Autonomic Nervous System

Question 1

Concisely describe the autonomic nervous system?

Answer 1

The autonomic nervous system (ANS) consists of afferent and efferent pathways in the central and peripheral nervous systems.

The sensory afferents transmit sensory information from visceral structures, and chemical and mechanical data from chemoreceptors and pressure receptors.

The efferent pathway contributes to the control of cardiac muscle, smooth muscle and glands.

It is split into 2 main divisions the sympathetic and parasympathetic NS.

Question 2

Describe the efferent pathway in terms of neurones?

Answer 2

The efferent pathway consists of 2 neurones the pre and post ganglionic neurones, which synapse in an intermediary ganglion.

The cell bodies of the preganglionic neurones are in the brain or spinal cord. Their axons are myelinated slow conducting B fibres.

Each preganglionic cell anastomoses with 8-9 postganglionic cells to produce the diffuse responses characteristic of the ANS.

The axons of the postganglionic neurones are mainly unmyelinated C fibres.

Question 3

Where does the sympathetic efferent pathway originate from?

Answer 3

Thoracolumbar region

The preganglionic efferent neurones originate in the spinal column from T1-L2.

The preganglionic cell bodies of the neurones are found in the lateral horns of the spinal cord.

Question 4

In the sympathetic efferent pathway where do the preganglionic neurones synapse?

Answer 4

Synapse in one of 3 ways:

1. Most synapse in the paravertebral sympathetic chain: a series of sympathetic ganglia found alongside the vertebral column from the base of the skull to the coccyx.
2. Some pass through the sympathetic chain, via the splanchnic nerves, to synapse in a peripheral autonomic ganglion in the abdomen or pelvis, e.g. the coeliac ganglion, superior mesenteric ganglion or inferior mesenteric ganglia
3. A few preganglionic fibres synapse directly with chromaffin cells in the medulla of the adrenal gland

Question 5

Where does the parasympathetic efferent pathway originate from?

Answer 5

It is said to have a craniosacral outflow, as the parasympathetic efferents originate from cranial nn III, VII, IX and X, as well as the spinal roots S2-S4.

Question 6

In the parasympathetic efferent pathway where do the preganglionic neurones synapse?

Answer 6

Cranial nerves III, VII and IX that supply the head and neck synapse in four discrete intermediary ganglia: the ciliary, sphenopalatine, submaxillary or otic ganglion.

Efferent neurones from the vagus and sacral roots synapse at ganglia located in the visceral walls of the viscera that they are supplying.

Question 7

How are the head, neck and pelvic regions supplied by the sympathetic NS?

Answer 7

There is no direct sympathetic outflow to the head, neck or pelvis.

Neurones travel from the thorco-lumbar sympathetic chain to cervical or sacral ganglia.

Question 8

What is the stellate ganglion?

Answer 8

The inferior cervical ganglion is often fused with the first thoracic ganglion to form the stellate ganglion at the level of C6, just in front of the neck of the first rib.

Question 9

Pain fibres run with the parasympathetic/synpathetic nn. Clinically why is this relevant?

Answer 9

Blocking the sympathetic pathways can be used therapeutically to treat sympathetic overactivity, sympathetically mediated pain, or visceral pain.

Question 10

What is the clinical use of a thoracic symapthetctomy?

Answer 10

Thoracic sympathectomy is used for the treatment of severe Reynaud’s phenomenon or hyperhidrosis (sweaty palms).

Question 11

What is the clinical use of a lumbar symapthetctomy?

Answer 11

Lumbar sympathectomy is used for circulatory insufficiency of the lower limb or phantom limb pain.

Question 12

What is the clinical use of a coeliac plexus block>

Answer 12

Coeliac plexus block is used for the pain associated with upper GI malignancy or acute or chronic pancreatitis.

Question 13

What are the 2 main neurotransmitters in the Autonomic NS?

Answer 13

Acetylcholine and Noradrenaline

Question 14

From which neurones are ACh secreted?

Answer 14

All preganglionic neurones

All postganglionic parasympathic neurones

Postganglionic sympathetic neurones that innervate sweat glands, skeletal muscle and blood vessels

Question 15

From which neurones are Noradrenaline secreted?

Answer 15

All postganglionic sympathetic neurones, except those that supply: sweat glands, skeletal muscle and blood vessels.

Question 16

What are the 2 types of acetylcholine receptors, where are they found and what class of membrane receptor do they belong?

Answer 16

Muscarinic receptors (M1-M5)
Found in smooth muscle, glands, heart and brain.
G-protein linked membrane receptors.
Actions mimicked by muscarine and blocked by atropine.

Nicotinic receptors.
Found in sympathetic ganglia and in the motor end plates of skeletal muscle.
Ligand gated ion channels.
Actions mimicked by nicotine and unaffected by atropine.

Question 17

What type of drug is neostigmine?

Answer 17

It is an anticholinesterase, it stops the break down of ACh which increases levels and therefore competes with the muscle relaxants at the motor end plate.

Explains why cannot reverse with neostigmine immediately as there will be too much relaxant bound for the ACh to effectively compete.

Question 18

How are adrenergic receptors categorised and what is the clinical significance?

Answer 18

There are 2 main types of adrenergic receptors alpha and beta.

These are further subdivided into alpha 1 and 2 and beta 1 and 2.

All the receptors are G protein linked.

α1, β1 and β2 are all post synaptic

α2 can be pre or post synaptic.

They are categorised as 1 or 2 based on their secondary messengers and effect site.

Noradrenaline has a greater affinity for α receptors and adrenaline has a greater affinity for β receptors (but also has α activity).

Question 19

Describe the effects of the parasympathetic NS on the cardiovascular system and which receptors are responsible?

Answer 19

Heart:
SA node: B1 decrease HR
Ventricles B1 parasympathetic decrease contractility.

Arterioles:
Coronary:

Question 19

Describe the effects of the parasympathetic NS on the cardiovascular system and which receptors are responsible?

Answer 19

Heart:
SA node B1 decrease HR
Ventricles B1 parasympathetic decrease contractility.

Arterioles:
Coronary α1 α2 dilation
Skeletal α1 dilation

Question 20

Describe the effects of the sympathetic NS on the cardiovascular system and which receptors are responsible?

Answer 20

Heart:
SA node B1 increase HR
Ventricles B1 increase contractility

Arterioles:
Coronary α1 α2 constriction B2 dilation
Skeletal α1 constriction B2 dilation
Skin α1 constriction

Veins:
α1 α2 constriction
B2 dilation

Think of Noradrenaline, it mainly acts as a alpha agonist, and its used as a potent vasoconstrictor.

Question 21

Describe the effects of the parasympathetic NS on the lungs and GI tract and which receptors are responsible?

Answer 21

Lungs:
Bronchi: B2 constriction
Glands α1 stimulation

GI tract:
Motility: α1 α2 B2 increased
Sphincters α1 B2 relaxation

Question 22

Describe the effects of the sympathetic NS on the lungs, GI tract, skin and which receptors are responsible?

Answer 22

Lungs:
Bronchi: B2 dilation
Glands: α1 inhibition B1 stimulation

GI tract:
Motility: α1 α2 B2 decreased
Sphincters: α1 B2 constriction

Skin α1 piloerection, M sweating

Question 23

What are the effects of the autonomic nervous system on the eye?

Answer 23

Parasympathetic NS supplies the sphincter of iris which causes pupillary constriction (miosis)

Sympathetic NS supplies radial muscle of the iris which produces pupillary dilation (mydriasis)

Question 24

How do atropine and adrenaline effect the pupil?

Answer 24

Both cause mydriasis (dilation).
Atropine does this as it is an antimuscarinic (aka antagonises the parasympathetic NS)

Adrenaline does this by causing stimulation of the sympathetic NS.

Question 25

Describe the mechanism of action of: ephidrine?

Answer 25

Combined alpha and beta agonist (weak) and stimulates release of NA.

Question 26

Describe the mechanism of action of: isoprenaline?

Answer 26

Beta agonist

Question 27

Describe the mechanism of action of: Phenylephirine?

Answer 27

Alpha 1 agonist

Question 28

Describe the mechanism of action of: Labetalol, metaprolol and propranolol?

Answer 28

Labetalol: combined alpha and beta blocker

Metaprolol: selective B2 blocker

Propanolol: non selective B blocker

Question 29

Describe the mechanism of action of: clonidine?

Answer 29

Alpha 2 agonist.

Question 30

Describe the mechanism of action of: phentolamine?

Answer 30

Alpha blocker.

Can be used to treat erectile dysfunction (sympathetic NS causes veins to constrict via alpha receptors, therefore blocking this will cause dilation)

Question 30

Describe the mechanism of action of: phentolamine?

Answer 30

Alpha blocker.

Can be used to treat erectile dysfunction (sympathetic NS causes veins to constrict via alpha receptors, therefore blocking this will cause dilation)

Question 31

What are the side effects of a high spinal block and why?

Answer 31

Normal spinal:
Blood pressure falls due to associated sympathetic block that produces widespread vasodilation (T4-L2).

High spinal:
More profound hypotension due to greater sympathetic block and bradycardia from loss of sympathetic supply to the heart (T1-4).