1. BIO 1414 Human Anatomy & Physiology II
Unit 3 Autonomic Nervous System and Senses
Part 2
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By: Robert F. Allen, Professor of Biology

2. Autonomic Nervous System (ANS)
Involuntary or visceral nervous system
Regulates the activity of:
Cardiac Muscle (Heart)
Smooth Muscle ( In Hollow Organs)
Blood Vessels
Digestive System
Bronchioles
Sphincters
Glands
Adrenal
Digestive glands
The Autonomic Nervous System (ANS) * is sometimes referred to as the involuntary or visceral nervous system. It is involuntary because we do not exercise conscious control of its activities. * It regulates the activity of viscera such as cardiac muscle of the heart, * smooth muscle including that in the walls of the hollow organs of the body * such as:
* the blood vessels,
* the digestive system,
* bronchioles and
* sphincters.
The Autonomic Nervous System also regulates the secretory activity of glands * such as the adrenal glands * and the digestive glands. * *

3. ANS Divisions Parasympathetic: Sympathetic: digestion of food
“Rest & Digest”
Reduces energy use
Promotes:
digestion of food
storage of energy
elimination of wastes
homeostasis
Sympathetic:
“Fight or Flight”
Activated during emergencies, exercise or vigorous physical activity
Revs up body to respond to situations that upset homeostasis
The Autonomic Nervous System has two divisions. The first of these is referred to as the sympathetic division. * The sympathetic branch of the ANS is commonly referred to as the “fight or flight” branch, * as it is activated during emergencies, exercise or vigorous physical activity. * Its purpose is the rev up the body to respond to situations,such as anger or fear, * that upset homeostasis . *
The parasympathetic division of the ANS * has the opposite effects on its target organs. * It is sometimes referred to as the “rest or digest” branch, * as it strives to reduce energy consumption while promoting activities such as
* the digestion of food,
* the storage of energy,
* the elimination of wastes and
* general homeostasis. *

4. Sympathetic - Origin Thoracolumbar
Nerve fibers originate between T1 & L2
The Sympathetic division of the ANS is sometimes referred to as the “thoracolumbar” branch * because its nerve fibers * arise from the central nervous system (spinal chord) between the first thoracic and the second lumbar vertebrae. * *

5. Parasympathetic - Origin
Craniosacral
Nerve fibers emerge from brain & sacrum
The Parasympathetic division of the ANS is sometimes referred to as the “craniosacral” division because its nerve fibers arise from the Central Nervous System directly from the brain in the cranium * and from spinal chord between the sacral vertebrae. *

6. Sympathetic Innervation of Visceral Targets
Short, lightly myelinated preganglionic neurons
Long, unmyelinated postganglionic neurons
Ganglia close to spinal cord
Spinal Cord
The target organs whose activities are regulated by the ANS are innervated by two neurons in tandem which synapse in ganglia * between the Central Nervous system and the target organ. The first ( preganglionic neuron) arises from the CNS * and carries information to the ganglion, * while the second carries information from the ganglion to the target organ (postganglionic neuron). *
The particular arrangement of these neurons that is characteristic of the Sympathetic division, includes a short, lightly myelinated preganglionic neuron and a much longer, unmyelinated postganglionic neuron which reaches from the ganglion to the target organ. Since the preganglionic neurons are short, the ganglia in which the sympathetic neurons synapse are located very close to the spinal cord itself. * Thus the innervation of visceral targets in the sympathetic division include a short preganglionic neuron, a ganglion that is close to the spinal cord and a much longer postganglionic neuron that reaches out to the target organ. *

7. Sympathetic Neurotransmitters
Preganglionic neurons -
Cholinergic = ( release acetylcholine )
As far as neurotransmitters are concerned, the preganglionic neurons of the Sympathetic division of the ANS release the neurotransmitter, acetylcholine (ACH). * Thus the preganglionic neurons are said to be “cholinergic”. *
An exception to the general rule that two neurons are necessary to innervate target organs is the adrenal gland. When stimulated, the preganglionic neuron carries information directly to the adrenal gland, and releases acetylcholine. * The ACH causes the adrenal gland to release its neurotransmitters epinepherine (adrenaline) & norepinepherine (noradrenaline) directly into the blood. When blood carrying these neurotransmitters reaches target organs with receptors for epinepherine & norepinepherine, the target organ responds to the stimulus. *

8. Sympathetic Neurotransmitters
Postganglionic neurons:
release norepinepherine at target organs
ie. Adrenergic
Adrenal medulla:
releases epinepherine & norepinepherine into blood
ie. Adrenergic
Sympathetic postganglionic neurons * which release norepinephrine to stimulate target organs * are referred to as “adrenergic” neurons. *The term comes from the words noradrenaline & adrenaline which were previously used in place of the currently used names norepinephrine and epinephrine.
* As the medulla of stimulated adrenal glands * release both epinephrine and norepinephrine * into the blood, they are considered to be “adrenergic” as well. * Thus we see that a unique characteristic of the Sympathetic division of the ANS is that its preganglionic neurons are always cholinergic while its postganglionic neurons are adrenergic.*

9. Adrenergic Receptors Located only on sympathetic target organs
Adrenergic receptors which bind with and respond to norepinephrine or epinephrine * are found exclusively on sympathetic target organs. * While adrenergic receptors which react to norepinephrine * delivered via postganglionic neurons stimulate very precise responses by target organs, * receptors which are stimulated by epinephrine and norepinephrine * delivered via the blood from the adrenal medulla stimulate more generalized effects. * *
Respond only to norepinepherine released by postganglionic neurons (precise effects) or
Epinepherine & norepinepherine released by adrenal medulla into blood (general effects)

10. Adrenergic Receptor Types
Alpha 1:
In walls of blood vessels leading to places other than skeletal muscles, brain & lungs.
Not on heart (cardiac muscle)
Alpha 2:
On membranes of platelets.
Beta 1:
On heart (cardiac muscle) & kidneys
Beta 2:
On coronary arteries, bronchioles & on smooth muscle walls of digestive & urinary systems
While adrenergic receptors all react to norepinephrine and epinephrine, they react differently, based on the specific receptor class they belong to. Alpha receptors are generally excited when bound with either epinepherine or norepinepherine. Alpha 1 type receptors * are located primarily in the walls of blood vessels leading to places other than to the skeletal muscles, the brain or to the lungs and on spincters in visceral organs. * * Alpha 1 receptors are not found on cardiac muscle of the heart. * Alpha 2 receptors are located on the membranes of platelets which have to do with blood clotting. *
Beta receptors may be either be excited or depressed when they bind with epinephrine or norepinephrine depending on the specific type of Beta receptor. Beta 1 receptors, * are found on the cardiac muscle of the heart and on the kidneys. Beta 2 receptors * are located in the walls of bronchioles leading to the lungs, on the coronary arteries and on smooth muscles in the walls of the digestive and urinary organs. *

11. Adrenergic Receptor Effects
Alpha 1:
Excites (constricts) smooth muscles in certain blood vessels & in spincters directing blood to skeletal muscles
Dilates pupils.
Alpha 2:
Promotes blood clotting
Beta 1:
Cardiac Muscle Increases heart rate & strength
Beta 2:
Depresses (dilates) smooth muscle in bronchioles & coronary arteries increasing blood flow to heart and air flow to lungs.
When bound to epinephrine or norepinephrine, Alpha 1 receptors become excited and cause constriction of the blood vessels and spincters they are located on. * This causes vasoconstriction of blood vessels directing the majority of blood flow to the skeletal muscles to support increased activity. Alpha 1 receptors in the kidneys stimulate the release of renin when stimulated. In the eyes, excited Alpha 1 receptors causes dilation of the pupil for better close vision. Excited Alpha 2 receptors promote blood clotting in case of injury.
Beta 1 receptors * on the cardiac muscle of the heart * increase heart rate and strength when stimulated. This increases cardiac output to support increased physical activity during an emergency (fight or flight). Beta 2 receptors * are depressed when they bind with epinephrine or norepinephrine. * Since they are located on smooth muscle cells lining the bronchioles and coronary arteries, their effect is to cause dilation which increases the flow of O2 to the lungs and increases the flow of blood to the myocardium of the heart. *

12. Parasympathetic Innervation of Visceral Targets
Ganglia close to or on target organs
Preganglionic neurons - long
Post ganglionic neurons - short
The Parasympathetic division of the ANS is characterized having the ganglia * very close to or on the target organs. * As a result, the preganglionic neurons * must be long to reach from the spinal cord to the ganglia. The postganglionic neurons * are relatively short, since they only must reach from the ganglia to the target organ the ganglia are already near or on. *

13. Parasympathetic Neurotransmitters
Preganglionic neurons release acetylcholine = Cholinergic
The only neurotransmitter that the receptors of the Parasympathetic division respond to is acetylcholine. * Because all preganglionic neurons in the Parasympathetic and the Sympathetic branches of the ANS release acetylcholine, * they are all considered to be cholinergic neurons. *

14. Parasympathetic Neurotransmitters
Postganglionic neurons release acetylcholine = Cholinergic
Postganglionic neurons * associated with the Parasympathetic division of the ANS also release the neurotransmitter acetylcholine. * Thus they are also considered to be cholinergic. One of the unique characteristics of the Parasympathetic division of the ANS is that both the preganglionic and the postganglionic neurons are cholinergic. *

15. Cholinergic Receptors
Found on skeletal muscle cells regulated by motor neurons.
If you will recall, * skeletal muscles are stimulated to contract by motor neurons * * which release the neurotransmitter acetylcholine. * Thus the motor neurons are cholinergic. Thus the receptors * on skeletal muscle cells which bind with acetylcholine are considered to be cholinergic receptors. *
Motor Neuron

16. Cholinergic Receptors
Found on dendrites & cell bodies of postganglionic neurons of both sympathetic and parasympathetic divisions of ANS.
* Since the preganglionic neurons of both the Sympathetic and the Parasympathetic divisions are cholinergic, * the receptors located on the dendrites and cell bodies of postganglionic neurons must be cholinergic receptors as well. * Recall that the receptors located on the target organs * which are regulated by the Parasympathetic division also are cholinergic. *
Found on parasympathetic target organs.

17. Cholinergic Receptor Types
Nicotinic:
On skeletal muscle cells
On postganglionic dendrites & cell bodies in both sympathetic & parasympathetic
Almost always excite
Muscarinic:
On all target organs of parasympathetic
May excite or decrease activity depending on target
Just as there were Alpha and Beta types of adrenergic receptors, there are two types of cholinergic receptor types as well. * Nicotinic receptors are found on skeletal muscle cells * as well as on the dendrites and cell bodies * of postganglionic neurons in both the Sympathetic and Parasympathetic divisions of the ANS. * When bound with acetylcholine, these almost always excite. *
Muscarinic receptors * are only found on the target organs affected by the Parasympathetic division of the ANS. * Muscarinic receptors which bind to acetylcholine * may either stimulate activity or decrease activity depending on the target organ. *

18. Muscarinic Receptor Effects
Cardiac Muscle - Slows heart rate and strength of contraction
Digestive System - Increases digestive activity including secretions & peristalsis.
Increases flow of blood to liver, pancreas & digestive organs by vasodilation of appropriate vessels.
Eye - Causes constriction of Iris
To give you specific examples of the varying effects of acetylcholine on muscarinic receptors, consider the following.
* Acetylcholine binding to muscarinic receptors on cardiac muscle cells of the heart slow down the heart rate and decrease the strength of contractions. * This has the effect of reducing cardiac output to restore the bodies normal resting rate of activity.
* When acetylcholine binds to muscarinic receptors on glands and smooth muscles associated with the digestive system, the effect is to excite or increase the secretion of glands and the contractions of the smooth muscle cells in the walls of the digestive organs to promote peristalsis.
* Stimulated muscarinic receptors on arterioles leading to the liver, pancreas and other digestive organs dilate to increase the flow of blood to those organs during times when digestive activities are needed to digest and store materials for future use.
* Stimulated muscarinic receptors on the smooth muscles of the iris cause the iris to constrict, thus reducing the size of the pupil and allowing less light into the eye. *

19. Blocking Agents
Interfere with stimulatory or depressing effects of neurotransmitters by blocking the receptors on target organs.
Blocker
Blocking agents are chemicals which bind to the receptors on target organs and prevent the normal neurotransmitter from binding. * Here you can see an illustration of a receptor on the membrane of a cell. * When a neurotransmitter molecule binds with the receptor, the cell will react. * Blocking agents bind with and cover up the binding site on the receptor * * so that the neurotransmitter cannot bind. * Thus the neurotransmitter molecule will not have its normal effect on the blocked cell. *
Normal neurotransmitter can’t bind with receptor because blocker covers the binding site.

20. Adrenergic Blockers
Block receptor binding sites preventing the binding of epinepherine or norepinepherine
Beta 1 blockers on heart
prevent heart rate increase & arrhythmias in cardiac patients without interfering with other sympathetic effects.
Examples:
Acebutolol (Sectral), Metoprolol (Lopressor)or Inderal.
Adrenergic blocking agents * block the receptor binding sites to prevent epinephrine and norepinephrine from binding. * Beta 1 blockers on the cardiac muscles of the heart * prevent the increase in heart rate and accompanying arrhythmias that could be compromising to cardiac patients. * Examples of Beta 1 blockers * include acebutolol (Sectral) * and Metoprolol (Lopressor).*

21. Adrenergic Blockers Alpha 1 blockers
Decrease blood pressure in patients with hypertension without interfering with other sympathetic effects.
Example:
Phentolamine
Adrenergic blockers don’t allow epinephrine or norepinephrine to bind with adrenergic receptors by binding with their binding sites. * Alpha 1 blockers * are used to decrease blood pressure in patients with chronic hypertension without interfering with other sympathetic effects by binding only Alpha 1 receptors. * An example of a common Alpha 1 blocker is Phentolamine. * *

22. Cholinergic Blockers Muscarinic blockers
Block parasympathetic effects on target organs
Example:
Atropine
Used topically during eye exams to dilate pupils
Sometimes used prior to surgery to reduce salivation & respiratory secretions
Cholinergic blocking agents interfere with the binding of acetylcholine with cholinergic receptors. * Since muscarinic receptors are exclusively located on the target organs effected by the Parasympathetic division of the ANS, they effect only Parasympathetic responses. * An example of a muscarinic blocking agent is Atropine, * which is used topically during eye exams to dilate the pupil of the eye. * Atropine us sometimes also used prior to surgery to reduce salivation and respiratory secretions.*

23. Acknowledgements Most of the figures used in this presentation came from the Benjamin Cummings Digital Library Version 2.0 for Human Anatomy & Physiology, Fifth Edition. Other figures came from public domain internet sources and software in the possession of the author.