Lecture 16 - ANS: The Autonomic Nervous System

Question 1

Three divisions of the autonomic nervous system

Answer 1

Sympathetic autonomic pathways
Enteric nervous system
Parasympathetic autonomic pathways

Question 2

Sympathetic autonomic pathways

Answer 2

Anatomically the output of this system is from thoracic to lumbar (nothing in the sacral region and nothing at the top)
Modified sympathetic ganglion at adrenal medulla such that the postsynaptic cells that would ordinarily would release more adrenaline on to the target end order, the adrenal medulla is a modified collection of those post ganglionic sympathetic cells that release a modified form of noradrenaline called adrenaline into the circulation which has many sympathetic effects
Some ganglia are not associated with the spinal cord (coeliac ganglion, superior mesenteric ganglion, interior mesenteric ganglion)
Inervation of blood vessels, swear glands and skin of trunk
Chain of ganglia that run parallel to the spinal cord

Question 3

Ganglion associated with sympathetic autonomic pathways

Answer 3

superior cervical ganglion
stellate ganglion
preganglionic sympathetic neurons (thoraco-lumbar)
Coeliac ganglion
superior mesentric ganglion
inferior mesenteric ganglion
ganglia on abdominal arteries (prevertebral ganglia)
Sympathetic trunk (paravertebral ganglia)

Question 4

SKM blood vessels and sympathetic

Answer 4

Sympathetic innervation of skeletal muscle blood vessels in dogs and cats, but not in humans

Question 5

Enteric nervous system

Answer 5

Many neurons in the gut

Question 6

Parasympathetic autonomic pathways

Answer 6

Outflow at higher regions of the spinal cord i.e. the cervical resigns
Major output is the tenth cranial nerve called the vagus never which is a large fibre with multiple branches, has efferent fibres going to end organs and also has afferent fibres going into he system, innervates the bod from top to bottom
There can be dual innervation by both the sympathetic and parasympathetic autonomic pathways - generally work. In opposite ways in different situations

Question 7

Ganglion associated with parasympathetic autonomic pathways

Answer 7

preganglion parasympathetic neurons (sacral)
otic ganglion
sphenopalatine ganglion
ciliary ganglion
preganglionic parasympathetic neurons (cranial)

Question 8

Vagus nerve is part of

Answer 8

parasympathetic autonomic pathways

Question 9

Many organs have what …

Answer 9

Many organs have dual innervation by sympathetic and parasympathetic NS, which act in opposite directions (e.g. ↑ or ↓ heart rate).

Question 10

Visceral afferents are in …

Answer 10

Visceral afferents are in the vagus nerve, or enter CNs through dorsal roots.

Question 11

In gut….

Answer 11

In gut, the sympathetic, parasympathetic and enteric nervous systems act together for fine control of gastrointestinal functions.

Question 12

Parasympathetic function

Answer 12

Parasympathetic = rest and digest (and propagate), ‘faint or freeze’ (some animals play bad, depends on the wiring of the animal’s brain)
Responsible for reproductive functions as well

Question 13

Sympathetic function

Answer 13

Sympathetic = fight or flight

Question 14

The running of sympathetic and parasympathetic systems

Answer 14

Both systems are always active, not just in emergencies or after a big meal
These systems are running together rather than in competition with each other

Question 15

ANS - sympathetic features

Answer 15

Widely distributed, to salivary glands, eye, skin, viscera, muscles.
First synapse in paravertebral (sometimes called sympathetic chain) and prevertebral ganglia, post- ganglionic fibres innervate organs.
Except adrenal medulla - supplied by preganglionic sympathetic Ach fibres direct from CNS, releases NE, E. (Releases neurotransmitter right into the body fluid rather than on to an end organ)
Another exception are the sweat glands which have ACh instead of NE
Sympathetic = short preganglionic in CNS, long post ganglionic in PNS, uses ACh and then uses NE on the effector muscle

Question 16

ANS - parasympathetic features

Answer 16

Distributed less widely, to viscera (mainly via vagus nerve), salivary glands, eye, but not to skin or muscles.
First synapse is in or near the target organ.
Different outflow to sympathetic - cervical and sacral region
ACh acts on muscarinic AChR receptors which are different to nicotinic receptors which open and cause a depolarisation of the membrane (inotropic), here they are metabotrophic and are coupled biochemically to the functions of the end organ

Question 17

Sweat glands and sympathetic innervation

Answer 17

Normal sympathetic is NE

Except ACh is used at sweat glands (sudomotor fibres), vasodilator fibres

sympathetic vasodilator innervation of skeletal muscle blood vessels in dogs and cats but not humans

Question 18

Adrenal medulla supplied by

Answer 18

by sympathetic ACh fibres, releases NE, E into circulation

Question 19

Sympathetic neurons

Answer 19

Short preganglionic fibres
Long post ganglionic fibres
Myelinated first, second unmyelinated
ACh first, NE second

Question 20

Parasympathetic neurons

Answer 20

Long preganglionic fibres
Short postganglionic fibres
Mylinated first, second unmyelinated
ACh for both

Question 21

Major target in gut is

Answer 21

enteric nervous system

Question 22

Parasympathetic, somatic and sympathetic neurons affected by …

Answer 22

Note = cigarette chemicals bind to cholinergic nicotinic receptors found in ganglia of both branches of the autonomic nervous system
Poisonous mushrooms … effect muscarinic receptors, poison, first symptom would be uncontrollable sweating (affecting sweat glands)

Question 23

ENS - the multi-layered ‘brain’ in the gut

Answer 23

Contains about as many neurons as the spinal cord ! (108 sensory neurons, motor neurons, interneurons), throughout the GI tract.
Multiple neurotransmitters and receptors.

Responds to local chemical and mechanical stimulation.
Within the gut wall there are receptors that respond by action potential generation to distension of the wall and also to the presence of food in the gut

Regulates motility, secretion, and endocrine signaling, through local reflexes.
Its own homeostatic mechanisms but coordinated by inputs of parasympathetic and sympathetic nervous systems
Activity is autonomous, but is influenced by parasympathetic and sympathetic inputs, that indirectly increase and decrease intestinal activity respectively.

The ENS controls motility, by influencing smooth muscle excitability
ACh released by ENS motor neurons excites smooth muscle (through muscarinic AChRs), VIP inhibits smooth muscle contraction
Controlling overall membrane potential it is possible to control the force of these contractions and the number of contractions
VIP has the opposite effect to muscarine
At first minimal calcium entry and then there is sufficient calcium entry and sufficient depolarisation to drive action potential generation

Question 24

AChRs and muscarinic receptors

Answer 24

All somatic and parasympathetic and sympathetic have nicotinic AChRs EXCEPT the postganglionic neuron that released ACH in the parasymapthetic division is a muscarinic AChRs

Question 25

ENS responds to

Answer 25

Responds to local chemical and mechanical stimulation.
Within the gut wall there are receptors that respond by action potential generation to distension of the wall and also to the presence of food in the gut

Question 26

ENS regulates ..

Answer 26

Regulates motility, secretion, and endocrine signaling, through local reflexes.
Its own homeostatic mechanisms but coordinated by inputs of parasympathetic and sympathetic nervous systems

The ENS controls motility, by influencing smooth muscle excitability
ACh released by ENS motor neurons excites smooth muscle (through muscarinic AChRs), VIP inhibits smooth muscle contraction
Controlling overall membrane potential it is possible to control the force of these contractions and the number of contractions
VIP has the opposite effect to muscarine
At first minimal calcium entry and then there is sufficient calcium entry and sufficient depolarisation to drive action potential generation

Question 27

Local ENS reflexes …

Answer 27

propel food etc along the gut

Question 28

Local ENS reflexes propel food etc along the gut

Answer 28

Contraction above (oral), relaxation below (anal). (Normally !)
Occurs in response to sensory neurons
This system can occur in reverse which is what happens when you vomit
ATP acts as a neurotransmitter as well and makes up feel sleepy

What might be a side effect of SSRI treatment for depression?
Excessive 5HT may contribute to GI disease, e.g. the unpleasant sensations and abnormal gut behaviour of Irritable Bowel Syndrome.

Question 29

muscarinic receptors

Answer 29

Muscarinic receptors are G-coupled protein receptors involved in the parasympathetic nervous system. The only exception to these receptors is the sweat glands, which possess muscarinic receptors but are part of the sympathetic nervous system.