Muscle spindle

• 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.