Muscle Spindle & Stretch Reflex

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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.