Human Anatomy and Physiology, Books a la Carte Edition: Muscles and Muscle Tissue and the Muscular System Flashcards

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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

no contraction.


As voltage is increased the threshold is reached

the minimum voltage necessary to generate an action potential and cause


At threshold or higher, a stimulus causes a quick cycle of contraction and relaxation called

a twitch


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
components occur.


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
stimulation frequency.
c. Twitch strength depends on how stretched the muscle was just before stimulation
(length–tension relationship).

d. Twitches vary with the temperature of the muscle; warmer muscle contracts more
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

contrasting effects


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

stronger twitches
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 stimulus arrives
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 summation.
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 by the
tetanus toxin.
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.


Muscle Metabolism

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


Anaerobic fermentation

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


Aerobic respiration

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 ATP.
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.


Muscle fatigue

is the progressive weakness and loss of contractility that results from prolonged use of
the muscles


Fatigue has multiple causes.

a. The accumulation of K+ in the ECF lowers the membrane potential.
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
muscles occurs.


Muscle fibers can be classed according to their

physiological characteristics


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 twitch
(~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 acid systems.
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
eye movement.


Humans have far more muscular strength than is normally used, and muscles can generate

more tension
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
certain size.
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
the muscles.

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
system function.


Cardiac and Smooth Muscle

are termed involuntary muscles because they are usually not subject to
conscious control.


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,
notched ends


Each myocyte is joined to several others at its ends through linkages called

intercalated discs


An intercalated disc has

electrical gap junctions


Damaged cardiac muscle is repaired by

fibrosis (scaring)


Cardiac muscle contains a built-in pacemaker that rhythmically sets off a wave of

electrical excitation.


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.


Smooth muscle

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 relaxing
smooth muscle in the arteries while contracting smooth muscle in the


Stretch alone sometimes causes smooth muscle

to contract


Muscular Dystrophy

is a group of diseases that cause progressive weakness and loss of muscle mass


Myasthenia Gravis

a chronic autoimmune disorder in which antibodies destroy the communication between nerves and muscle, resulting in weakness of the skeletal muscles.