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.
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