IPHREHAB : MUSCLE SPINDLE

IPHREHAB

MUSCLE SPINDLE

§Muscle structure is innervated by both sensory and motor neuron axons. When a skeletal muscle with intact nerve supply is stretched, this reflex is called as stretch reflex.

§Stimulus is stretch and response is contraction and the sense organ is muscle spindle. Example is knee jerk

§ The muscle spindle's functions are to send proprioceptive information about the muscle to the central nervous system, and to respond to muscle stretching.

§Muscle spindles are encapsulated by connective tissue capsule, and are aligned parallel to extrafusal muscle fibers, unlike Golgi tendon organs, which are oriented in series. 

§Muscle spindles are found within the fleshy portions of muscles, embedded in so-called extrafusal muscle fibres. They are composed of 3-10 intrafusal muscle fibres, of which there are three types:

§Dynamic nuclear bag fibres (bag1 fibres)

§Static nuclear bag fibers (bag2 fibres)

§Nuclear chain fibers and the axons of sensory neurons.

§There are two nuclear bag fibers per spindle whereas there are four or more nuclear chain fibers per spindle.

 Sensory Endings

Two kinds in each spindle

Primary or annulospiral endings are the terminations of rapidlly conducting Ia afferent fibers (innervates all three types of intrafusal fibers)

Secondary or flower spray endings are terminations of group II sensory fibers, located nearer the ends of intrafusal fibers but only on nuclear chain fibers.

Motor nerve supply

Motor nerve supply of their own

They make synapses at either or both of the ends of the intrafusal muscle fibers and regulate spindle sensitivity.

3-6μm in dia.

Constitute 30% fibers of ventral root.

Aγ group, also called as gamma efferent of Leksell, and go exclusively to spindles.

In addition, β motor neurons, innervates both intrafusal and extrafusal fibers.