Muscle spindle & Golgi tendon organ

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1. MUSCLE SPINDLE & GOLGI TENDON ORGAN BY : MMANTTHAN PUROHIT

2. WHAT IS MUSCLE SPINDLE ??? Basically, muscle spindle is the major sensory organ of muscle and is sensitive to quick and sustained stretch. It is one of the neurophysiological properties of contractile tissue, which may influence the muscle’s response to stretch. The main function of the muscle spindle is to receive and convey information about changes in the length of the muscle and the velocity of the length changes.

3. DEFINITION MUSCLE SPINDLE is defined as, ÏT IS A SPINDLE SHAPED PROPRIOCEPTOR SITUATED IN THE SKELETAL MUSCLE. IT IS FORMED BY MODIFIED SKELETAL MUSCLE FIBERS CALLED INTRAFUSAL FIBERS.

4. MUSCLE SPINDLE CONSIST OF... Muscle spindles are small, encapsulated receptors composed of afferent sensory fiber endings, efferent motor fiber endings, and specialized muscle fibers called INTRAFUSAL FIBERS. Each spindle contains 6-14 intrafusal fibers, which are of 2 types : NUCLEAR BAG FIBERS NUCLEAR CHAIN FIBERS NUCLEAR BAG FIBERS contains the nucleus in the central portion of the receptor and it gives the bag like structure. APPROX 2-5 NUCLEAR CHAIN FIBERS look like the chain, nuclei are arranged in the form of a chain. APPROX 6-10

5. INNERVATION OF MUSCLE SPINDLE… The muscle spindle is innervated by both sensory & motor nerve fibers. SENSORY SUPPLY : GROUP 1a FIBERS( primary sensory endings) GROUP 2 FIBERS( secondary sensory endings) The primary ending supplying both the nuclear bag fibers as well as nuclear chain intrafusal fibers are stimulated when the muscle spindle is stretched. Secondary endings innervate the receptor portion of mainly nuclear chain fiber, the nerve ending respond mainly to sustained stretch, therefore measure the length.

6. INNERVATION OF THE MUSCLE SPINDLE... MOTOR SUPPLY : The end portion of the muscle fiber is supplied by gamma efferent neurons. Gamma motor neurons are of 2 types. GAMMA-S GAMMA-D GAMMA-S fibers excite the nuclear chain fibers and enhance the static response in the muscle spindle. GAMMA-D fibers excite the nuclear bag fiber and enhance the dynamic responses in the muscle spindle.

7. FUNCTION OF THE MUSCLE SPINDLE… The muscle spindle fibers are sensitive to the length and the velocity of lengthening of the muscle fibers. They send messages to the brain about the state of stretch of the muscle fiber. When the muscle fiber shortens, the spindle stop sending messages because they are no longer stretched. When the signal decreases, the higher centers send a message to intrafusal muscle fibers in the spindle to shorten so that they once again are able to respond to the length change in the muscle spindle.

8. STATIC RESPONSE When the muscle spindle is stretched, the receptors which is present in that spindle is activated and it sends the impulses to the nervous system. It sends the messages to the nervous system through group 1a and group 2 fibers. Whenever the muscle spindle is stretched slowly, the proportion of the impulse transmit impulses many more minutes is called static response. It occurs due to the stretching of the nuclear chain fibers because it supplied by both the group 1a and group2 nerve fibers.

9. DYNAMIC RESPONSE If the muscle spindle structures stretched suddenly the nerve ending is stimulated powerfully and it is called dynamic response of spindle. It occurs when the nuclear bag fibers stretched because it is innervated by type 1a fibers.

10. MUSCLE SPINDLE AS THE RECEPTOR ORGAN FOR STRETCH REFLEX Stimulation of muscle spindle elicits the stretch reflex. Intrafusal fibers are situated parallel to the extrafusal 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 nerve fibers to alpha motor neurons in the spinal cord. Alpha motor neuron in turn send motor impulses to muscle through their fibers and cause contraction of extrafusal fibers.

11. ROLE OF MUSCLE SPINDLE IN THE MAINTENANCE OF MUSCLE TONE Muscle tone is defined as the state of continuous and passive partial contraction of muscle with certain vigor and tension. It is also defined as resistance offered by the muscle to stretch. Gamma motor neurons and muscle spindle are responsible for the development and maintenance of muscle tone. Development of muscle tone :

12. DEVELOPMENT OF MUSCLE TONE Impulses from the gamma motor neuron cause contraction of end portion of intrafusal fibers. This stretches and activates the central portion of the intrafusal fiber, which initiates the stretch reflex action for the development of muscle tone by discharging the impulses. Impulses from the central potion of the intrafusal fibers pass through primary sensory nerve fibers and reach the anterior gray horn of the spinal cord. These impulses stimulates the alpha motor neurons in the anterior gray horn. Alpha motor neuron in turn send impulses to extrafusal fibers of the muscle through spinal nerve fibers. These impulses produce partial contraction of the muscle fibers resulting in development of muscle tone.

13. GOLGI TENDON ORGAN

14. DEFINITION GOLGI TENDON ORGAN is defined as, It is a sensory organ located near the musculotendinous junction of extrafusal fibers. Which senses any change in the tension of the muscle.

15. STRUCTURE OF GTO GOLGI TENDON ORGAN is situated in the tendon of skeletal muscle near the attachment of the extrafusal muscle fibers. It is placed in series between the muscle fibers and the tendon. Golgi tendon organ is formed by a connective tissue capsule.

16. NERVE SUPPLY TO GTO Sensory nerve fiber supplying the GTO belongs to 1b type. Normally, the responses from the spinal cord or from the CNS are mostly relaxation of the muscle. When tension develops in a muscle, the GTO fires, inhibit alpha motor neuron activity, and decreases tension in the muscle-tendon unit being stretched.

17. FUNCTIONS OF THE GOLGI TENDON ORGAN GOLGI TENDON ORGAN gives response to the change in the force or tension developed in the skeletal muscle during contraction. ROLE OF GTO IN FORCEFUL CONTRACTION : During the powerful contraction, tension in the muscle increases and stimulates golgi tendon organ, which discharge the sensory impulses. Impulses are transmitted by 1b sensory nerve fiber to an inhibitory interneuron in the spinal cord. Inerneuron , in turn causes the development of IPSP in motor neurons, which supply the muscle.

18. 2) Role of GTO in inverse stretch reflex : Inverse stretch reflex is the sudden decrease in resistance due to relaxation when a muscle is stretched excessively. Inverse stretch reflex is actually the inhibition of contraction due to excessive stretching. So, it is also called autogenic inhibition. MECHANISM : Excessive stretch to the muscle leads to the activation of the GTO, which send afferent impulses which cause : Stimulation of inhibitory internuncial neuron, which in turn inhibits alpha motor neuron of the stretched muscle resulting in relaxation. Stimulation of excitatory internuncial neuron, which in turn activates alpha motor neuron of antagonistic muscle. It leads to contraction of antagonistic muscle and relaxation of stretched muscle.

19. 3) ROLE OF GTO IN LENGHENING REACTION : When tension increases during muscular contraction caused by stretch reflex, the GTO is activated. It causes the development of spinal reaction, which is called the lengthening reaction.

20. NEUROPHYSIOLOGICAL RESPONSE OF MUSCLE TO STRETCH When a stretch force is applied to muscle-tendon unit either quickly or over a prolonged period of time, the primary and secondary afferent nerve fibers of intrafusal muscle fibers sense the length changes and activate the extrafusal fiber via alpha motor neurons in the spinal cord, thus activating a stretch reflex and increasing a tension in the muscle being stretched. The increased tension causes resistance to lengthening and in turn, it is thought to compromise the effectiveness of the stretching procedure.

21. When stretch reflex is activated in a muscle being lengthened, decreased activity in the muscle on the opposite side of the joint, referred as reciprocal inhibition. In contrast, the GTO, which monitors tension in the muscle fibers being stretched, has an inhibitory impact on the level of muscle tension in the muscle tendon unit in which it lies, particularly if the stretch force is prolonged. This effect is called autogenic inhibition. Originally, the GTO was thought to fire and inhibit muscle activation only in the presence of high levels of muscle tension as a protective mechanism. However, the GTO has since been shown to have a low threshold for firing(fires easily), so it can continuously monitor and adjust the force of active muscle.

22. QUIZ TIME What is the difference between muscle spindle and GTO??? What is the motor supply of extrafusal fibers ??? What is reciprocal inhibition ??? What is autogenic inhibition ??? What is stretch reflex ??? What is inverse stretch reflex ???