Autonomic Nervous System

Allie Davis Jimmy Johnson

General Overview of Autonomic Nervous System

The purpose of the ANS is to control and maintain involuntary/voluntary actions in the body.

Activation Centers of ANS:

-Spinal cord, brainstem, and hypothalamus

-Parts of limbic cortex

-Visceral reflexes

-Sympathetic and Parasympathetic nervous system

Sympathetic Nervous System

Plays a key role in maintaining homeostasis during stress, it is the “flight-or-fight” system of the body.

This is done by accelerating heart rate, constricting blood vessels, and raising the blood pressure.

For this information to be communicated, it uses pre/post-ganglionic neurons.

-Preganglionic Neurons: -Postganglionic Neurons:

Cholinergic Has a stimulatory effect Long fibers of the axon Non-myelinated

Myelinated Targets neurons in ganglion/adrenal medulla Located in ganglia

Shorter fibers Located in CNS Effect depends on the signal from which target

More synapses Targets smooth muscle, heart, glands, misc.

organs

It uses norepinephrine as a neurotransmitter

-Adrenal Medulla

releases epinephrine and norepinephrine

It is target dependant, just as the postganglionic as well

Targets smooth muscle, glands, heart, misc.

Stimulation of sympathetic nerves to the adrenal medulla causes large

quantities of epinephrine/norepinephrine to be released.

Indirectly stimulates organs

Destruction of the Adrenal Medulla has little effect on the operation of

the sympathetic nervous system

Parasympathetic

Nerves (fig. 60-3)

Sympathetic

Nerves (fig.

60-1)

Parasympathetic Nervous System

This system plays key roles in maintaining homeostasis during relaxation of the body. It is the “rest-and-

digest” and “feed-and-breed” portion of the body.

Preganglionic Postganglionic

starts in the CNS Long Axons Cell body on ganglion

short axons

myelinated Cholinergic No myelination

cholinergic

Stimulates Targets ganglion targets smooth

muscle, glands, heart, misc. organs

target dependent on effect of neuron

Vagus Nerve (X)

The “wandering” nerve.

The vagus nerve is the only cranial nerve to extend beyond the head and neck.

It is a mixed nerve, which means it can have efferent and afferent signals.

It’s efferent signals are mostly parasympathetic.

Emerges from the medulla oblongata.

Effects on the ANS and drugs used in rabbit procedure

Neurotransmitters and receptors of the ANS

- Main neurotransmitters

- acetylcholine

- norepinephrine

- Secretion fibers

- cholinergenic

- secrete acetylcholine

- all preganglionic neurons in sympathetic/parasympathetic

- almost all postganglionic of parasympathetic

- almost all nerve endings of PNS secrete acetylcholine

- adrenergic

- secrete norepinepherine

- most postganglionic sympathetic neurons

- almost all nerve endings of SNS secrete norepinephrine

Cholinergic fibers (acetylcholine receptors)

- Muscarinic:

- found on all effector cells stimulated by postganglionic cholinergic neurons

- either sympathetic or parasympathetic

- Nicotinic:

- located between preganglionic and postganglionic neurons

- sympathetic and parasympathetic

- also present in many somatic nerve endings (muscular junctions)

Adrenegic fibers (norepinephrine receptors)

- Beta receptors:

- Beta1, Beta2, and Beta3 receptors

- Alpha receptors:

- Alpha1, and Alpha2 receptors

- Norepinephrine:

- Mainly excites alpha receptors, beta receptors to a lesser extent

- Epinephrine:

- Excites both types of receptors approximately equally, beta receptors to greater

extent than norepinephrine

Adrenergic fibers (norepinephrine receptors) Alpha receptor system effects:

- vasoconstriction

- iris dilation

- intestinal relaxation

- intestinal sphincter contraction

- bladder sphincter contraction

- inhibits neurotransmitter release

Beta receptor system effects:

- vasodilation

- cardioacceleration

- increased myocardial strength

- intestinal relaxation/uterus relaxation

- bronchodilation

- calorigenesis

- glycogenolysis

- lipolysis

Pharmacology of autonomic receptors

- Sympathomimetic drugs:

- Direct - cause same effects as sympathetic stimulation

- epinephrine

- methoxamine

- Indirect - cause release of norepinephrine from storage vesicles in nerve endings

- ephedrine

- tyramine

- amphetamine

Sympathomimetic drugs cont.

- Adrenergic blockage

- synthesis/storage of norepinephrine in nerve endings

- reserpine

- release of norepinephrine from nerve endings

- guanethidine

- sympathetic alpha receptors

- phenoxybenzamine, phentolamine

- sympathetic beta receptors

- propranolol, metoprolol

- sympathetic nerve impulses through ganglia

- hexamethonium

Pharmacology of autonomic receptors cont.

- Parasympathomimetic (cholinergic drugs)

- Direct -

- cholinesterase destroys acetylcholine before reaching effector organs

- drugs not destroyed produce similar effects

- drugs act directly on muscarinic type cholinergic receptors

- pilocarpine

- methacholine

Parasympathomimetic drugs cont. - Indirect -

- Anticholinesterase drugs - inhibit acetylcholinesterase, stopping destruction of acetylcholine at

nerve endings

- neostigmine

- pyridostigmine

- ambenonium

- Antimuscarinic drugs -

- block action of acetylcholine on muscarinic type cholinergic receptors only

- Atropine

- homatropine

Pharmacology of autonomic receptors cont.

- drugs blocking/stimulating postganglionic autonomic neurons

- Postganglionic stimulation -

- nicotinic drugs

- drugs stimulating postganglionic neurons

- nicotine

- methacholine

- Ganglionic stimulation -

- block impulse from preganglionic neurons to postganglionic neurons in PNS & SNS

- block acetylcholine stimulation

Neurotransmitters and receptors of the ANS

- Synthesis of neurotransmitters

- Sympathetic/parasympathetic nerve endings touch or pass by target cells

- Variscosities

- enlargements near target cells housing transmitter vesicles of acetylcholine or

norepinephrine to be synthesized or stored

- action potentials reach terminal fibers, increasing calcium permeability allowing for

variscosities and terminal endings to release acetylcholine/norepinephrine

Acetylcholine

- Chemical reaction for synthesis:

- Acetyl-CoA + Choline -> choline acetyl-tranferase -> Acetylcholine

- post secretion (deconstruction):

- performs nerve signal transmission for a few seconds

- split into acetate ion and choline by acetylcholinesterase

- choline is transported back to terminal nerve ending to form more acetylcholine

Norepinephrine

- chemical reactions for synthesis:

- begins in axoplasm of terminal nerve ending, completed in secretory vesicle

- 1. Tyrosine -> hydroxylation -> dopa

- 2. dopa -> decarboxylation -> dopamine (dopamine transported to vesicles)

- 3. dopamine -> methylation -> norepinephrine

- adrenal medulla turns ~80% of norepinephrine into epinephrine

- 4. norepinephrine -> methylation -> epinephrine

Norepinephrine

- Post secretion (deconstruction):

- removed from secretory site 3 ways

- 1. reuptake into nerve endings through active transport ~50-80% removal

- 2. diffusion into blood accounting for removal of most remaining norepinephrine

- 3. destruction of small amount left over by tissue enzymes

- monoamine oxidase (in nerve ending)

- catechol- O - methyl transferase (present in all tissues)

Patterns of ANS stimulation

- Discharge

- mass discharge:

- almost all sympathetic portions activated simultaneously as a unit.

- most frequently during activation of hypothalamus during fight or flight stimulation

- parasympathetic:

- usually specific localized responses

- very specific

Patterns of ANS stimulation

- discrete discharge:

- activation of isolated portions of sympathetic nervous system

- heat regulation - sweating, and blood flow

- “local reflexes” - sensory afferent fibers → peripheral nerves → sympathetic

ganglia/spinal cord,

- ex.) heating skin causing vasodilation, enhancing sweating, cooling is

opposite

- gastrointestinal functions, don’t enter spinal cord- sympathetic reflexes in gut →

paravertebral ganglia → gut.

Adrenal Medulla

- Function:

- produces and releases epinephrine/norepinephrine into blood (80% epinephrine, 20% norepinephrine

- NT’s released at same instance of sympathetic activation

- substitutes for/supports sympathetic stimulation

- epinephrine/norepinephrine stimulate areas not innervated by sympathetic fibers

Autonomic Tone

- Tone:

- continuous sympathetic/parasympathetic basal rate of activity

- allows single nervous system to increase/decrease organ activity

- ex.) systemic arterioles are constricted to ½ their max diameter

- sympathetic stimulation increases constriction

- decreased sympathetic stimulation dilates more than ½ diameter

- Medullae control of tone:

- resting secretion rate of epinephrine/norepinephrine in considerable quantities