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.

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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)
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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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