top of page

Muscle Spindle & Stretch Reflex

Extrafusal vs Intrafusal Fibers
  • Extrafusal fibers are the main, force-generating cells of the muscle.

  • Intrafusal fibers are modified muscle cells dispersed among extrafusal fibers.

Muscle Spindle
  • is a complex of intrafusal fibers along with their nerve supply.

  • are distributed throughout the belly of the muscle.

  • is a sensory structure → senses the length and changes in the length of the muscle

Structure of Muscle Spindle

  • 3-10 mm long.

  • Made up of

    • intrafusal muscle fibers and

    • their nerve supply

Intrafusal Muscle Fibers
  • Each muscle spindle has 3-12 intrafusal fibers.


  • are pointed.

  • are attached to glycocalyx of extrafusal fibers.

  • have contractile elements i.e. actin and myosin → contract when stimulated.

  • are innervated by γ (gamma) motor nueron.

Central Region:

  • has few or no contractile elements → does not contract.

  • serves the sensory function

  • contains nuclei.

  • Based on arrangement of nuclei, the intrafusal fibers are divided into two types:

    • Nuclear bag fibers:

      • have nuclei concentrated in wide central portion like a bag.

      • There are 1-3 nuclear bag fibers in a muscle spindle.

    • Nuclear chain fibers:

      • have nuclei aligned in a chain.

      • there are 3-9 nuclear chain fibers in a muscle spindle.

Nerve Supply

Sensory Innervation

  • Types:

    • Primary Sensory Endings

      • Encircles central region of all intrafusal fibers i.e. nuclear bag fibers as well as nuclear chain fibers.

      • are type Ia fibers.

    • Secondary Sensory Endings

      • Mainly innervate nuclear chain fibers.

      • Lie on one for both sides of primary endings.

      • are type II fibers.

  • Function

    • Sense length and changes in length of muscle.

    • Stimulated by stretching of central region of the muscle spindle.

      • Such stretching of central region occurs during:

        • Lengthening of the entire muscle or

        • Contraction of only the contracile ends of the muscle spindle.

    • When the muscle spindle is stretched → firing rate of sensory neurons increases.

    • On shortening → the firing decreases.

Motor Innervation

  • Innervates contractile ends of intrafusal fibers.

  • is a γ (gamma) motor nueron.

    • Note: Extrafusal fibers are supplied by α (alpha) motor neurons.

Stretch Reflex

  • The main role of muscle spindle is in stretch reflex.

  • When a muscle is stretched suddenly, excitation of muscle spindle causes reflex contraction of the same muscle.

Sudden stretching of the muscle


Central region of muscle spindle gets stretched


Firing of sensory nerve endings increases


Signal travels through the sensory neuron


Reaches the spinal cord


Stimulates the α (alpha) motor neuron to the same muscle


Stimulation of extrafusal fibers


Muscle contraction


Prevents further stretching of the muscle

↓ Prevents damage by sudden stretch on the muscle.

  • Upon sudden unstretching the above events occur in opposite magnitude → reflex relaxation of the muscle.

Reciprocal Innervation

A branch of sensory neuron also stimulates an inhibitory interneuron


The inhibitory interneuron inhibits motor neuron to antagonistic muscle


Relaxation of antagonistic muscle


Minimizes the antagonistic force on the main muscle


Increases the effectiveness of stretch reflex.


Next >>


bottom of page