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The autonomic nervous system comprises two parts—the sympathetic
ramus and are then distributed with the branches of that nerve.
and the parasympathetic. The former initiates the ‘fight or flight’ reac-
B They may pass to adjacent arteries to form a plexus around them
tion while the latter controls the body under more relaxed conditions.
and are then distributed with the branches of the arteries. Other
Both systems have synapses in peripheral ganglia but those of the sym- A They may pass back to the spinal nerve as a grey (unmyelinated)
pathetic system are, for the most part, close to the spinal cord in the gan- fibres leave branches of the spinal nerves later to pass to the arter-
glia of the sympathetic trunk whereas those of the parasympathetic ies more distally.
system are mostly in the walls of the viscera themselves and are micro- C They may pass directly to the viscera in distinct and sometimes
scopic (except for the four macroscopic ganglia in the head and neck named branches such as the cervical cardiac branches of the cer-
described below). Thus the sympathetic preganglionic fibres are re- vical ganglia.
latively short compared to the parasympathetic fibres (Fig. 53.1). If the sympathetic trunk is divided above T1 or below L2, the head
• Sympathetic outflow (Fig. 53.2): the anterior rami of spinal nerves and neck or the lower limb will lose all sympathetic supply.
T1 to L2 or 3. The fibres leave these spinal nerves as the white rami Loss of the supply to the head and neck will produce Horner’s syn-
communicantes and synapse in the ganglia of the sympathetic trunk. drome. There will be loss of sweating (anhidrosis), drooping of the
• Parasympathetic outflow: this comprises: upper eyelid (ptosis) and constriction of the pupil (myosis) on that side.
• Cranial outflowafibres travel as ‘passengers’ in the cranial nerves Details of the sympathetic system in the various regions are given in
3, 7, 9 and 10/11 and synapse in one of the four macroscopic the appropriate chapters, but Table 53.1 summarizes the autonomic
peripheral ganglia of the head and neck. supply to the most important regions and viscera.
• Sacral outflowatravel in sacral nerves S2, 3 and 4.
The parasympathetic system
The sympathetic system • The cranial outflow:
• The sympathetic trunk: from the base of the skull to the tip of the III The oculomotor nerve carries parasympathetic fibres to the
coccyx where the two trunks join to form the ganglion impar. The trunk constrictor pupillae and the ciliary muscle, synapsing in the ciliary
continues upwards into the carotid canal as the internal carotid nerve. ganglion.
• Superior cervical ganglion: represents the fused ganglia of C1, 2, 3 VII The facial nerve carries fibres for the submandibular and sublin-
and 4. gual glands (which synapse in the submandibular ganglion) and for the
• Middle cervical ganglion: represents the fused ganglia of C5 and 6. lacrimal gland (which synapse in the sphenopalatine ganglion).
• Inferior cervical ganglion: represents the fused ganglia of C7 and 8. IX The glossopharyngeal nerve carries fibres for the parotid gland
It may be fused with the ganglion of T1 to form the stellate ganglion. which synapse in the otic ganglion.
For courses of the pre- and postganglionic fibres see Fig. 53.3. X/XI The vagus and cranial root of the accessory carry fibres for the
• Preganglionic fibres: when the white (myelinated) rami reach the thoracic and abdominal viscera down as far as the proximal two-thirds
sympathetic trunk they may follow one of three different routes: of the transverse colon, where supply is taken over by the sacral out-
1 They may synapse with a nerve cell in the corresponding ganglion. flow. Synapses occur in minute ganglia in the cardiac and pulmonary
2 They may pass straight through the corresponding ganglion and travel plexuses and in the walls of the viscera.
up or down the sympathetic trunk, to synapse in another ganglion. • The sacral outflow:
3 They may pass straight through their own ganglion, maintaining From the sacral nerves S2, 3 and 4, fibres join the inferior hypogastric
their preganglionic status until they synapse in one of the outlying plexuses by means of the pelvic splanchnic nerves. They go on to
ganglia such as the coeliac ganglion. One exceptional group of supply the pelvic viscera, synapsing in minute ganglia in the walls of
fibres even pass through the coeliac ganglion and do not synapse the viscera themselves. Some fibres climb out of the pelvis around the
until they reach the suprarenal medulla. inferior mesenteric artery and supply the sigmoid and descending colon
• Postganglionic fibres: after synapsing, the postganglionic fibres and the distal one third of the transverse colon.
may follow one of three different routes:

Table 53.1 The autonomic system.
Region Origin of connector fibres Site of synapse
Sympathetic
Head and neck T1–T5 Cervical ganglia
Upper limb T2–T6 Inferior cervical and 1st thoracic ganglia
Lower limb T10–L2 Lumbar and sacral ganglia
Heart T1–T5 Cervical and upper thoracic ganglia
Lungs T2–T4 Upper thoracic ganglia
Abdominal and pelvic T6–L2 Coeliac and subsidiary ganglia
viscera
Parasympathetic
Head and neck Cranial nerves 3, 7, 9, 10 Various parasympathetic macroscopic ganglia
Heart Cranial nerve 10 Ganglia in vicinity of heart
Lungs Cranial nerve 10 Ganglia in hila of lungs
Abdominal and pelvic Cranial nerve 10 Microscopic ganglia in walls of viscera
viscera (down to transverse colon)
S2, 3, 4 Microscopic ganglia in walls of viscera

The autonomic nervous system 121