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The Autonomic Nervous System

Def: The ANS consists of all visceral motor neurons innervating smooth muscle, cardiac

muscle and glands.Chapter 60

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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 vary from one body system to another. Effects may be stimulatory or inhibitory.

– Location– Length of pre and

postganglionic fiber– Neurotransmitters used

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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

Location and neurotransmitters used by Sympathetic/Parasymp

sympathetic

parasympathetic

preganglionic neuron

postganglionic neuron

nicotinic receptors muscarinic receptors

adrenergic receptors

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 cavernosa)– these nerves are called cholinergic nerves

• EXCEPTION FOR SYMPATHETIC:– Cholinergic: mACH; used in body wall.– Vasodilation of vessels in brain and

skeletal muscle– Piloerector muscles– Sweat glands– Nitric oxide: vasodilation of vessels in

brain, skeletal muscle

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Sympathetic Nervous System“Thoracolumbar”

Figure 60-1;Guyton & Hall

When active?Length of pre and postganglionic nerve?Location of cell bodies of preganglionic nerve?Location of cell bodies of postganglionic nerve?Neurotransmitters released?Adrenal medulla

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Lipolysis and thermogenesisAdipose tissue 3

Inhibitory, relaxation of resp. tract to get dilation

Respiratory tract, GI, glands, hepatocytes

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Increased heart rate and inotropyHEART, kidney, adipocytes

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Presynaptic control/ release of NTVascular endothelium 2c

Increase vasoconstriction (increased MI mortality w/polymorphism)

Vascular endothelium 2b

Lipid metabolismVascular endothelium and adipocytes

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Increased BP, vasoconstrictionArteries, heart 1B

Increased inotropy, closure of GI sphincters, vasoconstriction, pupil dilation

Iris, intestine, heart and arteries

1aActionsWhere found?Receptor

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ParasympatheticNervous System“cranio-sacral”

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

occulomotor nerve - fibers to the pupillary sphincters and ciliary musclefacial nerve - fibers to lacrimal and submandibular glandglossopharyngeal nerve - fibers to parotid glandvagus nerve - motor inputs to visceral organssacral segments - fibers to descending colon, rectum, bladder and genitalia

Figure 60-3;Guyton & Hall

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)

• 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)

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 stimulates alpha receptors– isoproterenol stimulates both beta1 and beta2 receptors– albuterol stimulates only beta2 receptors

• some drugs act indirectly by increasing the release of norepi from its storage terminals– ephedrine, amphetamine, pseudoephedrine,

Pharmacology of the Sympathetic Nervous System

• drugs that block the effect of norepinephrine

• alpha blockers– phentolamine – Cheap Viagra!

• beta blockers– beta1 and 2 -

propranolol

Pharmacology of the Parasympathetic Nervous System

• parasympathomimetic drugs– muscarine– pilocarpine

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

• cholinesterase inhibitors– neostigmine, potentiates the

effect of acetylcholine• antimuscarinic drugs

– atropine blocks the effect of acetylcholine on effector cells