The Sympathetic and Parasympathetic Divisions

Why does SNS engage epinephrine/norepinephrine (at the effector organ) while the PNS uses acetylcholine?

The sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS) are two components of the autonomic nervous system (ANS) that work to regulate various processes in the body. One key difference between the SNS and the PNS lies in the neurotransmitters they use to transmit signals at the effector organs.

The SNS engages epinephrine (also known as adrenaline) and norepinephrine (noradrenaline) as its primary neurotransmitters released at the effector organ. This system is often referred to as the “fight-or-flight” response as it prepares the body for emergency situations or stress by increasing heart rate, dilating blood vessels to muscles, and initiating other physiological changes that promote survival.

Epinephrine and norepinephrine are produced by a part of the adrenal glands called the adrenal medulla. When the SNS is activated, nerve impulses travel to the adrenal medulla, signaling it to release these neurotransmitters into the bloodstream. From there, they bind to adrenergic receptors located on the target organ, initiating the appropriate response. The effects of epinephrine and norepinephrine include increased heart rate, enhanced blood flow to muscles, and heightened alertness.

On the other hand, the PNS primarily uses acetylcholine as its neurotransmitter at the effector organ. This system is often called the “rest-and-digest” response as it promotes relaxation, digestion, and other maintenance functions of the body.

Acetylcholine is released by preganglionic neurons of the PNS, which synapse with postganglionic neurons near or inside the target organ. The acetylcholine then binds to cholinergic receptors on the target tissues, stimulating specific responses. The effects of acetylcholine in the PNS include decreased heart rate, stimulation of digestion, and relaxation of blood vessels.

While both the SNS and PNS are important for maintaining homeostasis in the body, their different neurotransmitters allow them to have opposing effects. The SNS prepares the body for immediate actions in response to perceived threats, while the PNS conserves energy and promotes long-term well-being. This balance between the two systems helps the body maintain equilibrium under various physiological conditions.

More Answers: