Download Autonomic Nervous System

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
Myelinated
Targets neurons in ganglion/adrenal medulla
Shorter fibers
More synapses
Located in CNS
Long fibers of the axon
Non-myelinated
Located in ganglia
Effect depends on the signal from which target
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-anddigest” and “feed-and-breed” portion of the body.
Preganglionic
Postganglionic
starts in the CNS
short axons
Long Axons
myelinated
Cholinergic
Cell body on ganglion
No myelination
cholinergic
Stimulates
muscle, glands, heart, misc. organs
Targets ganglion
target dependent on effect of neuron
targets smooth
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
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
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
Adrenergic fibers (norepinephrine receptors)
Alpha receptor system effects:
Beta receptor system effects:
- vasoconstriction
- iris dilation
- intestinal relaxation
-
vasodilation
-
cardioacceleration
-
increased myocardial strength
-
intestinal relaxation/uterus relaxation
-
bronchodilation
-
calorigenesis
- intestinal sphincter contraction
- bladder sphincter contraction
- inhibits neurotransmitter release
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
Sympathomimetic drugs cont.
- Adrenergic blockage
-
synthesis/storage of norepinephrine in nerve endings
-
-
release of norepinephrine from nerve endings
-
-
guanethidine
sympathetic alpha receptors
-
-
reserpine
phenoxybenzamine, phentolamine
sympathetic beta receptors
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
Parasympathomimetic drugs cont.
- Indirect -
Anticholinesterase drugs - inhibit acetylcholinesterase, stopping destruction of acetylcholine at
nerve endings
-
neostigmine
-
pyridostigmine
-
ambenonium
- Antimuscarinic drugs -
Pharmacology of autonomic receptors cont.
- drugs blocking/stimulating postganglionic autonomic neurons
-
Postganglionic stimulation -
nicotinic drugs
-
drugs stimulating postganglionic neurons
-
nicotine
-
methacholine
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
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
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
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
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
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
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