What Is Acetylcholine?

Acetylcholine is a neurotransmitter. Neurotransmitters are chemical signals made by neurons to send information to associated receptors. These receptors receive or block signals and process them into cellular action. Acetylcholine can do both: It can stimulate or block responses (excite or inhibit) for desired physiological effects.

Acetylcholine signals your muscles to contract. It also plays a role in cognition and can affect behaviors such as motivation and attention. An imbalance in acetylcholine can cause conditions that are related to these functions like Parkinson's disease and Alzheimer's disease.

What Is Acetylcholine?

Acetylcholine is a neurotransmitter found in the central nervous system (CNS) and peripheral nervous system (PNS). It plays an important role in many of your body's functions, including muscle movement, memory and cognition, REM sleep, and attention and learning.

Naturally occurring acetylcholine was first identified in 1914 by British physiologist Sir Henry Dale from London. It was named after its structure. Acetylcholine is made of acetic acid (ethanoic acid) and choline (a nutrient similar to B vitamins).

More than two decades later, Dale and Otto Loewi from Graz shared the Nobel Prize in Physiology or Medicine for their work on chemical neurotransmission.

Acetylcholine's Function

Acetylcholine is released from cholinergic nerve synapses and acts on presynaptic (transmitter) and postsynaptic (receiver) acetylcholine receptors.

Dilates Blood Vessels

Acetylcholine plays a role in regulating blood pressure. When blood flows, it creates friction that can be seen on image signaling technology focusing on the endothelium, the cell barrier between your blood and blood vessel wall.

One ex vivo (outside of a living body) animal study on rats' main arteries found that this friction triggers the release of acetylcholine, which triggers calcium release from your endothelial cells, nitric oxide production (a known vasodilator, which relaxes or dilates blood vessels), and artery relaxation.

Human clinical trials are still needed before science can fully explain how acetylcholine works to dilate blood cells in the human body.

Contracts Smooth Muscles

Smooth muscles are those lining the walls of organs and tubular structures, including the intestine, bladder, airway, uterus, blood vessels, and stomach. Acetylcholine in the neuromuscular junction (located between the motor nerve and skeletal muscle) acts on nerve fibers, sending messages from the brain to targeted muscles, signaling them to respond with movement.

Here’s how it works, according to studies conducted on mice:

• Acetylcholine released from nerve endings will bind to acetylcholine receptors on your smooth muscle’s surface, causing sodium channels to open. This allows action potential to travel along cells, which triggers a process that opens the L-type calcium channel. 
• Calcium is released and binds to calmodulin, which regulates motor proteins with roles in muscle contraction.
• Calmodulin then binds to kinase myosin light-chain kinase, stimulating phosphorylation (molecule attachment) of myosin light chain, which leads to muscle contraction.  

Causes Erections

The penis is made of smooth muscle that is actually contracted in its flaccid state. As stimuli increase blood flow to the area, the cholinergic receptors on the endothelial cells inside the penis are activated by acetylcholine. This offers a relaxing effect, allowing for erection to occur.

Slows Heart Rate

Acetylcholine is the predominant neurotransmitter in the parasympathetic nervous system. When your heart rate increases beyond what's normal, acetylcholine is released to slow your heart rate and contractions until it goes back to baseline.

Stimulates Secretions

Acetylcholine also works on cholinergic muscarinic receptors in organ systems to stimulate secretions by all glands receptive to parasympathetic nerve impulses. 

Examples include:

• Digestive glands
• Salivary glands
• Exocrine sweat glands

Imbalance and Deficiency

Acetylcholine sends messages along nerve cells through the nervous systems. All of your body’s movements depend on this communication. This means any disruption to acetylcholine functioning compromises this process and can result in illness.

Acetylcholine in the brain also plays crucial roles in memory and cognitive functioning. As such, it is associated with higher brain functions and some neurodegenerative brain diseases.

Alzheimer's Disease

People with Alzheimer's disease have low levels of acetylcholine. It may be because the plaques that form in the brains of people with Alzheimer's disease increase the activity of chemicals that break down acetylcholine.

Parkinson's Disease

Parkinson's disease affects the muscles and causes involuntary movements and tremors. Research suggests that an imbalance of acetylcholine may play a role in the development of this condition.

Myasthenia Gravis

Myasthenia gravis is an autoimmune condition where the body's immune system mistakenly attacks acetylcholine receptors. People with this condition experience weakening of the muscles in the arms, hands, fingers, legs, and neck. 

Toxins and Venom

Organophosphates are insecticides that interfere with the enzyme that breaks down acetylcholine. Long-term exposure to these chemicals can cause an increase in acetylcholine and problems such as anxiety, memory loss, headache, and nausea and vomiting.

The venom of the black widow spider also affects acetylcholine. After a black widow bite, acetylcholine levels in the body increase, which can cause symptoms like severe muscle contractions, muscle spasms, and paralysis.

Medications That Affect Acetylcholine

Acetylcholine receptors can be targeted and manipulated with medications to adjust how your body functions in a disease state.

Cholinergic nerve receptors are those that receive and bind with acetylcholine. They can be found all over the body, including in muscle tissue. If there is any issue with these receptors or the appropriate release and uptake of acetylcholine, abnormal muscle function may result. In such cases, anticholinergic drugs may be necessary.

Anticholinergics are available by prescription to help treat conditions like:

• Urinary incontinence or overactive bladder: They work on the abnormal uterine contraction that causes the sensation of needing to urinate.
• Asthma or other obstructive respiratory disorders: They may have a protective effect on airway inflammation and airway changes due to pathology.
• Symptoms of Parkinson’s disease: They work on involuntary movements like jerks.  
• Gastrointestinal issues like diarrhea: They can inhibit gastrointestinal contractions. 
• Poisoning by toxins such as organophosphates, a class of man-made chemicals that are poisonous to insects and mammals: Certain poisons can work on the same receptors as acetylcholine and cause choline toxicity. Anticholinergics work to restore the normal process.

Summary

Acetylcholine is a neurotransmitter that has a wide range of functions in the body, from muscle movement to memory and cognition. An imbalance of this neurotransmitter can lead to a number of health conditions, including Alzheimer's disease and Parkinson's disease. Certain toxins and venoms can also interfere with acetylcholine, causing severe health problems. 

Medications that affect acetylcholine are used to treat conditions ranging from asthma and urinary incontinence to Parkinson's disease symptoms.

A Word From Verywell

Researchers are only just beginning to understand acetylcholine's role in diseases like Alzheimer's and Parkinson's. There is no cure for these conditions, but medications that affect the balance of this important neurotransmitter can help ease symptoms. Talk to your healthcare provider about whether or not these treatments are right for you.