بسم الله الرحمن الرحيم

1. بسم الله الرحمن الرحيم ﴿و ما أوتيتم من العلم إلا قليلا﴾ صدق الله العظيم الاسراء اية 58

2. Physiology of CNS Reflex Action By Dr. Abdel Aziz M. Hussein Lecturer of Physiology Member of American Society of Physiology

3. Reflex Action Def: • It is an automatic (involuntary) specific response of an organ caused by an adequate sensory stimulus

4. Pathway of Reflex Action (1) Stimulus (2) Receptor sensory neuron (3) CNS (spinal cord) motor neuron (4) Effector (5) Response

5. Pathway of Reflex Arc

6. Significance of Reflexes • Physiological importance: • Differs according the type of reflex. • Some reflexes are protective such as flexion withdrawal reflex , and others are supportive such as stretch reflex, so its importance differs according to the type of reflex. • Clinical importance: • They test the integrity of the pathway (afferent , center in CNS, and efferent)

7. Classification of Reflexes

8. Local Reflexes • Their centers are present outside CNS • Examples: • Their centers may be present in; • Dorsal root ganglia (DRG) as those responsible for the flare and allodynia (1ry hyperalgesia) • Enteric nervous system as those regulating the GIT activities • Prevertebral sympathetic ganglia as those responsible for inhibitory enterogastric reflexes

9. Local axon reflex

10. Systemic Reflexes

11. Conditioned Reflexes • Are acquired reflexes i.e. need previous experience or training • Need intact cerebral cortex → their centers are present in cerebral cortex. • Initiated by visual, olfactory, auditory stimuli or even thinking of the stimulus • Play a role in regulation of G.I.T., respiratory and cardiovascular functions

12. Conditioned Reflex

13. Pathway of Conditioned Reflex

14. Unconditioned Reflexes • They are inherent (inborn) reflexes • Don’t need intact cerebral cortex → their centers are not present in cerebral cortex • Don’t need training. • According to the site of their centers, they include; • Hypothalamic reflexes • Midbrain reflexes • Pontine reflexes • Medullary reflexes • Spinal reflexes

15. Unconditioned Reflexes Hypothalamic Midbrain Pontine Medullary Spinal

16. Unconditioned Reflexes Hypothalamic reflexes include reflexes of regulation of food intake, body temperature, etc… Midbrain reflexes include pupillary light reflex and righting reflexes Medullary reflexes include reflexes of regulation of CVS function e.g. baroreceptor reflex and GIT functions such as mastication, vomiting and GIT secretions and motility

17. Spinal Reflexes

18. Spinal Reflex 1) Def., • Are reflexes their centers are present in the spinal cord. 2) Classification: • Are classified into 3 types; A) Superficial spinal reflexes: • Their receptors are present on body surface (skin). • Examples; • Scratch reflex • Flexion withdrawal reflex • Abdominal reflex • Cremasteric reflex. • Plantar reflex.

19. Spinal Reflex 2) Classification: B) Deep spinal reflexes: • Their receptors are present in deep structures e.g. ms, bone and joints. • e.g. Ms Stretch reflex C) Visceral spinal reflexes: • Their receptors are present in visceral organs. Examples; • i) Micturition reflex: → is initiated by distension of urinary bladder. • ii) Defecation reflex: → is initiated by distension of rectum.

20. Spinal Reflex A) Somatic reflexes: • Mediated by somatic efferent supplying skeletal ms

21. Spinal Reflex B) Autonomic reflexes: • Mediated by autonomic efferent supplying blood vessels, glands and visceral organs

22. Spinal Reflex Pathway or Arc Its pathway consists of 3 main components; Afferents Interneurons Efferents

23. Afferent Neurons Are monopolar neurons which present in the DRG. The axon of each neuron divides into 2 branches; Peripheral branch → terminates in sensory receptors Central branch → enters into the spinal cord.

24. Afferent Neurons The central branch divides into several terminals which ends on; Gray matter of spinal cord e.g. posterior horn cell as MSN,SGR ,etc…,interneurons and anterior horn cells Ascend or descend to higher or lower segments respectively

27. Afferent Neurons Functions: a)Conduction of the sensory signals to the spinal centers to excite the subsequent neurons in the reflex pathways.

28. Afferent Neurons Functions: b)Divergence of the incoming sensory signals into wider areas in the NS.

29. Afferent Neurons

30. Interneurons Are small highly excitable neurons Are located in the gray matter between afferent neurons and the efferent neurons.

31. Interneurons All reflex arcs include interneurons exceptthe stretch reflex i.e. monosynaptic

32. Interneurons • Some of them are excitatory and the others are inhibitory.

33. Functions of interneurons Interneurons form different types of circuits that perform the following functions;

34. Interneurons Circuits

35. 1.Divergence A single input (afferent) neuron or (more commonly) interneuron divide to give several collaterals and reach a larger number of efferent neurons Significance : Help in spread of a single afferent signal to a large No. of postsynaptic neurons in the spinal cord .

36. 1.Divergence • May be on the same side and may be on the opposite side.

37. 1.Divergence

38. 2.Convergence Multiple collaterals from input (afferent) neurons or more commonly interneuronsconverge (collect) on a relatively fewer number of output (efferent) neurons Significance: This help spatial summation of EPSPs → discharge of impulses on the postsynaptic neurons.

39. 2.Convergence May come from single or multiple sources

40. 3.After-discharge Circuits Def. It is prolonged (continuous) discharge from an efferent neuron even after stoppage of stimulation of the afferent nerves Mechanism: Because the afferent impulses do not reach efferent directly but through interneurons circuits There are 2 types of these circuits; Open interneuron circuits Closed interneuron circuits.

41. 3.After-discharge Circuits

42. 3.After-discharge Circuits A) Parallel ( Open ) Chain Circuits: In this circuit an afferent neuron stimulates an efferent neuron both directly and indirectlythrough an interneuron which is anatomically arranged in parallel with the afferent neuron.

43. 3.After-discharge Circuits A) Parallel ( Open ) Chain Circuits: Impulses from inputs not reach to output at the same time due to delay 0.5 ms at each synapse Duration of discharge depends upon the No. of interneurons

44. 3.After-discharge Circuits Principal Interneurons Efferent 1 2 Afferent B) Closed-chain (Reverberating) Circuits: 3 Collaterals

45. 3.After-discharge Circuits 1. Fatigue of synapse B) Closed-chain (Reverberating) Circuits: The activity of these circuits stop by either; --- 2. Inhibitory interneurons

46. 3.After-discharge Circuits B) Closed-chain (Reverberating) Circuits: Examples : Reticular activating system (RAS) Wakefulness depends upon the activity of RAS (contains many reverberating circuits) Single sensory stimulation causes activation of RAS for long time (16-18 hours). RAS, in turn stimulate the cerebral cortex which by its turn re-stimulate it & so on. RAS activity continues till fatigue of the synaptic transmission occurs and then sleep occurs.

47. 4.Inhibitory Circuits In this circuit, an excitatory input is converted into an inhibitory output. Types: 2 types of these circuits; Lateral inhibitory circuits Recurrent inhibitory circuits

48. 4.Inhibitory Circuits A) Lateral inhibitory circuits: Inhibitory interneuron Collateral less active neurons More active neurons

49. 4.Inhibitory Circuits A) Lateral inhibitory circuits: In this circuit the afferent neuron activates an inhibitory interneuron which in turn inhibit the adjacent efferent neurons. Site: Ascending sensory pathways Importance: These circuits are important in focusing on or sharpening of the most important sensation.

50. 4.Inhibitory Circuits B) Recurrent inhibitory circuit: Nerve fiber gives a collateral branch which excites (via Ach) an inhibitory neuron which in turn, inhibits (via glycine) the original neuron as well as the surrounding neurons. Glycine Ach