63. Plexuses of the autonomic nervous system in the thorax, abdominal and pelvic cavities - for-mation, position, organs innervated by them

Introduction

Autonomic plexuses are networks of sympathetic and parasympathetic fibres and ganglia around the great vessels and viscera of the thorax, abdomen and pelvis. They are formed from sympathetic trunk branches, splanchnic nerves and vagal and pelvic splanchnic (parasympathetic) fibres, and they distribute these fibres to the organs they supply. This topic covers their formation, position and the organs that receive their fibres.

Where the fibres come from

• Preganglionic sympathetic neurons lie exclusively in the lateral horn of the thoracic and upper two or three lumbar segments of the spinal cord. Their axons leave in the ventral roots and join the spinal nerve. Postganglionic cell bodies lie either in the sympathetic chain or in the plexuses (coeliac, superior mesenteric, inferior mesenteric) around the main branches of the abdominal aorta.
• The sympathetic part is organised as the sympathetic chain (cervical, thoracic, lumbar and coccygeal parts) and the prevertebral ganglia.

• Preganglionic fibres enter the chain via the white rami communicantes and then ascend and synapse in a higher paravertebral ganglion, descend, synapse at the same level (postganglionic fibres leave through the grey ramus communicans and re-enter the same anterior ramus), or travel through without synapsing.
• Fibres passing through without synapsing are usually from T5 to L2(3); they combine to form a splanchnic nerve, which synapses in a prevertebral ganglion, and the postsynaptic fibres go to the abdominal and pelvic viscera through a visceral motor nerve plexus.

• Cervical part (3 ganglia: superior, largest; middle, smallest and absent in some; inferior, often fused with the first thoracic ganglion). The superior ganglion sends the internal carotid nerve (to the eye structures, the pterygopalatine artery and the internal carotid artery), the external carotid nerve (smooth muscle of the arteries), the nerve to the pharyngeal plexus (joining vagal and glossopharyngeal branches), nerves to cranial nerves II, III, IV, VI and IX, grey rami to C1-C4, and the superior cardiac branch. The middle ganglion gives grey rami to C5-C6, thyroid branches (along the inferior thyroid artery, to the larynx, trachea, pharynx and upper oesophagus) and the middle cardiac branch. The inferior ganglion gives grey rami to C7, C8 and T1, branches to the subclavian and vertebral arteries and the inferior cardiac nerve. All cardiac branches contribute to the cardiac plexus.

Sympathetic nerves travel alongside the arteries.