ 323
Skeletal muscles are a
collection of individual muscle fibers.
A. The entire
muscle is surrounded by a connective tissue sheath called the epimysium.
connective tissue sheath, called the perimysium, divides the muscle into groups
of muscle fibers called fascicles.
muscle fibers are surrounded by a thin connective tissue sheath called the
D. All the
connective tissues are connected to each other so that when a muscle fiber
contracts its force is transmitted directly to the tendon.
When a skeletal muscle is
stimulated, it is able to generate force and shorten.
E. A single
stimulus, resulting in the generation of a single action potential, produces a
rise and fall in tension called a twitch.
If an action potential is
generated by a stimulus, the resulting twitch is an all‑or‑none
The strength of the
muscle contraction can be increased by:
Increasing the number of
muscle fibers stimulated.
Increasing the frequency
Altering the length of
the muscle fiber so it is closer to its optimal contractile length.
F. If the stimulus
is repeated so that a train of action potentials is produced, the twitches
The higher the frequency
of stimulation, the greater the summation.
If the frequency is high
enough, the tension rises smoothly to a maximum contraction called a tetanic
G. The contractile
proteins of the muscle fiber are connected to the bones by a series of structures
called collectively the series elastic component (SEC) of the muscle.
When the muscle contracts
(shortens) the SEC is stretched.
The SEC must be stretched
before force can be transmitted to the bones.
The SEC can not be
stretched all the way out during a single twitch.
The amount of force
transmitted to the bone depends on how much the SEC is stretched.
The SEC is stretched more
during summation than it is during a single twitch.
The SEC is stretched all
the way out during a tetanic contraction.
Muscle fibers are divided
into functional units called motor units.
H. A motor unit
consists of all the muscle fibers innervated by a single alpha motoneuron.
Each muscle fiber is
innervated by a single branch of the alpha motoneuron.
When an action potential
is generated by an alpha motoneuron, all of the muscle fibers it innervates are
The central nervous system controls the force of a muscle
contraction by varying:
The number of alpha
motoneurons activated and
The frequency of
discharge in each of the alpha motoneurons.
Muscle contractions are
called isometric or isotonic.
A.. In an isotonic contraction the force generated
by the muscle is just equal (or slightly greater) than the load on the muscle.
As a result, the muscle
The contraction is called
an isotonic contraction because the force (tension) produced by the muscle
during the shortening remains constant.
J. In an isometric
contraction, the force generated by the muscle is less than the load on the
As a result, the muscle
does not shorten.
The contraction is called
an isometric contraction because the overall length of the muscle remains
V.. The structure of muscle fibers has been
analyzed using electron and light microscopy.
muscle fibers extend from one end of the muscle to the other and are continuous
with the tendons on both ends.
L. The muscle
fibers are divided longitudinally into subunits called myofibrils.
The myofibrils are about
1 micron in diameter.
Myofibrils are separated
from each other by a system of longitudinal tubules.
M. The myofibrils
are divided into function units called sarcomeres.
Sarcomeres are bounded on
each end by a perpendicularly oriented band of protein called the Z‑line.
The contractile proteins
within a sarcomere are organized into two sets of filaments.
The thick filaments
(about 1.5 microns long) are in the center of the sarcomere.
The thin filaments (about
1 micron long) are attached to the Z‑lines on both ends of the sarcomere.
The thin filaments slide
over the thick filaments when the length of the muscle fiber changes.
The arrangement of the
filaments and Z‑lines gives skeletal (and cardiac) muscle its striated
The thick filaments in
the center of the sarcomere form a dark region called the A‑band.
The light region between
the A‑bands of two neighboring sarcomeres is called the I‑band. The I‑band contains thin filaments and
is bisected by the Z‑line.
The thin filaments
partially overlap the thick filaments.
As a result, the periphery of the
A‑band is darker than the center.
The lighter, central region of the A‑band, is called the H‑band.
During contraction the
thick and thin filaments slide across each other.
N. The thick
filaments are composed primarily of the protein, myosin.
Myosin is a large
filamentary protein divided into a tail and head region.
The tail region is
embedded within the thick filament.
The head region, called a
cross bridge, projects out of the thick filament towards the thin filament.
When the muscle contracts
the cross bridges attach to the thin filaments, bend towards the center of the
sarcomere, and cause the thin filaments to slide over the thick filaments.
O. The thin
filaments are composed of three proteins: actin, tropomyosin, and troponin.(see
Actin is a small globular
Two chains of individual
actin molecules are intertwined to form the backbone of the thin filament.
When the cross bridges
attach to the thin filaments they bind to the actin molecules.
Tropomyosin is a
filamentary protein that lies along the groove between the two actin chains.
P. When the muscle
is at rest, the tropomyosin chain prevents the cross bridges from binding to
Q. When the muscle
is activated, the tropomyosin moves from its resting position and allows the
cross bridges to bind with actin.
Troponin is a globular
protein attached to both actin and tropomyosin. It moves the tropomyosin from its resting position when the
muscle is activated.
R. During each
cross bridge cycle, the myosin cross bridges attach to actin, bend towards the
center of the sarcomere, detach, and return to their original perpendicular
One molecule of ATP is
used each time the cross bridge cycles.
The ATP molecule and the
ATPase which removes the phosphate from the ATP are both bound to the cross
When the cross bridge
binds to actin, the ATPase is activated, and ATP is converted to ADP.
The energy liberated from
ATP when it is split into ADP and phosphate is used to bend the cross bridge.
After the cross bridge
bends and the ADP dissociates from it, a new molecule of ATP becomes attached.
The cross bridge detaches
from the actin after the new molecule of ATP binds to it.
S. The sliding
filament theory of muscle contraction can be used to explain the length‑tension
The force of muscle
contraction varies with the resting length of the muscle.
The muscle is able to
generate the greatest amount of tension when it is at its optimal length (Lo).
The optimal length of a
muscle fiber is the length at which all the cross bridges on the thick filament
are opposite, or overlap, actin molecules on the thin filaments.
Muscles are usually at
their optimal length when relaxed.
Increasing or decreasing
the resting length of the muscle reduces the number of cross bridges that can
bind to actin and thus reduces the amount of tension the muscle can generate.
Calcium plays an
essential role in the regulation of muscle contraction.
T. When skeletal
muscle is relaxed, tropomyosin prevents the myosin cross bridges from binding
When troponin is
activated, it moves tropomyosin out of the way, allowing myosin and actin to
Troponin is activated
when calcium binds to it.
U. Under resting
conditions, the intracellular concentration of calcium (10-7 M) is too
low for it to bind to troponin.
However, when an action
potential is produced on the skeletal muscle membrane, the intracellular
concentration of calcium rises.
concentration (10‑5 M) is high enough to activate all of the
troponin in the muscle fiber.
As a result, tropomyosin
moves out of the way and cross bridge cycling begins.
Cross bridge cycling
continues until the calcium is removed from the intracellular fluid.
The resting concentration
of calcium is kept low by a system of longitudinal tubules called the
sarcoplasmic reticulum (SR).
V. Calcium pumps
(see figure 6‑15) that are similar to the Na‑K pump but transport
calcium instead of sodium and potassium are located on the SR membrane and
remove calcium from the intracellular fluid.
W. The calcium is
stored in the terminal cisternae of the SR.
X. The terminal
cisternae come in close contact with a set of membrane invaginations called the
depolarization of the transverse tubules by an action potential causes calcium
to be released from the terminal cisternae.