What is that? /Definition
Acetylcholine is one of the most important neurotransmitters both in humans and in many other organisms. In fact, acetylcholine already occurs in unicellular organisms and is considered to be a very old substance in the history of development. At the same time, it is the longest known neurotransmitter (it was first experimentally proven in 1921), which is one of the reasons why it has been intensively studied until today.
Chemically, acetylcholine (abbreviated ACh) belongs to the group of biogenic amines and plays an enormously important role in the central as well as in the peripheral and vegetative nervous system. However, it is best known for its function as a transmitter at the motor end plate (neuromuscular end plate), where it mediates the voluntary contraction of skeletal muscles. Its role in the learning process and the development of memory is also widely discussed. Apart from this, it is considered certain that it is involved in the development of pain sensation and the maintenance of our day-night rhythm, as well as in the control of motor functions in the brain. In addition, acetylcholine not only functions as a messenger substance in the nervous system, but also as a hormone in the bloodstream, where it is involved in regulating heart rate and blood pressure.
Action of acetylcholine
Since acetylcholine is one of the most widespread messenger substances in the human body, its effect on the organism is very extensive. Especially in its function as an important neurotransmitter of all major nervous systems ACh has a wide range of tasks. For example, at the neuromuscular endplate it serves, among other things, to transmit excitation from the nerves to the muscle by binding to the nicotinic acetylcholine receptor, which causes the muscle to contract.
It is also an essential component of the excitation conduction in the autonomic nervous system. Here, acetylcholine transmits impulses from the first to the second neuron both in the parasympathetic (parasympathetic nervous system) and the sympathetic system (sympathetic nervous system). On the other hand, in the case of the parasympathetic nervous system, it is also responsible for the connection of the second neuron with the respective target organ.
The autonomic nervous system is responsible for all involuntary functions of the internal organs. The parasympathetic nervous system in particular ensures a resting metabolism. In relation to the effect of acetylcholine, this ultimately means a slowing down of the heart rate and lowering of blood pressure, a narrowing of the bronchial tubes, stimulation of digestion and also functions such as increased salivation and narrowing of the pupils.
In the central nervous system, in turn, it is associated with many cognitive functions. Among other things, it is involved in learning processes, the formation of memory and probably also in the development of drive. This can be seen in the consequences of Alzheimer’s disease, in which nerve cells that produce acetylcholine are destroyed. In addition, ACh as a hormone in the bloodstream has an effect on our circulatory system. Here it has a blood pressure-lowering effect mainly by dilating blood vessels far from the body.