Sympathetic Nervous System: Anatomy & Pathways

Main role of the sympathetic nervous system

Prepares the body for emergency situations and helps maintain homeostasis during physical activity.

Sympathetic nervous system and physical activity

Maintains homeostasis during physical work, including minor activities such as standing.

Sympathetic response to standing

Temporarily changes heart rate and blood vessel diameter to counteract gravity's effects on blood pressure.

Orthostatic hypotension

Drop in blood pressure that can occur with standing and may cause a person to fall.

Sympathetic nervous system and emotions

Mediates visceral responses to strong emotions such as fear, excitement, rage, and embarrassment.

Origin of sympathetic preganglionic neurons

Preganglionic neuron cell bodies begin in the lateral horns of the thoracic and lumbar spinal cord.

Sympathetic preganglionic pathway out of spinal cord

Preganglionic axons leave the spinal cord through the anterior root with axons of somatic lower motor neurons.

Sympathetic pathway after anterior root

Preganglionic axons travel with the spinal nerve and anterior ramus before branching into the white rami communicantes.

White rami communicantes

Small nerves containing myelinated sympathetic preganglionic axons that enter sympathetic chain ganglia.

White rami communicantes myelination

White rami communicantes contain myelinated axons.

Sympathetic chain ganglia

Series of ganglia along the vertebral column containing most sympathetic postganglionic neuron cell bodies.

Sympathetic chain ganglia location

Located along the vertebral column near the spinal cord.

Extent of sympathetic chain ganglia

Extend beyond the thoracic and lumbar spinal cord from the superior cervical ganglion to the inferior sacral ganglion.

Postganglionic sympathetic neuron cell bodies

Most are located in the sympathetic chain ganglia.

Collateral ganglia

Sympathetic ganglia located away from the sympathetic chain and nearer the target organs.

Preaortic ganglia

Collateral ganglia located near the aorta.

Splanchnic nerves

Preganglionic sympathetic axons that pass toward collateral ganglia near organs of the abdominopelvic cavity.

Major collateral ganglia

Celiac ganglion, superior mesenteric ganglion, and inferior mesenteric ganglion.

Celiac ganglion

Collateral sympathetic ganglion associated with splanchnic nerve pathways to abdominopelvic organs.

Superior mesenteric ganglion

Collateral sympathetic ganglion associated with splanchnic nerve pathways to abdominopelvic organs.

Inferior mesenteric ganglion

Collateral sympathetic ganglion associated with splanchnic nerve pathways to abdominopelvic organs.

Three possible sympathetic preganglionic pathways

Ascend or descend and synapse in a different chain ganglion; synapse in the same chain ganglion; or pass through the chain and synapse in a collateral ganglion.

Sympathetic pathway 1

Preganglionic axon ascends or descends the sympathetic chain and synapses with a postganglionic neuron in a different chain ganglion.

Sympathetic pathway 2

Preganglionic axon synapses with a postganglionic neuron in the same sympathetic chain ganglion.

Sympathetic pathway 3

Preganglionic axon passes through the sympathetic chain ganglion and enters the abdominopelvic cavity, where it synapses with a postganglionic neuron in a collateral ganglion.

Gray rami communicantes

Small nerves through which postganglionic sympathetic axons from sympathetic chain ganglia rejoin spinal nerves.

Gray rami communicantes myelination

Gray rami communicantes contain unmyelinated postganglionic axons.

Postganglionic pathway after gray ramus

Postganglionic axons rejoin and travel with spinal nerves to reach their target cells.

Postganglionic pathway from collateral ganglion

Postganglionic axons travel directly from the collateral ganglion to their target cells.

Sympathetic pathway to a chain ganglion

Lateral horn → anterior root → spinal nerve → anterior ramus → white ramus communicans → sympathetic chain ganglion.

Sympathetic pathway back to a spinal nerve

Sympathetic chain ganglion → gray ramus communicans → spinal nerve → target.

Sympathetic pathway to a collateral ganglion

Preganglionic axon passes through the sympathetic chain → travels toward the abdominopelvic cavity → synapses in a collateral ganglion → postganglionic axon travels to target.

White vs. gray rami

White rami carry myelinated preganglionic sympathetic axons into the sympathetic chain; gray rami carry unmyelinated postganglionic sympathetic axons out of the chain to rejoin spinal nerves.

Preganglionic vs. postganglionic sympathetic axons

Preganglionic axons travel from the CNS to an autonomic ganglion; postganglionic axons travel from the ganglion to target cells.