Muscle spindles are small, capsule-bound stretch receptors lying parallel to ordinary muscle fibres in nearly all skeletal muscles. Each contains a few specialised intrafusal fibres (nuclear bag and nuclear chain fibres) wrapped by sensory endings. They signal muscle length and how fast it is changing.
| Location | Within skeletal muscles throughout the body, lying parallel to the extrafusal fibres |
|---|---|
| Parts |
|
| Nerve Supply | Type Ia and type II sensory fibres; Gamma motor neurons (fusimotor supply) |
| Blood Supply | Intramuscular arterial branches of the host muscle |
Sense muscle length and rate of stretch, providing position sense and driving the stretch reflex that helps control posture and movement.
Primary (Ia) endings respond to the speed of stretch and secondary (II) endings to static length. Stretching a muscle activates its spindles and triggers the stretch reflex, tested clinically as tendon reflexes such as the knee jerk. Gamma motor neurons keep spindles sensitive as the muscle shortens. Spindles are especially dense in muscles needing fine control, such as hand and neck muscles, and are a key target of proprioceptive training.
After upper motor neuron damage, such as stroke or spinal cord injury, exaggerated spindle-driven stretch reflexes cause velocity-dependent stiffness and brisk reflexes.
Loss of spindle and other proprioceptive input, for example from large-fibre neuropathy or dorsal column disease, causes unsteady movement worse with eyes closed.
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