Overview
The paired phrenic nerves arise in the neck from C3-5 nerve
roots and course down through the thorax, giving off sensory fibres, to reach
the diaphragm, where they supply motor and sensory fibres to their respective
hemidiaphragm.
Gross Anatomy
Originates mainly from the anterior primary rami of C4, with
contributions from C3 and C5
Arising between the anterior and middle scalene muscles, it courses
around the lateral border of anterior scalene, moving from its lateral to
medial edge
It enters the mediastinum between the subclavian vein,
anteriorly, and the subclavian artery, posteriorly
Sensory supply to mediastinal pleura
Left and right phrenic nerves then follow different courses
through the thorax: The left courses over the arch of the aorta, anterior to
the left Vagus nerve (CN X) and pulmonary artery then runs laterally through
the pericardium (sensory supply) across the left ventricle. It reaches the underside
of the diaphragm via its muscular portion near the apex of the heart.
Right descends along the lateral surface of the superior
vena cava, pericardium of the right atrium (sensory supply), and inferior vena
cava. It travels through the caval orifice with IVC to reach the underside of
the diaphragm.
Approximately two thirds of fibres to the diaphragm are
motor, the remaining third providing sensory supply to diaphragmatic pleura
& peritoneum, and the diaphragm itself.
Clinical Anatomy
Injury
to one of the phrenic nerves results in paralysis of the ipsilateral
hemidiaphragm, with the other half unaffected.
Injection
of local anaesthetic around the phrenic nerve where it lies anterior to the
anterior scalene produces a phrenic nerve block, causing a temporary paralysis
of the hemidiaphragm (e.g. – in thoracic surgery)
Irritation
of the diaphragmatic peritoneum (e.g. – subphrenic abscess, cholecystitis)
classically refers pain to the C4 dermatome- shoulder tip pain.
Quick Anatomy
Key Facts
Quick-look box – Phrenic nerve |
|
Origin |
C3, 4, 5 ventral rami |
Supply |
· Mediastinal pleura (SENSORY) · Pericardium (SENSORY) · Diaphragmatic pleura (SENSORY) · Diaphragmatic peritoneum (SENSORY) · Diaphragm (MOTOR) |
Aide-Memoire
Remember:
“C3, 4, 5 keeps the diaphragm alive!”
Summary
The phrenic nerves descend from neck
through thorax where they provide sensory supply to adjacent structures before
reaching the diaphragm. Here they supply sensory fibres to adjacent pleura and
peritoneum, as well as being the sole motor supply to their respective half of the
diaphragm. Insult, either traumatic or iatrogenic, to one phrenic nerve
therefore will result in paralysis of the diaphragm on the affected side.
References
Ellis H
& Mahadevan V. 2010. Clinical Anatomy
12th ed. Oxford: Wiley-Blackwell
Whitaker RH
& Borley NR. 2010. Instant Anatomy
4th ed. Oxford: Wiley-Blackwell
Sinnatamby
CS. 2006. Last’s Anatomy 11th
ed. China: Churchill Livingstone
Moore KL & Dalley AF. 2006. Clinically Oriented Anatomy 5th
ed. Baltimore: Lippincott Williams & Wilkins