Human Anatomy and Physiology, Books a la Carte Edition: Muscles and Muscle Tissue and the Muscular System Flashcards
A nerve–muscle preparation, such as the gastrocnemius and sciatic nerve from a frog attached to stimulating electrodes, can be used to record a chart of stimulation and muscle contraction called a
A weak (subthreshold) electrical stimulus causes
As voltage is increased the threshold is reached
the minimum voltage necessary to generate an action potential and
At threshold or higher, a stimulus causes a quick cycle of contraction and relaxation called
A delay, or latent period, of about 2 milliseconds occurs between the onset of the stimulus and the onset of the twitch
During this time excitation, excitation–contraction coupling, and
tensing of elastic
The force generated is called internal tension
it does not show up on the myogram because it causes no shortening of the muscle.
Once elastic components are taut, the muscle begins to produce external tension
this is called the contraction phase of the twitch.
The resisting load in a preparation is the sensor of the recording apparatus, so
the movement is recorded on the myogram.
In the body, the resisting load is usually a
As the Ca2+ level falls, muscle tension declines during the relaxation phase.
The muscle is quicker to contract than it is to relax
Although electrical excitation of a muscle fiber obeys an all-or-none
law, muscle fibers do not exhibit
all-or-none twitches in response to excitation
Twitches vary in strength for a number of reasons:
a. Twitch strength varies with stimulation frequency, stimuli arriving close together
produce stronger twitches than those arriving far apart.
b. Twitches vary with the concentration of Ca2+ in the sarcoplasm, which can vary with
c. Twitch strength depends on how stretched the muscle was just before stimulation
d. Twitches vary with the temperature of the muscle; warmer muscle
e. Twitches are weaker when the pH of the sarcoplasm falls below normal, producing
f. Twitches vary with the state of hydration of the muscle, which affect overlap between
filaments and ability of myosin to form cross-bridges with actin.
______________ must be able to contract with variable strength for different tasks,
Stimulus intensity and stimulus frequency have
At threshold, a weak twitch occurs, and if voltage is increased, twitches are stronger.
i. Higher voltages excite more and more nerve fibers in the motor
nerve and thus
stimulate more motor units.
ii. This effect is called recruitment or multiple motor unit (MMU) summation.
Even at constant voltage, a higher frequency of stimulation produces
than does a lower frequency.
Up to 10 stimuli per second, a muscle produces an identical twitch
for each stimulus
recovers fully between twitches
Between 10 and 20 stimuli per second, the muscle recovers fully between twitches, but
each twitch develops more tension than the one before it. This
pattern is called treppe or
the staircase phenomenon.
At higher stimulus frequency (20–40 stimuli per second) each new
before the previous twitch is over. Each new twitch “piggybacks” on the previous one
and generates higher tension
i. This phenomenon is called temporal summation or wave
ii. It produces a state of sustained fluttering contraction called incomplete
At still higher frequency (40–50 stimuli per second) the muscle has
no time to relax at
all and the twitches fuse into a smooth, prolonged contraction called complete tetanus.
i. This state should not be confused with the disease tetanus caused
ii. Complete tetanus rarely if ever occurs in the body.
Contraction does not always mean the shortening of a muscle
it may mean only that the muscle is
producing internal tension
Isometric contraction is contraction without a change in length
Isometric contraction of antagonistic muscles at a joint maintains joint stability.
Isotonic contraction is contraction with a change in length but no change in tension
Isotonic contraction moves a load as the muscle shortens.
All muscle contraction depends on ATP, and the supply of ATP depends
on the availability of oxygen
and organic energy sources such as glucose and fatty acids.
two main sources of ATP synthesis are
anaerobic fermentation and aerobic respiration
allows the cell to produce ATP in the absence of oxygen, but
yield is limited and lactic acid, a toxic end product, is a major factor in muscle fatigue
produces more ATP and less toxic end products, but requires a
continual supply of oxygen.
Immediate energy such as that needed for a 100 m dash relies on
oxygen stored in myoglobin
The muscle borrows phosphate groups from other molecules and
transfers them to
ADP to form ATP. Two enzymes systems control these transfers
i. Myokinase transfers phosphate from one ADP to another to form
ii. Creatine kinase obtains phosphate from creatine phosphate (CP) and donates
it to ADP to make ATP.
ATP and CP, collectively called the phosphagen system
provide nearly all the energy
used for short bursts of intense activity, such as sprinting for 6 seconds.
As the phosphagen system is exhausted, the muscles shift into
anaerobic fermentation for short-
term energy until cardiopulmonary function can catch up with the oxygen demand.
a. During this period, muscles obtain glucose from the blood and from
their own stored
b. The pathway from glycogen to lactic acid, called the glycogen–lactic acid system,
produces enough ATP for 30 to 40 seconds of maximum activity.
Aerobic respiration produces much more ATP and
is a very efficient means of meeting
the ATP demands of prolonged exercise.
is the progressive weakness and loss of contractility that results
from prolonged use of
Fatigue has multiple causes.
a. The accumulation of K+ in the ECF lowers the membrane
b. ADP/Pi accumulation.
c. Lactic acid lowers the pH of the sarcoplasm, impairing Ca ion handling.
d. Fuel depletion; as muscle glycogen and blood glucose decline less ATP is synthesized.
e. Electrolyte loss through sweating can alter electrolyte balance of the extracellular fluid
and reduce muscle excitability.
f. The CNS fatigues by processes not yet understood, so that less signal output to the
Muscle fibers can be classed according to their
Slow oxidative (SO), slow-twitch, red, or type I fibers have
relatively abundant mitochondria,
myoglobin, and blood capillaries and a deep red color.
a. SO fibers do not fatigue easily and exhibit a relatively long
(~100 msec) in response to a single stimulus
b. Examples are the soleus muscle of the calf and the postural muscles.
Fast glycolytic (FG), fast-twitch, white, or type II fibers are
adapted for quick responses but not
for fatigue resistance.
a. They are rich in enzymes of the phosphagen and glycogen–lactic
b. Their SR releases and reabsorbs Ca2+ quickly.
c. FG fibers are poorer in mitochondria, myoglobin, and blood capillaries than SO fibers,
so they are relatively pale.
d. They produce twitches as short as 7.5 msec.
e. Examples are the gastrocnemius of the calf, biceps brachii of the arm, and muscles of
Humans have far more muscular strength than is normally used, and muscles can generate
than the bones and tendons can withstand.
Muscular strength depends on anatomical and physiological factors:
a. Muscle strength is primarily determined by muscle size.
b. Fascicle arrangement contributes to strength. Pinnate muscles such as the quadriceps
femoris are stronger than parallel muscles such as the sartorius, which in turn are stronger
than circular muscles such as the orbicularis oculi.
c. Large motor units produce stronger contractions than small ones.
d. Recruitment, or multiple motor unit (MMU) summation produces a stronger muscle
e. Temporal summation of action potentials causes stronger contraction.
f. The length–tension relationship contributes in that a muscle resting at optimum length
can contract more forcefully.
g. Fatigue contributes in that fatigued muscles contract more weakly.
Resistance exercise, such as weight lifting, can stimulate muscle
growth even if only done a few
minutes at a time a few times a week.
a. Growth results
primarily from cellular enlargement, not cell division.
b. Myofibrils grow thicker and split longitudinally when they reach a
c. Muscle fibers are incapable of mitosis, but some evidence indicates that they may also
split longitudinally as they enlarge.
Endurance (aerobic) exercise, such as jogging and swimming, improves
the fatigue resistance of
a. Slow-twitch fibers produce more mitochondria and glycogen and
acquire a greater
density of blood capillaries with endurance exercise.
b. This form of exercise also improves skeletal strength, increases red blood cell count
and oxygen transport capacity, and enhances cardiovascular, respiratory, and nervous
Cardiac and Smooth Muscle
are termed involuntary muscles because they are usually not subject
Cardiac muscle cells
are also called cardiocytes; cardiac muscle is limited to the heart,
where its function
is to pump blood.
Cardiac muscle is striated
like skeletal muscle but has shorter and thicker cells with uneven,
Each myocyte is joined to several others at its ends through linkages called
An intercalated disc has
electrical gap junctions
Damaged cardiac muscle is repaired by
Cardiac muscle contains a built-in pacemaker that rhythmically sets off a wave of
Cardiac muscle uses aerobic respiration almost
Smooth muscle myocytes are named smooth due to
lack of striations
Smooth muscle is composed of
myocytes with a fusiform shape
There is only one nucleus
The sarcoplasmic reticulum is scanty, and there are
no T tubules.
Thick and thin filaments are both present, but they are not aligned
and produce no visible
Z discs are absent and in their place are protein plaques on the
inner plasma membrane and
protein masses called dense bodies
Most smooth muscle is innervated by autonomic nerve fibers that can
trigger or modify its contraction.
The nerves have contrasting effects in different locations, such as
smooth muscle in the arteries while contracting smooth muscle in the
Stretch alone sometimes causes smooth muscle
is a group of diseases that cause progressive weakness and loss of muscle mass
a chronic autoimmune disorder in which antibodies destroy the communication between nerves and muscle, resulting in weakness of the skeletal muscles.