Physiology of the Autonomic Nervous system (ANS)

1. Physiology of the Autonomic Nervous system (ANS) • Dr. Taha Sadig Ahmed

3. A.1. The autonomic nervous system: • is regulated by the Hypothalamus • It regulates activities of : • (1) Visceral muscles : which are involuntary , and include • cardiac muscle , and • smooth moscle in arterioles and the gastro-intesinal tract (GIT) . • (2) Glands ( e.g. , sweat glands and GIT glands ) • It consists of 2 subdivisions: Figure 11-2

4. A.2 The autonomic nervous system pathways: • The autonomic division consists of two efferent neurons in series Effector, postganglionic, neurons for the autonomic nervous system (ANS) are different from the neurons we have previously discussed. The synapse between the ANS neuron and the target cell is called theneuroeffector junction. Fig. 11-4

5. The efferent autonomic system consists of 2 effector neurons arranged in series: • Preganglionic nerves : arise from the spinal cord • Postganglionic nerves : arise from ganglia . The synapse between the ANS neuron and the target cell is called theneuroeffector junction. Fig. 11-4

6. Higher control of autonomic function • Sympathetic functions regulated by posterior hypothalamus • Parasympathetic functions regulated by anterior hypothalamus

7. The ANS has 2 subdivisions : sympathetic and parasympathetic

8. Effect of sympathetic stimulation • Sympathetic part of ANS is active during fear, anxiety, sever pain preparing the body for fight or flight • Promoting mechanisms which increase energy & metabolism

9. Effect of parasympathetic stimulation • Parasympathetic system activities is related to relaxed state • Vegetative function: • Feeding • Resting • Part of the sexual function

10. Pathways of the Autonomic Nervous System • Each of the 2 divisions of the ANS system sends neurons to each of the organs shown (except the adrenal, which receives input from only the Sympathetic division only . • One division of the ANS will excite the organ while the other will inhibit it. • Nerves of the Sympathetic system exit the spinal cord in the thoracic and lumbar regions of the spinal cord. Therefore , the Sympathetic is called : “ Thoraco-lumbar Outflow “ • Nerves of the Parasympathetic system exit at the brain stem and sacral region of the spinal cord. Therefore , the Parasympathetic division is called : “ Cranio-sacral Outflow “ • The preganglionic neurons of both divisions synapse in ganglia . • From ganglia a second “ Postganglionic “ nerve arises and travels to the effector (target) organ .

11. Sympathetic (Thoracolumbar) System The preganglionic neurons are located in the CNS. The postganglionic neurons (located in the sympathetic ganglia) are distant from their target tissues . There is greater divergence . The ratio (pre/post) being = 1/10 ; This divergence implies diffuse ( generalized )adrenergic actions .

12. Parasympathetic (Craniosacral ) system : The preganglionic neurons are located in the CNS. The postganglionic neurons (located in parasympathetic ganglia) are either close to or embedded in their targets tissues .The neurotransmitter between the preganglionic and the postganglionic neuron is Ach. The receptors on the postganglionic neurons are of the cholinergic nicotinic type. The neurotransmitter secreted by the postganglionic neuron is Ach. The receptors on the target are of the muscarinic cholinergic type. There is little divergence. The ratio (pre/post) = 1/3. The limited divergence results in more specific cholinergic actions.

14. Effect of sympathetic & parasympathetic stimulation

19. It should be noted that • Under physiological conditions , nearly all sympathetic and parasympathetic activities are opposite ( contradictory ) to each other ; however , the two systems cooperate (& may act in unison/accord ) in • (1) salivary secretion ( sympathetic  scanty secretion rich in enzymes , parasympathetic  Watery , copious secretion ) , • (2) Emotional stress : parasympathetic  increased tear and nasal secretions , sympathetic  increased heart-rate , BP , etc • (3) Sexual intercourse : parasympathetic  erection of the penis or clitoris , sympathetic  ejaculation or orgasm

20. Autonomic Neurotransmitter • All preg. Fibres (sympathetic and parasympathetic ) secrete acetylcholine at the ganglia . • All postganglionic parasympathetic fibers secrete acetylcholine at target organs . • Most postganglionic sympathetic fibers secrete norepinephrine at target organs . • Postganglionic sympathetic fibers to sweat gland & blood vessels of skeletal muscles release acetylcholine • All epinephrine in the bloodstream comes from the adrenal medulla . Postganglionic sympathetic nerves can not synthesize epinephrine from its precursor which is norepinephrine .

21. Actions of Norepinephrine and Epinephrine Adrenergic( Alpha and Beta ) Receptors • Norepinephrine acts on both alpha and beta receptors , but it acts much better on alpha than beta receptors . • Epinephrine excites both α & β equally .

22. Distribution of Adrenergic Receptors • αis found in : • Iris • Blood vessels , especially arterioles • GIT • β is found in : • Heart • Bronchioles • Skeletal muscle • GIT

23. Noteworthy Effects of Alpha or Beta Receptor Stimulation by Norepinephrine or by Epinephrine

24. The Heart

31. Figure 11-3 Figure 11-2

33. Adrenal Medulla

35. Neurotransmitters of ANS (cont.) • All preganglionic fibers ( sympathetic and parasympathetic) secrete Acetylcholine at the ganglia • All postganglionic parasympathetic fibers secrete Acetylcholine

36. Neurotransmitters of ANS (cont.)

37. Neurotransmitters of ANS (cont.) • Most postganglionic sympathetic fibers secrete norepinephrine • Postganglionic sympathetic fibers to sweat gland & blood vessels of skeletal muscles release acetylcholine

38. Adrenergic receptors • Noradrenaline mainly excite α (and β to a lesser extent) • Adrenaline excite both α & β equally

40. Adrenergic receptors • αis found in : • Iris • Blood vessels • GIT • β is found in : • Heart • Skeletal muscle • GIT

41. Cholinergic receptors • Divided into : • Nicotinic ( in all ganglia ) • Muscarinic ( in effector tissues )

42. Adrenergic receptors blockers • α blockers: • Prazosin (α 1) • Yohimbine (α 2) • β blockers: • Propranolol ( both β1 & β2) • Atenolol (β 1)

43. Drugs that you should know • Epinephrine ( adrenaline ) . • Salbutamol ( ventolin ) . • Propranolo ( inderal ) . • Atenolol ( tenormin ) . • Atropine .

44. Cholinergic receptors • Muscarinic receptors are found on all effector cells stimulated by postganglionic parasympathetic • And postganglionic cholinergic sympathetic • Nicotinic receptors are found in the synapses between pre- & postganglionic (both sympath. & parasympath.)

45. Cholinergic receptors • Drugs acting on cholinergic receptors:Hexamethonium (block both types)Atropine (block muscarinic receptors)

46. Sympathetic (Thoracolumbar): The neurotransmitter between the preganglionic and the postganglionic neuron is Ach. The receptors on the postganglionic neurons are of the cholinergic nicotinic type. The neurotransmitter secreted by the postganglionic neuron is NorEpinephrine (NE). The receptors on the target are adrenergic receptors (alpha or beta). There is a great divergence. The ratio (pre/post) = 1/10. The divergence implies diffuse adrenergic actions

47. Parasympathetic (Craniosacral) The neurotransmitter between the preganglionic and the postganglionic neuron is Ach. The receptors on the postganglionic neurons are of the cholinergic nicotinic type. The neurotransmitter secreted by the postganglionic neuron is Ach. The receptors on the target are of the muscarinic cholinergic type. There is little divergence. The ratio (pre/post) = 1/3. The limited divergence results in more specific cholinergic actions.

48. Comparison between beta 1 and beta 2 adrenergic receptors

49. Comparison between alpha 1 and alpha 2 adrenergic receptors