1. 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)

2. Muscle spindle Bundles of encapsulated, specialized fibers –“Intrafusal” Contractile polar regions Passive equatorial region –Isolated from surrounding structure Sensory afferents

3. Spindle Innervation Gamma motor innervation Primary (Ia) annulospiral equatorial ending Secondary (II) polar ending Mechanical filtering

4. Spindle Response Primary ending adapts rapidly, velocity sensor Secondary ending adapts slowly, length sensor Directionally asymmetric

5. Spindle Response Depends on neuromuscular set Depends on L,V, A Increasing Central drive Muscle stretch Spindle response Spindle response (Hz) Length Increasing Stretch 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.

6. Stretch reflex circuitry Spindle afferent synapses with homonymous motorneuron Inhibits antagonist motorneuron Length controller –Postural mechanism Myotatic unit

7. 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”

8. Golgi Tendon Organ Interwoven collagen and neural fibrils Series connection between fiber & tendon Active force sensor –Fibers pulls collagen –Collagen deforms neuron

9. GTO Response Twitch Response Tetanic Response Instantaneous GTO frequency Muscle Force 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.

10. 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

11. Renshaw interneurons Activated by recurrent collaterals of motor axon Inhibit homonymous motor pool Drive dependent inhibition

12. Higher spinal organization Locomotion Sensory information may entrain CPG Positive feedback

13. 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