The sympathetic nervous system is the part of theautonomic nervous systemthat prepares the body to react to situations ofstress or emergency. It is mainly formed by adrenergic fibers and tends toincreasethe contractility of smooth or cardiac muscle. But in smooth muscle, sympathetic stimulation willdecreasetone and contractility.

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

• 增加的压力荷尔蒙水平

• 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 differentneurotransmittersare released in the sympathetic nervous system:acetylcholine(ACh)from the preganglionic fiber and去甲肾上腺素(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.