The nerves of the thorax

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