Introduction to the Autonomic Nervous System

Question 1

What is the peripheral nervous system?

Answer 1

Efferent outputs from the CNS (i.e. any nerves coming out from the brain and spinal cord to the rest of the body rather than the nerves travelling within the brain and spinal cord) - spinal nerves and cranial nerves are peripheral nerves

Question 2

What are the the branches of the peripheral nervous system?

Answer 2

Autonomic nervous system

Somatic nervous system

Neuroensocrine system

Question 3

What is the neuroendocrine system?

Answer 3

The interaction between the nervous system and the endocrine system - mainly the hypothalamic-neurohypophyseal system (e.g. hypothalamic-pituitary-thyroid axis) as hypothalamic neurones have neurosecretions which hormones to regulate the secretions of other hormones

Question 4

Which limb of the autonomic nervous system has more of an effect on the liver?

Answer 4

Sympathetic – causes hepatic gluconeogenesis and glycogenolysis for to provide glucose and therefore energy for ‘fight or flight’ (not much effect of PNS on liver)

Question 5

Which limb of the autonomic nervous system dominates the lungs and the eyes at rest?

Answer 5

Parasympathetic

Lungs – causes partial bronchoconstriction

Eyes – causes partial pupil constriction

Partial constriction (i.e. partial smooth muscle contraction) allows a change in both directions when necessary

Question 6

Which branch of the ANS dominates the heart at rest?

Answer 6

Parasympathetic - to slow heart down from its intrinsic HR due to SAN alone which is 120bpm

Question 7

Which branch of the ANS are the arterioles (and blood vessels in general) innervated by?

Answer 7

ONLY sympathetic - vasodilation caused by reduced SNS firing (not PNS firing)

Question 8

Describe the relationship between baroreceptor firing and parasympathetic discharge.

Answer 8

Baroreceptors are stimulatory to the parasympathetic nerves i.e. an increase in baroreceptor firing stimulates (higher BP) an increase in parasympathetic firing and an increase in sympathetic inhibition

Question 9

Describe the differences between sympathetic responses and parasympathetic responses.

Answer 9

Parasympathetic – discrete and localised (1:1 pre:post) Sympathetic – coordinated and divergent (1:20 pre:post) All the postganglionic fibres emerge from the sympathetic trunk, allowing for a coordinated, divergent response which is necessary to allow lots of things happening at the same time during a ‘fight or flight’ (e.g. increased HR as well as hepatic glycogenolysis)

Question 10

What is the enteric nervous system?

Answer 10

It is like a little brain responding to various types of stimuli within the GI tract - operates independently of CNS (and ANS which originates from brain and spinal cord) The ANS does project onto this though - ANS can modify the ENS (e.g. PNS stimulates, SNS inhibits)

Question 11

How does the ENS work?

Answer 11

Has its own sensory, inter- and motor neurones which allow it to function indpently Sensory neurones - connected to mucosal chemoreceptors to detect chemical substances in gut lumen or tension in gut wall caused by food Interneurones - relay information from sensory neurones to submucosal or myenteric plexus (motor neurone network) Motor neurones - release substances to contract (by ACh or substance P) or relax (by vasoactive intestinal peptide or nitric oxide) smooth muscle depending on presence of the stimulus

Question 12

What type of receptor are nicotinic receptors?

Answer 12

Type 1 – ionotropic receptors (ion-channel linked) They are VERY fast transmission (chemical to electrical)

Question 13

What type of receptor are muscarinic receptors?

Answer 13

Type 2 – G-protein coupled receptors Slower than Type 1 receptors

Question 14

Describe the difference in the transmission through nicotinic and muscarinic receptors.

Answer 14

Nicotinic is much faster

Question 16

Where are nicotinic receptors found and what do they respond to?

Answer 16

On all autonomic ganglia, the adrenal medulla the NMJ - they respond to ACh

Question 17

Where are muscarinic receptors found and what do they respond to?

Answer 17

On effectors responding to the PNS - they respond to ACh EXCEPTION: sweat glands have muscarinic receptors - SNS causes sweating via ACh

Question 18

What effect does parasympathetic dominance at rest have on the: lungs, gut, bladder and eyes?

Answer 18

Lungs – partial bronchoconstriction Gut – increased gut motility Bladder – increased urinary frequency Eyes – short-sightedness (near vision) ‘REST AND DIGEST’

Question 19

Where are the subtypes types of muscarinic receptor found?

Answer 19

M1 – present in neural tissue (in the brain and autonomic ganglia) M2 – present in the heart M3 – present in exocrine glands and smooth muscle M4 and M5 subtypes exist too NOTE: all subtypes present in brain to some extent

Question 20

Why would giving a ganglion blocker at rest cause constipation?

Answer 20

At rest the parasympathetic nervous system is dominant, which increases gastric motility. This means that a ganglion blocker will knock out this effect and cause constipation.

Question 20

What type of receptor are all adrenergic receptors?

Answer 20

G-protein coupled

Question 20

Where are adrenergic receptors found and what do they respond to?

Answer 20

On effectors responding to the SNS - they respond to adrenaline and noradrenaline

Question 21

Which adrenergic receptors are responsible for the sympathetic control of vasculature?

Answer 21

Alpha 1 – vasoconstriction (responds to NA) Beta 2 – vasodilation (responds to adrenaline)

Question 22

What are the subtypes of adrenoceptors?

Answer 22

α1 α2 β1 β2

Question 23

What is the general process of formation of a neurotransmitter?

Answer 23

Precursor (could be from diet) converted to NT by enzymatic action

Question 24

Describe the synthesis of acetylcholine.

Answer 24

It is formed from acetyl CoA and choline by choline acetyltransferase It is broken down by acetylcholinesterase

Question 24

Describe the synthesis of Noradrenaline.

Answer 24

Tyrosine → DOPA (enzyme: tyrosine hydroxylase) DOPA → dopamine (enzyme: DOPA decarboxylase) Dopamine → Noradrenaline (enzyme: dopamine beta-hydroxylase) This last step takes place in a vesicle

Question 26

What are the two uptake and breakdown mechanisms of noradrenaline?

Answer 26

Uptake 1 – neuronal tissue – broken down by Monoamine Oxidase (MAO) Inside the pre-synpatic neurone, it can also be recycled (i.e. repackaged back into vesicles) Uptake 2 – extraneuronal tissue (i.e. non-neuronal tissue; effector cell like smooth muscle cell, liver) – broken down by Catechol-O-Methyl Transferase (COMT)

Question 27

Which neurotransmitter is cleared more quickly from the synapse?

Answer 27

ACh because acetylcholinesterase is present in the synapse so it can just be broken down there NA is present for a bit longer in the synapse because it has to be transported into tissue first before it can be broken down, which takes a bit longer