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Lecture 15

PSL300H1 Lecture Notes - Lecture 15: Posterior Grey Column, Tendon Reflex, Withdrawal Reflex


Department
Physiology
Course Code
PSL300H1
Professor
Michelle French
Lecture
15

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PSY300H1F L15; Oct. 17, 2011
Need efferents to stop out hair cells from amplifying,
otherwise getting ringing in ears
Reflexes
• Basic element of motor control in motor system
• ‘bending back’ of a sensory stimulus within the CNS to
produce a motor response – sensory-motor linkage at
basic level
• perform a specific task; often a feedback loop
regulating force, position, etc.; called up often, otherwise
suppressed
Reflex Loop
Signal sent by afferents to dorsal horn in spinal cord
or sensory nuc in brainstem (involving cranial
nerves)
Then directed to motor nuc to specifc muscles to get
motor response in periphery of body
Stretch reflex
• Stimulus: passive stretch of a muscle by applied load
or contraction of antagonist muscle;
• most important reflex, often unaware of; ex. Bang on
knee quadriceps by doctor
• Response: active contraction of muscle; e.g. patellar
tendon reflex
• very sensitive(can detect 1-2 µ) and fast due to muscle
spindle afferents (so fastest in NS), and monosynaptic
connection to motoneurons (only sensory afferent that
goes straight to motorneurons) – Grp 1a, 2
Get motor signal to stabilize load on arm
Stretch reflex function
• Essential for stabilizing posture; reflex strongest in
postural muscles; constantly detecting stretches and
making adjustments to keep posture upright; ex. If doze
off makes you stand up
• latencies (delays) of about 25 ms for forearm muscles,
37 ms for ankle extensors (longer conduction distance,
still short delay)
• parallel multisynaptic paths through spinal cord, and a
transcortical path – sustains reflex for longer, branch
sent up dorsal column medial meniscus system to cortex
• during movement, reflex suppressed– when actually
want to get out off posture and move, inhibit the stretch
reflex
Golgi tendon reflex
•Stimulus: active tension/contraction in muscle
• Response: relaxation or reduction of tension
• GTO afferents (Ib) synapse on interneurons in
intermediate zone of spinal cord – slightly slower than 1a
but still fast, less direct access
•Ib-interneurons inhibit α-motoneurons (perform
contraction vs gamma when sensitize and don’t do
physical work) of same muscle – so opposite effect on
muscle – so inhibit contraction
Over-exerting bicep muscle, over-contract trying to
maintain stability; negative-feedback/inhibitory most
of the time
Motorneurons inhibited to lower muscle activity to
basic level, functions in opposite direction to muscle
spindle (stretch relex sees as not enough, golgi
tendon reflex sees as too much)
Golgi tendon reflex properties
• Prevents movement
• acts in concert with stretch reflex to stabilize posture,
just operate in dif directions
• suppressed when net motion is desired
• in certain conditions reflex output is reversed (positive
feedback instead of negative); e.g. stance phase of
walking Over contracting biceps,
object brought up too high
1b afferents activated in
over-contracted muscle
Triceps stretched – slight
upward movement stretch
on opposite side, activate
muscle spindles, volley in 1a
afferents stretch reflex
causes small contraction in
stretched muscle
1b afferents in
overcontracted muscle cause
inhibition so loosen up to
stabilize position
So same overall function
Flexion withdrawal reflex
• Actually conscious of, but happens very rarely
• Stimulus: noxious injury of limb
• Response: flexion of joints proximal to stimulus (not
purely extension since distal limbs extend to stimulus)
pull limb away from site of injury
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