<h2>Muscle Spindle</h2>

Muscle spindles are sensory receptors embedded within the skeletal muscle. They respond rapidly to change in length of the muscle and convey this information to the CNS

Structure of muscle spindle
Each muscle spindle has three important components

• Specialized intrafusal fibres (static and dynamic nuclear bag fibres and static nuclear chain fibre types) with contractile ends and non-contractile centre

• Group Ia and II sensory nerve fibers.

Group Ia sensory fibers (annulospiral endings) that innervate both nuclear bag and nuclear chain fibers. They are dynamic, rapidly adapting sensory fibers and as such are sensitive to the rate of change in length of the muscle. 
Group Ia afferent fibers end directly on α-motor neurons supplying the extrafusal(the main skeletal muscle fibers) fibers of the same muscle. 
Group II sensory fibers innervate nuclear chain fibers. They are slowly adapting sensory nerve fibers that convey information about muscle length rather than the rate of change of length

• Efferent nerves (gamma motor neurons) supplying the contractile ends of the intrafusal fibres. Dynamic γ-motor neurons innervate dynamic nuclear bag fibers; static γ-motor neurons innervate combinations of chain and static bag fibres.

Role of muscle spindle

• Being in parallel with the extrafusal muscle (the main skeletal muscle fibers that are supplied by alpha motor neurons) fibers, when the main muscle is passively stretched, the spindles are also stretched, referred to as “loading the spindle.” This causes reflex contraction of the extrafusal fibers in the skeletal muscle due to firing of the afferent spindle fibers.
• On the contrary, muscle spindle afferent fibers characteristically stop firing when the muscle is made to contract by electrical stimulation of the α-motor neurons to the extrafusal fibers because the muscle shortens while the spindle is unloaded. 

'Thus, the muscle spindle and its reflex connections form a feedback mechanism that functions to maintain muscle length.