(1) For proprioception, there is a very special receptor organ to monitor the degree of stretch of the muscle. These receptor organs, called muscle spindles or stretch receptors, are distributed within the fleshy belly of each skeletal muscle. In effect, the muscle spindles are parallel to striated muscle fibers of the skeletal muscles. Therefore, as the muscle fibers contract or are stretched, the muscle spindle detects relative muscle length.
(a) The afferent neuron from the muscle spindle is known as the annulospiral neuron because its terminal is coiled. Due to this coiling, it is a spring-like apparatus which can be stretched or compressed according to the condition of the muscle. The annulospiral neuron travels to the CNS by way of the appropriate nerve. It continuously carries information about the specific state of the muscle.
(b) An annulospiral neuron from a muscle in one of the limbs, in particular, synapses directly on the motor neuron that carries commands back to the same muscle. This motor neuron is called the alpha motor neuron. Together, the annulospiral neuron and the alpha motor neuron make up the stretch (monosynaptic) reflex. Due to this reflex, there is a proportionate increase in the tension of a muscle as it stretches.
(2) Another stretch receptor associated with the skeletal muscle is the Golgi tendon organ. As its name implies, this organ is located within the tendon of the muscle. The Golgi tendon organ is located in the tendon near its attachment to the muscle fibers. Thus, it detects relative muscle tension. Its threshold is higher than that of the muscle spindles; in other words, there must be proportionately more contraction before it puts out a signal. Thus, when the muscle has been stretched excessively and might be subject to injury, its afferent neuron carries the message to the CNS. This results in relaxation of the muscle.
(3) The pathway for the conscious sensation of these stretches uses the same structures as the deep touch general sense.
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