24A05: Exam Report

Outline the anatomy of the sympathetic nervous system including:
(i) The origin (15% of marks).
(ii) Fibre types and their course (70% of marks).
(iii) Receptors and neurotransmitters (15% of marks).

54% of candidates passed this question

The question provided the headings that candidates would be expected to provide information on and those that used these headings to structure their answers provided good overall responses.

A definition of the SNS and effects of activation was not required.

Candidates were expected to outline SNS anatomy with respect to origin (pre-ganglionic neurons in the grey matter of the lateral horn of the spinal cord, T1 to L2), fibre types including a description of short pre-ganglionic myelinated b fibres, the sympathetic chain and the long post-ganglionic unmyelinated C fibres.

Receptors and neurotransmitters included nicotinic Ach receptors in the ganglion and adrenergic alpha and beta receptors on target organs/vessels.

A brief discussion of the arrangement at the adrenal medulla was also expected.

M1i / 24A05: Outline the anatomy of the sympathetic nervous system

The sympathetic nervous system is the accelerative component of the autonomic nervous system.

(i) Origin:

  • sympathetic nervous impulses originate centrally, with significant coordination of output from the hypothalamus and rostral ventrolateral medulla, responding the both peripheral and central stimuli
  • Pre-ganglionic fibres originate in the lateral horn of the spinal cord between T1 and L2/3

(ii) Fibre types & course

  • Pre-ganglionic fibres are short, type B (myelinated) fibres and emerge from the spinal cord in white rami communicans
  • They synapse with post ganglionic fibres in either paravertebral (sympathetic chain) or pre-vertebral ganglia (i.e. coeliac ganglia)
  • Post ganglionic fibres are longer & unmyelinated C fibres, they exit the ganglia through grey rami communicans
Diagram of sympathetic nervous system structure and connections.
  • The “sympathetic chain” between T1-L2/3 is the chain of ganglia from which post-ganglionic sympathetic fibres supply;
    • Cervical (head and neck) supply – i.e. pupilary dilation
    • Thoracic (heart/lungs/thoracic vessels) – i.e. tachycardia/bronchial dilation
    • Lower thoracic (upper GI tract)
    • Lumber (lower GI tract)
    • Sacral (Pelvic viscera)
  • Importantly, sympathetic stimulation of the adrenal gland is an exception, where myelinated pre-ganglionic fibres directly innervate the adrenal medulla

(iii) Receptors & neurotransmitters

  • Neurotransmission at the ganglia;
    • ACh released from pre-ganglionic neuron
    • Acts on nicotinic receptor
  • Neurotransmission at the effector site;
    • Noradrenaline released from post-ganglionic neuron
    • Acts on adrenergic receptors
      • Alpha 1 (GqPCR – vasoconstrictive)
      • Alpha 2 (GiPCR – inhibitory)
      • Beta 1 (GsPCR – positive ino/chronotropy)
      • Beta 2 (GsPCR – smooth muscle relaxation – bronchi/uterus)
    • Exceptions:
    • Neurotransmission at sweat glands/muscle vasculature/hairfollicles: ACh acts on muscarinic receptors (rather than noradrenaline as per typical post-ganglionic transmission)
    • Neurotransmission at adrenal medulla: as per pre-ganglionic synapse – ACh released

References

  • Basic Physiology for Anaesthetics (Huang, Matthews)
  • Principles of Physiology for the Anaesthetist (Power and Kam)

Author: Brodie Farrow