Question 1

Outline the anatomy and physiology of the parasympathetic nervous system.

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College Answer

An efficient way to answer this question was to describe the anatomy and physiology of both cranial and sacral sections together. High scoring answers included an outline of the relevant nerves, the various ganglia, neurotransmitters and physiological effects. Some candidates described the cellular basis of Nicotinic, Muscarinic and M1-M5 receptors which didn't attract marks.

Discussion

Some might point out that the cellular basis of receptor function is a part of what one might conventionally mean by "physiology" and so should have formed a part of the marking rubric. Trying to guess what the examiners wanted leads one to an answer like this:

Structural organisation of the parasympathetic nervous system:

  • Central control structures: amygdala, insular cortex and anterior cingulate cortex, which project mostly to the hypothalamus, which then projects to the brainstem cranial nerve nuclei
  • Parasympathetic preganglionic neurons reside in the brainstem and the sacral spinal cord:
    • Edinger-Westphal nucleus of the midbrain
    • Superior salivary nucleus in the pontine tegmentum
    • Inferior salivary nucleus in the lowermost dorsal pons 
    • Dorsal vagal nucleus in the medulla
    • Nucleus ambiguus, also in the medulla
    • Intermediolateral laminae (V-VII) at the S2-S4 sacral spinal cord segments. 
  • Sympathetic preganglionic fibres
    • Lightly myelinated B fibres in cranial nerve and pelvic nervi erigendes
    • Transmission is reasonably fast, eg. vagus measures about 15 m/s
    • Long fibres (whereas sympathetic preganglionic fibres are short)
  • Parasympathetic ganglia:
  • Parasympathetic postganglionic fibres are unmyelinated and short
    • Transmission here is cholinergic and muscarinic
  • Physiological effects:
    • Miosis (contraction) of the pupillary sphincter muscle
    • Contraction of the ciliary muscle for far vision
    • Lacrimal gland secretion
    • Significant reduction in heart rate, atrial contractility, action potential duration, conduction velocity of the AV node, ventricular contractility
    • Increased motility and tone of the stomach
    • Relaxation of gastric and intestinal sphincters
    • Contraction of the gallbladder
    • Bladder detrusor muscle contraction, and relaxation of the trigone sphincter
    • Erection
    • Generalised secretion of the sweat glands (not just sweaty palms, but all over)

References

Jänig, Wilfrid. The integrative action of the autonomic nervous system: neurobiology of homeostasis. Cambridge University Press, 2022. 

Nilsson, Stefan. "Comparative anatomy of the autonomic nervous system." Autonomic Neuroscience 165.1 (2011): 3-9.

Phillips, Colin, and Katherine Ower. "Anatomy of the Sympathetic and Parasympathetic Nervous System." Pain: A Review Guide (2019): 9-14.

Kalamida, Dimitra, et al. "Muscle and neuronal nicotinic acetylcholine receptors: structure, function and pathogenicity." The FEBS journal 274.15 (2007): 3799-3845.

Papke, Roger L. "Merging old and new perspectives on nicotinic acetylcholine receptors." Biochemical pharmacology 89.1 (2014): 1-11.

Skok, Vladimir I. "Nicotinic acetylcholine receptors in autonomic ganglia.Autonomic Neuroscience 97.1 (2002): 1-11.

Caulfield, Malcolm P. "Muscarinic receptors—characterization, coupling and function." Pharmacology & therapeutics 58.3 (1993): 319-379.