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Central Nervous System

1 out of 9

Muscle spindle function is: (AIIMS MAY 2009)

A Length
B Stretch

C Touch
D Temperature

Ans. A Length
  1. There are two types of intrafusal fibers in mammalian muscle spindles. The first type contains many nuclei in a dilated central area and is therefore called a nuclear bag fiber.
  2. Typically two nuclear bag fibers occur per spindle: nuclear bag fiber 1 with a low level of myosin ATPase activity and nuclear bag fiber 2 with a high level of myosin ATPase activity. The second fiber type, the nuclear chain fiber, is thinner and shorter and lacks a definite bag.
  3. There are two kinds of sensory endings in each spindle. The primary (annulospiral) endingsQ are the terminations of rapidly conducting group Ia afferent fibersQ. One branch of the Ia fiber innervates nuclear bag fiber 1, whereas another branch innervates nuclear bag fiber 2 and nuclear chain fibers. The secondary (flower-spray) endings are terminations of group II sensory fibersQ and are located nearer the ends of the intrafusal fibers but only on nuclear chain fibers.

  1. The nerves from the endings in the nuclear bag region show a dynamic responseQ;ie, they discharge most rapidly while the muscle is being stretched and less rapidly during sustained stretch.
  2. The nerves from the primary endings on the nuclear chain fibers show a static responseQ;ie, they discharge at an increased rate throughout the period when a muscle is stretched. Thus, the primary endings respond to both changes in length and changes in the rate of stretch.

Functions of Muscle spindle

a. Sensing and Maintaining normal muscle length (By Length servo and Follow up servo mechanism)

b. Stretch ReflexQ

c. Muscle tone Q (Due to Gamma efferent discharge) Q

d. Body Posture (Postural reflex Qat spinal cord level)

So both stretch and length are functions of muscle spindle but the primary function is maintenance of length.

Pneumonic: Golgi tendon organ is for tension (t for tension, t for tendon) Muscle spindle is for length