Question 4

Outline the anatomy and physiology of the parasympathetic nervous system

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College Answer

Generally there was a lack of detailed knowledge, incorrect facts and at times confusion between 
the sympathetic and parasympathetic nervous system functions. A lack of anatomical detail was 
common (the origin of preganglionic cell bodies was not described clearly, and parasympathetic 
ganglia were not often named and located). It was expected an answer would mention the 
central role of Acetylcholine as a neurotransmitter at preganglionic and post ganglionic neurons 
in the parasympathetic system. Target organs were identified correctly but the exact action was 
not specified e.g. pupillary constriction vs. dilatation, GI sphincter/bladder - contraction vs. 
relaxation. Detail concerning receptor physiology was not required.
This is a question covering a core topic that no candidate passed. An overview of the 
arrangement and function of the autonomic nervous system is provided in several core 
physiology texts, including Ganong and Guyton.

Discussion

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.