Functions of muscle spindle

 FUNCTIONS OF MUSCLE SPINDLE

Muscle spindle gives response to change in the length of the muscle. It detects how much the muscle is being stretched and sends this information to central nervous system (CNS) via sensory nerve fibers. The information is processed in CNS to determine the position of different parts of the body. By detecting the change in length of the muscle, the spindle plays an important role in preventing the overstretching of the muscles.

Muscle spindle has two functions:

1. It forms the receptor organ for stretch reflex

2. It plays an important role in maintaining muscle tone.

1. Role of Muscle Spindle in Stretch Reflex

Stretch reflex

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.

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.

Physiologic tremor

Physiologic tremor is the continuous discharge of actions potentials with low voltage and ineffective

frequency from primary and secondary sensory nerve fibers of muscle spindle at resting condition.

Physiological tremor plays an important role in the feedback regulation of muscle length.

2. Role of Muscle Spindle in the Maintenance of Muscle Tone

The state of continuous and partial contraction of the muscle is called muscle tone. It is due to the continuous discharge of impulses from gamma motor neurons. Gamma motor neurons innervate the intrafusal fibers. Motor impulses from gamma motor neurons stimulate the intrafusal fibers of muscle spindle, which in turn sends sensory impulses back to spinal cord. Now the alpha motor neurons in spinal cord are activated resulting in contraction of extrafusal fibers of muscle.  When the frequency of discharge from gamma motor neurons increases, activity of muscle spindle is increased and the muscle tone also increases.