Introduction to the autonomic nervous system

Question 1

What is the autonomic nervous system?

Answer 1

• Sub-section of peripheral nervous system that regulates involuntary processes from CNS to peripheral organs e.g. heart rate, digestion, respiration

Question 2

What are the 3 divisions of the autonomic nervous system?

Answer 2

• Sympathetic nervous system - links CNS to peripheral organs
• Parasympathetic nervous system - links CNS to peripheral organs
• Enteric nervous system - controls GI tract

Question 3

Give some examples of what organs the ANS innervates and their functions

Answer 3

• ANS innervates many cell types/organs, controls many functions:
  
  • Heart:
    
    • SAN cells- regulates heart rate
    • AVN cells- regulates electrical conduction through heart
    • Cardiac myocytes- regulate contractility of heart
  • Smooth muscle:
    
    • ANS contracts or relaxes smooth muscle cells
  • Gland cells:
    
    • AND causes secretions from glandular cells e.g. saliva, sweat, insulin

Question 4

Describe the anatomy of the ANS

Answer 4

• 2 efferent (away from) neurones arranged in series conducting electrical activity from CNS to peripheral tissue/organ
• These two neurones are called the pre-ganglionic nerve and the post-ganglionic nerve
• The autonomic ganglion (ganglia=plural) = Group of neuronal cell-bodies lying outside CNS

Question 5

Compare the pre-ganglionic and post-ganglionic fibres of the parasympathetic nerves

Answer 5

Have long pre-ganglionic fibre, short post-ganglionic fibre

Question 6

Where do the parasympathetic pre-ganglionic fibres arise from?

Answer 6

Arise from cranial nerves 3,7,9 and 10 and the sacral spinal cord 2,3,4

Question 7

Where do the parasympathetic pre-ganglionic fibres synapse with the post-ganglionic neurones?

Answer 7

Synapse with postganglionic neurones at parasympathetic ganglia of the head or near the wall of an organ supplied by vagus or sacral nerves. The vagus nerve contains about 75% of all parasympathetic fibres.

Question 8

What neurotransmitter do the parasympathetic pre-ganglionic fibres release, and what receptor do they act at?

Answer 8

• Pre-ganglionic fibres release Ach , which acts at nicotinic receptors within the post-ganglionic membrane

Question 9

What neurotransmitter do the parasympathetic post-ganglionic fibres release, and what receptor do they act at?

Answer 9

Post-ganglionic fibre releases ACh which acts at muscarinic (Mus) receptors on the target organs

Question 10

What neurotransmitter do non-adrenergic-non-cholinergic (NANC) nerves release?

Answer 10

Chemicals such as NO (nitric oxide)

Question 11

What are the functions of the parasympathetic nervous system?

Answer 11

• Functions of parasympathetic:
  
  • Slows heart rate and lowers blood pressure
  • Vasodilates gut and stimulates gut motility and secretions
  • Constricts pupil and contracts ciliary muscles to allow closer focus
  • Increase secretion from salivary glands

Question 12

What are some organs only innervated by parasympathetic nerves?

Answer 12

• Some organs only innervated by parasympathetic nerves:
  
  • Pancreas
  • Secretory cells of stomach
  • Lungs (though airways contain Beta adrenoceptors, modulate by circulating adrenaline released by sympathetic on adrenal glands)

Question 13

Compare the pre-ganglionic and post-ganglionic fibres of the sympathetic nerves

Answer 13

Short pre-ganglionic fibre, long post-ganglionic fibre

Question 14

Where do the sympathetic pre-ganglionic fibres arise from?

Answer 14

• Lateral horns of grey matter from T1-L2/3 (thoracolumbar region)

Question 15

Where do the sympathetic pre-ganglionic fibres synapse with the post-ganglionic neurones?

Answer 15

Pre-ganglionic neurones synapse onto post-ganglionic neurones at paravertebral or prevertebral ganglia.

They directly synapse onto chromatin cells of adrenal medulla

Question 16

What neurotransmitter do the sympathetic pre-ganglionic fibres release, and what do they act at?

Answer 16

• Release Ach, which acts at nicotinic receptors within the post-ganglionic membrane

Question 17

What neurotransmitter do the sympathetic post-ganglionic fibres release, and what receptor do they act at?

Answer 17

• Release NA (noradrenaline) which acts at alpha or beta adrenoceptors on the target organ
• • Stimulation of pre-ganglionic sympathetic fibres release adrenaline (80%) and noradrenaline (20%) from adrenal glands
• No post-ganglionic fibres are involved in innervation of adrenal glands

Question 18

What is the exception in the neurotransmitter released by the post-ganglionic fibres of the sympathetic nervous system innervating the sweat glands?

Answer 18

Sympathetic nerves release Ach at the sweat glands, which acts at muscarinic receptors (Mus)

Question 19

What are some organs only innervated by sympathetic nerves?

Answer 19

• Sweat glands
• Kidney
• Blood vessels

Question 20

What are the functions of the sympathetic nervous system?

Answer 20

• Increase heart rate + contractility and vasodilates coronary arteries. Constricts vessels to skin and intestines
• Causes bronchodilation and increases blood flow to lungs
• Vasodilates to skeletal muscles, relaxes ciliary muscle to lens
• Inhibits gut peristalsis and constricts intestinal and urinary sphincters
• Dilates pupils
• Increase secretions from salivary gland

Question 21

Summarise the events of chemical transmission at the synapse

Answer 21

• Synthesis of neurotransmitter (NT)
• Storage of NT in vesicles
• Arrival of AP at synaptic terminal
• Terminal depolarises, activation of VGCC, Ca2+ influx
• Ca2+- dependent release of NT
• NT binds to receptor and induces response
• Uptake/metabolism of NT back into pre-synaptic membrane

Question 22

What neurones innervate the adrenal medulla and what receptor do they act on?

Answer 22

Cholinergic preganglionic sympathetic neurons

Ats at nicotinic receptors