Autonomic Nervous System

Question 1

What is the ANS?

Answer 1

Motor neurons controlling the physiology of organs (largely unconscious)
Subgroup of the motor afferents in the PNS
consists of the sympathetic (fight or flight), parasympathetic (rest and digest) and the enteric systems.

Question 2

How is the Sympathetic nervous system (SNS) organised?

Answer 2

segmentally: has segmental chain ganglia (paravertebral sympathetic chain)
Segmented according to the thoracic and lumbar aspects in the organisation in the body
No axons from brain of cervical and sacral spinal cord
Three midline ganglia (prevertebral as in front of vertebrae) supplying the gut
Celiac ganglia
Superior mesenteric ganglia
Inferior mesenteric ganglia
Preganglionic neurons have cell bodies in the spinal cord and send out axons to the ganglia
Short preganglionic and long postganglionic nerves
(see diagram)

Question 3

How is the Parasympathetic nervous system (PSNS) organised?

Answer 3

Output in cranial nerves and sacral spinal cord
No segmental ganglia
Ganglia close to target organs
Long preganglionic and short postganglionic nerves

Question 4

What is the main neurotransmitter in the SNS?

Answer 4

noradrenaline (NA)

although input from preganglion neuron is cholinergic

Question 5

What is the main neurotransmitter in the PSNS?

Answer 5

acetylcholine (ACh)

Question 6

what are autonomic nerve junctions?

Answer 6

autonomic nerves do not end in synaptic junctions (as in the somatic NMJ)
where the axons enter a tissue they contain swellings called varicosities
which form a series of enlargements where neurotransmitter is released along the axon and over a large area of the effector tissue

Question 7

What else is released at the synapse?

Answer 7

neuromodulators are released in addition to neurotransmitter

Sympathetic:
noradrenaline (NA)
ATP
Neuropeptide Y (NPY)

Parasympathetic
ACh
Vasoactive Intestinal Polypeptide (VIP)
(ATP and Nitrous Oxide)

Enteric inhibitory
ATP
NO
VIP

Enteric excitatory
ACh
Substance P

Question 8

What postganglionic sympathetic neuron is an exception to the rule of expressing NA?

Answer 8

Sweat glands release ACh

late phenotypic switch in embryonic development from NA to ACh

Question 9

What are the roles of the sympathetic nerves

Answer 9

Piloerection
Peripheral vasoconstriction
Lipolysis
Fight or flight

Question 10

What is the paravertebral chain of sympathetic ganglia?

Answer 10

Organised in a rostral to caudal fashion
Superior cervical ganglia (most rostral) innervates head
Middle cervical and stellate ganglia to heart and lungs
Thoracic chain ganglia innervate thoracic tissue, hair cells and vasculature
Prevertebral ganglia to abdominal and pelvic organs

Question 11

How are the sympathetic ganglia innervated

Answer 11

Somatosensory system emerge from the ventral horn
Meet sensory counter parts emerging from the dorsal horn
Soma of preganglionic neurons localised in the lateral horn
‘Hitchhike’ on to motor neurons exiting the spinal cord
Enter the sympathetic ganglia via the white ramus (preganglionic neurons are myelinated)
Synapses onto sympathetic postganglionic neuron
Which exits the ganglia via the grey ramus (ramus = branch)
Some preganglionic nerves terminate in the ganglia.
Some pass along the chain before terminating
Some pass to pre-vertebral/midline ganglia via the splanchnic nerves
The main pre -vertebral ganglia innervate the gut and are supplied by the greater, lesser, and least splanchnic nerves
Greater: The Celiac ganglion supplies the foregut
Lesser: The Superior mesenteric ganglion supplies the midgut
Least: The Inferior mesenteric ganglion supplies the pelvic organs

Question 12

What are the parasympathetic outputs?

Answer 12

Parasympathetic preganglionic supply emerges with cranial nerves:
III Oculomotor - the eye - iris
VII Facial - tear and salivary glands
IX Glossopharyngeal - parotid salivary gland
X Vagus - heart, lungs and viscera

Question 13

How do the SNS and PSNS control the pupil reflex?

Answer 13

Stimulation of the oculomotor parasympathetic nerves stimulates post-ganglion fibres from the ciliary ganglion that innervate the iris constrictor muscles
Oculomotor nerve (pink) splits into two branches
Inferior branch converges onto the ciliary ganglion which contain parasympathetic nerves that innervate the iris
Parasympathetic input to the ciliary ganglion constricts the pupil decreasing light passage to the retina
Opiates Cause Parasympathetic Stimulation of the eye (ciliary ganglion): making Pinpoint pupils
Sympathetic stimulation causes dilation of the pupil increasing light passage to the retina
Sympathomimetic drugs such as Amphetamines dilate pupils (cycloplegic effect)

Question 14

What happens if the sympathetic superior cervical ganglion is lesioned?

Answer 14

Horner’s syndrome
Pupil constricted
Eyelid droops (Ptosis)
Dry flushed facial skin
Caused by damage to axons in sympathetic tract somewhere between the ganglion and the target organ
Carotid artery lesion can lead to Horner due to anatomical relation due to sympathetic nerves stemming from the superior cervical ganglia and target organs

Question 15

What does the VII facial nerve do?

Answer 15

Controls lacrimation independently of why its happening
Parasympathetic innervation of the Pterygopalatine ganglion regulates secretions of the lacrimal gland and nasal mucosa,
Facial nerve also synapses into a different ganglia:
Parasympathetic innervation of the Submandibular ganglion regulates secretions submandibular & sublingual salivary glands (not the parotid gland)

Question 16

What does the IX Glossopharyngeal nerve do?

Answer 16

to otic ganglion for parotid gland – controls salivary secretion
Parasympathetic stimulation of the otic ganglion leads to increased secretion of saliva from the parotid glands

Question 17

how can cardiovascular changes happen?

Answer 17

Cardiovascular effects are driven by CNS control centres
In turn project down from brain to spinal cord, down to lateral horn where preganglionic neurons innervate the stellate ganglia which in turn innervate the heart
Increased input causes the heart rate increases, force of contraction increases, conduction increases

Question 18

How does sympathetic activation mediate respiratory changes?

Answer 18

Main effect of sympathetic activation on the lungs is bronchodilation which leads to increased respiratory capacity

Question 19

How does the fight or flight response function?

Answer 19

Simulation of the adrenal medulla by the greater splanchnic nerve
This nerve receives descending inputs from the brain down the spinal cord
Upon that signal there is a massive systemic release of noradrenaline and adrenaline

Question 20

What diseases phenocopy the fight or flight response and why?

Answer 20

Adrenal medulla tumours
paragangliomas
massive increase adrenal medulla which leads to increase of catecholamines in circulation
Chromaffin cells and SNS neurons have a common Embryonic origin
Release same neurotransmitters

Question 21

What effect does sympathetic stimulation have on the body?

Answer 21

Pupils - dilate
Hear rate - increases
Blood vessels - peripheral vasoconstriction and muscle vasodilation
kidney - renin secretion
brochi - increased bronchodilation
liver - glycogenolysis and gluconeogenesis
adipocytes - lypolysis
salivary galnds - increased mucin so increased viscosity of saliva
GI tract - contraction of sphincters

Question 22

What does the vagus nerve X do ?

Answer 22

Parasympathetic supply to thorax and abdomen

control gut reflexes & motility

Question 23

baroreflex

Answer 23

sensory nerves that run along the vagus control blood pressure
Although this homeostatic baroreflex has been described for more than 80 years, the molecular identity of baroreceptor mechanosensitivity is only 2-years old.
Sensory neurons sitting in the petrosal ganglia express Piezo 2 - mechanosensory channels expressed in sensory neurons
Responsible for control of blood pressure and heart rate - stimulation leads to sharp decrease
Mechanosensitivity relies on sensory neurons that project to the vasculature
Piezo2- expressing neurons in pretrosal & nodose ganglia that sense stretch in the aortic arch and carotid sinus And control BP & HR

Question 24

micturition reflex

Answer 24

Sensory nerves sense bladder stretch and send signals to CNS → SNS
Stimulation (Beta3ergic receptor) of the smooth (detrusor) muscle of the bladder wall, resulting in contraction.
PSNS = retention (opposite)
Voluntary relaxation of sphincter muscles via pudendal

Question 25

male genital ‘refex’

Answer 25

Requires cognition
Afferent via spinal and autonomic nerves
PSNS stimulation onto helicine arteries → dilation = Erection
SNS stimulation leads to emission of seminal fluid and closure of bladder sphincter = ejaculation
Pudendal (spinal) control over penile muscles causes ejaculation

Question 26

defaction reflex

Answer 26

Normally, the anal sphincters are closed by sympathetic tonic stimulation.
Detection of a full sigmoid rectum via visceral sensory afferents
PNS stimulation of bowel smooth muscle, inhibition of tonic sympathetic control of sphincters, voluntary (spinal) opening of sphincters
why/how?
Parasympathetic nerve terminals (varicosities) from the vagus nerve innervating ganglion cells in the myenteric plexus of the gut wall

Question 27

What is the enteric nervous system

Answer 27

Intrinsic nerve networks (plexi) in the intestinal wall
Submucosal plexus
Myenteric plexus

Question 28

Peristalisis

Answer 28

Food in gut causes release of serotonin by EC cell
Stimulates submucosal plexus which synapses on myenteric plexus
Stimulates myenteric plexus leads to bowl movement
Optogenetic stimulation of calretinin expressing myenteric neurons promotes peristalsis