Download Motor Control Spinal Reflexes

PHYSIOLOGY 1
LECTURE 20A
SKELETAL MUSCLE
SPINAL REFLEXES
MOTOR CONTROL SPINAL
REFLEXES
• Objectives: The student should learn:
• 1.
MOTOR CONTROL SPINAL
REFLEXES
•
Introduction:
– Most movements of the body are
controlled at the level of the spinal column
by activating a series of preprogrammed
spinal reflexes (Walking, running, etc.).
This allows the brain to direct motion but
not be involved in the necessity of
repetitive commands unless an emergency
arises.
MOTOR CONTROL SPINAL
REFLEXES
• Thus, the brain initiates the motion and at the
end brings it to a stop or changes the pace but
is not consciously active for each step. In order
to accomplish this spinal cord makes use of
reverberatory (oscillatory) circuits leading into
the skeletal muscle reflex circuits.
MOTOR CONTROL SPINAL
REFLEXES
II. Organization of the Spinal Cord for Motor Function
A. Gray matter – integrative area for the spinal cord
1. Posterior root – sensory signals
Sensory signals travel two pathways after entering
the spinal cord
a. Branch one of the sensory nerve terminates
almost immediately in the grey matter – local
segmental reflexes
b. b. Branch two of the sensory nerve transmits
signals to higher levels of the nervous system –
sensory relay neurons (propriospinal fibers)
MOTOR CONTROL SPINAL
REFLEXES
• 2. Neurons of the Grey Matter
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a. Anterior Motor
neurons – two types
• 1. Alpha motor neurons – very large
control extrafusal fibers (motor units)
• 2. Gamma motor neurons – smaller
control intrafusal motor fibers of the
spindles
MOTOR CONTROL SPINAL
REFLEXES
• b. Interneurons – Small very highly
excitable, provide interconnections and
are responsible for most reflex control –
types
• 1. Renshaw cells – inhibitory cells –
provide for lateral inhibition to sharpen
focus and increase central tendency
MOTOR CONTROL SPINAL
REFLEXES
• 2. Multisegmental connections from one
spinal cord level to another – Propriospinal
Fibers
• More than half of all spinal nerves –
provide for Multisegmental reflexes
coordinating simultaneous movements of
forelimbs and hind limbs.
MOTOR CONTROL SPINAL
REFLEXES
• III Skeletal Muscle Reflexes
• A. Muscle Stretch Reflex – simplest of the
muscle reflexes
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1. Sudden stretch of a skeletal muscle
•
2. Activation of a muscle spindle
• 3. Group 1a fibers from muscle spindle activate
alpha motor neuron
• 4. Alpha motor neuron contracts stretched
skeletal muscle
MOTOR CONTROL SPINAL
REFLEXES
• 5. Dynamic stretch reflex and static stretch reflex
• a. Dynamic stretch reflex – This reflex opposes any
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sudden change in muscle length and is controlled by
the muscle spindle primary sensory endings (1a fibers)
1] Sudden stretch = contraction
2] Sudden unload in contracted state = relaxation
3] Occurs over a fraction of a second from sudden
change
MOTOR CONTROL SPINAL
REFLEXES
• b. Static stretch reflex – This is a weaker
secondary reflex following the dynamic
stretch reflex but continues for up to a
minute. It tends to cause the degree of
muscle contraction to remain constant
unless the nervous system wills otherwise
MOTOR CONTROL SPINAL
REFLEXES
• 6. Damping function of the stretch reflex
(Signal averaging function) –
• a. Nervous signals from the motor cortex and
spinal cord vary in both frequency and intensity
which would produce a very jerky muscle
contraction if not modified.
• b. The stretch reflexes tend to dampen or
average these signals and thus, produce a
smooth muscle contraction
MOTOR CONTROL SPINAL
REFLEXES
• 7. Role of Muscle Spindle in Voluntary Motor Activity
• a. Coactivation Alpha motor neuron and gamma
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motor neuron
1] Both extrafusal and intrafusal muscle fibers
contract simultaneously
2] Sensitivity of the muscle spindles is maintained
3] Damping function is enhanced
4] Keeps muscle within ideal operating lengths
MOTOR CONTROL SPINAL
REFLEXES
• b. Stabilizes body Position during tense muscle
action
• 1] Reflex contracts both agonist and
antagonist muscles in response to muscle
spindle activation which tends to lock joints in a
stable position to best support the body
• 2] In performing a delicate task requiring fine
motor control the reflex locks major joints
providing a solid stable platform for the
execution of the task
MOTOR CONTROL SPINAL
REFLEXES
• 8. Clinical Applications of the stretch reflex
• a. Determine degree of spinal facilitation of the
spinal cord centers
• 1] Knee jerk reflex – (Patellar reflex)
• a] Muscle jerk exaggerated – upper level
facilitation high
• b] Muscle jerk low – upper level facilitation low
MOTOR CONTROL SPINAL
REFLEXES
• 2] Clonus – Oscillation of muscle jerks – Given
certain circumstances the muscle jerks can
oscillate
• a] Ankle clonus – a person standing on tip toes
suddenly drops to a flat footed stand stretching
the gastrocnemius muscles stretch reflex which
then causes contraction of the muscle to lift the
body again this can repeat several times
• b] Clonus usually only occurs when the cord is
highly facilitated in the first place
MOTOR CONTROL SPINAL
REFLEXES
• B. Golgi Tendon Reflex
• 1. Golgi tendon organ senses muscle tension
and tends to redistribute load over the entire
muscle group limiting chance of injury to any
small group of fibers
• 2. Dynamic and static responses
• a. Dynamic response – initiatial response to
increased tension is an increase in firing rate
(Initiatial motor units bear the entire load)
MOTOR CONTROL SPINAL
REFLEXES
• b. Static response - once the initial response
is over firing rates tend to drop back toward
normal as the load is distributed over the entire
muscle group
• c. Mechanism - Golgi reflex is inhibitory – thus
when fibers which are not involved in the
original lift are not inhibited and become
stimulated due to stretch then they pick up part
of the load – redistribution
MOTOR CONTROL SPINAL
REFLEXES
• C. Flexor Reflex and the Withdrawal Reflexes
• 1. Flexor reflex – usually elicited by stimulation of
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pain endings (Pin prick, heat, or a wound).
a. Stimulation of pain or pressure endings on the
limbs
b. Activation of interneurons in spinal cord
1] Multisegmental connections – stimulation of flexor
Alpha Motor neurons and interconnections to higher
levels of the nervous system
2] Renshaw cells – Inhibition of the extensor alpha
motor neurons
MOTOR CONTROL SPINAL
REFLEXES
• c. Activation of flexor muscles and inhibition of
extensor muscles – The withdrawal pattern
depends upon the sensory nerve stimulated
(principal of local sign – the body part afflicted
will be withdrawn from the pain stimulus using
whichever nerves and muscles which can best
complete the job)
• d. Limb withdraws from contact
MOTOR CONTROL SPINAL
REFLEXES
• 2. Crossed Extensor Reflex • Very shortly after a flexor reflex the
opposite limb begins to extend in order to
push the offending object away from the
body the crossed extensor reflex
MOTOR CONTROL SPINAL
REFLEXES
• D. Reflexes of Posture and Locomotion
• 1. Positive Supportive Reaction - Pressure on
the footpad causes the limb to extend against
the pressure
• a. Complex interneuron activation result in
activation of both flexors and extensors
straighten the limb and lock the joint
MOTOR CONTROL SPINAL
REFLEXES
• b. Magnet reaction – pressure on one
side of the foot causes the limb to extend
toward that side helping to keep the
animal upright
MOTOR CONTROL SPINAL
REFLEXES
• 2. Cord Righting Reflexes – An animal
placed on its side will make uncoordinated
movements until the spinal cord is in the
upright position.
MOTOR CONTROL SPINAL
REFLEXES
• 3. Stepping and Walking Movements –
The act of walking is a rhythmical set of
motions that are initiated by the motor
cortex but then continued in the spinal
cord by reverberatory (oscillatory) circuits
and mutually inhibitory circuits between
limbs.
MOTOR CONTROL SPINAL
REFLEXES
• a. Motor cortex command
• b. Reverberatory circuit firing initiated
• c. Leading leg flexor alpha motor neuron
and gamma motor neurons are excited
while extensor alpha motor neuron and
gamma motor neurons are inhibited.
MOTOR CONTROL SPINAL
REFLEXES
• d. Leading leg lifts and moves forward
• e. Trailing leg extensor alpha motor
neuron and gamma motor neurons are
excited while flexor alpha motor neuron
and gamma motor neurons are inhibited.
• f. Trailing leg extends and moves
backward
• g. Body moves forward