It essentially consist of modified muscle fibres called intrafusal fibries. The end of these fibries are striated and inverted by γ – motor neurons.
I A afferent fibers innervate the central part of these fibers ( annulospiral ending ).
The motor neurone is excited. The impulse passes out via the axons of these neurones and reach the neuro-muscular junction ,in the extrafusal fibers . The muscle fibres are excited by the release of neurotransmitter,acetylcholine. Muscle action potential develops. It excites the muscle and muscle contracts.