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9b structure and function of muscle spindle - Coggle Diagram
9b
structure and function of muscle spindle
Structure
It is a macroscopic structure, 4 mm long, spindle shaped and is widely distributed within the muscle.
Its number depends on the type of the
muscle, such as
-small distal muscles
which help in delicate control,
contain more muscle spindle
(150/gm);
-while
large muscles
contain
lesser
number (5/gm).
Each muscle spindle contains 8-12 muscle fibers enclosed in a connective tissue ‘capsule’, called the
intrafusal muscle fibers
,
while the regular contractile units of muscle are called the
extrafusal muscle
fibers.
The intrafusal fibers are in parallel with rest of the muscle
fibers
because the ends of the ‘capsule’ of the spindle are attached to the tendons at either end of the muscle or to the sides of the extrafusal fibers.
The central area of the spindle is 80-120μm wide and in this region the intrafusal fibers are surrounded by fluid contained in the capsule.
Functions
Muscle spindle is the receptor of stretch reflexes which are fundamental reflexes to regulate the posture.
By Length Servo Mechanism.
It is a system of negative feedback
device that
operates to maintain muscle length
during body movements.
The muscle spindle compares the length of extrafusal fibers with intrafusal fibers and there by control the length of extrafusal fibers.
'Follow up Servo' Mechanism.
By 'Follow up Servo' Mechanism. Muscle spindle also acts in regulation of posture by sustained contraction of the muscle.
Higher Control of Stretch Reflex
Since the connection between the afferent and efferent neurons is in the CNS (brain or spinal cord), therefore, activity of the reflex arc can be modified by higher centres.
However the balance between the two will determine the normal reflex activity.
γ-motor neuron activity is regulated to a large degree by descending tracts from a number of areas in the brain.
Thus, the sensitivity of the muscle spindle and hence the threshold of the stretch reflexes in various parts of the body can be adjusted and shifted to meet the needs of the postural control.
The brain areas that facilitate and inhibit stretch reflexes generally act by increasing or decreasing muscle spindle sensitivity.
Increasing
Facilitatory reticular formation area which is located in the brain stem discharges spontaneously in response to afferent input like
reticular activating system.
This increases discharge of γ-motor
neurons and stretch reflex become hyperactive.
Vestibular nucleus acts by a direct action on the α-motor neurons
(Proof: After cutting afferent nerve i.e., deafferentation of the muscle, if we stimulate the facilitatory reticular formation area, stretch reflex disappears; however, if stimulate the vestibular
nucleus, muscle contraction occurs.)
decreasing
Inhibitory reticular formation area acts by inhibiting γ-efferent neuron discharge, thereby decreases the spindle sensitivity.
It does not discharge spontaneously.
Fibers from the cerebral cortex and cerebellum reflexly inhibit the stretch reflex, they act by stimulating the inhibitory reticular formation.
Inhibitory area in the Basal ganglia may act through inhibiting facilitatory reticular formation or by stimulating the inhibitory area in the motor cortex.