motor nervous system : Stretch reflex

Detailed description of structure and function of muscle spindle and stretch reflex

1. Dr : Dina Merzeban Dept of Physiology Motor nervous system Stretch reflex

2. What is a reflex? Response to a stimulus Stimulus Response Task: Write down 3 reflexes .

3. What is a reflex? Afferent nerve Central c o n n ec t i on s Efferent nerve Receptor Stimulus Response Effector organ

4. Stretch reflex This is a basic reflex present in the spinal cord Stimulus: muscle stretch Response: contraction of the muscle Receptors: stretch receptors located in the muscle spindle .

5. skeletal muscle two types of muscle fibres extrafusal normally contracting fibres intrafusal non contractile fibres present inside the muscle spindle lie parallel to extrafusal fibres contains stretch receptors .

6. E x t r a f usal fibre I n t r a f u sal fibre

7. Contractile a r eas Stretch receptor

8. Nerve supply Sensory to intrafusal fibre: Ia afferent II afferent Motor: to extrafusal fibre  motor neuron to intrafusal fibre  motor neuron .

9. Ia afferent nerve  motor neuron o n e syna p se musc l e st r e tch musc l e contraction Stretch reflex

10. When a muscle is stretched stretch receptors in the intrafusal fibres are stimulated via type Ia afferent impulse is transmitted to the spinal cord  motor neuron is stimulated muscle is contracted Monosynaptic Neurotransmitter is glutamate

11. Str e tch Reflex

12. Stretch Reflex - Knee Jerk

13. nuclear bag fibre primary (Ia) afferent supplies annulospiral ending in the centre nuclear chain fibre primary (Ia) and secondary (II) afferent supplies flower spray ending . two types of intrafusal fibres

14. Ia afferent fibre II afferent fibre nuclear bag fibre  m o t o r neuron nuclear chain fibre  m o t o r neuron

15. Plate ending Trail ending Annulospiral ending Flowerspray ending

16. Importance of stretch reflex detects muscle length and changes in muscle length .

17.  motor neuron cell body is located in the anterior horn motor neuron travels through the motor nerve supplies the intrafusal fibres ( c on t rac t i le e l em e n ts a t e i t he r en d ) .

18.  motor neuron  motor neuron  motor neuron

19. When  motor neuron is active extrafusal fibres are contracted muscle contracts when  motor neuron is active intrafusal fibres are contracted stretch receptors are stimulated stretch reflex is activated impulses will travel through Ia afferents alpha motor neuron is activated muscle contracts .

20. at rest m u s c le stretched a c t i v e  m o t o r neuron I a I a  Ia afferents are stimulated stretch reflex is initiated .

21.  motor neuron activity active all the time - mild contraction Maintain the sensitivity of the muscle spindle to stretch modified by the descending pathways descending excitatory and inhibitory influences sum effect is generally inhibitory in nature .

22. Alpha gamma co-activation gamma motoneurons are activated in parallel with alpha motoneurons to maintain the firing of spindle afferents when the extrafusal muscles shorten Prevent unloading

23. Stretch reflex 2 types -Response that is transmitted: Dynemic: -when there is change in the length of the spindle receptor (stretching of the sensory receptor area of the muscle spindle by stretching of the muscle spindle or the whole muscle). Detect Change in length. -transmitted by the primary fiber Aα type Static continuous information about the length of the muscle (not the change in length). transmitted by both the primary Aα and secondary (Aβ and Aγ)

24. S TATIC AND DYNAMIC RESPONSE OF MUSCLE SPINDLE AFFRENTS Static response is the discharge at any constant length of the muscle. The greater the muscle length greater is the stretch in the spindle and the higher is the static response of the spindle affrents. Both the primary (Iα) and secondary II spindle affrents gives static or length sensitive responses. The dynamic response of a spindle affrents refer to the discharge during stretch of the muscle. If the spindle affrents gives greater response during a fast stretch than it dose during a slow stretch (velocity different but distance of stretch same) it is said to poses a dynamic response component. Only the primary spindle affrents gives a dynamic or velocity sensitive response.

25. S TATIC AND D YNAMIC FUSIMOTOR NEURONS Dynamic fusimotor fiber increase the dynamic response of the primary spindle affrents (Iα) and have little or no effect on secondary. Static fusimotor fibers increases the static response of both the primary and secondary spindle affrents. However the effect of static fusimotor fiber on primary spindle affrents is less marked than their effect on the secondary. Static fusimotor fiber terminate as trail endings (mostly present in nuclear chain fibers). Experiment using depletion of muscle glycogen as an index of muscle fiber activity have shown that repetitive stimulation of the static fusimotor fiber result primarily in chain fiber glycogen depletion. Dynamic fusimotor stimulation produces mostly bag fiber glycogen depletion.

26. Significance of stretch reflex Maintain muscle length Control of voluntary movement Muscle tone

27. Servoassistant function: alpha and gamma motor neurons are coactivated during voluntary movements Damping function Role of stretch r eflex in the control of voluntary movement

28. Muscle tone Def.: continuous alternating reflex subtetanic contraction of muscle fibers Cause : continuous stretch of muscle spindle (rest) Short dist. Between origin and insertion Gravity Gamma efferent discharge No fatigue!

29. Function : Posture against gravity Background for voluntary movement Regulation of body temp. Venous and lymph return

30. Inverse stretch reflex When the muscle is strongly stretched Golgi tendon organs are stimulated Via type Ib afferents impulse is transmitted to the spinal cord inhibitory interneuron is stimulated  motor neuron is inhibited muscle is relaxed .

31. G OLGI T ENDON O RGAN

32. Inverse Stretch Reflex

33.  motor neuron Undue stretch Golgi tendon organ muscle re l ax a t i o n Ib afferent nerve inhibitory interneuron Inverse stretch reflex

34. Importance of inverse stretch reflex detects muscle tension .

35. Supraspinal regulation Facilitatory areas Facilitatory reticular formation(pons) Area 4 Neocerebellum Vestibular nuclei Inhibitory areas Inhibitory reticular formation (medulla) Area4s Paleocerebellum Basal ganglia Red nucleus

36. Disorders of muscle tone Abnormalities of the tone : Hypertonia – Pyramidal hypertonia (Spasticity) Extrapyramidal hypertonia (Rigidity) Hypotonia

37. Pyramidal hypertonia (Spasticity) Spasticity – a motor disorder characterized by velocity- dependent increase in muscle tone with exaggerated tendon jerks, resulting from hyperexcitability of the stretch reflex. Pyramidal hypertonia is most pronounced in the muscle groups most used in voluntary movements.

38. C LASP KNIFE REFLEX Seen in decerebrate rigidity On stretching the muscle beyond a point causes Ib affrent inhibitory discharge from GTO which reflexly inhibits homonymous stretched muscle

39. Spa s ticity Physiologic evidence suggests that interruption of reticulospinal projections is important in the genesis of spasticity. In spinal cord lesions, bilateral damage to the pyramidal and reticulospinal pathways can produce severe spasticity and flexor spasms, reflecting increased tone in flexor muscle groups and weakness of extensor muscles.

40. Spasticity - EDX There will be increased H reflexes, identified with an increase of maximum amplitude H reflex compared to the M wave – H/M ratio. Increased F wave amplitude.

41. Spasticity – The Mechanism α- motoneuron excitability- enhanced H:M ratio and F-wave amplitude suggest enhanced excitability of α- motoneuron. γ- motoneuron excitability – causes increased spindle sensitivity to stretch, augmenting the Ia afferent response to stretch, and exaggerates the stretch reflex.

42. Spasticity – the mechanism Recurrent inhibition –recurrent collateral axons from motoneurons activate Renshaw cell, which inhibit α- motoneurons. Changes in recurrent inhibition plays a role in the pathophysiology of spasticity. Reciprocal inhibition- During active contraction, it is necessary to inhibit MNs supplying the antagonist muscle(s),at the same rate ( Sherrington’s law of reciprocal innervation). This is to prevent their reflex contraction in response to stretch. Presynaptic inhibition

43. Clinical correlation In cortical and internal capsular lesions, the controlling drive on the inhibitory center in the medullary brain stem is lost and so in absence of inhibitory influence of lateral RST originating from this center, facilitatory action of ventral RST becomes unopposed. This results in spastic hemiplegia with antigravity posturing, but flexor spams are unusual.

44. Clinical correlation - Spinal lesions 1. Incomplete (partial) myelopathy involving lateral funiculus may affect CST only to produce paresis, hypotonia, hyporeflexia, and loss of reflexes. (Peterson et al., 1975) If lateral RST is involved in addition, unopposed ventral RST activity then results in hyper-reflexia and spasticity (similar to cortical or capsular lesions).

45. Clinical correlation - Spinal lesions 2. Severe myelopathy with involvement of all the four descending pathways produces less marked spasticity compared to isolated lateral cord lesion because of lack of unopposed excitatory influences of ventral RST. Neuroplasticity of the spinal cord in the form of receptor supersensitivity of neurons to a loss of synaptic input and sprouting of axon terminals are also responsible for hypertonicity in complete myelopathy with delayed reorganization after a variable period of spinal shock

46. Clo n us Clonus is the phenomenon of involuntary rhythmic contractions in response to sudden sustained stretch. A sudden stretch activates muscle spindles, resulting in the stretch reflex. Tension produced by the muscle contraction activates the Golgi tendon organs, which in turn activate an „inverse stretch reflex‟, relaxing the muscle. If the stretch is sustained, the muscle spindles are again activated, causing a cycle of alternating contractions and relaxations.

47. Cerebellum and muscle tone The cerebellum does not seem to have a direct effect on muscle tone determining spinal reflex pathways as there is no direct descending cerebellospinal tract. The cerebellum mainly influences muscle tone through its connections with the vestibular and brain stem reticular nuclei. Pure cerebellar lesions classically produce hypotonia.

48. Cerebellum and muscle tone Gamma motor neurons selectively depressed Alpha motor neurons can respond to inflow from spindles to produce tendon jerk. Associated corticospinal tract involvement produces varying degrees of spasticity as seen in spino- cerebellar ataxia (SCA).

49. Extrapyramidal hypertonia (Rigidity) Rigidity is characterized by an increase in muscle tone causing resistance to externally imposed joint movements. It does not depend on imposed speed and can be elicited at very low speeds of passive movement. It is felt in both agonist and antagonist muscles and in movements in both directions.

50. Extrapyramidal hypertonia (Rigidity) 'Cogwheel' rigidity and 'leadpipe' rigidity are two types. 'Leadpipe' rigidity results when an increase in muscle tone causes a sustained resistance to passive movement throughout the whole range of motion, with no fluctuations. 'Cogwheel' rigidity occus in association with tremor which presents as a jerky resistance to passive movement as muscles tense and relax. Basal ganglia structures are clearly implicated in pathophysiology of rigidity.

51. Extrapyramidal hypertonia (Rigidity) Nurophysiology 1. Reflex origin of rigidity Enhanced tonic reflex activity ( a stimulus produces a prolonged discharge of motor neurons causing sustained muscle contraction). The phasic stretch reflex (monosynaptic) is not responsible for rigidity. 2. Segmental and supraspinal influences α- motoneurons and possibly cortical excitability is enhanced in rigidity. Recurrent Renshaw cell inhibition is normal.

52. Extrapyramidal hypertonia (Rigidity) It has been suggested that the distribution of higher facilitatory influence between flexor and extensor motoneurons may be unequal in pyramidal and approximately equal in extrapyramidal type. 3. Inadequate voluntary relaxation.

53. Dystonia Characterized by abnormal muscle spasm producing distorted motor control and undesired postures. A principle finding is the loss of cortical inhibition. Failure of “surround inhibition”. Brain activates a specific movement and simultaneously inhibits unwanted movements.

54. Hypotonia Hypotonia may affect muscle resistance to passive movement and/or its extensibility. Aetiological types of hypotonia : Nerve trunk and root lesion A lesion of anterior horn Cerebellar lesions Cerebral lesions