The sympathetic nervous system is constantly active to maintain homeostasis in the body, but has a very specific response mechanism against perceived threats.
The fight or flight response to perceived threats enables humans to either fight off the threat or run away quickly. Blood flow increases to the heart and skeletal muscle, while blood flow decreases to the GI tract and kidneys. Additionally, breathing and heart rates increase to meet oxygen demands of physical stress, and pupils dilate to maintain clear vision.
In the autonomic nervous system, all preganglionic neurons use acetylcholine as a neurotransmitter.
Sympathetic postganglionic neurons release norepinephrine as a neurotransmitter to excite or inhibit other cells.
The sympathetic and parasympathetic nervous systems have antagonistic effects to each other, so only one can be active at the same time.
The parasympathetic nervous system involves stimulating excretory and digestive organs and has antagonistic effects to the sympathetic nervous system. Innervation occurs primarily via the vagus nerve.
The rest and digest response of the parasympathetic nervous system stimulates organs that help the body eat and relax. Blood flow to digestive and excretory organs increases, while blood flow to the heart and skeletal muscle decreases. Ventilation and heart rates decreases.
The parasympathetic postganglionic neurons use acetylcholine as a neurotransmitter rather than norepinephrine. Thus, in the parasympathetic system, the primary neurotransmitter used for all neurons is acetylcholine.
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