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THE AUTONOMIC
NERVOUS SYSTEM
LECTURE 12
CH 9
Neural Control of Involuntary Effectors
• The autonomic nervous system helps regulate cardiac,
smooth muscle and glands
• Impulses are conducted from the CNS to a second
autonomic neuron. And then the second neuron
innervates the involuntary effector.
Visceral Effector Organs
The autonomic nervous system regulates all the organs.
Common features of organs regulated autonomically:
1) a built-in muscle tone – they maintain a resting ‘tone’
2) denervation hypersensitivity – they may become more
sensitive to regulation when nerves are damaged
3) They may contract without autonomic input
4) The autonomic system is like an accelerator or brake
Divisions of the Autonomic Nervous System
Sympathetic
- thoracolumbar division – preganglionic fibers exit spinal cord
at spinal nerves T1-L2
- synapse with paravertebral ganglia (sympathetic chain)
Fig. 9.2 The Sympathetic Chain of Paravertebral Ganglia
Sympathetic Neuron Pathways
Sympathetic Division
Divergence – preganglionic fibers branch out to
postganglionic fibers at different levels of the chain
Convergence – a postganglionic neuron can receive info from
many preganglionic
nerves.
Mass activation – all postganglionic sympathetic neurons
can be activated simultaneously for fight or flight
Sympathetic Collateral (prevertebral) Ganglia
These are places
where preganglionic
neurons
synapse
if they do not
synapse in the
paravertebral
chain; they form
splanchnic nerves
E.G. celiac, superior, mesenteric, inferior mesenteric ganglia
which innervate digestive, urinary, reproductive
Note: dual innervation of organs
by both systems
Adrenal Glands
• Two functionally different glands: medulla and
cortex
• Medulla – originates from neural crest; innervated
by preganglionic sympathetic fibers which trigger
the secretion of epinephrine into the blood
• Cortex – secretes steroid hormones
• Sympathoadrenal system –
stimulation of adrenal
medulla by mass activation of
sympathetic nervous system
Question
What happens to blood vessels of the digestive
tract during sympathetic activation?
What happens to skeletal muscles?
Lungs?
Parasympathetic Division
Craniosacral Division
Preganglionic fibers originate
in the brain (midbrain, pons,
medulla) and in sacrum; they
extend to terminal ganglia,
which are inside the organs
they stimulate.
Terminal ganglia supply
postganglionic fibers to
synapse with effector cells.
Functions of the Autonomic Nervous System
Sympathetic:
Fight or flight
Epinephrine/norepi
released from
postganglionic neurons
Can be regulated as a
“mass system” or finely
tuned
Purpose: increase ATP
Parasympathetic
Rest and digest
Acetylcholine released
from postganglionic
neurons
Not activated as a whole
Opposite of sympathetic
Adrenergic and Cholinergic Synaptic Transmission
Sympathetic and
parasympathetic –
preganglionic neurons
release Acetylcholine
Sympathetic
postganglionic neurons
release epi [adrenergic]
Parasympathetic
postganglionic neurons
release Ach [cholinergic]
Exception: some
sympathetic fibers to
skeletal muscle and sweat
glands release ACh
Response to Adrenergic Stimulation
Both excitatory and inhibitory effects can be produced in different
tissues by the same neurotransmitter because different tissues
have different receptors.
α(alpha) -adrenergic receptors stimulate a rise in cytoplasmic
Calcium
α1– causes vasoconstriction by increasing Ca2+
α2 – they are activated by norepi, but then cause a negative
feedback reduction in epi levels (p. 254)
clonidine (drug) - α2 receptors in the brain  lowers sympathoadrenal
system
β(beta) -adrenergic receptors stimulate the production of cAMP in
the target cell.
β1 – increase heart rate;
β2 – vasodilation; bronchodilation
Response to Cholinergic Stimulation
The effects of ACh depend on the nature of the ACh
receptor:
Nicotinic – located in CNS and neuromuscular junction
and in autonomic ganglia; always excitatory; always
cause an inflow of Na+
Muscarinic – located in visceral organs; excitatory or
inhibitory; G-protein coupled and can activate
different membrane enzymes. Subtypes of
muscarinic receptors exist
The effects of ACh depend on nature of the Ach receptor:
Nicotinic –always excitatory; always cause an inflow of Na+
Muscarinic –excitatory or inhibitory; Subtypes exist
Other Autonomic Neurotransmitters
Non-adrenergic, non-cholinergic
Nitric oxide – causes vasodilation of penis, causes
vasodilation of cerebral arteries (parasympathetic);
sometimes called a paracrine regulator
VIAGRA blocks the breakdown
of cyclic GMP
Antagonistic Effects
“Antagonistic” means the two systems produce
opposite effects.
Example: the heart – epi speeds it up/ACh slows it
Complementary and Cooperative Effects
Complementary – both systems produce similar effects
e.g. salivary glands –
Cooperative – systems produce different effects that work
together to promote a single action.
e.g. erection of the penis/clitoris – arousal is
parasympathetic/orgasm is sympathetic
e.g. urinary bladder -
Organs Without Dual Innervation
Some organs receive only sympathetic innervation:
1) Adrenal medulla
2) Arrector pili
3) Sweat glands
4) Blood vessels of skin
Regulation is achieved by increases or decreases in firing rate
of sympathetic fibers.
E.g. During exercise, increased sympathetic activity dilates
blood vessels in the muscles and stimulates sweat glands
(which secrete bradykinin to stimulate dilation of surface
blood vessels)
Control of the Autonomic Nervous System by Higher Brain Centers
Medulla oblongata
Hypothalamus
Limbic system (contains the cingulate gyrus, hypothalamus,
fornix, hippocampus, amygdala) – emotional control of
autonomic activation (fainting, blushing, racing heartbeat)
Cerebellum – control of motion sickness, nausea