Unit of stretch reflex is -

Ans. is 'd' i.e., Reflex arc"A reflex arc is the basic functional unit of the nervous system which can perform an integrated neural activity. The simplest form is monosynaptic stretch reflexReflex arc of stretch reflexo When a muscle is stretched, the spindles (especially central stretch sensitive part of spindles) with in it also get stretched. Stretching of the muscle spindles increases the afferent nerve activity from muscles spindles. The increased activity is conveyed monosynaptically to the alpha motor neurons in the spinal cord. This alpha motor neurons supply the muscle fibers (extrafusal fibers) and stimulation of the alpha motor neurons lead to contraction of muscle (extrafusal fibers). Contraction of the extrafusal fibers release (i.e., reduces) the stretch on the intrafusal fibers (muscle spindle) and the reflex stops.o Stretching a muscle increases its length. The increase in length is detected by muscle spindles. The muscle spindles set up stretch reflex that causes contraction of the muscle, i.e., decrease in length. Thus the increase in length is corrected.o Depending upon the type of afferent stimulated, stretch leads to two type of responses: -i) Dynamic response:# Afferents respond with a frequency proportional to the rate at which the fiber is stretched. Afferents originating from nuclear bag fibers show this type of response.ii) Static response:# Afferents respond to the absolute length of the spindle. Afferents originating from nuclear chain fibers show this type of response.o Stretch reflex is the only known example of a monosynaptic reflex, i.e., has only single synapse, that is located in the anterior horn of spinal cord -->> Lowest most level of integration of stretch reflex is spinal cord.