Proprioceptors its types and functions

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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