In: Anatomy and Physiology
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.