18.3: The Sympathetic Nervous System

The sympathetic nervous system—one of the two major divisions of the autonomic nervous system—is activated in times of stress. It prepares the body to meet the challenges of a demanding circumstance while inhibiting essential body functions—such as digestion—that are a lower priority at the moment.

The “Fight or Flight” System

As a student, you may have had the experience of walking into class and finding a surprise exam that you were not expecting. In the moment of realization, you may sense your gut tighten, your mouth goes dry, and your heart starts to race all of a sudden. These are signs of the sympathetic system taking over in preparation to react. While you may not be in immediate danger, the system has evolved to facilitate immediate reaction to stress or threats: blood is directed away from the digestive system and skin to increase energy supplies to muscles. Furthermore, the heart rate, and blood flow increase, and pupils dilate to maximize visual perception. At the same time, the adrenal gland releases epinephrine into the circulatory system. Your body is now primed to take action, whether that means to swiftly flee from danger or fight whatever threat may be at hand.

Control of the Sympathetic System

The sympathetic nervous system can be activated by various parts of the brain, with the hypothalamus playing a particularly important role. Sympathetic instructions from the central nervous system are sent from preganglionic neurons in the thoracic and lumbar parts of the spinal cord out to the peripheral nervous system—where they have their ultimate effects—through three main pathways.

The first pathway involves preganglionic neurons that connect to a nearby chain of ganglia—clusters of neuronal cell bodies—that runs along each side of the spinal column, often called the sympathetic chain. Here, the signal activates postganglionic neurons that travel to target organs or glands to regulate their function. Their effects include dilation of the pupils—allowing more light to enter the eyes; relaxation of airways—bringing in more oxygen, and constriction of blood vessels and increased heart rate—pushing more blood to muscles.

In the second pathway, preganglionic neurons send signals to ganglia farther from the spinal cord, near the target organs. Here, they activate postganglionic neurons that control the activity of organs such as the stomach, pancreas, liver, intestines, bladder, and genitals. Effects of this pathway include inhibiting digestion and urination, and stimulating orgasms.

In the third pathway, a small group of preganglionic neurons travels from the spinal cord directly to the adrenal gland, where they stimulate the secretion of the hormones epinephrine and norepinephrine into the bloodstream—helping to mediate the stress response throughout the body.

Chronic Stress

The sympathetic nervous system evolved to allow animals to best respond to immediate threats such as the sudden encounter with a predator or rival. While humans have left most of those primal fears behind, the body’s sympathetic system can still respond to stressful situations in the very same manner as when encountering a fierce savannah cat. This includes the release of the hormones epinephrine—also called adrenaline—and cortisol.

Under normal conditions, the system is briefly activated, and these hormones are not present in the circulatory system for long. However, modern stressors in our daily life can over-activate this system and expose the body for longer periods, which can have debilitating effects. These can include anxiety, depression, heart disease, weight gain, and sleeping disorders. The demands of university life can be challenging, so it is critical to engage in healthy stress management. Some strategies for managing stress include healthy eating, getting an adequate amount of sleep, maintaining social relationships, and seeking professional counseling when necessary.

The sympathetic nervous system prepares the body to take action during times of threat, stress or excitement, including activating the fight or flight response.

It is one of the major divisions of the autonomic nervous system which controls involuntary functions by regulating the activity of smooth muscle in visceral organs, cardio muscle in the heart, and certain glands such as sweat glands.

When the sympathetic nervous system is activated, preganglionic neurons in the thoracic and lumbar spinal cord send signals to the peripheral nervous system. Some send their signals to a chain of ganglia, clusters of neuronal cell bodies that runs down the side of the spinal column.

From here, postganglionic neurons transmit the signal to target tissues causing responses such as increased heart rate and constriction of blood vessels bringing more blood to muscles.

Other preganglionic neurons send signals to postganglionic neurons in ganglia close to their target organs where they have effects such as inhibiting digestion in the stomach and excretion from the bladder, processes that are low priority in times of stress. 

A third group of preganglionic neurons travels directly to the adrenal gland where they stimulate the production of hormones such as epinephrine, that mediate the stress response throughout the body.