📝 STANDARD DESCRIPTION:
Phrenic Nerve:
- Origin: Ventral branches of C5, C6, C7 cervical spinal nerves.
- Course: Runs down through the thorax on the side of the mediastinum.
- Supply: SOLE motor nerve to the DIAPHRAGM.
- Memory: 'C3-4-5 keeps the diaphragm alive' (in ox more like C5-6-7).
Sympathetic Trunk in Thorax:
- Chain of ganglia along the necks of the ribs.
- Receives white rami from thoracic spinal nerves.
- Forms Cardiac plexus around the heart base (with vagus).
- Contributes to the Greater Splanchnic Nerve → goes to abdomen → coeliac ganglion.
- Sympathetic effect: SPEEDS the heart, DILATES bronchi (opposite of vagus).
📚 DETAILED DESCRIPTION:
NERVE TO THE DIAPHRAGM + SYMPATHETIC SUPPLY OF CHEST
PHRENIC NERVE — THE 'BREATHING NERVE':
The phrenic is a special nerve because it travels far from its origin. It is formed in the neck from the ventral branches of cervical spinal nerves C5, C6 and C7 (in ox; C5-C7 in horse and dog as well, sometimes C4 contributes).
COURSE: The phrenic runs down the side of the neck → enters the thoracic inlet → passes through the cranial mediastinum on the side of the pericardium (covering of the heart) → continues caudally and ends by spreading into the diaphragm.
SUPPLY: The phrenic is the only motor nerve to the diaphragm. Without it, the diaphragm cannot contract.
WHY CERVICAL ORIGIN? Embryologically, the diaphragm starts as a sheet of muscle in the neck region; it then migrates back to its adult position, dragging its nerve supply along with it. That is why a 'belly muscle' is supplied by 'neck nerves'.
SYMPATHETIC TRUNK IN THORAX:
This is a chain of small swellings (ganglia) and connecting cords lying along the heads/necks of the ribs, just under the parietal pleura.
• Receives white communicating rami from each thoracic spinal nerve (carrying preganglionic sympathetic fibres).
• Sends grey communicating rami back to all spinal nerves (carrying postganglionic fibres for blood vessels and sweat glands of the body wall).
• Sends branches forward to form the cardiac plexus around the heart base, working together with vagus branches there.
• Sends branches caudally that gather into the greater splanchnic nerve, which pierces the diaphragm and goes to the coeliac ganglion in the abdomen.
SYMPATHETIC EFFECTS in chest:
• Heart: speeds up rate, increases force of contraction.
• Bronchi: dilates them (open airways) — opposite of vagus.
This is why drugs that mimic sympathetic action (like adrenaline) are used in shock or asthma-like conditions.
// COMPARATIVE ANALYSIS
| DOG |
Phrenic from C5-C7. Cervical disc disease in front of C5 can paralyse the diaphragm. |
| HORSE |
Phrenic injury is rare but devastating — animal cannot breathe properly. |
CLINICAL RELEVANCE
Phrenic Paralysis: If only one phrenic is damaged → that half of the diaphragm rises during breathing instead of falling (paradoxical movement) → mild breathing difficulty.
If both are damaged → fatal respiratory failure unless ventilator support is given.
Hiccups = sudden involuntary contractions of the diaphragm caused by phrenic nerve irritation.
