KIN 425,
Concepts of Strength & Conditioning
Chapter 2,
Neuromuscular Anatomy and adaptations to Conditioning
Can you make a motor neuron
selectively choose which muscle fibers it stimulates and fires?
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There is no such thing as a motor neuron stimulus that causes only some
of the fibers to contract.
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Once an action potential passes a motor neuron
all the fibers it innervates will contract.
Do individual motor neurons
control more than one type of muscle fiber?
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No. A motor unit is composed
of muscle fibers that are all of the same type.
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Motor units can be classified the same as muscle fibers, Type I – slow
twitch or Type II – fast twitch.
What are the characteristics of
a fast-twitch motor unit?
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Type IIa compared to IIb
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IIa has greater capacity for aerobic metabolism, and more capillaries
surrounding them than Type IIb, which gives them a greater resistance to fatigue.
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Fast-twitch units develop force rapidly
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Fast-twitch units relax rapidly and thus have a short twitch time.
What
are the characteristics of a slow-twitch motor unit?
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Develops force slowly
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Relaxes slowly and thus has a long twitch time.
What are the two ways muscle
force can be graded?
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One is by varying the frequency
of activation. If a motor
unit is activated once, the twitch that arises does not produce a great deal of
force. Increasing the frequency of
activation so that the twitch forces are allowed to summate, however, results in
greater force developed by the motor unit.
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The other means of achieving an increase in force is varying the number
of motor units activated, a process
known as recruitment.
Increases in force output are achieved by recruiting additional motor
units.
What are proprioceptors and
where are they located?
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Proprioceptors are specialized sensory receptors located within joints,
muscles, and tendons.
What do proprioceptors do?
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Because these receptors are sensitive to pressure and tension, they relay
information concerning muscle dynamics to the conscious and subconscious parts
of the central nervous system.
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Proprioceptors send information concerning kinesthetic sense to the
brain.
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Most of the proprioceptive information is processed at subconscious
levels. In our normal environments
we don’t have to dedicate conscious activity toward maintaining posture or
position of body parts.
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Proprioceptors are specialized sensory receptors that provide the CNS
with information needed to maintain muscle tone and perform complex coordinated
movements.
What proprioceptors provide
feedback information concerning muscle length and the rate of change in length?
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Muscle Spindles
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Muscle spindles are located within the muscles, coiled around modified
fibers called intrafusal muscle fibers and surrounded by normal extrafusal
muscle fibers.
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When the muscle lengthens, spindles are stretched.
This stretch activates the sensory neuron of the spindle, which sends an
impulse to the spinal cord, where it synapses with motor neurons, and these
motor neurons activate the muscles they innervate and a forceful contraction
takes place - This process is
called the myotatic stretch-reflex and also referred to as the
stretch-shortening-cycle (will cover in more detail in later weeks).
What proprioceptors provide
feedback information concerning the amount of tension developed within a muscle?
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Golgi Tendon Organs (GTOs)
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GTOs are located in tendons, and attached with extrafusal muscle fibers.
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GTOs are activated when an active muscle develops enough tension to cause
it’s attached tendon to stretch.
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When an extremely heavy load is placed on a muscle, reflexive inhibition
mediated by the GTOs causes the muscle to relax and lose tension (a protective
mechanism from development of excessive force).
With
training we develop the ability to override this inhibition and the GTOs become
less sensitive.