Sympathetic nervous system

The sympathetic nervous system is the part of the autonomic nervous system that prepares the body to react to situations of stress or emergency. It is mainly formed by adrenergic fibers and tends to increase the contractility of smooth or cardiac muscle. But in smooth muscle, sympathetic stimulation will decrease tone and contractility.

It is also called the “fight, flight or freeze” response, and our bodies react in a certain way:

• Increase of stress hormone levels

• The heart pumps more blood into the muscles so that more energy is being produced

• Oxygen demand rises because more blood needs to be oxygenated, so the level of respiration rises

• Pupils dilate for better vision

• The activity of the gastrointestinal system slows down

• Increase in sweating

It consists of preganglionic and postganglionic fibers, both connected in the ganglia. The preganglionic fibers are short, while the postganglionic ones are long (Figure 1).

Figure 1. Schematic representation of smooth or cardiac muscle sympathetic control.

Two different neurotransmitters are released in the sympathetic nervous system: acetylcholine (ACh) from the preganglionic fiber and norepinephrine (NE) from the postganglionic fiber to the end organ. ACh binds to nicotinic receptors in the postganglionic fiber, while the NE binds to adrenergic receptors in the end tissue.