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Portrait of The plexus

Science · Medicine & the body

The plexus

Latin for a braid or plait

Medicine — the nervous system and its nerve networks

A plexus is a braided network of intersecting nerves — the word is Latin for a plait — where the fibres of several spinal roots are shuffled and recombined before travelling on as named nerves. The nervous system divides into the central system (brain and spinal cord) and the peripheral system, whose 31 pairs of spinal nerves form four great somatic plexuses: the cervical, the brachial, the lumbar and the sacral. There are autonomic ones too, above all the coeliac or solar plexus, a sunburst of fibres behind the stomach, and the enteric networks woven through the wall of the gut.

How to spot them

  • ▸The word is Latin for a braid or plait, from 'plectere', to weave
  • ▸Four great somatic examples: cervical (C1–C4), brachial (C5–T1), lumbar (L1–L4), sacral (L4–S4)
  • ▸The brachial one runs roots, trunks, divisions, cords, branches — five stages, in that order
  • ▸The coeliac version, behind the stomach, is the one a boxer aims for
  • ▸31 pairs of spinal nerves and 12 pairs of cranial nerves make up the peripheral nervous system

Key facts

A braid of the ventral rami of C5 to T1, passing between the scalene muscles and under the clavicle to supply the arm

The brachial network — the one most asked about

Roots, trunks, divisions, cords, branches — five stages memorised by every medical student

The arrangement of the brachial network; the cords are named lateral, posterior and medial for their position round the axillary artery

The musculocutaneous, axillary, radial, median and ulnar nerves, all emerging from a single braid

The five terminal branches of the brachial network

Erb–Duchenne palsy and the 'waiter's tip' posture from injury to the upper roots; Klumpke's claw hand from the lower

Upper (C5–C6) and lower (C8–T1) injuries, often at birth or from a fall on the shoulder

A sunburst of autonomic fibres behind the stomach around the coeliac trunk, the target of a winding blow

The coeliac or solar network, the largest autonomic one in the body

Auerbach's myenteric and Meissner's submucosal networks in the wall of the gut

The enteric nervous system, sometimes called the second brain

The sciatic nerve, the thickest in the body, emerging from the braid of L4 to S3

The sacral network

The choroid network in the ventricles, which secretes the cerebrospinal fluid

A vascular rather than nervous braid, but it bears the same name

Key ideas

  • Fibres from a single spinal root end up in several different named nerves, which is why a root injury and a nerve injury produce quite different patterns of weakness.
  • The thorax is the exception: the intercostal nerves run straight out as segmental nerves and form no braid, because the ribs impose a simple segmental plan.

Quick-fire facts

  • •The nervous system divides into central (brain and spinal cord) and peripheral; the peripheral divides into somatic and autonomic, and the autonomic into sympathetic and parasympathetic
  • •There are 31 pairs of spinal nerves — 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal — and 12 pairs of cranial nerves
  • •The brachial network is formed from the ventral rami of C5–T1: five roots, three trunks, six divisions, three cords, five terminal branches
  • •Its terminal branches are the musculocutaneous, axillary, radial, median and ulnar nerves
  • •Erb–Duchenne palsy follows upper trunk (C5–C6) injury and gives the 'waiter's tip' arm; Klumpke's palsy follows C8–T1 injury and may bring Horner's syndrome
  • •The cervical network (C1–C4) gives the phrenic nerve, from C3, C4 and C5, which supplies the diaphragm
  • •The coeliac or solar network sits at the level of T12/L1 behind the stomach, around the coeliac trunk; 'solar' refers to its radiating fibres, and a blow there causes diaphragmatic spasm — being winded
  • •The sacral network gives the sciatic nerve (L4–S3), the largest and thickest nerve in the body, and the pudendal nerve (S2–S4)
  • •The lumbar network (L1–L4) gives the femoral and obturator nerves
  • •The enteric nervous system comprises the myenteric (Auerbach's) and submucosal (Meissner's) networks and contains hundreds of millions of neurons
  • •The choroid plexus in the brain's ventricles produces cerebrospinal fluid
  • •Charles Sherrington coined the word 'synapse' in 1897; Golgi and Cajal shared the 1906 Nobel Prize for the structure of the nervous system
  • •The vagus, cranial nerve X, is the longest cranial nerve; its name is Latin for 'wandering'

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