Muscle Spindle

Muscle spindle is a stretch receptor that is found within the belly of a muscle.  It monitors how stretched that particular muscle is and initiate the stretch reflex to prevent injury caused by over-stretching and to maintain posture.

Below is a picture of a muscle in transverse section showing the difference between a peripheral nerve (left) and the muscle spindle (right).  The spindle capsule, intrafusal muscle fiber and nerve fiber of gamma motorneurons with the nucleus of Schwann cell could be seen.

Muscle t.s. showing peripheral nerve (left) and muscle spindle (right) (TM: 400x, picture taken with a Zeiss Planapo 40/1.0 Oil on Sony A7ii)
Muscle Spindle t.s. showing capsule, intrafusal fibers, nuclei of Schwann cell and the gamma motorneuron it surrounds, orientation same as above (TM: 630x, picture taken with a Zeiss Planapo 63/1.4 Oil on Sony A7ii)

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