1. Autonomic Nervous System (ANS) n The ANS consists of motor neurons that: n Innervate smooth and cardiac muscle and glands n Make adjustments to ensure optimal support for body activities n Operate via subconscious control n Have viscera as most of their effectors

2. ANS in the Nervous System  

3. ANS Versus Somatic Nervous System (SNS) n The ANS differs from the SNS in the following three areas n Effectors n Efferent pathways n Target organ responses

4. Effectors n The effectors of the SNS are skeletal muscles n The effectors of the ANS are cardiac muscle, smooth muscle, and glands

5. Efferent Pathways n Heavily myelinated axons of the somatic motor neurons extend from the CNS to the effector n Axons of the ANS are a two-neuron chain n The preganglionic (first) neuron has a lightly myelinated axon n The ganglionic (second) neuron extends to an effector organ

6. Neurotransmitter Effects n All somatic motor neurons release Acetylcholine (ACh), which has an excitatory effect n In the ANS: n Preganglionic fibers release ACh n Postganglionic fibers release norepinephrine or ACh and the effect is either stimulatory or inhibitory n ANS effect on the target organ is dependent upon the neurotransmitter released and the receptor type of the effector

7. Comparison of Somatic and Autonomic Systems  

8. Divisions of the ANS n ANS divisions: sympathetic and parasympathetic n The sympathetic mobilizes the body during extreme situations n The parasympathetic performs maintenance activities and conserves body energy n The two divisions counterbalance each other

9. Role of the Parasympathetic Division n Concerned with keeping body energy use low n Involves the D activities – digestion, defecation, and diuresis n Its activity is illustrated in a person who relaxes after a meal n Blood pressure, heart rate, and respiratory rates are low n Gastrointestinal tract activity is high n The skin is warm and the pupils are constricted

10. Role of the Sympathetic Division n The sympathetic division is the “fight-or-flight” system n Involves E activities – exercise, excitement, emergency, and embarrassment n Promotes adjustments during exercise – blood flow to organs is reduced, flow to muscles is increased n Its activity is illustrated by a person who is threatened n Heart rate increases, and breathing is rapid and deep n The skin is cold and sweaty, and the pupils dilate

11. Sympathetic Outflow n Arises from spinal cord segments T 1 through L 2 n Sympathetic neurons produce the lateral horns of the spinal cord n Preganglionic fibers pass through the white rami communicantes and synapse in the chain (paravertebral) ganglia n Fibers from T 5 -L 2 form splanchnic nerves and synapse with collateral ganglia n Postganglionic fibers innervate the numerous organs of the body

12. Sympathetic Trunks and Pathways n The paravertebral ganglia form part of the sympathetic trunk or chain n Typically there are 23 ganglia – 3 cervical, 11 thoracic, 4 lumbar, 4 sacral, and 1 coccygeal

13. Sympathetic Trunks and Pathways n A preganglionic fiber follows one of three pathways upon entering the paravertebral ganglia n Synapse with the ganglionic neuron within the same ganglion n Ascend or descend the sympathetic chain to synapse in another chain ganglion n Pass through the chain ganglion and emerge without synapsing

14. Pathways with Synapses in Chain Ganglia n These fibers innervate sweat glands and arrector pili muscles n Rami communicantes are associated only with the sympathetic division

15. Pathways to the Head n Preganglionic fibers emerge from T 1 -T 4 and synapse in the superior cervical ganglion n These fibers: n Serve the skin and blood vessels of the head n Stimulate dilator muscles of the iris n Inhibit nasal and salivary glands

16. Pathways to the Thorax n Preganglionic fibers emerge from T 1 -T 6 and synapse in the cervical chain ganglia n These fibers innervate the heart via the cardiac plexus, as well as innervating the thyroid and the skin n Postganglionic fibers emerge from the middle and inferior cervical ganglia and enter nerves C 4 -C 8 n Postganglionic fibers directly serve the heart, aorta, lungs, and esophagus

17. Pathways with Synapses in Collateral Ganglia n These fibers (T 5 -L 2 ) leave the sympathetic chain without synapsing n They form thoracic, lumbar, and sacral splanchnic nerves n Their ganglia include the celiac, the superior and inferior mesenterics, and the hypogastric

18. Pathways to the Abdomen n Sympathetic nerves innervating the abdomen have preganglionic fibers from T 5 -L 2 n They travel through the thoracic splanchnic nerves and synapse at the celiac and superior mesenteric ganglia n Postganglionic fibers serve the stomach, intestines, liver, spleen, and kidneys

19. Pathways to the Pelvis n Preganglionic fibers originate from T 10 -L 2 n Most travel via the lumbar and sacral splanchnic nerves to the inferior mesenteric and hypogastric ganglia n Postganglionic fibers serve the distal half of the large intestine, the urinary bladder, and the reproductive organs

20. Pathways with Synapses in the Adrenal Medulla n Fibers of the thoracic splanchnic nerve pass directly to the adrenal medulla n Upon stimulation, medullary cells secrete norepinephrine and epinephrine into the blood

21. Segmental Sympathetic Supplies  

22. Visceral Reflexes n Visceral reflexes have the same elements as somatic reflexes n They are always polysynaptic pathways n Afferent fibers are found in spinal and autonomic nerves

23. Visceral Reflexes  

24. Referred Pain n Pain stimuli arising from the viscera are perceived as somatic in origin n This may be due to the fact that visceral pain afferents travel along the same pathways as somatic pain fibers  

25. Neurotransmitters and Receptors n Acetylcholine (ACh) and norepinephrine (NE) are the two major neurotransmitters of the ANS n ACh is released by all preganglionic axons and all parasympathetic postganglionic axons n Cholinergic fibers – ACh-releasing fibers n Adrenergic fibers – sympathetic postganglionic axons that release NE n Neurotransmitter effects can be excitatory or inhibitory depending upon the receptor type

26. Cholinergic Receptors n The two types of receptors that bind ACh are nicotinic and muscarinic n These are named after drugs that bind to them and mimic ACh effects

27. Nicotinic Receptors n Nicotinic receptors are found on: n Motor end plates (somatic targets) n All ganglionic neurons of both sympathetic and parasympathetic divisions n The hormone-producing cells of the adrenal medulla n The effect of ACh binding to nicotinic receptors is always stimulatory

28. Muscarinic Receptors n Muscarinic receptors occur on all effector cells stimulated by postganglionic cholinergic fibers n The effect of ACh binding: n Can be either inhibitory or excitatory n Depends on the receptor type of the target organ

29. Adrenergic Receptors n The two types of adrenergic receptors are alpha and beta n Each type has two or three subclasses (  1,  2,  1,  2,  3) n Effects of NE binding to: n  receptors is generally stimulatory n  receptors is generally inhibitory n A notable exception – NE binding to  receptors of the heart is stimulatory

30. Effects of Drugs n Atropine – blocks parasympathetic effects n Neostigmine – inhibits acetylcholinesterase and is used to treat myasthenia gravis n Tricyclic antidepressants – prolong the activity of NE on postsynaptic membranes n Over-the-counter drugs for colds, allergies, and nasal congestion – stimulate  -adrenergic receptors n Beta-blockers – attach mainly to  1 receptors and reduce heart rate and prevent arrhythmias

31. Drugs that Influence the ANS  

32. Interactions of the Autonomic Divisions n Most visceral organs are innervated by both sympathetic and parasympathetic fibers n This results in dynamic antagonisms that precisely control visceral activity n Sympathetic fibers increase heart and respiratory rates, and inhibit digestion and elimination n Parasympathetic fibers decrease heart and respiratory rates, and allow for digestion and the discarding of wastes

33. Sympathetic Tone n The sympathetic division controls blood pressure and keeps the blood vessels in a continual state of partial constriction n This sympathetic tone (vasomotor tone): n Constricts blood vessels and causes blood pressure to rise as needed n Prompts vessels to dilate if blood pressure is to be decreased n Alpha-blocker drugs interfere with vasomotor fibers and are used to treat hypertension

34. Parasympathetic Tone n Parasympathetic tone: n Slows the heart n Dictates normal activity levels of the digestive and urinary systems n The sympathetic division can override these effects during times of stress n Drugs that block parasympathetic responses increase heart rate and block fecal and urinary retention

35. Unique Roles of the Sympathetic Division n Regulates many functions not subject to parasympathetic influence n These include the activity of the adrenal medulla, sweat glands, arrector pili muscles, kidneys, and most blood vessels n The sympathetic division controls: n Thermoregulatory responses to heat n Release of renin from the kidneys n Metabolic effects

36. Thermoregulatory Responses to Heat n Applying heat to the skin causes reflex dilation of blood vessels n Systemic body temperature elevation results in widespread dilation of blood vessels n This dilation brings warm blood to the surface and activates sweat glands to cool the body n When temperature falls, blood vessels constrict and blood is retained in deeper vital organs

37. Release of Renin from the Kidneys n Sympathetic impulses activate the kidneys to release renin n Renin is an enzyme that promotes increased blood pressure

38. Metabolic Effects n The sympathetic division promotes metabolic effects that are not reversed by the parasympathetic division n Increases the metabolic rate of body cells n Raises blood glucose levels n Mobilizes fat as a food source n Stimulates the reticular activating system (RAS) of the brain, increasing mental alertness

39. Localized Versus Diffuse Effects n The parasympathetic division exerts short- lived, highly localized control n The sympathetic division exerts long-lasting, diffuse effects

40. Effects of Sympathetic Activation n Sympathetic activation is long-lasting because NE: n Is inactivated more slowly than ACh n Is an indirectly acting neurotransmitter, using a second-messenger system n And epinephrine are released into the blood and remain there until destroyed by the liver

41. Levels of ANS Control n The hypothalamus is the main integration center of ANS activity n Subconscious cerebral input via limbic lobe connections influences hypothalamic function n Other controls come from the cerebral cortex, the reticular formation, and the spinal cord

42. Levels of ANS Control  

43. Hypothalamic Control n Centers of the hypothalamus control: n Heart activity and blood pressure n Body temperature, water balance, and endocrine activity n Emotional stages (rage, pleasure) and biological drives (hunger, thirst, sex) n Reactions to fear and the “fight-or-flight” system

44. Embryonic Development of the ANS n Preganglionic neurons are derived from the embryonic neural tube n ANS structures in the PNS – ganglionic neurons, the adrenal medulla, and all autonomic ganglia – derive from the neural crest n Nerve growth factor (NGF) is a protein secreted by target cells that aids in the development of ANS pathways

45. Developmental Aspects of the ANS n During youth, ANS impairments are usually due to injury n In old age, ANS efficiency decreases, resulting in constipation, dry eyes, and orthostatic hypotension n Orthostatic hypotension is a form of low blood pressure that occurs when sympathetic vasoconstriction centers respond slowly to positional changes