Covering, Support, and Movement of the Body
T tubules are invaginations of the sarcolemma that run between
the terminal cisterns of the SR. ±ey allow an electrical stimulus
to be delivered quickly deep into the cell.
Sliding Filament Model of Contraction
According to the sliding ﬁlament model, cross bridge (myosin
head) activity of the thick ﬁlaments pulls the thin ﬁlaments
toward the sarcomere centers.
Physiology of Skeletal Muscle Fibers
Regulation of skeletal muscle cell contraction involves
(a) generating and transmitting an action potential along the
sarcolemma and (b) excitation-contraction coupling.
An end plate potential is set up when acetylcholine released by a
nerve ending binds to ACh receptors on the sarcolemma, causing
local changes in membrane permeability which allow ion ﬂows
that depolarize the membrane at that site.
±e ﬂow of current from the locally depolarized area spreads
to the adjacent area of the sarcolemma, opening voltage-gated
channels, which allows Na
inﬂux. ±ese events generate
the action potential. Once initiated, the action potential is self-
propagating and unstoppable.
±en as the action potential travels away from a region, Na
channels close and voltage-gated K
channels open, repolarizing
In excitation-contraction coupling the action potential is
propagated down the T tubules, causing calcium to be released
from the SR into the cytosol.
Sliding of the ﬁlaments is triggered by a rise in intracellular
calcium ion levels. Troponin binding of calcium moves
tropomyosin away from myosin-binding sites on actin, allowing
cross bridge binding. Myosin ATPases split ATP, which energizes
the power strokes. ATP binding to the myosin head is necessary
for cross bridge detachment. Cross bridge activity ends when
calcium is pumped back into the SR.
Muscular System; Topic: Sliding Filament Theory, pp. 18–29.
Contraction of a Skeletal Muscle
A motor unit is one motor neuron and all the muscle cells it
innervates. ±e neuron’s axon has several branches, each of which
forms a neuromuscular junction with one muscle cell.
A motor unit’s response to a single brief threshold stimulus is a twitch.
A twitch has three phases: latent (preparatory events occurring),
contraction (the muscle tenses and may shorten), and relaxation
(muscle tension declines and the muscle returns to its resting length).
Graded responses of muscles to rapid stimuli are wave summation
and unfused and fused tetanus. A graded response to increasingly
strong stimuli is multiple motor unit summation, or recruitment. ±e
type and order of motor unit recruitment follows the size principle.
Isotonic contractions occur when the muscle shortens
(concentric contraction) or lengthens (eccentric contraction) as
the load is moved. Isometric contractions occur when muscle
tension produces neither shortening nor lengthening.
Muscular System; Topic: Contraction of Motor Units, pp. 1–11.
±e energy source for muscle contraction is ATP, obtained from
a coupled reaction of creatine phosphate with ADP and from
aerobic and anaerobic metabolism of glucose.
When ATP is produced by anaerobic pathways, lactic acid
accumulates and ionic imbalances disturb the membrane potential.
To return the muscles to their resting state, ATP must be produced
aerobically and used to regenerate creatine phosphate, glycogen
reserves must be restored, and accumulated lactic acid must be
metabolized. Oxygen used to accomplish this repayment is called
excess postexercise oxygen consumption (EPOC).
Only about 40% of energy released during ATP hydrolysis powers
contractile activity. ±e rest is liberated as heat.
Muscular System; Topic: Muscle Metabolism, pp. 1–7.
Force of Muscle Contraction
±e force of muscle contraction is aﬀected by the number and
size of contracting muscle cells (the more and the larger the
cells, the greater the force), the frequency of stimulation, and the
degree of muscle stretch.
In twitch contractions, the external tension exerted on the load is
always less than the internal tension. When a muscle is tetanized,
the external tension equals the internal tension.
When the thick and thin ﬁlaments are optimally overlapping, the
muscle can generate maximum force. With excessive increase or
decrease in muscle length, force declines.
Velocity and Duration of Contraction
Factors determining the velocity and duration of muscle
contraction include the load (the greater the load, the slower the
contraction) and muscle ﬁber types.
±e three types of muscle ﬁbers are: (1) fast glycolytic
(fatigable) ﬁbers, (2) slow oxidative (fatigue-resistant) ﬁbers,
and (3) fast oxidative (fatigue-resistant) ﬁbers. Most muscles
contain a mixture of ﬁber types. ±e fast muscle ﬁber types can
interconvert with certain exercise regimens.
Adaptations to Exercise
Regular aerobic exercise gives skeletal muscles increased
eﬃciency, endurance, strength, and resistance to fatigue.
In skeletal muscle, resistance exercises cause hypertrophy and
large gains in strength.
Immobilizing muscles leads to muscle weakness and severe atrophy.
Improper training and excessive exercise result in overuse
injuries, which may be disabling.
Microscopic Structure of Smooth Muscle Fibers
A smooth muscle ﬁber is spindle shaped and uninucleate, and has
Smooth muscle cells are most o²en arranged in sheets. ±ey lack
elaborate connective tissue coverings.
±e SR is poorly developed and T tubules are absent. Actin
and myosin ﬁlaments are present, but sarcomeres are not.
Intermediate ﬁlaments and dense bodies form an intracellular
network that harnesses the pull generated during cross bridge
activity and transfers it to the extracellular matrix.
Contraction of Smooth Muscle
Smooth muscle ﬁbers may be electrically coupled by gap junctions.
ATP energizes smooth muscle contraction, which is activated by a
calcium pulse. However, calcium binds to calmodulin rather than
to troponin (which is not present in smooth muscle ﬁbers), and
myosin must be phosphorylated to become active in contraction.
Smooth muscle contracts for extended periods at low energy cost
and without fatigue.
Neurotransmitters of the autonomic nervous system may inhibit or
stimulate smooth muscle ﬁbers. Smooth muscle contraction may also
be initiated by pacemaker cells, hormones, or local chemical factors
that inﬂuence intracellular calcium levels, and by mechanical stretch.