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MOTOR NEUROPHYSIOLOGY
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Motor output is of two types:
 Reflexive.
 Involuntary, and voluntary.
Some would add as a subdivision of reflex responses rhythmic responses;such as
swallowing, chewing, scratching, and walking, which are largely involuntary but subject to
voluntary adjustment and control.
Much is still unknown about the control of voluntary movement.
To move a limb, for example, the brain must plan a movement, arrange appropriate motion
at many different joints at the same time, and adjust the motion by comparing plan with
performance.
The motor system “learns by doing,” and performance improves with repetition. This
involves synaptic plasticity.
 Muscles containing many type I fibers called red muscles (darker than other
muscles), respond slowly and have a long latency, are adapted for long, slow,
posture-maintaining contractions (The long muscles of the back ).
 White muscles, which contain mostly type II fibers, have short twitch durations and
are specialized for fine, skilled movement. The extraocular muscles and some of the
hand muscles.
-The axial muscles are concerned with postural adjustments and gross movements.
-The distal limb muscles are those that mediate fine, skilled movements.
SOMATIC MOTOR SYSTEMS
The human skeleton is a system of levers that are moved by contraction of skeletal
muscles.
Muscle contraction only occurs in response to
action potentials in alpha motor neurons, which
originate in the ventral gray matter of the spinal
cord and brainstem nuclei.
A. Functions
1. Posture: maintain appropriate body position against external forces
2. Movement: change of body position to accomplish desired act
3. Coordination: control pattern and sequence of muscle contraction for smooth, effective
action
B. Initiation of Function
1. Involuntary: motor act, initiated by specific internal or external stimulus, generally
stereotyped,don’t need conscious will.
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]MOTOR SYSTEMS, POSTURE, SOMATIC SENSATION, [
2. Voluntary: sometimes initiated without any obvious stimulus, often complex and varied,
requires conscious will.
Note: voluntary and involuntary are not separate systems, since they involve shared
effectorsorgans and pathways, and interact with each other
C. Effector Structures
1. skeletal muscle: regular skeletal muscle fibers
2.neurons innervating skeletal muscle fibers:
 spinal cord: alpha motor neuronsin ventral horn, innervating muscles of movement
andposture
 cranial nerves: motor nuclei in the brainstem controlling;
1) Eye movement
2) Mastication (chewing)
3) Muscles of facial expression
4) Muscles of the soft palate and larynx
5) Muscles supporting the head
6) Tongue muscles
D. Influences on Motor Neurons (Motoneurons);
1. Reflexes: simple, stereotyped, involuntary movements (e.g. flexion reflex, myotatic
reflex)
2. Brain stem nuclei (tone)
3. Brain stem pattern generators: more complex movements, oftenvoluntarily initiated (e.g.
chewing, swallowing, walking)
4. Motor cortex; essential for voluntary activity
5.cerebellum(coordination, balance, posture/tone)
6. Basal ganglia (elaboration of movement & suppression of unwanted movements)
SPINAL CONTROL OF MOVEMENT
There are three inputs to alpha motor neurons
1. Upper motor neurons from the cortex or brainstem
are important mediators of voluntary movements and
are mainly inhibitory in nature.
2. Spinal interneurons may be excitatory or inhibitory .
3. Sensory neurons from muscle proprioceptors
provide feedback about muscle length and tension.
PROPRIOCEPTORS:
 Sensory information about body position and its relationship to the environment is
relayed from the vestibular, visual, and auditory systems, and is conveyed via three
major tracts:
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■ The tectospinal tract receives input from the retina and visual cortex(direct
the head and eyes to move toward a selected object in the visual field).
■The vestibulospinal tract provides one of the links between the sensors for
balance and the extensor muscles, which are important for maintaining posture.
■ The reticulospinal tract consists of two antagonistic pathways. A balance
between the activities of these pathways facilitates fine control of posture
through actions on the extensor muscles of the lower limb.
 A large amount of somatosensory information
comes from receptors(e.g.,Pacinian corpuscles) in
the skin and joint capsules.
 Sensory information and feedback is needed at
every stage of motor control to ensure smooth,
coordinated, and accurate movements.
In addition, muscles contain two types of
proprioceptors that provide feedback about muscle
length and tension:
1. intrafusal&extrafusal muscle fibersspecialized
muscle fibers for stretch (length) and the speed with
which muscle length is changing.
2. Golgi tendon organs are sensory nerve terminals
that are encapsulated within tendons,provide information about muscle force.
THE MYOTACTIC REFLEX (muscle stretch);
The myotacticreflex is demonstrated by tapping a tendon with a
reflex hammer; for example, when testing the knee jerk reflex.
■ Tapping the patellar tendon causes a small degree of stretch
in the quadriceps muscle, which results in the generation
ofaction potentials in afferents neuronsfrom muscle spindles.
■ A monosynaptic reflex arc afferents neuronssynapse
directly on the motor neurons.
The physiologic function of the monosynaptic
myotactic reflex is to resist gravity.
■A polysynaptic reflex arc there are inter
neurons between afferent &efferent neurons.
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RECIPROCAL INHIBITION
In many cases, muscles work in antagonistic pairs (e.g., the biceps and triceps muscles of
the arm).
When a muscle produces movement by shortening, it is
referred to as an agonist,muscles that oppose the action of the
agonist are antagonists.
(e.g., flexion of the elbow), the contraction of biceps is
accompanied by relaxation of its antagonist, the triceps muscle;
this phenomenon is known as reciprocalinhibition and is
mediated by spinal interneurons.
THE REVERSE MYOTACTIC REFLEX
The reverse myotactic reflex is mediated by Golgi tendon organs
 It is the reflex inhibition of muscle contraction stimulated by active contraction of the
muscle itself.
 protects a muscle from a potentially damaging overload during extreme contraction.
 under normalloading conditions it regulates muscle tension during a sustained
contraction.
For example, when picking up a fragile object such as an egg, a steady force that is not too
powerful must be applied.
THE FLEXOR WITHDRAWAL REFLEX
Spinal interneurons are not always inhibitory,
flexor withdrawal response, which protects the affected limb by rapidly removing it from an
injurious stimulus.
Thesteps of the flexor withdrawal reflex
responsecan be illustrated, for example,
when a painful stimulus is applied to one leg:
■ Pain receptors are activated at the site of
stimulation.
■ Afferent pain fibers enter the dorsal root
and send collaterals to several spinal
segments.
■ Excitatory interneurons that synapse with
alpha motor neurons serving flexors are
stimulated. Contraction of flexors removes
the limb from the aversive stimulus.
■
Reciprocal
inhibition
suppresses
contraction of the extensors of the affected limb.
■ In this example, where one leg would suddenly be lifted, the person would rarely fall
down because the postural support of the other limb would be simultaneously increased.
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]MOTOR SYSTEMS, POSTURE, SOMATIC SENSATION, [
CORTICAL CONTROL OF MOTOR FUNCTIONS:
There are numerous connections between the cortical areas related to motor function.
THE BASAL GANGLIA:initiation of voluntary movements.
THE CEREBELLUM:necessary adjustments to achieve smooth coordinated movements.
Functional classification is often used, based on the evolution of cerebellar functions:
■ The vestibulocerebellumcontributes to thecontrol of balance and eye movements.
■ The spinocerebellumcontrol of posture and walking and modulates spinal reflexactivity.
■ The cerebrocerebellumis essential in the decisions regarding the detailed sequence
and timing of muscle contractions needed to effect a coordinated movement.
E. Terms and Definitions
Tone; muscle tension in the absence of voluntary activity
synergist muscles; muscles effecting the same action at a joint
antagonist muscles; muscles performing opposing actions at a joint (e.g. flexors and
extensors)
hypertonia / hypotonia;increased / reduced muscle tone compared to normal
dystonia; tonically rigid posture
hyper- / hyporeflexia; increased / reduced reflex response
tremor; involuntary, small, rapid motions
rigidity; hypertonia resisting passive movement
asthenia; weakness, loss of strength
paresis; partial loss of voluntary control
paralysis; complete loss of voluntary control
Paraplegia; paralysis of the lower half of the body
Quadriplegia; paralysis from the neck down
Hemiplegia; paralysis of one side of the body
Ataxia; awkward, uncoordinated movement
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