Sympathetic Nervous System - Structure - Chain (2024)

Thesympathetic nervous system (SNS) is one of the two main divisions of the autonomic nervous system (ANS). It acts in conjunction with the parasympathetic nervous system (PNS) to maintain homeostasis within the body.

The collective action of the sympathetic nervous system can be described as “fight or flight”. It dominates during periods of stress or urgency and helps to rapidly prepare the body for intense physical activity.

In this article, we shall look at theanatomy of the sympathetic nervous system – its components, actions, and clinical correlations.

Anatomical Structure

The sympathetic nervous system consists of the following components:

• Pre-ganglionic fibres – these originate within the thoracic and lumbar divisions of the spinal cord. They end by synapsing with post-ganglionic fibres at peripheral ganglia.
• Peripheral ganglia– groups of neuronal cell bodies where the pre-ganglionic and post-ganglionic fibres synapse.
• Post–ganglionic fibres – these begin at the peripheral ganglia and carry sympathetic fibres to the target organs.

We shall now look at each component of the sympathetic nervous system in more detail.

Pre-Ganglionic Neurones

The pre-ganglionic neurones originate in the thoracic and lumbar segments of the spinal cord. They areshort, myelinated, and release acetylcholine.

They exit the spinal cord via the anterior spinal nerves and then pass through white ramus communicantes to enter the sympathetic trunk.

Within the sympathetic trunk, the sympathetic fibres may take one of three actions:

• Synapse with post-ganglionic neurones – occurring within a sympathetic ganglion at the same spinal level.
• Ascend or descend within the sympathetic chain – to synapse with post-ganglionic neurones at a higher or lower spinal level.
• Pass through the chain without synapsing – will later synapse at a prevertebral ganglion or the adrenal medulla (these fibres are known as splanchnic nerves).

Peripheral Ganglia

The peripheral ganglia are groups of neuronal cell bodies where the pre-ganglionic and post-ganglionic fibres synapse.

In the sympathetic nervous system, they are organised into two main structures:

• Sympathetic trunks – two chains of interconnected ganglia that lie either side of the vertebral column.
• Prevertebral ganglia – three ganglia arranged around the major branches of the abdominal aorta (coeliac, superior mesenteric and inferior mesenteric ganglia).

Sympathetic Chain

The sympathetic chain (sympathetic trunk or paravertebral ganglia) are 22 pairs of interconnected neuronal cell bodies which run parallel either side of the vertebral column – extending from the base of the skull to the coccyx.

The sympathetic chain may be broken down into regions, each of which has multiple ganglia:

• Cervical – three ganglia (superior/middle/inferior) – see here to read about the cervical ganglia in more detail.
• Thoracic – eleven or twelve ganglia
• Lumbar – four ganglia
• Sacral – four or five ganglia
• Coccyx – a single, ganglion impar

Prevertebral Ganglia

The prevertebral ganglia (pre-aortic ganglia or collateral ganglia) are a collection of cell bodies located anterior to the abdominal aorta.

They are supplied by splanchnic nerves –pre-ganglionic neurons which have passed through the sympathetic chain without synapsing.

• Coeliac ganglion – located at base of coeliac trunk. It receives input from thoracic pre-ganglionic neurons from T5-T9 via the greater splanchnic nerve. It supplies liver, gallbladder, stomach, spleen, pancreas.
• Superior mesenteric ganglion – located near the base of the superior mesenteric artery. It receives input from lower thoracic pre-ganglionic neurons from T10-T11 via the lesser splanchnic nerve. It supplies small intestine and colon.
• Inferior mesenteric ganglion – located near the base of inferior mesenteric artery. It receives input from pre-ganglion neurons from T12 via the least splanchnic nerve. It supplies the kidney, bladder, colon, uterus, ovaries and testes.

Adrenal Medulla

The adrenal medulla is a modified sympathetic ganglion.

Although a target organ, the pre-ganglionic fibres synapse directly in the medulla (having passed through the sympathetic chain and coeliac ganglion without synapsing).

Upon sympathetic stimulation, the adrenal medulla secretes adrenaline and noradrenaline into the bloodstream – where they have a more longer lasting hormonal effect on target organs.

Post-Ganglionic Neurones

Thepost-ganglionic neuronesoriginate in the peripheral ganglia and carry signals to the target organs.

They travel to their target organs either as distinct, named nerves, alongside blood vessels (arteries in particular) or within autonomic plexi.

Their axons are typically long and unmyelinated. They release noradrenaline (with the exceptions of neurones synapsing at sweat glands and erector pili muscles in skin, which release acetylcholine).

* Exception of nerves to sweat glands and erector pili muscles where acetylcholine is released

Sympathetic Actions

The sympathetic nervous system acts on a range of systems within the human body:

• Eyes – dilatation of the pupils.
• Skin – sweat secretion, contraction of erector pili muscles.
• Cardiovascular – vasodilatation of vessels to skin, skeletal muscles, heart, lungs and brain, vasoconstriction of blood to gastrointestinal tract and kidneys; increased heart rate.
• Endocrine – secretion of adrenaline and noradrenaline from adrenal medulla, ADH secretion from pituitary gland, decreased insulin secretion from pancreas.
• Respiratory – bronchodilatation, mucous secretion.
• Digestive – decreased activity, constriction of sphincters in gastrointestinal tract, inhibition of secretory glands, glycogenolysis and gluconeogenesis in the liver, decreased exocrine secretion from the pancreas.
• Skeletal – increased force of contraction, glycogenolysis.
• Adipose tissue – lipolysis.
• Urinary – renin secretion from kidneys, relaxation of urinary bladder and constriction of internal urethral sphincter.
• Reproductive – glandular secretion and ejaculation in males; relaxation of non-pregnant uterus or increased glandular secretion and contraction of pregnant uterus in females.