Nerve signal transmission is accomplished through bioelectrical impulses generated by neurons and transported from one neuron to another until the signal reaches its destination. The transport depends on the action of neurotransmitters, substances that are transmitted from one neuron to another through the synapses and cause an excitatory or inhibitory effect on the postsynaptic neuron. One such neurotransmitter, and the first to be identified, is acetylcholine. It was first identified in 1914 by Henry Hallett Dale and later confirmed as Otto Loewi’s neurotransmitter. For their work, they received in 1936 the Nobel Prize in physiology and medicine.
It is an ester of acetic acid and choline with the chemical formula CH3COOCH2CH2CH2N+(CH3)3. It is synthesized in certain neurons by the enzyme choline acetyltransferase from choline and acetyl-CoA. After that, acetylcholine is sent along the axon to the terminal button, where it will be stored until its utilization and release into the synaptic space. Organic mercury compounds have a high affinity for sulfhydryl groups, which is why they are attributed to the dysfunctional effect of the enzyme choline acetyltransferase. This inhibition can lead to acetylcholine deficiency, contributing to the symptomatology of motor dysfunctions.
It should be noted that to form acetylcholine, the body needs choline molecules, which must necessarily come from the diet. Meat, egg yolks, and soy are the foods richest in this molecule. Similarly, glucose is needed to form the neurotransmitter.
Function in the Human Body
- Muscle Control – This is the main function of acetylcholine. This neurotransmitter is the one that allows muscle contractions (and relaxations), both voluntary and involuntary. Walking, running, jumping, breathing, grasping objects, lifting weights, standing, eating… None of this would be possible without the role of acetylcholine, which helps to transmit commands from the brain to the muscles.
- Decreased Heart Rate – Acetylcholine has an inhibitory function on cardiovascular activity, slowing the heart rate and reducing blood pressure. This is essential since, otherwise, the neurotransmitters that stimulate the cardiac rhythm would cause an overexcitation, with all the health problems that derive from hypertension.
- Stimulation of the Intestinal Movement – In the case of the digestive system, acetylcholine has an excitatory function. It stimulates the movement of the intestinal muscles to promote the passage of food and increase the action of these intestines.
- Stimulation of the REM Sleep Phase – Acetylcholine plays a very important role in regulating sleep cycles. This neurotransmitter is essential for entering the REM sleep phase, which is the time when, in addition to dreaming, memories are consolidated, the mood is balanced, and learning what we have experienced is promoted. Although, the mechanisms by which this happens are still not very clear.
- Regulation of Hormone Synthesis – Acetylcholine is also important in controlling the action of various endocrine glands, i.e., the structures of the body specialized in synthesizing hormones. This neurotransmitter stimulates the synthesis of vasopressin (contracts blood vessels) and reduces that of prolactin (stimulates milk production in mammals), among other functions.
- Promotion of Neuroplasticity – Acetylcholine is very important at the brain level as it promotes the interconnection between neurons, thus promoting the consolidation of memories, learning, memory, motivation, attention span, etc. Problems with this neurotransmitter have been associated with the development of Alzheimer’s disease.
- Consolidation of Memories – As we have said, acetylcholine is very important when it comes to consolidating memories. It encourages neurons to interconnect in such a way that specific events are stored in short and long-term memory.
- Perception of Pain – Acetylcholine is also essential in the transmission of nerve impulses from the sensory organs to the brain, especially when we experience pain. Therefore, this neurotransmitter is very important in the perception of pain.
- Decreased Bladder Capacity – As with the cardiac muscles, acetylcholine causes inhibition of bladder muscle activity, preventing it from increasing too much in size. Thus, this neurotransmitter is essential in determining when we have the urge to urinate.
- Activation of The Senses Upon Wakening – Acetylcholine is very important in stimulating neuronal connections after opening the eyes in the morning, i.e., it “wakes up” the nervous system. Thus, this neurotransmitter allows the senses to start sending information to the brain as soon as we wake up.
Myasthenia Gravis Illustration
An imbalance in the concentration of acetylcholine and its receptors in the nervous system can lead to serious diseases such as Myasthenia and Dystonia:
Myasthenia – Myasthenia is a rare neuromuscular disease characterized by loss of strength and fatigue, symptoms that appear or worsen with exercise; the degree of weakness may vary throughout the day, with a clear worsening in the evening. The disease may manifest with different symptoms, alone or in combination:
– weakness preferentially of the ocular muscle groups in 90% of cases, resulting in palpebral ptosis (droopy eyelids).
– diplopia (double vision).
– dysarthria (difficulty articulating words).
– dysphagia (difficulty swallowing).
– difficulty chewing and loss of facial expression.
The lack of strength can extend to the arms, legs, and respiratory muscles, producing a generalized paralytic crisis, a very serious situation known as a myasthenic crisis, which requires hospitalization.
The disease evolves by outbreaks (alternating symptomatic periods with periods of improvement and spontaneous remission of symptoms).
Dystonia – Dystonias are movement disorders in which frequent muscle contractions cause twisting and involuntary repetitive movements that result in abnormal postures. These movements, which are involuntary and can sometimes be painful, can affect a single muscle, a group of muscles such as some of all the muscles of the arms, legs, or neck, or even the entire body. The decrease in intelligence and emotional imbalance is not characteristic of dystonia. Still, the pain can become so intense that it causes incapacity to work, make the person irritable, and even suffer some degree of secondary depression, often due to lack of timely diagnosis, adequate treatment, or some other or other types of causes that can be fatal.