Sympathetic nerves

At a glance

English

Sympathetic nerves

Latin (TA2)

Nervi sympathici

System

Nerve

Region

Neck

Side

Paired, left and right

About the sympathetic nerves

Sympathetic nerves make up one of two main autonomic divisions, the other being the parasympathetic. They govern functions that run without conscious control. The division is best known for preparing the body for effort or threat, the fight-or-flight response, but it is active at a low level all the time, holding blood pressure, body temperature and other settings steady.

Every sympathetic pathway uses two neurons. The first, preganglionic cell lies in the intermediolateral (lateral) grey column of the spinal cord from about T1 to L2 or L3, which is why the outflow is called thoracolumbar. Its short myelinated axon leaves through the anterior root, enters the spinal nerve and quickly turns off through a white ramus communicans into the sympathetic trunk. There it may relay at its own level, travel up or down the chain to relay at another ganglion, or pass through to a prevertebral ganglion, such as the coeliac or mesenteric ganglia, near the aorta. The second, postganglionic neuron then sends a long axon out to the target. Cervical ganglia serve the head and chest; coeliac and mesenteric ganglia serve the gut.

Preganglionic endings release acetylcholine onto nicotinic receptors. Most postganglionic endings release noradrenaline onto adrenergic receptors. Exceptions are sweat glands, which mostly receive acetylcholine; the adrenal medulla, whose chromaffin cells act as modified postganglionic cells and pour adrenaline into the blood; and renal vessels, where dopamine is also released. Typical effects include a faster, stronger heartbeat, widened airways and pupils, slower gut movement, sweating, raised hairs and narrowing of vessels in skin, gut and kidney, while vessels in working muscle, heart and brain tend to widen through beta-2 receptors. Blood is thus moved to the tissues that need it most.

Visceral pain fibres run back alongside sympathetic nerves and explain referred pain to the skin of the same spinal segments. In heart failure, sustained sympathetic drive supports blood pressure in the short term but worsens the condition over time, which is why beta-blockers are used.

Course and supply

Arises from

Preganglionic neurons in the lateral grey column of spinal segments T1 to L2 or L3; axons leave through the anterior roots and white rami communicantes.

Ends in

Postganglionic fibres from the paravertebral and prevertebral ganglia end on smooth muscle, cardiac muscle and glands throughout the body; some preganglionic fibres end directly on the adrenal medulla.

Supplies

Visceral motor supply to the heart, blood vessels, airways, gut, pupils, sweat glands and hair muscles, raising heart output and redistributing blood during effort or stress; visceral pain fibres travel with them.