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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.

Ends:

  • 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.

 

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