2.8 - Autonomic Nervous System

Question 1

What is the ANS in general?

Answer 1

• subdivision of the PNS
• not under conscious control - carries out many reflex responses to visceral stimuli
• controls non-skeletal peripheral function e.g. cardiac muscle, smooth muscle, internal organs, skin

Question 2

What types of neurones are involved in ANS that send information to and from the brain?

Answer 2

• somatic sensory - responds to external stimuli e.g. skeletal muscle responding to you walking
• visceral sensory - responds to internal stimuli
• path taken through cranial or spinal neurones e.g. enter spinal cord through dorsal horn (PNS)
• this information is then taken to the hypothalamus
• information then sent back out down efferent nervous system i.e. motor division - involves somatic motor and visceral motor neurones
• visceral motor neurones have two arms - parasympathetic and sympathetic nervous systems

Question 3

What do the sympathetic and parasympathetic nervous systems do?

Answer 3

• sympathetic - fight or flight - mobilisation and increased metabolism
• parasympathetic - rest or digest - routine maintenance and basal effects
• often innervate the same tissues and have opposing / antagonistic effects
• not always opposing effects e.g. SNS controls blood vessel tone - both constriction and dilation

Question 4

How do the sympathetic and parasympathetic systems interact with pupils, heart, stomach, lungs, liver, bladder?

Answer 4

• pupil - SNS causes dilation, PNS causes constriction
• heart - SNS increases rate and contractility, PNS decreases
• stomach - SNS decreases motility and secretions, PNS increases
• lungs - SNS causes dilation, PNS causes constriction
• liver - SNS increases glucose release, PNS increases bile release
• bladder - SNS causes relaxation, PNS causes contraction

Question 5

What do parasympathetic/sympathetic nervous systems react to?

Answer 5

• sensory information received in the integrating centre
• either PNS or SNS usually dominant
• e.g. sensory information relayed by baroreceptors determines parasympathetic/sympathetic control of heart rate
• baroreceptors detect high BP and sends this to hypothalamus, which sends signal to PNS neurones to lower BP and turns SNS off to stop further BP increase
• low BP = baroreceptors fire less often = less signal through visceral motor neurones = PNS effect drops and SNS switches on (disinhibition) and is now dominant

Question 6

Where are visceral motor neurones located?

Answer 6

• visceral motor nuclei originate in the hypothalamus
• hypothalamus signals to brainstem/spinal cord
• these visceral motor neurones project to the brainstem or spinal cord where they synapse with autonomic neurones (PNS/SNS)

Question 7

What do autonomic neurones consist of?

Answer 7

• two neurones: a pre-ganglionic (originating from brain/spinal cord) and a post-ganglionic neurone
• ganglion - a nerve cell cluster or group of nerve bodies

Question 8

What does the parasympathetic nervous system look like in terms of ganglions?

Answer 8

• long pre-ganglionic fibres
• short post-ganglionic fibres
• ganglions close to / embedded within effector tissues

Question 9

What does the sympathetic nervous system look like in terms of ganglions?

Answer 9

• short pre-ganglionic fibres
• long post-ganglionic fibres
• ganglions close to spinal cord

Question 10

How does the sympathetic nervous system have more control than parasympathetic nervous system?

Answer 10

• the ganglions for SNS exist in structures called sympathetic trunks - long chains running parallel to spinal cord
• lots of sympathetic nerves coming out of spinal cord synapse within these trunks - allows for mass activation of sympathetic nerves
• allows coordinated fight or flight response e.g. want heart rate to increase as lung constricts

Question 11

What is the exception to the two neurone arrangement in ANS?

Answer 11

• sympathetic innervation of adrenal gland
• no ganglion - just one sympathetic nerve that looks like a pre-ganglionic fibre

Question 12

What neurotransmitters do pre-ganglionic nerves of both SNS and PNS release, and what are the receptors?

Answer 12

• between pre- and post-ganglionic fibres, acetylcholine (ACh) is the NT for both SNS and PNS
• nicotinic ACh receptor

Question 13

What neurotransmitter is released from post-ganglionic nerve of PNS, and what are the receptors?

Answer 13

• ACh released between post-ganglionic fibre and effector organ
• detected by muscarinic ACh receptors

Question 14

What neurotransmitter is released from post-ganglionic nerve of SNS, and what is the exception?

Answer 14

• noradrenaline (NA) between post-ganglionic fibre and effector organ (into effector)
• exception - sweat glands, where ACh is the NT released

Question 15

What does the adrenal gland secrete?

Answer 15

• the adrenal gland secretes a hormone not a NT - releases adrenaline (and some NA) - secreted into bloodstream not synapse

Question 16

What is the name of the complex neural network within the gut?

Answer 16

• enteric nervous system - largely responds to gut function and stimuli received within the gut without engaging the brain

Question 17

What is the unusual situation with the lung in terms of innervation?

Answer 17

• there are parasympathetic nerves but no sympathetic neurones innervating the lung tissue
• however, lungs can bronchodilate so SNS clearly has some level of control over the lung - through production of adrenaline from adrenal gland secreted into blood, travels to lung and diffuses in causing bronchodilation

Question 18

What is the micturition reflex?

Answer 18

• in bladder, sensory information relayed to the brain is pressure - as bladder fills, pressure builds up
• pressure builds up (SNS on during this which contracts internal sphincter to keep it closed and prevent urine leaking out)
• when it gets to a certain point (bladder full), this information is sent to brain, PNS switched on and SNS switched off
• PNS contracts detrusor muscle which squeezes bladder and switched-off SNS causes internal sphincter to relax = urine leaves bladder
• somatic NS also gives voluntary level of control for when you want to empty bladder through control of another sphincter

Question 19

Which receptors mediate the effects at all autonomic ganglia?

Answer 19

• nicotinic acetylcholine (nACh) receptors - ion channel linked receptor that gives a fast response
• nACh receptors mediate responses to ACh released from preganglionic fibres at all autonomic ganglia
• found at all ANS ganglion - need that fast propagation of signal
• also mediate response to ACh released by sympathetic nerves innervating adrenal medulla for rapid adrenaline release
• mediate all fast excitatory and inhibitory transmission

Question 20

Which receptors mediate the effects of NT released from post-ganglionic fibres?

Answer 20

• G protein coupled receptors - muscarinic receptors for ACh and adrenergic receptors for NA
• G protein coupled receptors have a slower onset than ion channel linked receptor

Question 21

How is ACh made at synapses and released?

Answer 21

1. choline + acetyl CoA enzymatically converted by choline acetyl transferase –> ACh
2. packaged into vesicles
3. AP causes Ca2+ influx and exocytosis
4. NT release
5. receptor activation (muscarinic or nicotinic)
6. ACh rapidly degraded by acetylcholinesterase in the synapse - choline taken up into presynaptic terminal by choline uptake protein

Question 22

What happens if you block acetylcholinesterase?

Answer 22

• metabolism of ACh stopped
• accumulates in neurone and you have more powerful ACh effect

Question 23

How is NA made at synapses and released?

Answer 23

1. tyrosine converted to DOPA by tyrosine hydroxylase; DOPA converted to dopamine by DOPA decarboxylase
2. dopamine packaged into vesicles with dopamine beta hydroxylase –> NA is the product
3. AP causes Ca2+ influx and exocytosis
4. NT release
5. receptor activation (adrenergic)
6. removal of NT from synapse via uptake into pre-synaptic terminal (monoamine oxidase) or glial cell (COMT: catechol-O-methyltransferase); can be metabolised in synapse prior to uptake

Question 24

How is adrenaline made and released?

Answer 24

1. tyrosine converted to DOPA by tyrosine hydroxylase; DOPA converted to dopamine by DOPA decarboxylase
2. dopamine packaged into vesicles with dopamine beta hydroxylase –> NA is the product
3. NA converted to adrenaline in the cytoplasm by phenylethanol methyl transferase
4. AP causes Ca2+ influx and exocytosis
5. NT release
6. adrenaline diffuses into capillary and is transported to tissues in the blood

Question 25

If you take an opioid, what will happen to your pupils and why?

Answer 25

• constrict
• PNS activated

Question 26

Biosynthesis and metabolism - general sequence

Answer 26

1. precursor enzymatically converted to NT
2. packaged into vesicles
3. AP causes Ca2+ influx and exocytosis
4. NT release
5. receptor activation
6. removal of NT from synapse via uptake into pre-synaptic terminal or glial cell; can be metabolised in synapse prior to uptake