Acetylcholine is a neurotransmitter, a chemical released by nerve cells to send signals to other cells. Its name derives from its molecular structure: it is an ester of acetic acid and choline (ACh). It was the first neurotransmitter to be discovered, and for this reason it has been extensively studied. It is also the most abundant neurotransmitter and is present in both the central nervous system (CNS) and the peripheral nervous system (PNS).
Acetylcholine is the main neurotransmitter in the autonomic nervous system, which has important functions such as contracting smooth muscles, dilating blood vessels, increasing body secretions and slowing the heart rate..
Acetylcholine has both excitatory and inhibitory functions, which means that it can speed up and slow down nerve signals.
In the central nervous system, its function is mainly excitatory. It is responsible for modulating the functioning of various neurons in the areas of the brain that control motivation, arousal and attention. It is a key neurotransmitter for maintaining memory and promoting learning, as well as promoting brain neuroplasticity. Critical impairment of the cholinergic pathway in the CNS has been associated with the onset of Alzheimer's disease.
It also helps activate sensory functions upon awakening, helping people maintain attention and acting as part of the brain's reward system. Acetylcholine is essential for rapid eye movement (REM) sleep, when people have our dreams when we are sleeping.
In the brain, acetylcholine acts as a neuromodulator, meaning that instead of participating in direct synaptic transmission between specific neurons, it acts on a wide variety of neurons throughout the nervous system. Drugs and substances that disrupt the function of acetylcholine can have negative effects on the body and even lead to death. Examples of such substances are some types of pesticides and nerve gases.
Within the peripheral nervous system, acetylcholine is an important part of the autonomic nervous system, as it transmits signals between motor nerves and muscles, contributing to the contraction of cardiac, skeletal, and smooth muscles. Acts at neuromuscular junctions allowing motor neurons to activate muscle action.
For example, the brain could send a signal to move the left leg. The signal is carried through the nerve fibers to the neuromuscular junctions. Once there, the signal is transmitted by acetylcholine, triggering the desired response in those specific muscles..
Acetylcholine is responsible for controlling numerous bodily functions, since it acts on the preganglionic neurons of the sympathetic and parasympathetic systems.
In the cardiovascular system, it acts as a vasodilator, slowing the heart rate and contracting the heart muscle. In the gastrointestinal system, it works by increasing peristalsis in the stomach and the amplitude of digestive contractions. In the urinary tract, its activity focuses on reducing the capacity of the bladder and increasing the voluntary sensation of evacuation. It also affects the respiratory system by stimulating the secretion of all the glands that receive parasympathetic nerve impulses. In the central nervous system, acetylcholine appears to have multiple functions.
Because acetylcholine plays an important role in all muscle actions, drugs that influence this neurotransmitter can cause various degrees of movement disruption or even paralysis..
Acetylcholine imbalances can contribute to the development of myasthenia gravis, an autoimmune disorder that causes muscle weakness and fatigue.
In the PNS, acetylcholine is stored in vesicles at the ends of cholinergic neurons (acetylcholine producers). In the PNS, when a nerve impulse reaches the terminal of a motor neuron, acetylcholine is released at the neuromuscular junction. There it combines with a receptor molecule on the postsynaptic membrane (or endplate membrane) of a muscle fiber. This binding changes the permeability of the membrane, causing channels to open that allow positively charged sodium ions to flow into the muscle cell. If successive nerve impulses accumulate at a high enough frequency, sodium channels along the end plate membrane become fully activated, resulting in muscle cell contraction..
Acetylcholine is rapidly destroyed by the enzyme acetylcholinesterase and is therefore only briefly effective. Enzyme inhibitors (drugs known as anticholinesterases) prolong the life of acetylcholine. Such agents include physostigmine and neostigmine, which are used to help increase muscle contraction in certain gastrointestinal conditions and in myasthenia gravis. Other acetylcholinesterases have been used in the treatment of Alzheimer's disease.
The cholinergic portion of the brain is the area of the brain that produces acetylcholine. Damage to this part of the brain is linked to the development of Alzheimer's disease. Many people with Alzheimer's disease have altered levels of acetylcholine. Cholinesterase inhibitors are routinely prescribed to people with Alzheimer's disease in an effort to slow the development of this condition by preventing the breakdown of acetylcholine..
Acetylcholine also plays an important role in Parkinson's disease. Acetylcholine along with dopamine are the neurotransmitters that allow smooth muscle movements. When there is an imbalance between acetylcholine and dopamine, movements can be unstable and uneven, a hallmark of Parkinson's disease..
American Psychological Association. APA Concise Dictionary of Psychology. Washington, DC.