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The phrenic nerves
chea and oesophagus as well as providing a motor supply to all of the
The phrenic nerves arise from the C3, C4 and C5 nerve roots in the
muscles of the larynx (except cricothyroid) and sensory fibres to the
neck.
lower larynx.
• The right phrenic nerve (Fig. 9.1) descends along a near vertical • The recurrent laryngeal nerves supply the mucosa of the upper tra-
path, anterior to the lung root, lying on sequentially: the right brachio- • The vagi also contribute branches to the cardiac and pulmonary
cephalic vein, the superior vena cava, and the right atrium before pass- plexuses.
ing to the inferior vena caval opening in the diaphragm (T8). Here the
right phrenic enters the caval opening and immediately penetrates the The thoracic sympathetic trunk (Figs 9.2 and 9.3, and
diaphragm which it supplies. Chapter 53)
• The left phrenic nerve (Fig. 9.2) descends alongside the left subcla- • The thoracic sympathetic chain is a continuation of the cervical
vian artery. On the arch of the aorta it passes over the left superior inter- chain. It descends in the thorax behind the pleura immediately lateral to
costal vein to descend in front of the left lung root onto the pericardium the vertebral bodies and passes under the medial arcuate ligament of the
overlying the left ventricle. The left phrenic then pierces the muscular diaphragm to continue as the lumbar sympathetic trunk.
diaphragm as a solitary structure. Note: the phrenic nerves do not pass • The thoracic chain bears a ganglion for each spinal nerve; the first
beyond the undersurface of the diaphragm. frequently joins the inferior cervical ganglion to form the stellate gan-
• The phrenic nerves are composed mostly of motor fibres which supply glion. Each ganglion receives a white ramus communicans containing
the diaphragm. However, they also transmit fibres which are sensory preganglionic fibres from its corresponding spinal nerve and sends
to the fibrous pericardium, mediastinal pleura and peritoneum as well back a grey ramus, bearing postganglionic fibres.
as the central part of the diaphragm. Upper limb sympathectomy is used for the treatment of hyperhidro-
Irritation of the diaphragmatic peritoneum is usually referred to the sis and Raynaud syndrome. Surgical sympathectomy involves excision
C4 dermatome. Hence, upper abdominal pathology such as a perfor- of part of the thoracic sympathetic chain (usually for two interspaces)
ated duodenal ulcer often results in pain felt at the shoulder tip. below the level of the stellate ganglion. The latter structure must be
identified on the neck of the 1st rib.
The vagi Branches:
The vagi are the 10th cranial nerves (p. 145). • Sympathetic fibres are distributed to the skin with each of the thor-
• The right vagus nerve (Figs 9.3 and 3.2) descends adherent to the thor- acic spinal nerves.
acic trachea prior to passing behind the lung root to form the posterior • Postganglionic fibres from T1–5 are distributed to the thoracic
pulmonary plexus. It finally reaches the lower oesophagus where it visceraathe heart and great vessels, the lungs and the oesophagus.
forms an oesophageal plexus with the left vagus. From this plexus, • Mainly preganglionic fibres from T5–12 form the splanchnic nerves,
anterior and posterior vagal trunks descend (carrying fibres from both which pierce the crura of the diaphragm and pass to the coeliac and
left and right vagi) on the oesophagus to pass into the abdomen through renal ganglia from which they are relayed as postganglionic fibres to
the oesophageal opening in the diaphragm at the level of T10. the abdominal viscera (cf. fibres to the suprarenal medulla which are
• The left vagus nerve (Fig. 9.2) crosses the arch of the aorta and preganglionic). These splanchnic nerves are the: greater splanchnic
its branches. It is itself crossed here by the left superior intercostal (T5–10), lesser splanchnic (T10–11) and lowest splanchnic (T12).
vein. Below, it descends behind the lung root to reach the oesophagus They lie medial to the sympathetic trunk on the bodies of the thoracic
where it contributes to the oesophageal plexus mentioned above (see vertebrae and are quite easily visible through the parietal pleura.
Fig. 3.2).
The cardiac plexus
Vagal branches This plexus is for descriptive purposes divided into superficial and deep
• The left recurrent laryngeal nerve arises from the left vagus below parts. It consists of sympathetic and parasympathetic efferents as well
the arch of the aorta. It hooks around the ligamentum arteriosum and as afferents.
ascends in the groove between the trachea and the oesophagus to reach • Cardiac branches from the plexus supply the heart where they:
the larynx (p. 139). accompany coronary arteries for vasomotor control and supply the
• The right recurrent laryngeal nerve arises from the right vagus in the sinu-atrial and atrioventricular nodes for cardio-inhibitory and cardio-
neck and hooks around the right subclavian artery prior to ascending in acceleratory purposes.
the groove between the trachea and the oesophagus before finally • Pulmonary branches supply the bronchial wall smooth muscle (con-
reaching the larynx. trolling diameter) and pulmonary blood vessels for vasomotor control.

The nerves of the thorax 25