The muscle spindle is a complex, spindle shaped, encapsulated receptor. It monitors muscle length (nuclear chain fibres) and the rate of change of length, i.e. velocity of shortening (nuclear bag fibres). It is a stretch receptor for myo-tactic reflexes.
The muscle spindle maintains muscles at their most efficient length under variable loads through the gamma loop, which determines muscle tone. Gamma motor neurons control contraction of intrafusal muscle fibres, which are responsible for muscle spindle sensitivity. Spindles are abundant in the anti-gravity muscles, muscles of the neck and in the intrinsic hand muscles. The components of a muscle spindle are:
Intrafusal: nuclear bag fibres (2-5)
nuclear chain fibres (6-10) Extrafusal Sensory endings:
Secondary, or flower-spray: responds to static stretch; relatively insensitive to U
phasic stretch; located over the contracting poles of the nuclear chain fibres. 1
Spindle afferents inhibit the alpha motor neurons to antagonist muscles via g inhibitory internuncial neurons. 1
Motor nerve supply:
Gamma efferents of Leksell (small motor nerve system). The gamma motor neuron regulates the sensitivity of the sensory endings to applied stretch. Two histological types of gamma efferent fibre endings: Motor end-plate (plate endings on nuclear bag fibres); Trail endings on nuclear chain fibres. Two kinds of sensory nerve patterns: Dynamic; Static.
The Renshaw internuncial neuron or Renshaw cell is a short interneuron in the cord, which is activated by a recurrent collateral from an alpha motor neuron. It synapses with and inhibits other motor neurons. Its system constitutes a feedback loop. The process by which a physiological mechanism controls itself by feeding back information that reflexly governs the action is called a servomechanism.
e The Golgi tendon organ at the musculotendinous junction comprises collage-
01 motor neurons innervating the same muscle. The circuit is thus a negative o y feedback system regulating muscle tension.
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