NROC64H3 Chapter Notes - Chapter 13: Golgi Tendon Organ, Withdrawal Reflex, Stretch Receptor

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13 Apr 2012
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Chapter 13 - Spinal control of Movement:
The motor system consists:
consists of all our muscles and the neurons that control them
Can be divided into two part:
oThe spinal cord command and control of coordinated muscle contraction
oThe brain command and control of the motor programs in the spinal cord.
The somatic motor system:
Smooth muscle: lines that digestive tract, arteries and related structures and is innervated by
nerve fibers from the autonomic nervous system.
oPlays a role in peristalsis (the movement of material through the intestines) and the
control of blood pressure and blood flow.
oStriated muscle: cardiac and skeletal.
Cardiac muscle: the heart muscle, and it contracts rhythmically even in the
absence of any innervation. Innervation of the heart from the autonomic
nervous system functions to accelerate or slow down the heart rate.
Skeletal muscle: constitutes the bulk of the muscle mass of the body and
functions to move bones around joints, to move eyes within the head, to
control respiration, to control facial expression and to produce speech.
Each skeletal muscle is enclosed in a connective tissue sheath that, at
the ends of the muscle, forms the tendons.
oThe cells of skeletal muscle are called - muscle fibers.
Each fiber is innervated by a single axon branch from
the CNS
*** Because skeletal muscle is derived embryologically
from 33 paired somites, these muscles and the parts of
the nervous system that control them are collectively
called the somatic motor system.
*** Important: Somatic motor system - voluntary and generates behavior.
Remember: the elbow joint is formed where the humerus, is bound by fibrous ligaments to the radius
and ulna.
Movement in that direction that closes the joint is called - flexion
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obrachialis causes flexion. Its tendons insert into the humerus at one end and into the
ulna at the other.
oThe biceps brachii and the coracobrachialis (lies under the biceps) also cause flexion at
the joint.
*** These muscles are called synergistic because they work together.
Movement in the direction that opens the joint is called- extension
o( also called synergistic muscles) - triceps brachii and the anconeus
Somatic musculature:
Axial muscles - are responsible for movements of the trunk
Proximal muscles ( girdle) - move the shoulder, elbow, pelvis and knee
Distal muscles- move the hands, feet and digits ( fingers and toes)
*The axial musculature is very important for maintaining posture, the proximal musculature is critical
for locomotion and the distal musculature, particularly of the hands is specialized for the manipulation
of objects.
The Lower Motor Neuron:
The somatic Musculature is innervated by the somatic motor neurons in the ventral horn of the spinal
cord.
These cells are sometimes called lower motor neurons to distinguish them from the higher-
order upper motor neurons of the brain that supply input to the spinal cord.
Remember: *** only the lower motor neurons directly command muscle contraction.
The Segmental Organization of lower Motor Neurons:
The axons of lower motor neurons bundle together to form ventral roots
Each ventral root joins with a dorsal root to form a spinal nerve that exits the cord through the
notches between vertebrae.
Skeletal muscles are not distributed evenly throughout the body, nor are lower motor neurons
distributed evenly within the spinal cord.
For example, innervation of the more than 50 muscles of the arm originates entirely from
spinal segments C3-T1.
oThus, in this region of the spinal cord, the ventral horns appear swollen to
accommodate the large number of motor neurons that control the arm musculature.
Similarly spinal segments L1-S3 have a swollen ventral horn because this is where the motor neurons
controlling the leg musculature reside.
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The motor neurons that innervate distal and proximal musculature are found mainly in the
cervical and lumbar-sacral segments of the spinal cord, whereas those innervating axial
musculature are found at all levels.
The lower motor neurons are also distributed within the ventral horn at each spinal segment in
a predictable way, depending on their function.
oThe cells innervating the axial muscles are medial to those innervating the distal
muscles, and the cells innervating flexors are dorsal to those innervating extensors.
Alpha Motor Neurons:
** There are two categories of lower motor neurons of the spinal cord: alpha motor neurons and
gamma motor neurons.
Alpha motor neurons – directly trigger the generation of force by muscles.
One alpha motor neuron and all the muscle fibers it innervates collectively makeup the
elementary component of motor control: A motor unit.
The collection of alpha motor neurons that innervates a single muscle (i.e. the biceps brachii)
is called a motor neuron pool.
Graded Control of muscle contraction by Alpha Motor Neurons (AMN):
The first way the CNS controls muscle contraction is by varying the firing rate of motor
neurons.
oAn AMS communicates with a muscle fiber by releasing the neurotransmitter Ach at
the neuromuscular junction, the specialized synapse between a nerve and a skeletal
muscle.
oBecause of the high reliability of neuromuscular transmission, the Ach released in
response to one presynaptic action potential causes an excitatory postsynaptic potential
(EPSP) in the muscle fiber (sometimes also called an endplate potential) large enough
to trigger one postsynaptic AP.
A postsynaptic AP causes a twitch in the muscle fiber.
A sustained contraction requires a continual barrage of APs.
High frequency presynaptic activity causes temporal summation of the
postsynaptic responses as it does for other types of synaptic transmission.
Twitch summation increases the tension in the muscle fibers and smoothes the
contraction.
A second way the CNS grades muscle contraction is by recruiting additional synergistic motor
units.
oThe extra tension provided by the recruitment of an active motor unit depends on how
many muscle fibers are in that unit.
oIn the antigravity muscles of the leg (muscles that oppose the force of gravity when
standing upright), each motor unit tends to be quite large, with an innervation ration of
more than 1000 muscle fibers per single AMN.
Inputs to AMNs:
Lower motor neurons are controlled by synaptic inputs in the ventral horn.
oThere are three major sources of input to an AMN.
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