Question 20

Describe the anatomy of the sympathetic nervous system.

[Click here to toggle visibility of the answers]

College Answer

A definition of the sympathetic system, followed by a systematic description of the central  sympathetic centres; what happens at the spinal cord; the anatomy of the pre and post  ganglionic fibres would have been awarded with a pass mark. Additional information about  the sympathetic ganglia and the neurotransmitters involved would have rounded off a good  answer. Many answers lacked anatomical detail and described the actions (function) of the  sympathetic system which was not asked for.
Most answers lacked any structure. The most common reason for not passing this question  was that significant sections of the anatomy from central to peripheral were not mentioned. Most had a simple sketch understanding of the question asked but could not add enough of  the next layer to be awarded a pass mark.

Discussion

This SAQ has been repeated for the third time, and each time the pass rate had roughly doubled (from 11.1%, to 25%, and now to 50%). Modelling suggests that the next time it is repeated, the pass rate should be 100%. 

  • Central control structures: brainstem nuclei (hypothalamus, rostral ventrolateral medullary vasomotor centre, raphe nucleus and the nucleus tractus solitarius)
  • Spinal preganglionic neurons (cell bodies in intermediolateral nucleus of the spinal grey matter) send fibres to the ganglia
  • Sympathetic preganglionic fibres
    • Lightly myelinated B fibres via the ventral root to the white rami
    • White rami connect to sympathetic chain
    • Fibres also pass without synapsing through the sympathetic chain to give rise to splanchnic nerves that synapse with a distal ganglion:
  • Splanchnic nerves:
    • Greater splanchnic nerve from T5-9, to coeliac ganglion
    • Lesser splancnic nerve from T10 and T11, to coeliac ganglion
    • Least splanchnic nerve from T12, to renal ganglion
    • Lumbar splanchnic nerve from L1 and L2, to aortic plexus
    • Sacral splanchnic nerves, from T12-L2, to the inferior hypogastric plexus, the superior hypogastric plexus and the aortic plexus
  • Sympathetic chain contains
  • Sympathetic ganglia
    • Paravertebral ganglia: 24 on each side, forming the sympathetic chains.
      • one ganglion for each spinal level, except in the neck where there are only three (superior cervical ganglion, stellate ganglion and intermediate ganglion)
    • Prevertebral ganglia: coeliac plexus, aortic plexus and the superior hypogastric plexus; network of nervous tissue structures ventral to the abdominal aorta. 
  • Ganglionic neurotransmission: presynaptic terminals release acetylcholine, postsynaptic membrane carries N2 nicotinic receptors
  • Sympathetic postganglionic fibres are unmyelinated
    • The fibres that synapse with a ganglionic neuron send postganglionic fibres that return to spinal nerve via the grey rami, so called because they are darker and thinner, being composed of largely unmyelinated fibres.
    • These postganglionic fibres then carry on with other somatic nerves to innervate their peripheral targets (vessels, skin, sweat glands, etc)
  • Sympathetic nerve endings
    • Ultrastructure: "beaded strands", varicosities along the axon release the neurotransmitters in proximity (within 1-2 μm) of target organ/tissue
    • Mostly release noradrenaline
    • Minority (4%) are cholinergic - innervate the eccrine sweat glands
  • Sympathetic innervation of tissues and organs
    • ​​​​​​​Most receive both sympathetic and parasympathetic input
    • The tissues and organs that are managed exclusively by the sympathetic nervous system include:
      • The adrenal glands
      • The majority of the blood vessels
      • The pilomotor muscles in the skin (hair follicles),
      • Sweat glands

References

Furness, John B. "The organisation of the autonomic nervous system: peripheral connections." Autonomic Neuroscience: Basic and Clinical 130.1 (2006): 1-5. 

McCorry, Laurie Kelly. "Physiology of the autonomic nervous system." American journal of pharmaceutical education 71.4 (2007). 

Powley, Terry L. "Central control of autonomic functions: organization of the autonomic nervous system." Fundamental neuroscience. Academic Press, 2013. 729-747. 

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

Patel, Toral R. "Anatomy of the Sympathetic Nervous System." Nerves and Nerve Injuries (2015): 495-506.