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Lecture 15

Lecture 15


Department
Physiology
Course Code
PSL300H1
Professor
Doug Mac Kay
Lecture
15

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15 Reflexes October 15, 2007
Reflexes
Basic element of motor control
“bending back” of a sensory stimulus within the CNS to produce a motor response
perform a specific task – often a feedback loop regulating force, position of limbs, contraction,
etc.
Reflex Loop
Stretch reflex
Stimuluspassive stretch of a muscle by applied load or contraction of
antagonist muscle
o(passive because the force is external to the muscle)
Response – active contraction of muscle (negative feedback)
oe.g. patellar tendon reflex
fastest and very sensitive due to muscle spindle afferents (large
afferents, group 1a), and monosynaptic connection to motoneurons
(direct connection – no interneuronal processing)
Stretch reflex function
Essential for stabilizing posture
oreflex strongest in postural muscles – stretches are very minute
latencies of about 25 ms for forearm muscles, 37 ms for ankle extensors
olatency – time between the delivery of the stimulus and the onset
of the motor response
olonger conduction distance, longer latency
parallel multisynaptic paths through spinal cord (goes through
interneurons) and a transcortical path (signals carried back down spinal
cord to motor neurons)
during movement, reflex suppressed
oget rid of posture whenever you want to move
Golgi tendon reflex (GTO)
Stimulusactive tension/contraction in muscle
Response – relaxation or reduction of tension (negative feedback)
GTO afferents (Ib) synapse on interneurons in intermediate zone of spinal
cord (no direct connection)
Ib-interneurons inhibit a-motoneurons of same muscle
Golgi tendon reflex properties
Prevents movement
osuppressed when net motion is desired
Complementary to stretch reflex to stabilize posture
o(they’re just operating in the opposite directions)
in certain conditions, reflex output is reversed (positive feedback instead of
negative) [because of connections to interneurons]
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15 Reflexes October 15, 2007
ocan reroute the connections through the spinal cord to different interneurons
oe.g. stance phase of walking
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