Question 5

College Answer

Most candidates had a suitable structure to their answers, those without a clear organisation of thought tended to gain fewer marks. In many cases incorrect information or limited detail, particularly around the anatomical organisation prevented higher marks

Discussion

• 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

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.