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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