ICPP 12 - The ANS (Part 1)

Question 1

What is the role of the ANS?

What is it entirely regulated by?

Answer 1

• To control all involuntary (vegetative) functions, e.g.: HR, BP etc.
• It is entirely efferent (but regulated by afferent inputs)

Question 2

What are the key afferent/sensory inputs to the ANS and what do they monitor?

Answer 2

• Sensory neurones monitor CO2, O2 and nutrients in blood, as well as arterial pressure and GIT composition
• Chemoreceptors in carotid body monitor blood O2, CO2 and pH relaying to the CNS via the glossopharyngeal nerve
• Primary sensory neurones project onto second order sensory neurones in medulla oblongata

Question 3

When are the 2 branches of the ANS activated?

What are the consequences of activation?

Answer 3

• Sympathetic to stressful situations (fight or flight) - decreased digestion, decreased salivation, increased HR, BP, force of contraction, hyperventilation.
• Parasympathetic regulates basal activities (rest &
  digest)

Question 4

Where are the cell bodies of sympathetic and parasympathetic neurones arising from the CNS found?

Answer 4

Sympathetic = Thoracic and Lumbar regions (runs from T1-L3)
Parasympathetic = Medullary and Sacral regions

Question 5

Describe the general structure of sympathetic and parasympathetic neurones

e.g.: position of cell bodies, autonomic ganglia, myelination etc.

Answer 5

• Both have cell bodies in CNS
• Both have myelinated pre-ganglionic fibres
• Both have unmyelinated post-ganglionic fibres
• Both originate in the lateral horn of their respective CNS regions
• Shorter pre-ganglionic fibres in sympathetic, longer in parasympathetic
• Ganglia within/close to innervated tissues in parasympathetic, ganglia close to SC in sympathetic

Question 6

What neurotransmitter is released from both pre-ganglionic fibres?
What does this result in the activation of?

Answer 6

• ACh

- Post-ganglionic nAChR’s (ligand-gated ion channels)

Question 7

What neurotransmitters (most popular) are released by the post-ganglionic branches of the ANS? + What receptors do these act upon?

Answer 7

Parasympathetic = ACh, acting on mAChR's in target tissue (GPCR's)
Sympathetic = mostly noradrenaline (NA), acting on either alpha or beta-adrenoreceptors (a1,2,3 or b1,2)

Question 8

Which post-ganglionic sympathetic fibres are cholinergic rather than noradrenergic?
What transmitters are sometimes co-released with NA or ACh?

Answer 8

• Those innervating sweat glands and hair follicles.

- ATP, NO, 5-HT, Neuropeptides (non-adrenergic,non-cholinergic (NANC) transmitters.

Question 9

Why are sympathetic postganglionic neurones in adrenal glands different to the rest?

Answer 9

• They are differentiated to form neruosecretory chromaffin cells.
• These cells are considered postganglionic sympathetic neurones that dont project to a target tissue
• Instead they release adrenaline into the blood stream.

Question 10

What are the consequences of parasympathetic stimulation in the heart, smooth muscle and in glands?

Answer 10

Heart = bradycardia + reduced conduction velocity (via M2 receptors)

Smooth muscle = bronchiolar contraction, increased intestinal motility, bladder contraction & relaxation, penile erection (via M3 receptors)

Glands = Increased sweat/salivary/lacrimal secretion (via M1/M3 receptors)

+ NO release which causes blood vessel dilation (penile erection).

Question 11

What are the consequences of sympathetic stimulation in the heart, in smooth muscle and in glandss?

Answer 11

Heart = Positive inotropic and chronotropic effects (B1 receptors)

Smooth muscle = Arteriole contraction, bronchiolar relaxation, bladder contraction

Glandular = Increased (viscous) secretion