Autonomic nervous system

An introductory lecture to the autonomic nervous system.

1. Autonomic Nervous System
Csilla Egri, KIN 306 Spring 2012
Pain, fright, and being a mom; all ways of activating
the sympathetic nervous system

2. Outline
Function of the autonomic nervous system
(ANS)
Organization of the ANS
Major neurotransmitters
Classical
Nonclassical
Central control of ANS function
2

3. Function of the ANS
3
Autonomic Nervous System
Parasympathetic: Rest and
digest
Sympathetic: Flight or fight
• vasodilates vessels to the GI
tracts
• constricts the pupil
• constrict bronchioles
• stimulates salivary gland
secretions and peristalsis
• sexual arousal
• vasocontricts vessels to GI tract and
skin
• vasodilates vessels to skeletal
muscle and lungs
• dilates bronchioles
• increases heart rate and contractility
• dilates pupils
• inhibits peristalsis
• constricts intestinal sphincters and
urinary sphincter*
*so why can you pee your pants if you’re really scared?

4. Organization of the ANS
4
ANS efferents all originate in the CNS and
innervate tissues via a two-synapse pathway
Preganglionic neurons
columns of cells in the brain stem and spinal cord that
synapse on:
Postganglionic neurons
In peripheral ganglia interposed between CNS and
target cells
Check: What are
ganglia?

5. Organization
of the ANS
5
SNS preganglionic
neurons
Spinal cord
between T1-L3 in
the _________ horn
Exit thru ventral
roots
Synapse on many
paravertebral or
prevertebral ganglia
located far from
target organs
B&B Figure 15-4

6. Organization
of the ANS
6
PNS preganglionic
neurons
Nuclei of medulla,
pons, midbrain and
segments S2-S4 of
spinal cord
Synapse on many
distant terminal
ganglia close to
target organ
B&B Figure 15-4

7. ANS afferent division
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All visceral organs densely innervated by
afferent neurons
Monitor nociceptive input and mechanical and
physiological stimuli such as diameter of vessels,
blood gas concentrations, pH, glucose, and
temperature
Stimuli often don’t reach consciousness, instead
initiate ANS reflex arcs

8. Classical neurotransmitters of the
ANS
8
Both branches of the
ANS release Ach from
preganglionic neurons
that act on N
2
nicotinic
receptors
B&B Figure 15-8
Check: To what family of
receptors do N
2

receptors belong?

9. Classical neurotransmitters of the
ANS
9
NTs released by
postganglionic
neurons of each
branch differ
B&B Figure 15-8
Check: To what family of
receptors do muscarinic
ACh receptors and
adrenergic receptors
belong?

10. Classical neurotransmitters of the
ANS
10
Postganglionic neurons
PNS
• release ACh onto
muscarinic receptors
• results in stimulation or
inhibition of target cell,
depending on
downstream signalling
cascade
• response is slow and
prolonged
SNS
• most release
norepinephrine (NE) that
act on α or β adrenergic
receptors*
• neuroendocrine branch
innervating the adrenal
medulla has
postganglionic chromaffin
cells releasing epinephrine
(Epi) into the bloodstream
* Except for innervation of sweat glands, where SNS postganglionic neurons release
ACh that acts on muscarinic receptors

11. Adrenergic receptors
Agonists Typical target cell response
α
1
NE ≥ Epi Ciliary muscles of iris: pupil dilation
Blood vessels: vasoconstriction
α
2
NE ≥ Epi Can be located presynaptically as well, modulate NT release
β
1
Epi > NE Heart: increased heart rate and contraction
β
2
Epi > NE Bronchioles: bronchodilation
Skeletal muscle vasculature: vasodilation
β
3
Epi > NE Fat cells: lipolysis
Classical neurotransmitters of the
ANS
11
Acetylcholine receptors
Agonists Typical target cell response
N
2
ACh, nicotineTrigger EPSP on postganglionic neuron
M
1
-M
5
ACh,
muscarine
Varied, can be excitatory or inhibitory, heterogeneous
distribution
*N
1
receptors are not part of the ANS, they are part of the somatic nervous system and
initiate action potentials at the neuromuscular junction

12. Nonclassical neurotransmitters of
the ANS
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Some postganglionic SNS/PNS neurons
contain muscarinic receptors for Ach
Results in varied response in target cell to ACh
ANS can release numerous “nonclassical”
neurotransmitters
Colocalized in ANS axon terminals and can
participate in cotransmission with cholinergic or
adrenergic neurotransmitters
Examples: glutamate, glycine, substance P,
serotonin, neuropeptide Y, ATP, NO

13. Central control of ANS function
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ANS activity controlled by reflex pathways in spinal cord and
brainstem as well as descending controls originating from
higher autonomic brain regions
Reflex examples:
Pupillary light reflex
Light excites retinal cells which send connections to
PNS ganglia in the brainstem  constriction of pupil
size
Micturition reflex (overcome by potty training…
hopefully)
Distension of bladder sensed by mechanoreceptors
which send connections to pontine micturation centre
(PMC) in the pons  SNS input to bladder
decreases, PNS input increases  voiding of
bladder

14. Central control of ANS function
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Autonomic brain regions: Hypothalamus
Temperature regulation
Hypothalamic thermoreceptors have connections to SNS
Cold temp.  increased SNS activity  shivering,
increased metabolism, cutaneous vasoconstriction
Regulation of food intake
Hypothalamic glucoreceptors
Blood glucose falls  release of opioid peptides and
pancreatic peptides (enteric nervous system connections)
stimulate food intake
Emotional behaviour
Limbic system sends connections to hypothalamus and other
brain regions (ex. PMC)
Influence dietary behaviour
Can pee your pants when your scared!

15. Objectives
After this lecture you should be able to:
List the general functions of the ANS
Describe ANS neuronal connections
Compare and contrast SNS and PNS neurotransmitters
and receptors
Describe the role of central control of ANS function
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16. 16
1.What would happen if you administered a systemic
nicotinic receptor antagonist? What about a systemic
adrenergic antagonist?
2.Atropine, when applied as eye drops, mimics the effect
of the sympathetic nervous system by blocking
receptors of the parasympathetic branch. What is the
effect on the eye and what specific receptors is
atropine an antagonist?
Test your knowledge