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Proprioception
• Sense of place and position• Sensory afferents
– Muscle spindles
– Joint receptors – Cutaneous afferents
• Other mechanisms
– Force feedback (Golgi tendon organ) – Effort feedback (Renshaw/recurrent inhibition)
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Muscle spindle
• Bundles of encapsulated,specialized fibers
– “Intrafusal”
• Contractile polar regions• Passive equatorial region
– Isolated from surrounding
structure
• Sensory afferents
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Spindle Innervation
• Gamma motor innervation• Primary (Ia) annulospiral equatorial ending
• Secondary (II) polar ending
• Mechanical filtering
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Spindle Response
• Primary ending adapts rapidly, velocity sensor• Secondary ending adapts slowly, length sensor
• Directionally asymmetric
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Spindle Response
• Depends on neuromuscular set• Depends on L,V, A
IncreasingCentral drive
Musclestretch
Spindleresponse
S p i n d l e
r e s p o n s e
( H z )
Length
IncreasingStretch velocity
Primary ending
Time
Houk JC, Rymer WZ, and Crago PE.Dependence of dynamic response ofspindle receptors on muscle lengthand velocity. Journal of
Neurophysiology 46: 143-166, 1981.
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Stretch reflex circuitry
• Spindle afferent synapseswith homonymous
motorneuron
• Inhibits antagonistmotorneuron
• Length controller
– Postural mechanism
• Myotatic unit
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Stretch Reflex
• Coordination with muscle mechanics• Crossbridge mechanics provide short-range
stability
• Larger perturbations could be catastrophic
Reflexive muscle
Areflexive muscle“Short range” stiffness.
Crossbridge response
“Yield” Forcible
crossbridge detatchment
Reflex “compensation”
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Golgi Tendon Organ
• Interwoven collagen and neural fibrils• Series connection between fiber & tendon
• Active force sensor
– Fibers pulls collagen – Collagen deforms neuron
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GTO Response
Twitch Response Tetanic Response
Instantaneous
GTO frequency
Muscle
Force
Houk J and Henneman E. Responses ofGolgi tendon organs to active contractionsof the soleus muscle of the cat. Journal of
Neurophysiology 30: 466-481, 1967.
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GTO Circuitry
• Di-/Tri-synaptic inhibition of homonymous MN – Inhibitory interneuron
– Also modulated by 1a, cutaneous, joint, …
• Broad intermuscular distribution• Force dependent inhibition
– Loss-of-force excitation
– Reflex compensation for fatigue
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Renshaw interneurons
• Activated by recurrent collaterals of motoraxon
• Inhibit homonymous motor pool
• Drive dependent inhibition
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Higher spinal organization
• Locomotion• Sensory information may entrain CPG
• Positive feedback
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Locomotor feedback
• Phase dependent
response (fictive
locomotion,
decerebrate)
• Excitation of
Extensors duringflexor phase initiates
extensor phase
• During extensor
phase, extendsextensor phase
• Broad, multimuscular
response
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