Acetylcholine at the heart
As early as 1921 it was discovered that a chemical substance must be present which transmits the electrical impulse transmitted via the nerves to the heart. This substance was initially called vagus substance after the nerve whose impulse it transmits. Later it was chemically correctly renamed acetylcholine instead.
The nervus vagus, with its messenger substance acetylcholine, is an important branch of the parasympathetic nervous system, which, along with the sympathetic nervous system, belongs to the vegetative or nervous system. The nervous system is responsible for the control of involuntary bodily functions such as digestion. The parasympathetic nerves in particular ensure a resting or recovery metabolism, thus promoting digestion among other things.
The docking point for ACh here is the M2-receptor, a so-called muscarinic receptor. This knowledge has been used to develop a drug called atropine which blocks this receptor and thus counteracts the effect of the parasympathetic nervous system. This effect is called parasympathetic.
Atropine is used in emergency medicine, for example. A further effect of acetylcholine on the circulation, again corresponding to the function of the parasympathetic nervous system, is to relax the vascular muscles. This also results in a reduction in blood pressure.
A synapse is a neural connection between a neuron and another cell (usually another neuron, but often also a muscle, sensory or glandular cell). They are used to transmit and, in some cases, modify excitation, as well as to store information by adapting the structure of the synapse. The human body has about 100 trillion synapses.
A single neuron can have up to 200,000 synapses. The transmission of the electrical signal from one synapse to a second one is usually carried out chemically by means of neurotransmitters, including acetylcholine, which we will use as an example here. When an electrical signal reaches the synapse of neuron A, this leads to the release of acetylcholine from its storage sites within the synapse, the vesicles, into the synaptic cleft.
This gap is only about 20 to 30 nanometres wide and microscopically small. The acetylcholine subsequently diffuses to the synapse of neuron B and docks here to special receptors. This in turn leads to the formation of an electrical impulse in Neuron B, which is then transmitted.
After a short time ACh is degraded by the enzyme acetylcholinesterase and rendered ineffective. Its components choline and acetic acid are then reabsorbed into the synapse of Neuron A so that acetylcholine can be formed again. In addition to these chemical synapses, there are also electrical synapses, which are equipped with ion channels through which ions and small molecules can pass from one cell to another. The electrical impulse can therefore be transmitted directly between two or more cells.