Discuss the structure and function of muscle spindles and golgi tendon organs, explaining how spinal reflexes contribute to posture

Sanjay Manohar (Oxford 2015)

Spindle Tendon organ
Discovered Hassall 1851 Golgi
Weissmann 1861 structural description Ruffini-like
2-4mm fibrous capsule around lymph space 1mm
~6 intrafusal fibres: "parallel" series
1 per 500 extrafusal fibres 1 per 500 extrafusal
Ia afferent wrap around central area Ib afferent
large myelinated very fast moderately fast
contact spindle fibres free endings in tendon collagen
nuclear bag nuclear chain
thick thin
(Boyd cross-section diameter histograms)
peripheral II afferent
gamma_d gamma_s no efferent
d/dt (length) Length Force (tension)
predict future current state ?protective
dynamic / adapting static nonadapting
phasic tonic
Stretch / myotactic reflex

Monosynaptic (~1ms (synapse=0.5-0.9ms))

reciprocal innervation antagonist (inhib interneuron)

Clasp-knife / reverse myotactic

Inhibitory interneuron

Gamma: ~2 per spindle; account for 2/5 of efferent fibres (rest alpha)
Graph of spikes against stretch, for different gamma inputs Whitteridge 1959
 - gain; maintain sensitivity at all lengths; or "desired length"
Synapse onto all alpha for that muscle (~60). Small EPSP, spatio-temporal summation.
Voluntary muscle contraction => spindle firing => gamma / alpha coactivation

 - Important in disease states
 - synapse in clarke's column -> posterior spinocerebellar,
   (UL: accessory cuneate -> cuneocerebellar), but also to thalamus / cortex (proprioception)

Graph of Hunt & Kuffler 1951: 
 - stim alpha alone: silences afferents. alpha+gamma: constant response

Sherrington decerebrate cat (hypertonic/rigid)

- section dorsal root: abolish stretch reflex, but muscle movt remained intact abolishes vestibular postural reflex (head movt -> leg), but spindle discharge remains modulated by head position ==> Gamma efferents are responsible. - sectioned ventral roots: abolishes decerebrate rigidity Posture: automatic , active antigravity. Balancing, standing maintain centre of gravity in critical support area. - Stretch reflex increases effective stiffness - spindle as comparator, (desired minus actual) = error - large repertoire of more complex reflexss. seen in decerebrate suspended animals - Positive supporting: extend leg when it is pushed up - Buttress reaction & postural sway: extend limb when differential pressure to legs - Stepping reflex: several muscle groups - Tonic neck reflex: head-up extends forelimbs - flexor withdrawal + crossed extensor reflex - complemented by tactile, vestibular and visual information - "modulation" of reflexes; transcortical reflex.

Big picture

- how can automatic reflex action be combined with voluntary control? - how does postural muscle tone coexist with free movement - predictive coding: servo system that frees higher areas from low level jobs - hierarchical coding: delegation, simple jobs done "locally" = faster - integration with higher level information: tactile feedback; expectations (feedforward)