each skeletal muscle is innervated by at least ____ motor nerve which contains axons of up to hundreds of motor neurons.
one
the cell bodies of motor neurons are located in the _____
brain or spinal cord
the site where a nerve enters a muscle is called the _____
motor point
motor point
site where a nerve enters a muscle
each axonal branch supplies a _____ muscle cell.
single muscle cell
although a skeletal muscle is innervated by several neurons and the axonal branches of each neuron go to different muscle cells, each cell is usually innervated by only ____ motor neuron
one
motor unit
a motor neuron and all the muscle cells it innervates
electrical response to an action potential
depolarization and repolarization
physical response to an action potential
contraction and relaxation
muscle twitch
a motor unit’s physical response to a single stimulus from its neuron
a motor unit’s physical response to a single stimulus from its neuron is called a ___
muscle twitch
muscle twitches three periods
latent period
contraction period
relaxation period
the time between the arrival of a stimulus and the beginning of muscle shortening. during this time excitation contraction coupling occurs. once the cross bridges become active, tension begins to develop in the muscle
latent period
time between the beginning of muscle shortening and the development of peak tension
contraction period
time between the development of peak tension and the return of the muscle to its resisting length
relaxation period
the number of fast-twitch fibers in a muscle determines ___
twitch duration
twitch duration
how long a muscle twitch lasts
an ocular muscle is composed of mostly ____ twitch fibers and has a twitch duration of around _____ milliseconds
fast twitch fibers and has a twitch duration of around 10 milliseconds
the soleus muscle (a postural muscle in the lower leg) has a lot of _____ fibers and its twitch duration is around ______ milliseconds
slow twitch fibers and has a twitch duration of around 200 milliseconds
changing the strength of contraction is brought about by _____
force summation
force summation occurs in what two ways
- by increasing the number of motor units contracting simultaneously (called multiple fiber summation or motor unit recruitment)
- by increasing the frequency of contraction (called frequency summation)
multiple fiber summation or motor unit recruitment
number of motor units contracting simultaneously
frequency summation
frequency of contraction
the body produces multiple fiber summation by _____.
increasing the number of motor neurons sending signals to a muscle
in a weak contraction only a ____ motor units are active
only a few motor units are active in a weak contraction
in a stronger contraction more motor units are sending signals resulting in
resulting in more motor units being active or more muscle cells are contracting
the body produces frequency summation by increasing what
by increasing the frequency (number of signals per second) at which motor neurons send signals
frequency
number of signals per second
at lower frequencies individual twitches occur when
one after the other
as frequency increases there comes a point when another twitch occurs when
before the preceding twitch is over
why is the second twitch stronger?
because the muscle cells are contracting stronger
frequency summation can lead to ___
tetnus
tetnus
a sustained contraction
tetnus occurs at the frequency where ____________.
the successive contractions become so rapid they fuse together so there is no relaxation between stimuli
one reason a higher frequency of stimulation causes muscle cells to contract stronger is
because it increases intracellular levels of calcium ions
it is the binding of ____ ions to troponin that makes the binding sites available
calcium ions
an increased level of calcium reults in more binding sites available to participate in contraction which, in turn, causes what
an increase in contraction strength
increasing the stenth of stimulation will result in
muscle fiber summation
3 ways to increase muscle contraction strength
- muscle fiber summation
- frequency summation and tetnus
- treppe (staircase effect)
when a muscle has small motor units recruitment results in only a small increase in contraction strength. this allows what?
a fine degree of control over muscle tension
a muscle with large motor units results in a large increase in the amount of tension developed. such muscles can produce powerful contraction but with what?
with less precise control
when a well rested muscle is stimulated several times, there will be a slight increase in strength during the first few contractions. what is this called?
treppe or staircase effect
muscle tone or tonus
continual activity of some motor units maintains a low level of tension
true or false: even in a relaxed muscle some motor units are active
true
tonus in postural muscles allows what?
allows the body to remain upright
the loss of tonus results in…
collapse of the body or fainting
fatigue
occurs when a muscle is unable to contract
what is one factor that plays a major role in fatigue
increase hydrogen ions
an electromyogram is commonly used to….
determine the intensity and frequency of a muscle electrical response under varying conditions as well as which muscle are active during specific movements
displacement
the distance the transducer is moved
is there any difference between hands in the amount of time it took for fatigue to occur?
the non dominant hand fatigued faster
a muscle with a normal blood supply with fatigue ______ than a muscle with no blood supply
slower
which period of muscle twitch had the shortest duration?
latent period
which period of the muscle twitch had the longest duration?
relaxation period
the strength of contraction of a muscle twitch was _____ the strength of contraction of tetanus
less than
what is the frequency of stimulation that caused frequency summation?
5 stimuli per second
what is the frequency of stimulation that caused tetanus?
8 stimuli per second
