Costanzo | Physiology | Chapter 2 | Autonomic Nervous System | Study Guide
Chapter 2: Autonomic Nervous System Notes
I. Organization and General Features
The Autonomic Nervous System (ANS) is an efferent (motor) system that regulates involuntary functions such as heart rate, blood pressure, and digestion.
It consists of two primary divisions: the sympathetic and parasympathetic nervous systems, along with a third division, the enteric nervous system, located in the gastrointestinal tract.
Anatomic Arrangement: Both divisions use a two-neuron relay. Preganglionic neurons have cell bodies in the central nervous system (CNS), and postganglionic neurons have cell bodies in autonomic ganglia outside the CNS.
Neuroeffector Junctions: Unlike the discrete neuromuscular junction, autonomic postganglionic neurons form diffuse, branching networks with beads called varicosities, which serve as the sites for neurotransmitter synthesis and release.
Autonomic Centers: Coordinated regulation is managed by centers in the hypothalamus and brain stem (e.g., vasomotor, respiratory, and micturition centers).
II. Sympathetic vs. Parasympathetic Divisions
Sympathetic Division (Thoracolumbar):
Preganglionic neurons originate in the thoracic and lumbar spinal cord (T1–L3).
Axon lengths: Short preganglionic and long postganglionic axons.
Ganglia: Located in the paravertebral (sympathetic chain) or prevertebral regions.
Adrenal Medulla: A specialized sympathetic ganglion that secretes epinephrine (80%) and norepinephrine (20%) directly into the circulation during "fight or flight" responses.
Parasympathetic Division (Craniosacral):
Preganglionic neurons originate in the brain stem (CN III, VII, IX, and X) and sacral spinal cord (S2–S4).
Axon lengths: Long preganglionic and short postganglionic axons.
Ganglia: Located in or near the effector organs.
Overall function is restorative, serving to conserve energy.
III. Neurotransmitters and Receptors
All preganglionic neurons (both divisions) release Acetylcholine (ACh), which acts on nicotinic (N2) receptors on postganglionic cell bodies.
Parasympathetic postganglionic neurons are cholinergic, releasing ACh to act on muscarinic receptors in target tissues.
Sympathetic postganglionic neurons are generally adrenergic, releasing Norepinephrine (NE) to act on adrenoreceptors (α or β).
Exception:Sweat glands receive sympathetic cholinergic innervation (ACh acting on muscarinic receptors).
IV. Autonomic Receptor Mechanisms
Most autonomic receptors are G protein-linked, meaning they use G proteins to activate enzymes that produce second messengers.
Adrenoreceptors:
α1 Receptors: Found in vascular smooth muscle and sphincters; activation causes contraction via the Gq protein and the IP3/Ca2+ second messenger system.
α2 Receptors: Primarily inhibitory; found on presynaptic terminals. They use Gi proteins to inhibit adenylyl cyclase and decrease cAMP.
β1 Receptors: Located in the heart; activation increases heart rate and contractility by using Gs proteins to stimulate adenylyl cyclase and increase cAMP.
β2 Receptors: Located in bronchioles and GI/bladder walls; activation leads to relaxation/dilation via Gs proteins and increased cAMP.
Cholinoreceptors:
Nicotinic Receptors (N1,N2): Act as ion channels for Na+ and K+; activation results in rapid depolarization.
Muscarinic Receptors (M1–M5): Located in all parasympathetic target organs. Some (e.g., in the heart) act directly via G proteins to open K+ channels and slow the heart rate, while others use the IP3/Ca2+ mechanism.
V. Clinical Applications
Pheochromocytoma: A tumor of the adrenal medulla that secretes excessive catecholamines, leading to severe hypertension and "panic attacks" [155, 156b].
Horner Syndrome: Caused by loss of sympathetic innervation to the face, resulting in ptosis (drooping eyelid), miosis (constricted pupil), and anhidrosis (lack of sweating) [171b].
Autonomic Nervous System Study Guide
The Autonomic Nervous System (ANS) is an efferent system that regulates involuntary functions, such as heart rate and digestion, through a two-neuron relay involving preganglionic and postganglionic neurons. It consists of the sympathetic, parasympathetic, and enteric divisions. Coordinated regulation is managed by centers in the hypothalamus and brain stem.
I. Glossary of Key Terms
Adrenergic: Neurons or receptors that utilize norepinephrine or epinephrine.
Anhidrosis: Lack of sweating, a classic sign of Horner syndrome.
Autoreceptor: An α2 receptor on sympathetic postganglionic terminals that inhibits further norepinephrine release.
Cholinergic: Neurons or receptors that utilize acetylcholine (ACh).
Chromaffin Cells: Specialized cells in the adrenal medulla that secrete catecholamines into the blood.
Craniosacral: The anatomic origin of the parasympathetic division (Cranial nerves and S2–S4).
Miosis: Constriction of the pupil.
Mydriasis: Dilation of the pupil.
Thoracolumbar: The anatomic origin of the sympathetic division (T1–L3).
Varicosity: Beaded, branching networks on autonomic postganglionic neurons where neurotransmitters are synthesized and released.
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II. 50 Question-and-Answer Quiz
Questions
What are the two components of the efferent nervous system?
Which division of the ANS is called "thoracolumbar"?
Which division is called "craniosacral"?
Where are the cell bodies of preganglionic neurons located?
Where are the cell bodies of postganglionic neurons located?
Which cranial nerves carry parasympathetic fibers?
Which spinal segments represent the sympathetic origin?
Which division has short preganglionic and long postganglionic axons?
Which division has long preganglionic and short postganglionic axons?
Where are sympathetic ganglia typically located?
Where are parasympathetic ganglia typically located?
What neurotransmitter is released by all preganglionic neurons?
Which receptor is found on all postganglionic cell bodies?
What is the primary neurotransmitter of parasympathetic postganglionic neurons?
What is the primary neurotransmitter of sympathetic postganglionic neurons?
What is the major exception for sympathetic postganglionic neurotransmission?
What specialized sympathetic ganglion secretes directly into the blood?
What percentage of adrenal medulla secretion is epinephrine?
What amino acid is the precursor for catecholamines?
Which enzyme converts norepinephrine to epinephrine?
What tumor causes excessive catecholamine secretion?
Which receptor increases heart rate and contractility?
Which receptor causes dilation of the airways (bronchioles)?
Which receptor causes constriction of vascular smooth muscle in the skin?
Which receptor causes relaxation of the bladder wall (detrusor)?
Which receptor causes contraction of the bladder sphincter?
Which receptor is responsible for mydriasis?
Which receptor is responsible for miosis?
Which receptor mediates ejaculation?
Which receptor mediates erection?
What is the effect of α2 receptors on adenylyl cyclase?
What G protein is associated with α1 receptors?
What second messenger system is used by α1 receptors?
What G protein is associated with β1 and β2 receptors?
What second messenger is produced by β receptors?
What effect does β1 activation have on the kidney?
Which division is restorative and energy-conserving?
Which division coordinates the "fight or flight" response?
What are the three classic symptoms of Horner syndrome?
What is the target of scopolamine in treating motion sickness?
Which receptors are found on the motor end plate of skeletal muscle?
How many subunits make up a nicotinic receptor?
What ions flow through an open nicotinic receptor?
Which G protein inhibits adenylyl cyclase?
Where is the satiety center located?
What enzyme degrades cAMP to 5' AMP?
What is the effect of sympathetic stimulation on the GI wall?
What is the effect of parasympathetic stimulation on the GI wall?
Which receptor type is found in all parasympathetic effector organs?
What type of receptors are autoreceptors?
Answer Key
Somatic and Autonomic. 2. Sympathetic. 3. Parasympathetic. 4. CNS. 5. Autonomic ganglia. 6. CN III, VII, IX, X. 7. T1–L3. 8. Sympathetic. 9. Parasympathetic. 10. Paravertebral or prevertebral. 11. Near or in effector organs. 12. ACh. 13. Nicotinic (N2). 14. ACh. 15. NE. 16. Sweat glands (cholinergic). 17. Adrenal medulla. 18. 80%. 19. Tyrosine. 20. PNMT. 21. Pheochromocytoma. 22. β1. 23. β2. 24. α1. 25. β2. 26. α1. 27. α1 (radial muscle). 28. Muscarinic (sphincter muscle). 29. α. 30. Muscarinic. 31. Inhibition (decreases cAMP). 32. Gq. 33. IP3/Ca2+. 34. Gs. 35. cAMP. 36. Renin secretion. 37. Parasympathetic. 38. Sympathetic. 39. Ptosis, miosis, anhidrosis. 40. Muscarinic receptors. 41. Nicotinic (N1). 42. Five. 43. Na+ and K+. 44. Gi. 45. Hypothalamus. 46. Phosphodiesterase. 47. Relaxation (α2,β2). 48. Contraction. 49. Muscarinic. 50. α2.
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III. Essay Questions
Compare and contrast the anatomic and chemical organization of the sympathetic and parasympathetic nervous systems.
Focus: Discuss origins (thoracolumbar vs. craniosacral), axon lengths, location of ganglia, and the specific neurotransmitters/receptors used at both the preganglionic and postganglionic levels.
Explain the cellular mechanism of action for the α1 adrenoreceptor.
Focus: Describe the role of the Gq protein, activation of phospholipase C, the cleavage of PIP2 into IP3 and diacylglycerol, and the subsequent release of Ca2+ from the endoplasmic reticulum leading to contraction.
Describe the physiologic basis of the symptoms observed in Horner syndrome.
Focus: Explain how the loss of sympathetic innervation to the face leads to ptosis (drooping eyelid), miosis (constricted pupil), and anhidrosis (dry skin) based on the normal function of sympathetic receptors in those tissues
