Sympathetic nervous system

Autonomic (visceral) nervous system

The nervous system can be divided into central and peripheral nervous systems. The peripheral nervous system is further divided into the somatic and autonomic nervous systems.

The autonomic nervous system (ANS) is a functional division of the nervous system that controls involuntary actions of muscles, glands and internal organs (e.g. bowel movements). Together with endocrine glands, the ANS affects important body functions without the direct involvement of the cerebral cortex. In contrast, the somatic nervous system mediates voluntary responses of the body (e.g. skeletal muscle function) and it’s under the direct control of the cerebral cortex.

The ANS can be divided according to its location (central and peripheral parts) and function. Functionally, the ANS is divided into sympathetic (SNS) and parasympathetic (PSNS) nervous systems. They usually work antagonistically in the organs but in a well-integrated manner. It is the balance of the actions of both divisions that maintains a stable internal environment in the body.

The anatomical distinction between the sympathetic and parasympathetic divisions is given by the location of the presynaptic cell bodies and the types of nerves conducting presynaptic nerve fibers.

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General sympathetic pathway

The general sympathetic pathway can be simplified into the following components:

• Preganglionic neurons;
• Sympathetic ganglia;
• Postganglionic neurons.

The preganglionic neurons are located inside the thoracic and lumbar segments (T1-L2,3) of the spinal cord and their fibers (axons), which are called preganglionic fibers. These preganglionic fibers synapse with the postganglionic neurons inside the sympathetic ganglia, which are typically found near the vertebral column. These ganglia are actually a collection of cell bodies of postganglionic neurons. The postganglionic neurons give off long axons (postganglionic fibers) that leave the ganglia and project onto visceral effectors, where they release the neurotransmitter norepinephrine. Both preganglionic and postganglionic neurons are multipolar.

Preganglionic neurons

The cell bodies of the preganglionic neurons of the SNS are found only in the intermediolateral cell columns (ICLs) of the spinal cord, one on the left side and on the right. ICLs are part of the lateral horns of the gray matter of the thoracic (T1-12) and upper lumbar (L2 or L3) spinal cord segments, hence the alternative name “thoracolumbar” for the sympathetic division. This region consists of the visceral motor region of the spinal gray matter. You can think of the ICLs as longitudinal tubes passing through the respective lateral horns of the spinal cord. The preganglionic SNS cell bodies are organised somatotopically, meaning the arrangement of the cell bodies is a close representation to that of the body. Basically, the T1-6 cell bodies that are located superiorly innervate the head, upper limb and thoracic viscera. T7-11 located in the middle innervate the body wall and abdominal viscera, while T11-L2(3) located inferiorly innervate the lower limb and pelvic viscera.

The preganglionic fibers leave the ICLs and thus, the spinal cord through the anterior roots. They travel very briefly through the anterior rami of spinal nerves T1-L2(3), before leaving them and passing to the sympathetic trunks (more details later) through the white rami communicantes (white because nerve fibers are covered with white myelin).

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Sympathetic ganglia

Postganglionic neurons

The postganglionic compartment consists of postganglionic fibers travelling to effectors. The number of postganglionic fibers are greater than preganglionic ones. Approximately one preganglionic fiber synapses with at least thirty postganglionic fibers. After synapsing, postganglionic fibers leave the ganglia through gray rami communicantes and travel through the anterior and posterior rami of the spinal nerves. These rami carry the fibers all the way to the periphery and visceral components.

Ascending sympathetic fibers through the sympathetic trunk join peripheral nerves from C2-8 spinal nerves. These project onto effectors in the head, neck, upper limbs and thoracic cavity. For example, a cephalic arterial nervous branch leaves the superior cervical ganglion and projects onto the peri-arterial plexus on the carotid arteries. From here they project onto the dilator muscle of iris.

Descending sympathetic fibers through the sympathetic trunk join peripheral nerves from L3 to coccyx spinal nerves. These project onto the skin in the lower limbs, where they stimulate vasomotion, sudomotion and pilomotion.

Sympathetic fibers that enter and leave the trunk at the same level join peripheral nerves from T1-L2(3) spinal nerves. These project onto the body wall via cutaneous branches, but also via visceral motor nerves to sweat glands, smooth muscle and arrector pili muscles. Postganglionic fibers can also combine to form splanchnic nerves. These nerve types convey visceral efferent and afferent fibers to and from the viscera. Postganglionic fibers projecting onto thoracic viscera (e.g., heart, lungs, esophagus) pass through cardiopulmonary splanchnic nerves.

Function

The reach of the sympathetic system is extremely broad within the human body. It is a component of virtually all spinal nerves and peri-arterial plexuses, and sympathetic fibers innervate all the blood vessels, sweat glands, arrector pili and viscera. The only structures the sympathetic system does not reach are avascular structures, like nails and cartilage.

The sympathetic and parasympathetic divisions of the nervous system work in very close association, with contrasting, yet tightly coordinated effects. The sympathetic system is involved in energy-expending (catabolism), enabling the body to use energy appropriately to respond to stressful situations and emergencies, as in the “fight or flight” response. Activation of the sympathetic system results in pupil dilation, piloerection, vasoconstriction of cutaneous blood vessels, sweating, release of adrenaline, bronchodilation, increased cardiac contraction and reduced digestion.

During normal conditions, blood vessels are tonically maintained in a resting state of moderate vasoconstriction. If sympathetic signals are increased, vasoconstriction increases and vice-versa. However, in coronary vessels, skeletal muscles and vessels of the external genitalia, sympathetic stimulation results in vasodilation.

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