Sympathetic nervous system

The sympathetic nervous system (SNS) is a vital division of the autonomic nervous system. This system regulates many of the body's unconscious actions. It works alongside the parasympathetic and enteric nervous systems to maintain balance. The SNS is often described as being antagonistic to the parasympathetic nervous system. While the parasympathetic system handles "rest-and-digest" functions, the SNS manages the "fight or flight" response.

Even when you are not in danger, the SNS is constantly active. It works at a basic level to maintain homeostasis, which is the body's steady internal state. It plays major roles in regulating blood glucose, body temperature, cardiac output, and immune function.

To send signals, the sympathetic system uses two distinct types of neurons. The first type is the preganglionic neuron. These are relatively short cells that originate in the thoracolumbar division of the spinal cord. This specific area spans from the T1 level to the L2 or L3 vertebrae.

These neurons travel to a structure called a ganglion. A ganglion is a collection of nerve cell bodies that acts as a relay station. At the synapse, or the gap between cells, the preganglionic neuron releases a neurotransmitter called acetylcholine. This chemical activates nicotinic acetylcholine receptors on the next cell in the chain.

The second type of neuron is the postganglionic neuron. These neurons begin in the ganglion and extend long distances to reach target tissues. Once activated by acetylcholine, the postganglionic neuron releases norepinephrine. This chemical then binds to adrenergic receptors on the target organ or tissue. This binding causes the physiological changes we associate with the sympathetic response.

There are a few notable exceptions to this chemical pattern. For example, postganglionic neurons for sweat glands release acetylcholine instead of norepinephrine. However, in the palms and soles of the feet, they release norepinephrine to act on adrenergic receptors.

The organization of these nerves follows a specific path from the spinal cord. The axons leave the spinal cord through the anterior root. They pass through white rami connectors, which are named for the shiny white myelin sheaths around the axons. These connectors lead the nerves to either paravertebral ganglia or prevertebral ganglia. Paravertebral ganglia sit near the vertebral column. Prevertebral ganglia are located near the aortic bifurcation. This arrangement allows the system to reach almost every organ system in the body.

A unique part of this system involves the adrenal medulla, which is located in the suprarenal glands. The adrenal medulla acts like a modified sympathetic ganglion. Instead of a long postganglionic neuron, preganglionic neurons synapse directly with chromaffin cells in the gland. When stimulated, these cells release neurotransmitters directly into the bloodstream. They release a small amount of norepinephrine, but they primarily release epinephrine, also known as adrenaline. This release of epinephrine is a distinguishing feature of these specialized cells.

The effects of the sympathetic nervous system are widespread and rapid. During a sympatho-adrenal response, the system primes the body for intense physical activity. It can increase heart rate and the force of cardiac contractions. It dilates the bronchioles in the lungs to allow more oxygen into the body. It also causes pupillary dilation and perspiration.

To ensure muscles have enough energy, the SNS causes vasoconstriction in the skin and digestive tract. This shunts blood away from non-essential organs and toward the skeletal muscles and brain. In the muscles, the presence of beta-2 adrenergic receptors helps dilate blood vessels to increase flow.

This complex system of nerves and chemicals ensures survival during stress. By coordinating the heart, lungs, and blood vessels, the SNS prepares the body to act. It also manages internal signals through general visceral afferent fibers. These fibers carry sensory information from the internal organs back to the central nervous system. Understanding the sympathetic nervous system helps scientists understand how the body manages everything from sudden danger to daily biological stability.