Acetylcholine is a molecule released by nerves. It signals other nerves and muscles to perform a function. It is released by nerves to signal skeletal muscles in the arm to contract. It is also has functions in the brain, and in the part of the nervous system that runs without conscious control, called the autonomic nervous system. This nervous system has two basic divisions, and controls such processes as digestion and heart beat. It can change the heart rate, which can be too fast, a condition called tachycardia, or too slow, a condition called bradycardia.
Autonomic Nervous System
The autonomic nervous system controls unconscious functions in the body through two divisions: a "rest and digest" branch and a "fight or flight" branch. The "rest and digest" brand is the parasympathetic system and the "fight or flight" is the sympathetic system. In the heart, activation of the sympathetic system increases the heart rate, or leads to tachycardia. Activation of the parasympathetic system leads to a slowing of the heart. The parasympathetic nerves communicate directly with the heart by releasing acetylcholine.
Acetylcholine and Nerves
The nerves of the parasympathetic system synthesize acetylcholine and store it in little sphere-shaped containers called vesicles. When a signal arrives at the end of the nerve cell, the vesicle fuses with the outside of the cell and releases the acetylcholine. The acetylcholine travels a short distance to the target organ, such as the heart. There, it binds to proteins on the surface of the heart cells, causing chemical changes in the heart that slow it down.
Effects
Acetylcholine released from the parasympathetic system will slow the heart, increase movements in the digestive tract, constrict the pupils in the eyes and divert blood to the internal organs. The sympathetic nervous system does the opposite. It mostly releases norepinephrine instead of acetylcholine; in the heart this causes tachycardia.
Modification
Sometimes tachycardia, or a fast heart rate, is normal. For example, during exercise, or fear, in order to supply more blood and oxygen to the muscles for fight or flight. However, sometimes it is abnormal or harmful. Drugs that mimic acetylcholine, such as succinylcholine, can decrease the heart rate. Similarly, drugs that block the effects of acetylcholine, such as atropine or glycopyrrolate increase the heart rate, which can lead to tachycardia.
References
- "Physiology"; Linda S.Costanzo; 4th Ed 2008
- "Basic and Clinical Pharmacology"; Bertram Katzung, et al.; 11th Ed 2009
