michiganneurology

1. Autonomic Nervous System http://www.michiganneurology.com

2. Much of the text material is from, “Principles of Anatomy and Physiology, 12th edition” by Gerald J. Tortora and Bryan Derrickson (2009). I don’t claim authorship. Other sources are noted when they are used. Mapping of the lecture slides to the 13th edition is provided in the supplement.

3. Outline • Basic principles • More detail on motor control • Neurotransmitters and receptors • Physiological responses • Autonomic integration and control • Two medical conditions

4. Basic Principles

5. Autonomic Nervous System • The autonomic nervous system (ANS) responds to certain visceral sensations, and excites or inhibits effectors. • Effectors include smooth muscle, cardiac muscle, and endocrine and exocrine glands. • The ANS consists of sensory neurons, integrative centers, and motor neurons. • The ANS operates via lower- and higher-level reflex arcs, and almost always without conscious control. Visceral = pertaining to organs or tissue coverings of organs. Effector = a muscle, gland, or organ capable of responding to a stimulus, especially a nerve impulse (action potential). Chapter 15, page 546

6. Divisions • The output or motor components of the ANS are its sympathetic and parasympathetic divisions. • Most organs receive nerve impulses (action potentials) from both ANS divisions—this is known as dual innervation. • The divisions often, but not always, work in opposition to one another. Chapter 15, page 547

7. Divisions (continued) http://www.yesselman.com

8. Divisions (continued) • Nerve impulses from one division stimulate the organ to increase its activity (excitation). • Nerve impulses from the other division decrease the organ’s activity (inhibition). • For example, sympathetic activation increases and parasympathetic activation decreases heart rate. • In some instances, the two divisions work together, such as the male sexual response. Nerve impulses = action potentials. Chapter 15, page 547

9. Control • The ANS was originally named autonomic because it was thought to function autonomously. • Nuclei in the hypothalamus and brainstem, however, are involved in regulating ANS functions. Autonomous = self-governing; free of external influence or control. Chapter 15, page 547

10. Conscious Control • Due to the lack of sensory awareness, very few autonomic responses can be consciously altered. • Consider, however: • Practitioners of yoga and other meditative techniques can learn from long and diligent practice how to regulate some autonomic functions, such as heart rate. • Biofeedback using electronic monitoring can provide sensory feedback to enhance the ability to exert some conscious control of ANS functions. Chapter 15, page 547

11. Biofeedback http://counseling.ucr.edu

12. Sensory Input • Most sensory input to the ANS is from autonomic sensory neurons. • Many of these neurons are interoreceptors located in blood vessels, visceral organs, muscles, and nervous system. • Interoreceptors are sensory receptors that monitor the body’s internal environment. • They include chemoreceptors to monitor blood CO2 level, and mech-anoreceptors to detect stretch in the walls of hollow organs and blood vessels. Chapter 15, page 547

13. Sensory Input (continued) • Sensory signals from interoreceptors are usually not consciously per-ceived since they generally don’t reach the level of the cerebral cortex. • Intense activation of interoreceptors, however, can produce conscious sensations. • For example, inadequate coronary blood flow can result in chest pain known as angina pectoris. • Somatic (body) sensations can also affect the ANS—intense pain can produce changes in autonomic activity. Chapter 15, page 547

14. Other Sensory Inputs • The special senses acting through the limbic system can also affect autonomic responses. • For example, part of the reaction to an unexpected loud noise can include increased heart rate in a physiological response known as sympathetic arousal. Chapter 15, page 547

15. Motor Output • Autonomic motor neurons regulate visceral activities by either increas-ing (exciting) or decreasing (inhibiting) ongoing activities in effectors. • ANS motor responses include changes in the diameter of the pupils, changes in heart rate, and dilation and constriction of blood vessels. • Unlike skeletal muscle tissue, tissues innervated by the ANS can func-tion autonomously to some extent when their nerve supply is damaged. ANS effectors = cardiac muscle, smooth muscle, and endocrine and exocrine glands. Innervation = to supply an organ or a body part with nerves. Chapter 15, page 547

16. Motor Pathways • An autonomic motor pathway has two motor neurons positioned in series. • The cell body of the first neuron is located in the CNS—its myelinated axon extends to an autonomic ganglion located outside of the CNS. • The cell body of the second neuron is located in the autonomic gang-lion—its unmyelinated axon extends to an effector. • An exception to this rule is the axon of the first motor neuron extends directly to chromaffin cells in the adrenal medulla—there is no second neuron. Ganglion (plural, ganglia) = collection of cell bodies of neurons outside of the CNS. Chapter 15, page 547

17. Motor Pathways (continued) Neuron 1 Neuron 2 Axon Effector Synapse Cell body and dendrites (dendrites are not shown) Schematic diagram = a drawing intended to explain how something works.

18. Motor Pathways (continued) Central nervous system Autonomic ganglion Myelinated axon Unmyelinated axon Effector Smooth muscle Cardiac muscle Endocrine and exocrine glands Postganglionic neuron Preganglionic neuron

19. Neurotransmitters of the ANS • Somatic motor neurons release acetylcholine (ACh) at the neuromus-cular junctions (synapses) with skeletal muscles. • Autonomic motor neurons release either ACh or norepinephrine (NE) at their synapses. • More detail will be provided as we proceed with this lecture material. Chapter 15, page 547

20. More Detail on Motor Control

21. Motor Pathways • The cell body of an ANS preganglionic neuron is located in the brain or spinal cord. • Its axon—a small-diameter, myleninated type B fiber—exits the CNS as part of a cranial nerve or spinal nerve. • The axon extends to an autonomic ganglion outside of the CNS, where it synapses with a postganglionic neuron. Figure 15.1 Chapter 15, page 549

22. Motor Pathways (continued) • The axon of the postganglionic neuron is a small-diameter, unmyleni-nated type C fiber. • The axon synapses with an effector (smooth muscle, cardiac muscle, or gland). Figure 15.1 Chapter 15, page 549

23. Parasympathetic Preganglionic Neurons • In the parasympathetic division, cell bodies of preganglionic neurons are found in the nuclei of cranial nerves III, VII, IX, and X, and sacral segments 2 through 4. • The parasympathetic division is also known as the craniosacral divi-sion. Figure 15.3 Chapter 15, page 549

24. Sympathetic Preganglionic Neurons • The cell bodies of the preganglionic neurons are located in the lateral horns of the 12 thoracic segments and the first 2-to-3 lumbar segments of the spinal cord. • The sympathetic division is therefore also known as the thoracolumbar division. Figure 15.2 Chapter 15, page 549

25. Lateral Horn of Spinal Cord http://www.microscopy-uk.org.uk WM = white matter; GM = gray matter.

26. Autonomic Ganglia • The autonomic ganglia differ in location and structure in the two ANS divisions. • Parasympathetic division—the ganglia are located close to or in the walls of visceral organs. • Sympathetic division—the ganglia form an interconnected chain of cell bodies and axons (known as the ganglionic chain), which is in close proximity to the spinal cord. Figure 15.2 Figure 15.3 Chapter 15, page 549

27. Axon Lengths • Parasympathetic division—the preganglionic axons are long and the postganglionic axons are short. • Sympathetic division—the converse generally holds true: short pre-ganglionic axons and long postganglionic axons. Figure 15.2 Figure 15.3 Chapter 15, page 549

28. Comparison of Axon Lengths Sympathetic division: Long Short Effector Parasympathetic division: Long Short Effector Preganglionic neuron Postganglionic neuron

29. Parasympathetic Postganglionic Neurons • One parasympathetic preganglionic neuron can synapse with up to 4-to-5 postganglionic neurons. • Each postganglionic axon, however, only innervates only one effec-tor. • The arrangement enables parasympathetic responses to be localiz-ed to one or possibly a few organs. Figure 15.3 Chapter 15, page 552

30. Sympathetic Postganglionic Neurons • A sympathetic preganglionic neuron can synapse with 20 or more post-ganglionic neurons. • A postganglionic axon (actually, fiber) can innervate many different effectors. • The divergence helps explain why sympathetic activation affects much of the body simultaneously. Figure 15.2 Chapter 15, page 552

31. Neurotransmitters and Receptors

32. Cholinergic and Adrenergic Neurons • Autonomic neurons are cholinergic or adrenergic based on the neuro-transmitter synthesized and released at their synapses. • Receptors, which consist of proteins, are located in the plasma mem-brane of the postsynaptic neuron or effector cell. Cholinergic = acetylcholine (ACh) is the neurotransmitter. Adrenergic = norepinephrine (NE) is the neurotransmitter. Figure 15.7 Chapter 15, page 558

33. Cholinergic and Adrenergic Neurons (continued) Sympathetic division: Adrenergic (usually) Cholinergeric Effector Parasympathetic division: Cholinergeric Cholinergeric Effector Preganglionic neuron Postganglionic neuron Cholinergic = acetylcholine Adrenergic = norepinephrine

34. Cholinergic Neurons • Cholinergic neurons in the autonomic nervous system include: • All preganglionic neurons in the parasympathetic and sympathetic divisions. • All postganglionic neurons in the parasympathetic division. • Postganglionic neurons in the sympathetic division that innervate sweat glands in the skin. Figure 15.7 Chapter 15, page 558

35. Cholinergic Neurons (continued) • ACh, stored in the synaptic vesicles of the end buttons of axons, is released via exocytosis in response to action potentials. • ACh diffuses across the synaptic cleft and binds to the cholinergic receptors in the postsynaptic membrane to produce graded poten-tials. • Cholinergic receptors are classified as either nicotinic or muscarinic based on their functional properties. Figure 15.7 Chapter 15, page 558

36. Synapse http://thebrain.mcgill.ca

37. Tobacco Plant http://etc.usf.edu

38. Nicotinic Receptors • Nicotinic receptors respond to nicotine, a substance not naturally found in the human body. • Nicotine, when introduced at nicotinic synapses, binds to the post-synaptic receptors, and mimics the action of ACh. • Activation of nicotinic receptors produces depolarizing graded po-tentials. • The result is excitation of the postsynaptic cell so it is more likely to produce a response. Figure 15.7 Chapter 15, page 558

39. Nicotinic Receptors (continued) • Nicotinic receptors are found in: • Preganglionic neurons of the parasympathetic and sympathetic divisions. • Chromaffin cells of the adrenal medulla innervated by the sym-pathetic division. • Neuromuscular junctions in skeletal muscle (which are innervated by somatic motor neurons). Figure 15.7 Chapter 15, page 558

40. Amanita muscaria http://ciuperci.org Muscarine is found in some species of mushrooms, including Amanita muscaria. Its ingestion can result in intense parasympathetic responses, convulsions, and death.

41. Muscarinic Receptors • Muscarinic receptors are named for a mushroom toxin known as muscarine that mimics the action of ACh in binding to post-synap-tic receptors. • These receptors are found in smooth muscle, cardiac muscle, and glands innervated by postganglionic axons of the parasympathetic division. • Most sweat glands innervated by postganglionic axons of the sym-pathetic division also have muscarinic receptors. Figure 15.7 Chapter 15, page 558

42. Muscarinic Receptors (continued) • Stimulation of muscarinic receptors results in depolarizing (excita-tory) or hyperpolarizing (inhibitory) graded potentials based on the cell type. • For example, the binding of ACh to the muscarinic receptors in the digestive tract inhibits (relaxes) the smooth muscle sphincters. • In contrast, ACh excites the muscarinic receptors in the smooth muscle fibers of the iris of the eye, causing the smooth muscles to contract and decrease pupil size. Figure 15.7 Chapter 15, page 558

43. Adrenergic Neurons • Adrenergic neurons release norepinephrine, sometimes called noradrenalin. • Most postganglionic neurons in the sympathetic division are adrenergic—except for those that innervate sweat glands in the skin. • Norepinephrine is stored in synaptic vesicles and is released via exocytosis in response to action potentials from postganglionic neurons. Figure 15.7 Chapter 15, page 559

44. Adrenergic Neurons (continued) • Norepinephrine diffuses across the synaptic cleft and binds to the adrenergic receptors in the postsynaptic membrane of the effector cell. • A depolarizing or hyperpolarizing graded potential results, depend-ing on the cell type. Figure 15.7 Chapter 15, page 559

45. Norepinephrine and Epinephrine • Adrenergic receptors in the postsynaptic membrane bind norepineph-rine and epinephrine (a closely-related molecule of the catecholamine family). • Norepinephrine is released as a neurotransmitter in most postgan-glionic neurons of the sympathetic division • Epinephrine (and small amounts of norepinephrine) are released as hormones from the chromaffin cells of the adrenal medulla into gen-eral blood circulation. Chapter 15, page 559

46. Neurotransmitter Inactivation • The action of norepinephrine is terminated when it is inactivated by an enzyme (COMT or MAO) and is then reabsorbed by the end buttons of the neuron. • Norepinephrine persists in the synaptic cleft for a longer period of time than ACh because COMT and MAO are relatively slow-acting compared to acetylcholine esterase (AChE). • Thus, the effects triggered by norepinephrine are generally longer-lasting than those triggered by ACh. COMT = catechol-O-methyl transferase MAO = monoamine oxidase inhibitor Chapter 15, page 559

47. Alpha and Beta Receptors • Adrenergic receptors are classified as alpha () or beta () types. • The receptors are found on the postsynaptic membranes of effectors innervated by most postganglionic axons in the sympathetic division. Chapter 15, page 558

48. Alpha and Beta Receptors (continued) • Cells of most sympathetic effectors have either  or  receptors, but some cells have both types. • Norepinephrine stimulates  receptors more strongly than it stim-ulates  receptors. • Epinephrine, in comparison, is a strong stimulator of both  and  receptors. Chapter 15, page 559

49. Receptor Subtypes • Alpha and beta receptors have subtypes—1, 2, 1, 2, and 3. • The classification is based on the responses they elicit, and the selective binding of drugs that activate or block the receptors. • 1 and 1 receptors generally produce excitation, and 2 and 2 receptors produce inhibition of effector cells. • 3 receptors are limited to brown adipose cells where their activa-tion produces thermogenesis. Thermogenesis = the production of heat, especially within an animal body. Chapter 15, page 559

50. Receptor Agonists • Some drugs and natural substances selectively activate or block cholinergic and adrenergic receptors. • An agonist is a substance that binds to and activates receptors in the postsynaptic membrane to mimic the effect of the neurotrans-mitter or hormone. • Phenylephrine, a common ingredient in cold and sinus medications, is an agonist of 1 receptors. • The drugs constricts blood vessels in the nasal mucosa to reduce the production of mucus and relieve nasal congestion. Chapter 15, page 560