Phrenic Nerves: Anatomy, Function, And Conditions

Phrenic Nerves: Anatomy, Function, And Conditions

Describe the phrenic nerve in brief and discuss its applied anatomy.
Answer.

Phrenic Nerve Origin

Phrenic Nerve is formed by ventral rami of C3 to C5, with chief contribution being from C4.

Phrenic Nerve Course

Phrenic Nerve first descends obliquely on the anterior surface of scalenus anterior. Then it runs vertically downwards on the cervical pleura to enter thoracic cavity behind the 1st costal cartilage.

Phrenic Nerve Branches and distribution

• Motor branches to diaphragm
• Sensory branches to central part of the diaphragm, pleura, pericardium and peritoneum (subdiaphragmatic)

Phrenic Nerve Applied Anatomy

• Damage of phrenic nerve in the neck leads to paralysis of corresponding half of the diaphragm. The paralysed half of diaphragm becomes relaxed and pushed up into thorax by the positive intra-abdominal pressure. This leads to collapse of the lower lobe of the lung.
• The fibres of C5 instead of joining phrenic nerve at its commencement may join it at the thoracic inlet through a communication received from nerve to subclavius. This communication is referred to as accessory phrenic nerve. In phrenic avulsion, the accessory phrenic nerve if present should be cut, otherwise C5 fibres will escape and diaphragm may continue to function and defeat the whole purpose of phrenic crush.
• In pleurisy, the pain from diaphragmatic pleura may be referred to shoulder region, which receives nerve supply from some spinal segments as that of phrenic nerve, i.e. C3 and C4.