ANS Physiology

Master answer

Anatomy of SNS

• Posterior hypothalamus is the main site of sympathetic nervous outflow
• Receives input from cardiovascular centres of medulla and pons
• Sympathetic innervation from Sympathetic trunks
• Paired bundle of sympathetic neurons run lateral from vertebral bodies from T1 to L2
• Consists of two neurons in series
  • Short Pre-ganglionic neuron → Sympathetic ganglion → Long Post-ganglionic neuron

Origin

• Origin: Preganglionic fibres originate in the grey matter of the spinal cord lateral horn
• Nerve Fibres and Pathway: Leave the spinal cord through ventral roots (with spinal nerves)
  • Leave spinal nerves, travel as white rami communicantes (short myelinated B fibres)
  • Meet with the ganglia of the sympathetic trunk
    • Synapse with post-ganglionic neurons
• Neurotransmitter: Preganglionic neurotransmitter is acetylcholine
• Receptor: nicotinic receptors

Sympathetic trunk:

4 parts

• Cervical part
  • Head
  • Neck
  • Thorax
• Thoracic part (T1-T5)
  • Aortic plexus
  • Pulmonary plexus
  • Cardiac plexus
  • Thoracic splanchnic nerves
• Lumbar sympathetic ganglia
  • Coeliac plexus
• Pelvic part
  • Hypogastric plexus
  • Pelvic plexus

Post-ganglionic

• Nerve Fibres and Pathway: Post-ganglionic neurons leave the ganglia as grey rami communicantes
  • Long unmyelinated C fibres
  • Rejoin the spinal nerves to travel to target organs
• Neurotransmitter: Postganglionic neurotransmitter is noradrenaline
  • (acetylcholine in muscles, sweat glands and hair follicles)
• Receptor: Adrenergic alpha and beta receptors on target organs/vessels

Exception is adrenal medullary outflow:

• Modified post-ganglionic cells release adrenaline into circulation
• Innervated directly by pre-ganglion sympathetic fibres

Mooney / JC 2020

Answer by JC, Mooney