Stretch reflex

PHYSIOLOGY OF MOVEMENT AND COORDINATION OF MUSCLES

1. Stretch reflex Department of P hysiology School of Medical Sciences and Research Sharda University SANJOG BAM HUMAN BIOLOGIST Msc . Medical Physiology

2. Learning objectives Introduction Muscles fibers and motor innervation Muscles spindles and its innervation Control of γ efferent discharge Phasic stretch reflex Tonic stretch reflex and role in posture maintenance

3. INTRODUCTION Reflex is a relatively predictable, involuntary and stereotyped response to an eliciting stimulus Skeletal muscles with intact nerve supply gets stretched ,it contracts It is the fundamental reflex which plays an important role in control of body posture. It is highly developed in antigravity muscles. Changes in the stretch reflex are involved in actions commanded by the brain.

4. Knee jerk reflex Reaction time:19-20ms Central delay:0.6 – 0.9ms

5. Types of Muscle fibers Types of muscle fibers Extrafusal fibers Forms bulk of muscle are innervated by alpha-motor neurons Provide the force for muscle contraction Intrafusal fibers Smaller than extrafusal muscle fibers innervated by gamma-motor neurons Encapsulated in sheaths Runs in parallel with extrafusal fibers Stretch reflex refers to contraction of muscle when it is stretched

6. Motor innervation… Two types of motor neurons: α motor neurons γ motor neurons α Motoneurons : innervate extrafusal skeletal muscle fibers Large multipolar ,up to 70 µm diameter. Activation of α - motorneurons leads to contraction of extrafusal muscle fibers γ Motoneurons innervates intrafusal muscle fibers Constitute 30% of fibers(3-6 ) µm in ventral root function is to adjust the sensitivity of the intrafusal muscle spindles

7. Muscle spindle Muscles stretch receptors Found in almost all skeletal muscles, particularly concentrated in muscles that exert fine motor control. e.g: the small muscles of the hand and eyes They are also found in good number in muscles involved in control of posture, especially the muscles rich in slow twitch fibers type. Muscles spindles lie in parallel with the regular muscle fibers

8. Innervated part of the muscle spindle encased in a connective tissue capsule Respond to change in the muscles length and velocity of lengthening . consists intrafusal fibers 2-12 fibers in a muscle spindle Spindle or fusiform shaped organ 100µm in diameter and up to 10mm long.

9. Intrafusal fibers Individual intrafusal fibers are much narrower and do not run the length of the muscle. Specialized muscles fibers innervated both by sensory and motor axons. Central non-contractile part contains receptor whereas the peripheral parts contain contractile element. Morphologically , two types: 1. nuclear bag fibers 2. nuclear chain fibers

10. Nuclear bag fibers Larger Nuclei are bunched together centrally Functionally these are divided into two types: Bag fiber 1: low myosin ATPase activity and responds best in the dynamic phase of muscle stretch. Bag fiber 2: high myosin ATPase activity and responds best in the static phase of muscles stretch Nuclear chain fibers Thinner and shorter fibers Present by the side of the nuclear bag fiber and donot have a definite bag Nuclei arranged in a row in the form of a chain Usually, four or more nuclear chain fibers are present in a spindle.

11. sensory innervation of muscle spindles Group Ia fibers Forms single primary ending ( annulo spiral) in each spindle. 12- 20µm 72-120 m/sec Wraps around the center of the nuclear bag(dynamic and static) and nuclear chain fibers Sensitive to change in length and velocity of lengthening of the muscle. Group II fibers Forms up to eight secondary endings(flower spray). 6 - 12µm 36-72 m/sec Located adjacent to the center of the static nuclear bag and nuclear chain fibers Donot innevate the dynamic bag fibers Sensitive to absolute length

12. Motor innervation to muscle spindles γ - Motoneurons :do not receive input from primary sensory afferents. : two types Dynamic γ 1 neurons Supply the dynamic nuclear bag fibers . Increases the dynamic sensitivity of the group Ia ending. Plate ending Static γ 2 neurons Supply the static nuclear bag and nuclear chain fibers . Increases tonic level of activity in both group Ia and group II endings Also decreases the dynamic sensitivity of group Ia afferents Can prevent silencing of Ia afferents during muscle stretch Trail ending

13. Control of γ efferent discharge No direct contact with primary sensory afferents . descending pathways adjust the rate of γ neuron discharge. γ neuron increases sensitivity of muscle spindles. Sensory pathways also indirectly influences γ neuron. α - γ Colinkage

14. muscles spindle continues to discharge even when the muscle is in contracted state.

15. Phasic stretch reflex Elicited by stimulating the primary endings of the muscle spindle Stimulus : sudden stretch of the muscle In the spinal cord ,afferent fibers divides into two branches: One branch of Ia fiber directly terminate mono synaptically on the homonymous neurons Other branch terminate di synaptically via an Golgibottle neuron on the heteronymous motor neurons. Reciprocal innervation and reciprocal inhibition . Phasic reflex contraction results in rapid limb movement. The phasic stretch reflex mediate tendon jerk and control rapid correction in motor output .

16. Tonic stretch reflex Elicited by a sustained stretch of the muscles Receptors are both the primary and secondary endings. Group II afferents direct contact with α motor neurons, thus monosynaptic reflex Contribute to the muscle tone

17. Posture Tonic stretch reflex is important for regulation of posture Stretch reflex helps to restore and maintain the posture for a very long period.

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