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)

Muscle spindle

• Bundles of encapsulated, specialized fibers– “Intrafusal”

• Contractile polar regions• Passive equatorial region

– Isolated from surrounding structure

• Sensory afferents

Spindle Innervation

• Gamma motor innervation• Primary (Ia) annulospiral equatorial ending • Secondary (II) polar ending• Mechanical filtering

Spindle Response

• Primary ending adapts rapidly, velocity sensor• Secondary ending adapts slowly, length sensor• Directionally asymmetric

Spindle Response

• Depends on neuromuscular set• Depends on L,V, A

IncreasingCentral drive

Muscle stretch

Spindleresponse

Spin

dle

resp

onse

(Hz)

Length

IncreasingStretch velocity

Primary ending

Time

Houk JC, Rymer WZ, and Crago PE. Dependence of dynamic response of spindle receptors on muscle length and velocity. Journal of Neurophysiology 46: 143-166, 1981.

Stretch reflex circuitry

• Spindle afferent synapses with homonymous motorneuron

• Inhibits antagonist motorneuron

• Length controller– Postural mechanism

• Myotatic unit

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” Forciblecrossbridge detatchment

Reflex “compensation”

Golgi Tendon Organ

• Interwoven collagen and neural fibrils• Series connection between fiber & tendon• Active force sensor

– Fibers pulls collagen– Collagen deforms neuron

GTO Response

Twitch Response Tetanic Response

InstantaneousGTO frequency

MuscleForce

Houk J and Henneman E. Responses of Golgi tendon organs to active contractions of the soleus muscle of the cat. Journal of Neurophysiology 30: 466-481, 1967.

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

Renshaw interneurons

• Activated by recurrent collaterals of motor axon

• Inhibit homonymous motor pool• Drive dependent inhibition

Higher spinal organization

• Locomotion• Sensory information may entrain CPG• Positive feedback

Locomotor feedback

• Phase dependent response (fictive locomotion, decerebrate)

• Excitation of Extensors during flexor phase initiates extensor phase

• During extensor phase, extends extensor phase

• Broad, multimuscular response