Proprioceptors Notes

Proprioceptors

Proprioceptors is necessary to know about the proprioceptors to understand the maintenance of posture and equilibrium.

Proprioceptors Definition: Proprioceptors are the receptors, which give responses to change in the position of different parts of the body. These receptors are also called kinesthetic receptors.

Proprioceptors Situation: Proprioceptors are situated in labyrinth, muscles, tendons 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

Read And Learn More: Medical Physiology Notes

Muscle Spindle

spindle is a spindle-shaped proprioceptor situated in the skeletal muscle. It is formed by modified skeletal muscle fibers called intrafusal muscle fibers. Intrafusal muscle fibers are innervated by gamma motor neurons and the extrafusal muscle fibers are innervated by alpha motor neurons.

Structure Of Muscle Spindle

• The muscle spindle has a central bulged portion and two tapering ends. Each muscle spindle is formed by 5-12 intrafusal muscle fibers.
• All these fibers are enclosed by a capsule, which is formed by connective tissue. The intrafusal fibers are attached to the capsule on either end.
• The capsule is attached to either side of intrafusal fibers or the tendon of the muscle. Thus, the intrafusal fibers are placed parallel to the extrafusal fibers. The intrafusal fibers are thin and striated.
• The central portion of the intrafusal fibers does not contract because it has only a few or no actin and myosin filaments. So, this portion acts only as a receptor. Only the end portion of the intrafusal fibers can contract. The discharge from the gamma motor neurons causes the contraction of the intrafusal fibers.

Types of Intrafusal Fibers: The muscle spindle is formed by two types of intrafusal fibers

1. Nuclear bag fiber
2. Nuclear chain fiber.
3. Nuclear bag fiber: The central portion of this fiber is enlarged like a bag and contains many nuclei. Hence, it is called the nuclear bag fiber.
4. Nuclear chain fiber: In this fiber, the central portion is not bulged and the nuclei are arranged in the center in the form of a chain. The nuclear chain fiber is attached to the side of end portion of the nuclear bag fiber.

Nerve Supply To Muscle Spindle: The 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: It belongs to type lα (Aα) nerve fiber. Each sensory (afferent) nerve fiber has two branches. One of the branches supplies the central portion of nuclear bag fiber
  2. Secondary sensory nerve fiber: It is a type 2 (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, the nerve ending is called the flower spray ending.
• Motor Nerve Supply: The motor (efferent) nerve fiber supplying the muscle spindle belongs to gamma motor neuron (Aγ) type.
  • Motor nerve supply to nuclear bag fiber: The gamma motor nerve fiber supplying nuclear bag fiber ends as a motor end plate. This nerve ending is called a plate end. Functionally, it is known as dynamic gamma efferent (motor) nerve fiber.
  • Motor nerve supply to nuclear chain fiber: The gamma motor nerve fiber supplying the nuclear chain fiber divides into many branches, which form a network called trail ends. Functionally, it is known as static gamma efferent (motor) nerve fiber. Sometimes, it gives a branch to nuclear bag fiber also.

Functions Of Muscle Spindle

• The muscle spindle gives response to change in the length of the muscle. It detects how much the muscle is being stretched and sends the information to central nervous system via sensory nerve fibers.
• The information is processed in the central nervous system to determine the position of different parts of the body. By detecting the change in the length of the muscle, the spindle plays an important role in preventing the over-stretching of the muscles.
• Muscle spindle has two functions:
  1. It is the receptor organ for stretch reflex
  2. It plays an important role in maintaining the 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.
     • The extensor muscles, particularly the antigravity muscles exhibit a severe and prolonged contraction during stretch reflex. Stretch reflex plays an important role in maintaining posture.
   • Role of muscle spindle in the stretch reflex
     • Stimulation of muscle spindle elicits the stretch reflex. The 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 the spinal cord. Alpha motor neurons in turn send motor impulses to muscles through their fibers and cause contraction of extrafusal fibers.
   • Response of muscle spindle to stretch: When the muscle is stretched, the primary sensory nerve fibers from muscle spindle discharge impulses. This response is of two types
     1. Dynamic response
        • It is the response in which the primary sensory nerve fibers discharge rapidly. When there is a change in the length of the muscle by stretching, the primary sensory nerve fibers from nuclear bag fiber start discharging impulses very rapidly.
        • But, the discharge becomes less or nil during the continuous stretching of the muscle. The discharge of impulses start only if there is the change in the degree of stretching of the muscle. Thus, the response depends upon the rate of change in the length of the muscle.
     2. Static response
        • It is the response in which the 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 the length of the muscle as well as the rate of change in the length.
     3. Physiologic Tremor: Physiologic tremor is the continuous discharge of action potentials with low voltage and ineffective frequency by the primary and secondary sensory nerve fibers of muscle spindle at resting condition. Physiological tremor plays 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 the muscle tone. It is due to the continuous discharge of impulses from gamma motor neurons.
   • The gamma motor neurons innervate the intrafusal fibers. The motor impulses from gamma motor neurons stimulate the intrafusal fibers of the muscle spindle which in turn sends sensory impulses back to spinal cord.
   • Now the alpha motor neurons in spinal cord are activated resulting in the contraction of intrafusal fibers of the muscle. When the frequency of the discharge from gamma motor neurons increases, the activity of muscle spindle is increased and the muscle tone also increases.

Golgi Tendon Organ

Structure Of Golgi Tendon Organ: Golgi tendon organ is situated in the tendon of skeletal muscle near the attachment of intrafusal fibers. Golgi tendon organ is placed in series between the muscle fibers and the tendon. The Golgi tendon organ is formed by a group of nerve endings covered by a capsule. The capsule is formed by connective tissue.

Nerve Supply To Golgi Tendon Organ: The sensory nerve fiber supplying Golgi tendon organ belongs to lb type. The nerve fiber supplying the Golgi tendon organ ramifies into many branches. Each branch ends in the form of a knob.

Functions Of Golgi Tendon Organ: The Golgi tendon organ gives response to the change in the force or tension developed in the skeletal muscle during contraction. It is also the receptor for the inverse stretch reflex and the lengthening reaction and thereby prevents damage of muscle due to overstretching.

1. Role of Golgi Tendon Organ in Forceful Contraction
   • During powerful contraction, the tension in the muscles increases and stimulates Golgi tendon organ which discharges the sensory impulses.
   • The impulses are transmitted by lb sensory nerve fiber to an inhibitory interneuron in the spinal cord. The interneuron, in turn, causes the development of IPSP in the motor neurons, which supply the muscle. Now, the contraction of the muscle is inhibited.
2. Role of Golgi Tendon Organ in inverse Stretch Reflex
   • Inverse stretch reflex
     • Inverse stretch reflex is the sudden decrease in resistance due to relaxation (instead of contraction) when a muscle is stretched excessively.
     • It is also called inverse myotatic reflex and it is a polysynaptic reflex. Inverse stretch reflex is actually the inhibition of contraction due to excessive stretching. So, it is also called autogenic inhibition.
   • Mechanism of inverse stretch reflex: Excessive stretch of the muscle leads to activation of Golgi tendon organ which send afferent impulses which cause
     • Stimulation of inhibitory internuncial neurons which in turn inhibits the alpha motor neuron of the stretched muscle resulting in relaxation
     • Stimulation of excitatory internuncial neurons which in turn activates alpha motor neuron of the antagonistic muscle. It leads to the contraction of the antagonistic muscle and relaxation of the stretched muscle.
3. Role of Golgi Tendon Organ in Lengthening Reaction
   • When tension increases during muscular contraction caused by the stretch reflex, the Golgi tendon organ is activated. It causes development of a spinal reaction, which is called the lengthening reaction. It can be demonstrated in a decerebrate preparation.
   • In decerebrate rigidity, the extension of limbs is due to the spastic contraction of extensor muscles. It is because of increased discharge from gamma motor neurons, which facilitates the stretch reflex.
   • In a decerebrate animal, some resistance is offered when the arm is flexed at the elbow joint passively. That is, the arm cannot be flexed easily.
   • This type of resistance is offered because of the stretch reflex developed in the triceps muscle. However, if the forearm is flexed forcefully, the resistance to flexion is abolished suddenly, leading to quick flexion of the arm. It is called the lengthening reaction.
   • The lengthening reaction is due to the activation of the Golgi tendon organ. The sudden flexion of arm is called the Phillipson’s reflex or clasp knife reflex.

Pacinian Corpuscle

• Pacinian corpuscle is a mechanoreceptor that senses pressure and vibration. It is situated in the deeper layers of skin. It is also situated in the tissues surrounding the joints such as fascia over the muscle, tendons, and joint capsule. The pacinian corpuscles situated in these tissues are responsible for determining the position of joints.
• Since the pacinian corpuscle is a rapidly adapting receptor (phasic receptor) it is very sensitive to the quick changes in the position of joints. So it is believed to send information about joint movement to central nervous system.

Free Nerve Ending

Free nerve ending is the receptor for pain sensation situated in skin, muscles, tendon, fascia, and joints. As it is a slow-adapting receptor (tonic receptor) it is maximally stimulated at specific joint positions. So it is belived to send information about joint position to central nervous system.