Proprioceptors are sense receptors embedded in muscle. A special category of proprioceptors are stretch receptors (also called muscle spindles), specialized muscle cells containing 1a sensory neurons in the central region. 1A sensory neurons are the first part of a reflex arc forming synapses in the spinal cord with alpha motor neurons which enervate the very muscle in which the spindle is located. Additionally, there are gamma motor neurons that act directly on the spindle. Under the influence of motor centers in the brain or reflex circuits, gamma neutrons initiate the reflex arc, 'tricking' the muscle through its spindle into contraction.

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