Lecture 16 - ANS: The Autonomic Nervous System Flashcards
Three divisions of the autonomic nervous system
Sympathetic autonomic pathways
Enteric nervous system
Parasympathetic autonomic pathways
Sympathetic autonomic pathways
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
Ganglion associated with sympathetic autonomic pathways
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)
SKM blood vessels and sympathetic
Sympathetic innervation of skeletal muscle blood vessels in dogs and cats, but not in humans
Enteric nervous system
Many neurons in the gut
Parasympathetic autonomic pathways
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
Ganglion associated with parasympathetic autonomic pathways
preganglion parasympathetic neurons (sacral)
otic ganglion
sphenopalatine ganglion
ciliary ganglion
preganglionic parasympathetic neurons (cranial)
Vagus nerve is part of
parasympathetic autonomic pathways
Many organs have what …
Many organs have dual innervation by sympathetic and parasympathetic NS, which act in opposite directions (e.g. ↑ or ↓ heart rate).
Visceral afferents are in …
Visceral afferents are in the vagus nerve, or enter CNs through dorsal roots.
In gut….
In gut, the sympathetic, parasympathetic and enteric nervous systems act together for fine control of gastrointestinal functions.
Parasympathetic function
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
Sympathetic function
Sympathetic = fight or flight
The running of sympathetic and parasympathetic systems
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
ANS - sympathetic features
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
ANS - parasympathetic features
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
Sweat glands and sympathetic innervation
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
Adrenal medulla supplied by
by sympathetic ACh fibres, releases NE, E into circulation
Sympathetic neurons
Short preganglionic fibres
Long post ganglionic fibres
Myelinated first, second unmyelinated
ACh first, NE second
Parasympathetic neurons
Long preganglionic fibres
Short postganglionic fibres
Mylinated first, second unmyelinated
ACh for both
Major target in gut is
enteric nervous system
Parasympathetic, somatic and sympathetic neurons affected by …
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)
ENS - the multi-layered ‘brain’ in the gut
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
AChRs and muscarinic receptors
All somatic and parasympathetic and sympathetic have nicotinic AChRs EXCEPT the postganglionic neuron that released ACH in the parasymapthetic division is a muscarinic AChRs
ENS responds to
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
ENS regulates ..
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
Local ENS reflexes …
propel food etc along the gut
Local ENS reflexes propel food etc along the gut
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.
muscarinic receptors
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.
