Each muscle spindle fibre consists of a bundle of small, specialized skeletal muscle fibres called intrafusal muscle fibres.
IFM fibres are surrounded by larger extra-fusal muscle fibres, which are responsible for the resting muscle tone and, at greater levels of stimulation, for the contraction of the entire muscle.
The IFMs have two types of endings - nuclear bag (NB) and nuclear chain (NC).
Contractile end regions are innervated by gamma (γ) efferent fibres that maintain spindle sensitivity. Extrafusal fibres are innervated by alpha (α) efferent fibres.
Stretching the central portion of the intra-fusal fibre distorts the dendrites and stimulates the sensory neuron, increasing the frequency of action potential generation.
Compressing the central portion inhibits the sensory neuron, decreasing the frequency of action potential generation
The Basal Nuclei and Cerebellum are responsible for coordination and feedback control over muscle contractions, whether contractions are consciously or subconsciously directed.
All stretch reflexes are monosynaptic and ipsilateral
Reciprocal inhibition also occurs—IIa fibres synapse with interneurons that inhibit the motor neurons of antagonistic muscles.