12-27. "BODY SENSE"
a. General. Body sense is the combined information from a number of sensory
inputs. Second by second, these inputs keep the brain informed of the specific posture
of the body and its parts. Some of the senses involved include:
(1) Muscle sense (proprioception).
(2) Joint capsule sense.
(3) Integument senses.
(4) Special senses (eye, ear, etc.).
b. Proprioception (Muscle Sense).
(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
(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.