Physio Lecture 12 Autonomic Nervous System

Physio Lecture 12 Autonomic Nervous System

The ANS consists of all visceral motor neurons innervating smooth muscle, cardiac muscle and glands.

TERMINOLOGY

Skeletalmuscle is voluntary, so nerves that supply them are called somatic motor neurons. They are not part of the ANS.

Efferent nerves are those that go out from the CNS (motor neurons).

Afferent nerves go into the CNS (sensory neurons).

Somatic receptors are on voluntary targets (you can choose to move them). They are not part of the ANS.

Visceral receptors are on involuntary targets (like organs and glands).

A ganglion is a collection of cell bodies in the peripheral nervous system (PNS)

There are two branches of the ANS: Sympathetic and Parasympathetic.

SIMILARITIES BETWEEN SYMPATHETIC AND PARASYMPATHETIC

Sympathetic and parasympathetic both are part of ANS.

Both have visceral motor neurons.

Both have 2 neurons (pre and post ganglionic neurons).

The post-ganglionic neuron is the one that actually puts the NT on the involuntary muscle. The post synaptic neuron has the effect.

DIFFERENCES BETWEEN SYMPATHETIC AND PARASYMPATHETIC

Some effectors (the target organ or gland) can be dually innervated by sympathetic and parasympathetic nerves.

Note that some people urinate (parasympathetic response) when they are very scared (sympathetic system is dominant). This is a case of shock, where the sympathetic system becomes overwhelmed in your brain. Urination and defecation during extreme stress will still help a person escape a predator, due to the unwanted odor or surprising the predatory so the prey can escape.

For this lecture, you must know

1. The exact location of the pre and post ganglionic cell body
2. What NT are used by each neuron
3. What receptors are used on the involuntary effector
4. What would happen if the involuntary effector bound to the sympathetic NT or the parasympathetic NT. What are the sympathetic effects and parasympathetic effects?
5. Know the drugs on the PPT. If sympathomimetic, then sympathetic will gain more control. What would the effectors do? If it is an antagonist to the sympathetic system, then the parasympathetic would gain control, and know what would happen to HR, pupils, etc.

Sympathetic and Parasympathetic Divisions of the ANS

•How they are similar:

–Both divisions are part of the ANS

–Both have effects on smooth muscle, cardiac muscle and glands

•How they differ:

–Stimulation of effectors by each system varies from one body system to another. Effects may be stimulatory or inhibitory.

–Location

–Length of pre and postganglionic fiber

–Neurotransmitters used

How are the ANS neurons different from somatic motor neurons?

Somatic motor neurons are just one neuron (not two, like in the ANS).

Somatic motor neurons innervate skeletal muscle (ANS innervates involuntary muscles and glands).

Somatic motor neurons have their cell body always in the CNS (ANS can have one cell body in CNS, one in PNS).

The ANS consists of two neurons.

The first has its cell bodies within the CNS, and is called the preganglionic neuron.

The second has its cell bodies outside of the CNS (in a ganglion), and is called the post-ganglionicneuron.

Comparison of ANS and Somatic Motor Pathways

•Somatics

–Single neuron from spinal cord to effector

–One neuron innervates the effector cell

–NTS is acetylcholine (ACh)

•Autonomics

–Two neurons relay (ganglion) to effector

–Dual innervation of effectors

–NTS at ganglia (ACh); NTS at effector can be ACh or norepinephrine

Both parasympatheticand sympatheticpreganglionic neurons have their cell bodies located in the CNS. The parasympathetic system neurons are called cranio-sacral neurons because those are the two locations their cell bodies can be.In the brain, the cell bodies are located in the nuclei for CN 3, 7, 9, 10.

In the sacral regions, the cell bodies are located at S2, 3,4

The sympathetic neurons are called thoraco-lumbar neurons because those are the two locations their cell bodies can be.

PARASYMPATHETIC PATHWAY

The preganglionicparasympathetic neuron will always release Ach onto the post ganglionic neuron.

What receptor will always be on the second neuron? Ach receptors (Nicotinic ionotropic).

What NT is released by post-ganglionic neurons? Ach again.

What receptors are on the involuntaryeffectors (smooth muscles or glands)? Metabotropic (muscarinic) receptors. Metabotropic means that it uses the second messenger pathway.

What are first two vowels in parasympathetic? (A, A) reminds you that in the parasympathetic pathway, Ach and Ach is released in both neurons.

Thepreganglionicaxon is long in the parasympathetic system. It reaches almost to the effecter cells.

The postganglionic axon is short in the parasympathetic system. It is almost in the wall of the target cell, such as the heart or small intestine.Therefore, parasympathetic post- ganglions (terminal ganglia) are inside the walls of target organs.

Cranial 10 is THE parasympathetic division because it communicates with so many other nerves.

Cut CN X, cannotstimulate postganglionic nerves. CN X is the preganglion neuron.

SYMPATHETIC PATHWAY

The preganglionic sympathetic neuron will always release Ach onto the post ganglionic neuron.

What receptor will always be on the second neuron? Ach receptors (Nicotinic ionotropic).

So far, this is the same as parasympathetic, but here is where it differs:

What NT is released by sympathetic post-ganglionic neurons? Epinephrine or norepinephrine (not Ach).

What receptors are on the involuntary effectors (smooth muscles or glands)? Adrenergic metabotropic receptors. Metabotropic means that it also uses the second messenger pathway.

What are first two vowels in sympathetic? (A, E) reminds you that in the sympathetic pathway, Ach and Epi (or norepi) is released in both neurons.

The preganglionic axon is shortin the sympathetic system. Its cell body is in the spinal cord (Thoracolumbar ganglia). The cell bodies are in the dorsal or ventral grey matter in those regions.

One exception: When the cell bodies are in T1-L2, the cell bodies are in the lateral grey horn.

The axons of the preganglionic neurons synapse in ganglia that are close to the vertebral column (this string of ganglia are called the sympathetic trunk or chain). In those ganglia are the cell bodies of the postganglionic neurons.The postganglionic axon is long in the sympathetic system. It has to go from near the lumbar vertebrae, all the way to the target cell.

There are three other types of ganglia that are near the abdominal aorta. These are not part of the sympathetic trunk. They are called celiac, superior mesenteric, and inferior mesenteric ganglia.

Therefore, there are four locations of the cells bodies of post ganglionic sympathetic neurons:

1)Sympathetic chain

2)Celiac ganglia

3)Superior mesenteric

4)Inferior mesenteric

There are certain alpha adernergic receptors, when epi or norepi bind, which cause vasoconstriction. The betaadernergic receptors causesdilation (muscle relax). This is the type needed for coronary arteries and the Circle of Willis.

Characteristics of Sympathetic and Parasympathetic Function

•Post-ganglionic sympathetic nerves release norepinephrine at their nerve endings

–these nerves are called adrenergic nerves

•Pre and post-ganglionic parasympathetic nerves release acetylcholine at their nerve endings (Nitric oxide- corpora cavernosum)

–these nerves are called cholinergic nerves

•EXCEPTION FOR SYMPATHETIC:

–Cholinergic: mACH; used in body wall.

–Vasodilatation of vessels in brain and skeletal muscle

–Piloerector muscles

–Sweat glands

–Nitric oxide: vasodilatation of vessels in brain, skeletal muscle

Involuntary effectors are located in two places:

1) in the body wall

2)in the skin

1. sweat glands
2. arrector pili muscles
3. blood vessels in dermis

EXCEPTION TO THE RULE

Thesympathetic involuntary effectors that are located in the body wall have postganglionic receptors that are muscarinic Ach receptors instead of the usual epi/norepi receptors.

Other sweat glands, such as apocrine glands are truly innervated by the sympathetic system. They use norepi, and when triggered during fight or flight, the apocrine glands will secrete. Apocrine glands are located in the axilla, pubic hair, and mammary glands are modified apocrine glands. When you are under stress, these glands will secrete. They are not triggered by just being in hot weather.

Sympathetic Nervous System“Thoracolumbar”

When active?

Length of pre and postganglionic nerve?

Location of cell bodies of preganglionic nerve?

Location of cell bodies of postganglionic nerve?

Neurotransmitters released?

What receptor is expressed by preganglionic and postganglionic neurons (iono, nicotinic?)

Adrenal medulla

SYMPATHETIC (ADERNERGIC)RECEPTORS TO KNOW FOR THIS CLASS

There are many types of sympathetic adernergic receptors in the body. You only need to know alpha-1, Beta-1, beta-2, and beta-3. You will learn about alpha-2 receptors in pharmacology; they are quite complicated. Adernergic means that it binds to a molecule made by the adrenal glands (epi or norepi).

Alpa-1 receptorsconstrict blood flow to areas where it is not needed during fight or flight. They close down digestive processes, too. They also cause the iris to open the pupils.During fight or flight, the alpha-1 receptors are stimulating vasoconstriction on organs and tissues that are not needed at the time, and that helps elevate blood pressure. When a person is in psychogenic shock, their sympathetic system is overwhelmed, and shuts down, so the alpha one receptor is not being stimulated, and they lose blood pressure and faint.

By the way, the GI tract has its own set of neurons. Your gut can work on its own, without the parasympathetic input from the thoracolumbar nerves. The parasympathetic system just modifies the effect of the GI tract. That is why people with paralysis and spinal cord injuries can still digest their food. Irritable bowel syndrome may be from the GI neurons firing excessively. The GI tract has chemoreceptors that detect food substances that they don’t like. There are people that try to eat spicy food and get irritable bowel syndrome (gas and either diarrhea or constipation). The food can be just as hot going out as it was going in!

Beta-1 receptors are the dominant receptors on the heart. It causes vasoconstriction, which increases heart rate and blood pressure.

Beta 2 receptors are the most dominate receptors around the air passageway. Stimulation leads to bronchial dilation. They are also found on blood vessels that supply the heart, brain, skeletal muscles, and any other areas which need blood flow during fight or flight situations. When Beta 2 receptors bind epinephrine, those blood vessels vasodilate.

Beta 3 receptors stimulate lipolysis, which is the release of fatty acids from lipid tissues. If you had more of these receptors, or if there was a drug that could stimulate these receptors, you could lose fat more easily!

PARASYMPATHETIC RECEPTORS

3, 7, 9, and 10 nerves are preganglionic nerves. They do not directly innervate the involuntary effectors. They get close, but they release Ach onto the post-ganglionic neuron, which is in the wall of the effectors. For example, CN X does not innervate the heart. It innervates the wall of the heart, but it is not the neuron that releases NT onto the heart receptors. When CN III is stimulated, it will stimulate a post-ganglionic neuron in your eye, causing the pupils to constrict. CN VII will stimulate a neuron that will stimulate the lacrimal and submandibular glands to get tears or salivary secretion. You secrete 800 ml of saliva every day (almost one liter)! Who found that out?? CN X is considered THE parasympathetic pre-ganglionic neuron. It extends throughout the thoracic and abdominal cavities.CN X innervates the postganglionic neurons in the heart, lungs, stomach, small intestine, the ascending, colon, transverse colon, and the descending colon, until it turned into the colonic flexure, then CN X stops. The rest of the descending colon, rectum, urinary bladder, and gonads have sacral nerves (S2, 3, 4) as their preganglionic neurons.

Parasympathetic Nervous System “cranio-sacral”

Parasympathetic nerves originate from cranial nerves III, VII, IX, and X and the sacral spinal cord.

CN III, Occulomotor nerve - fibers to the pupillary sphincters and ciliary muscle

CN VII, Facial nerve - fibers to lacrimal and submandibular gland

CN IX, Glossopharyngeal nerve - fibers to parotid gland

CN X, Vagus nerve - motor inputs to visceral organs

Sacral segments - fibers to descending colon, rectum, bladder and genitalia

Effects of the ANS on the Organs

•eye

–sympathetic --pupillary dilation (alpha 1 receptor)

–parasympathetic--pupillary constriction and accommodation (focusing) of the lens (mACh receptor, metabotropic)

•glands of the body

–parasympathetic stimulate the nasal, lacrimal, salivary, and G.I. glands (mACh receptor)

–sympathetic stimulates the sweat glands (mACh receptor)

Effect of the Autonomic Nervous System on the Organs

•G.I. tract

–parasympathetic stimulates overall activity including G.I. smooth muscle

–sympathetic has very little effect

•heart

–sympathetic increases the rate and contractility

–parasympathetic decreases heart rate

•blood vessels

–Sympathetic causes vasoconstriction. Reduced sympathetic response accounts for most vasodilation.

–parasympathetic causes some vasodilation (e.g., penis)

–For the most part, blood vessels are under the control of sympathetic nervous system. Epinephrine and norepinephrine bind to the alpha receptors, and blood vessels are held in a general state of some vasoconstriction. If you need more vasoconstriction, more epinephrine is released to stimulate alpha receptors to give you more vasoconstriction. If you need more vasodilation, one way is to reduce sym output and smooth muscle will naturally relax.

To be prepared for third order test questions:

Q: What is the effect of alpha-1 receptors which are stimulated on the smooth muscle on the intestine?

A: Apha-1 receptors are sympathetic

Sympathetic system decreases gut motility.

The second messenger is cAMP

Q: If heart rate was increased, what receptor was stimulated?

A: Beta 1

Q: If heart rate is decreased, what receptor was stimulated?

This can be tricky. To decrease the action does not mean to inhibit it.

A: To decrease the heart rate, you stimulate mACh receptors.

Q: what are the second messengers to decrease the heart rate?

A: IP3, DAG

In the ANS, both the sympathetic and the parasympathetic systems are on at the same time, and are discharging. Which system has more control at this point in time? If it is sympathetic, you are having fearful or angry feelings.

Sympathetic and Parasympathetic “Tone”

•the basal rate of activity of each system

•this background activity allows for an increase or decrease in activity by a single system

–sympathetic tone normally causes about a 50 % vasoconstriction

•increasing or decreasing “tone” can change vessel diameter

–parasympathetic tone provides background G.I. activity

Adrenergic or sympathomimetic drugs act like norepinephrine

•these drugs have an effect which is much more prolonged than that of either norepinephrine

–Phenylephrine (pseudofed) stimulates alpha-1 receptors. Mucous is derived from plasma. When you are congested, your blood vessels there are engorged with blood. You have vasodilation when you are congested. When alpha-1 receptors are stimulated, cause vasoconstriction and congestion improves.

–isoproterenol stimulates both beta1 and beta2 receptors. Given for asthma, stimulates Beta 2 receptors in lung, causing bronchiole dilation. However, cannot be given to someone with hypertension because it also stimulates beta 1 receptors, causing faster heart beat and greater force of contraction. Want to use a drug that is more specific for asthma:

–albuterol stimulates only beta2 receptors, safe for asthmatic people who have high blood pressure.

•some drugs act indirectly by increasing the release of norepi from its storage terminals

–ephedrine, amphetamine, pseudoephedrine

–Causes your own natural norepi to be released in larger amounts, causing alpha-1 receptors to be stimulated, causing vasoconstriction, and therefore, less congestion. However, people are using boxes and boxes of pseudoephedrine to be used as a precursor for making methamphetamine (Meth). You can still buy it, but it is under lock and key, and you have to register in the computer system so that all stores know how many times per week you have bought this medicine.

Pharmacology of the Sympathetic Nervous System

•drugs that block the effect of norepinephrine

•alpha blockers

–phentolamine blocks apha 1 receptors, so it blocks receptors in smooth muscles, leading to vasodilation. This is what men used before Viagra; had to be injected. Now it comes in an oral form

–phentolamine is not the same as Viagra, which blocks the enzyme that destroys nitrous oxide.

•beta blockers

–beta1 and 2–propranolol: given to people with hypertension to reduce heart rate. If the person also has asthma, don’t use this, because it causes bronchiole constriction.

Pharmacology of the Parasympathetic Nervous System

•parasympathomimetic drugs

–muscarine (mushroom toxin)

–pilocarpine (another toxin, blocks sympathetic receptors, giving more control to parasym)

•Activates muscarinic Ach receptors, cause profuse sweating (why if sweat glands are controlled by sympathetic system?)

•cholinesterase inhibitors

–neostigmine, potentiates the effect of acetylcholine. The parasympathetic system uses Ach for its preganglionic and postganglionic receptors. A person on a cholinesterase inhibitor will give more control over the sympathetic nervous system. The Ach is left in the synaptic cleft longer, and this stimulates the mACh on the involuntary effectors.

•antimuscarinic drugs

–atropine blocks the effect of acetylcholine on effector cells. If we block it, the sympathetic system gains more control.

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