What is a neurotransmitter?

There are billions of neurons in our brain and trillions of synapses between neurons. Synapses are connections that transmit signals from one nerve cell to another. Usually, the extension of one neuron, called axon, synapses with the dendrite of the other neuron. Although the two neurons come very close to each other in this region, they do not touch, leaving a synaptic gap between them. The stimulus is transmitted through this space through molecules called neurotransmitters.

What is a neurotransmitter?

Neurotransmitters are molecules that chemically transmit the signal between neurons, thereby increasing or decreasing the electrical activity of cells for a short time. They are released when the nerve impulse reaches the synapse. The role of acetylcholine molecule in chemical signal transmission between neurons was first shown by Otto Loewi in 1921. There are neurotransmitters of varying sizes, from very small molecules to moderately large molecules to much larger molecules called peptides. They are stored in small vesicles called synaptic vesicles. When the stimulus comes, the vesicles empty their contents into the synaptic cleft between the two neurons. The released neurotransmitters bind to specific receptors (receptors) on the receiving neuron.

There is a wide variety of neurotransmitters and even more receptors in the nervous system. It can be difficult to understand that there are so many different types of neurotransmitters and receptors, considering the mere task of increasing and decreasing excitability. Wouldn’t two neurotransmitters, one stimulating and the other suppressing, be sufficient to perform functions? It turns out that the way neurotransmitters work is more complex than that. Many act on slow metabolic processes in neurons rather than trigger rapid stimulation or suppression. These slow processes regulate the long-term strength of synaptic connections. Neurotransmitters can also have long-term effects on the properties of the cell and synapse by turning on and off important genes in the cell nucleus. These changes may be important in processes such as learning and long-term memory formation.

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As time went on, the number of neurotransmitters discovered increased. The knowledge we have gained over the years has led to the change and expansion of the concept of neurotransmitter. For example, nitric oxide (NO) is one of the most interesting neurotransmitters. It is a poisonous gas in high concentration. It cannot be stored in vesicles, so it is released as soon as it is produced. NO can spread to a relatively large area in the brain, thus affecting a large number of neurons that are not directly connected to each other.

Conventional neurotransmitters share some common characteristics. They are stored in synaptic vesicles and are released by calcium ion entry triggered by action potential reaching the axon terminal. They act by binding to their specific receptor on the cell on the other side of the synapse. Conventional neurotransmitters are divided into two main groups as small molecules and neuropeptides.

Small Molecule Neurotransmitters

Glutamate, GABA and glycine are amino acids.

Dopamine, norepinephrine, epinephrine, serotonin and histamine, which are biogenic amines derived from amino acid precursors.

Purinergic neurotransmitters ATP and adenosine.

Acetylcholine is a key neurotransmitter at the neuromuscular junction and some other synapses, but is considered on its own as it does not conform to the above structural classifications.

neuropeptides

Neuropeptides consist of three or more amino acids and are larger than small molecule neurotransmitters. There are many neuropeptides available. Substance P carries pain signals, while endorphins and enkephalins suppress pain. Neuropeptide Y increases appetite and is effective in preventing seizures.

receptors

Some neurotransmitters are considered excitatory, while others are suppressive (inhibitory). For example, glutamate is the major excitatory neurotransmitter in the central nervous system. GABA is the main inhibitory neurotransmitter in the adult brain . Glycine is the main inhibitory neurotransmitter in the spinal cord. However, these distinctions are not always clear. Depending on its location, a neurotransmitter can act as either a stimulant or a suppressor. This is because there can be more than one receptor for a neurotransmitter. In this case, it is the type of receptor on the post-synaptic neuron that determines the effect. For example, while acetylcholine has a stimulating effect on skeletal muscles, it has a suppressive (pulse-slowing) effect on the heart. Because the receptors in these two places are different. Skeletal muscle has nicotinic receptors and cardiac muscle has muscarinic acetylcholine receptors.

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Receptors activated by neurotransmitters are of two main types. Ligand-dependent ion channels allow ion passage across the cell membrane in response as soon as the neurotransmitter is bound. Metabotropic receptors, on the other hand, are not ion channels themselves, but trigger signaling pathways that indirectly open and close these channels.

Unconventional Neurotransmitters

These are two groups as endocannabinoids and gas transmitters. The gaseous ones are NO and carbon monoxide (CO), which we mentioned earlier. These different neurotransmitters are not stored in synaptic vesicles and can also carry messages from postsynaptic neuron to presynaptic neuron. They can pass through the cell membrane and directly affect the molecules inside the cell.

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