Proprioceptors are the receptors, which detect and give response to movement and change in position of
different parts of the
body. These receptors are also called kinesthetic receptors.
Situation
Proprioceptors are situated
in labyrinth, muscles, tendon of the muscles, joints, ligaments and fascia.
Different Proprioceptors
1. Muscle spindle
2. Golgi tendon organ
3. Pacinian corpuscle
4. Free nerve ending
5. Proprioceptors in
labyrinth.
MUSCLE SPINDLE
Muscle spindle is a
spindle-shaped proprioceptor situated in the skeletal
muscle. It is formed by modified
skeletal
muscle fibers called intrafusal muscle fibers.
STRUCTURE OF MUSCLE
SPINDLE
Muscle spindle has a
central bulged portion and two tapering ends. Each muscle spindle is formed by 5 to
12 intrafusal muscle fibers. All these fibers are enclosed by a capsule, which is formed by
connective tissue. Intrafusal fibers are attached to the
capsule on either end. The capsule is attached to either side of extrafusal fibers or the tendon of the muscle. Thus, intrafusal fibers are placed parallel to the extrafusal fibers. Intrafusal fibers are thin and striated. Central
portion of the intrafusal fibers does not contract because it has only few
or no actin and myosin filaments. So, this portion acts only
as a receptor.
Only the
end portion of intrafusal fibers can contract. The discharge from gamma
motor neurons causes the contraction of intrafusal fibers.
Types of Intrafusal Fibers
Muscle spindle is formed
by two types of intrafusal fibers:
1. Nuclear
bag fiber
2. Nuclear
chain fiber.
1. Nuclear bag fiber
Central
portion of this fiber is enlarged like a bag and contains
many nuclei. Hence, it is called the nuclear bag fiber.
2. Nuclear chain fiber
In nuclear
chain fiber, central portion is not bulged and the nuclei are arranged in
the center in the form of a chain. Nuclear chain fiber
is attached to the side of end portion of nuclear bag fiber.
NERVE SUPPLY TO
MUSCLE SPINDLE
Muscle spindle is innervated
by both sensory and motor nerves. It is the only receptor in the
body, which has both sensory and motor nerve supply.
Sensory Nerve Supply
Each muscle spindle
receives two types of sensory nerve fibers:
1. Primary
sensory nerve fiber
2.
Secondary sensory nerve fiber.
1. Primary sensory nerve fiber
Primary
sensory nerve fiber belongs to type Iα
(Aα) nerve fiber. Each sensory (afferent) nerve fiber has two branches. One of the branches supplies the central portion of nuclear bag fiber. The
other branch
ends in central portion of the nuclear chain fiber. These branches end in the form of rings around central portion of nuclear bag and
nuclear chain fibers. Therefore,
these nerve endings are called annulospiral
endings.
2. Secondary sensory nerve fiber
Secondary
sensory nerve fiber is a type II (Aβ) nerve fiber. It innervates only the nuclear chain fiber and
ends near
the end portion of nuclear chain fiber like the petals of the flower. So, this
nerve ending is called flower spray ending.
Motor Nerve Supply
Motor
(efferent) nerve fiber supplying the muscle spindle belongs to gamma motor
neuron (Aγ) type.
Motor nerve supply to nuclear bag fiber Gamma motor nerve fiber supplying the nuclear bag
fiber ends
as motor end plate. This nerve ending is called plate
ending. Functionally, it is known as
dynamic
gamma efferent (motor) nerve fiber.
Motor nerve supply to nuclear chain fiber
Gamma
motor nerve fiber supplying the nuclear chain fiber divides into many branches,
which form a network called trail ending. Functionally, it is known as static gamma efferent (motor) nerve
fiber. Sometimes, it gives a branch to nuclear bag fiber also.
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