Question

Fully describe the events that occur within the monosynaptic muscke spindle stretch reflex. ( must include specific anatomical structures and different types if neurons)

Answer 1

Stretch reflex is the reflex contraction of muscle when it is stretched. It is also called myotatic reflex. It is a monosynaptic reflex and the quickest of all the  reflexes. Extensor muscles, particularly the antigravity  muscles exhibit a severe and prolonged contraction  during stretch reflex. Stretch reflex plays an important  role in maintaining posture.

Muscle spindle as the receptor organ for stretch reflex :
Stimulation of muscle spindle elicits the stretch reflex.
Intrafusal muscle fibers are situated parallel to the  extrafusal muscle fibers and are attached to the tendon of the muscle by means of capsule. So, stretching of the muscle causes stretching of the muscle spindle also.

This stimulates the muscle spindle and it discharges  the sensory impulses. These impulses are transmitted  via the primary and secondary sensory nerve fibers to
the alpha motor neurons in spinal cord. Alpha motor  neurons in turn send motor impulse to muscles through  their fibers and cause contraction of extrafusal fibers.

Response of muscle spindle to stretch
When the muscle is stretched, primary sensory nerve  fibers from muscle spindle discharge impulses. This  response is of two types:
i. Dynamic response
ii. Static response.

i. Dynamic response
Dynamic response is the response in which the primary  sensory nerve fibers discharge rapidly. When there is a  change in length of the muscle by stretching, primary  sensory nerve fibers from nuclear bag fiber start discharging impulses very rapidly. But, the discharge becomes less or nil during continuous stretching of the muscle. Discharge of impulses start only if there is change in degree of stretching of the muscle. Thus, the response depends upon rate of change in length of the muscle.

ii. Static response
Static response is the response in which impulses are  discharged rapidly and continuously throughout the  period of muscle stretch by primary sensory nerve fibers
of the nuclear chain fibers.
Thus, the muscle spindle gives response to change  in length of the muscle as well as rate of change in  length.