The Autonomic Nervous System

Introduction

motor system (NOT sensory!)

innervates heart, smooth muscle, glands

basic unit is two-neuron unit

(1) preganglionic neuron – body in CNS grey matter (intermediolateral horn); synapses in ganglia

(2) postganglionic neuron – body in ganglion; synapse into end organ

two divisions:

(1) sympathetic – fight or flight; thorocolumbar system

(2) parasympathetic – rest and digest; craniosacral system
anatomically, biochemically and functionally different

usually opposing effects

End Organ Effects of Autonomic Stimulation

End Organ	Predominant Tone	Effect of Ganglionic Blockade
Arterioles	Sympathetic-adrenergic	Vasodilatation, increased peripheral blood flow, Hypotension
Heart(SA node)	Parasympathetic-cholinergic	Tachycardia (HR > 100)
Iris	Parasympathetic-cholinergic	Dilation of the pupil
Ciliary muscle	Parasympathetic-cholinergic	Paralysis of accomodation (adjustment)
GI tract	Parasympathetic-cholinergic	Reduced tone and motility
Urinary bladder	Parasympathetic-cholinergic	Urinary retention
Sweat glands	Sympathetic-cholinergic	Absence of sweating

Anatomy

sympathetic

preganglionic neurons:

mylenated axons; white rami
pass out of ventral root
convergence and divergence with postganglionic

postganglionic neurons – arranged into 2 ganglia:

(1) paravertebral ganglia (chain ganglia)

arranged in chain along vertabrae
non myelinated; grey rami
paired
cervical region: head and heart
thoracic region: blood vessels, hair, sweat glands, etc.

(2) prevertebral ganglia

located surrounding viceral branches of aorta
may be named by associated artery i.e. celiac, inferior, superior messenteric ganglia
paired or unpaired
innervate abdominal and pelvic viscera - stomach, intestine, bladder

adrenal chromaffin cells

like postganglionic but no efferent axons
release norepinephrine and epinephrine into bloodstream
innervated by preganglionic neurons

parasympathetic

preganglionic neurons:

can be cranial or sacrally located cell bodies
mylenated
longer than sympathetic; synapse with ganglia close to organ
cranial: heart, head, glands in abdomen
vagus nerve: thoracic and abdominal viscera
sacral: pelvic viscera
less convergence

postganglionic neurons:

unmyelinated
most cell body in organ, short axon
also some in descrete cranial ganglia

Neurotransmitters

acetylcholine (ACh)

used at all junctions between pre- and postganglionic neurons
synapses at parasympathetic post-ganglionic and their effectors
also at neuromuscular junction
sympathetic synapse in sweat glands
synthesized from AcCoA and choline
stored in synaptic vesicles
receptors are cholinergic
inactivation with acetylcholine esterase

norepinephrine (noradrenaline)

synapse at sympathetic post-ganglionic and their effectors
chromaffin cells release epinephrine and norepinephrine
subset of catacholamines
synth from tyrosine
stored in vesicles
adrenergic receptors
inactivation: diffusion, re-uptake

Receptors

cholinergic – two types:

(1) nicotinic receptors (ligand-gated channels)

in all post-ganglionic neurons symp and para
also in neuromuscular junction of skeletal
types in muscle and ganglia differ in what they are stimulated by (products of different genes)
nicotine stimulates in low doses, inhibit in high
curare inhibits all; acetylcholine stimulates all

(2) muscarinic (G-protein coupled receptor)

in all effector cells of parasympathetic
also effectors of sympathetic that are cholinergic (i.e. sweat glands)
stimulated by muscarine; inhibit by atropine

adrenergic – in effector cells of most postganglionic sympathetic neurons; all are G-protein coupled receptors

several subtypes:

a₁: in smooth muscle and glands; activation usually excitatory

a₂: on nerve terminals (presynaptic receptors; inhibit release of norepiphrine (feedback inhibition); also found on cholinergic terminals

b₁: primarily heart; activation increases force and rate of heart

b₂: primarily in smooth muscle and glands; inhibit in smooth muscle

phenoxybenzamine blocks both a, propranolol blocks both b

Organ Responses

sympathetic has excitatory effects on cardiovascular: increase HR; vasoconstriction; increase BP (fight or flight); inhibitory effect on smooth muscle outside cardiovascular (dilation)

parasympathetic inhibits cardiovascular excitation, smooth muscle and glands outside CV (rest and digest)

eye

sympathetic:

radial muscle contracts (a₁ stim) Þ pupil dilation

ciliary muscle relaxes (b₂ stim) Þ less concave lens: far focus

elevation of eyelid

parasympathetic:

sphincter muscle contracts Þ constriction

ciliary muscle contracts Þ lens more concave: near focus

Horner’s syndrome: interupt sympathetic innervation of one side of head

constricted pupil, ptosis (eyelid droop), no face sweat, vasodilation of face

arterioles and veins

activate a₁: constriction

activate b₂: dilation

overall effect usually dilation

parasympathetic: no effect, can be activated with drugs