Neurons for "dummies"

Neurons are a special group of cells in the body that distribute information throughout the body. Using electrical and chemical signals, they help the brain coordinate all vital functions.

To simplify it, the tasks of the nervous system are to collect signals coming from the environment or from the body, assess the situation, decide how to respond to them (for example, change the heart rate), and also think about what is happening and remember it. The main tool for performing these tasks are neurons woven throughout the body by a complex network.

According to average estimates, the number of neurons in the brain is 86 billion, each of them is connected to another 1,000 neurons. This creates an incredible network of interaction. A neuron is the basic unit of the nervous system.

Neurons (nerve cells) make up about 10% of the brain, the rest are glial cells and astrocytes, whose function is to maintain and nourish neurons.

What does a neuron look like?

Three parts can be distinguished in the structure of a neuron:

· The neuron body (soma) – receives information. Contains the cell nucleus.

· Dendrites are short processes that receive information from other neurons.

· Axon is a long process that carries information from the neuron body to other cells. Most often, the axon ends with a synapse (contact) with the dendrites of other neurons.

A diagram of the structure of a neuron (here and further drawings from Wikipedia).

Dendrites and axons are called nerve fibers.

Axons vary greatly in length, from a few millimeters to a meter or more. The longest are the axons of the spinal ganglia.

Types of neurons

Classification of neurons can be carried out by several parameters, for example, by structure or function performed.

Types of neurons depending on function:

· Efferent (motor) neurons – carry information from the central nervous system (brain and spinal cord) to cells of other parts of the body.

· Afferent (sensitive) neurons – collect information from the whole body and carry it to the central nervous system.

·        Insertion neurons – transmit information between neurons, more often within the central nervous system.

How do neurons transmit information?

The neuron, receiving information from other cells, accumulates it until it exceeds a certain threshold. After that, the neuron sends an electrical impulse along the axon – an action potential.

The action potential is formed by the movement of electrically charged particles through the axon membrane.

At rest, the electric charge inside the neuron is negative relative to the surrounding intercellular fluid. This difference is called the membrane potential. It is usually 70 millivolts.

When the body of a neuron receives enough charge and it "shoots", depolarization occurs in the adjacent section of the axon – the membrane potential grows rapidly, and then falls in about 1/1000 seconds. This process triggers the depolarization of the adjacent section of the axon, and so on, until the pulse passes along the entire length of the axon. After the depolarization process, hyperpolarization occurs – a short-term state of rest, at which point the transmission of the pulse is impossible.

The action potential is most often generated by potassium (K+) and sodium (Na+) ions, which move through ion channels from the intercellular fluid into the cell and back, changing the charge of the neuron and making it positive at first, and then reducing it.

The action potential ensures the operation of the cell according to the principle of "all or nothing", that is, the impulse is either transmitted or not. Weak signals will accumulate in the body of the neuron until their charge is sufficient to transmit through the processes.

Myelin

Myelin is a thick white substance covering most axons. This coating provides electrical insulation of the fiber and increases the speed of the pulse passing through it .

Myelinated fiber versus unmyelinated fiber.

Myelin is produced by Schwann cells on the periphery and oligodendrocytes in the central nervous system. In the course of the fiber, the myelin sheath is interrupted – these are the interceptions of Ranvier. The action potential moves from intercept to intercept, which ensures rapid transmission of momentum.

Such a common and serious disease as multiple sclerosis is caused by the destruction of the myelin sheath.

How synapses work

Neurons and the tissues to which they transmit an impulse do not physically touch, there is always a space between the cells – a synapse.

Depending on the way information is transmitted, synapses can be chemical and electrical.

After the signal, moving along the process of the neuron, reaches the synapse, chemical substances – neurotransmitters (neurotransmitters) are released into the space between the two neurons. This space is called the synaptic cleft.

Diagram of the structure of the chemical synapse.

A neurotransmitter from a transmitting (presynaptic) neuron, entering the synaptic cleft, interacts with receptors on the membrane of the receiving (postsynaptic) neuron, triggering a whole chain of processes.

Types of chemical synapses:

· glutamatergic – the mediator is glutamic acid, has an excitatory effect on the synapse;

· GABA-ergic – the mediator is gamma-aminobutyric acid (GABA), has an inhibitory effect on the synapse;

· cholinergic – the mediator is acetylcholine, carries out neuromuscular transmission of information;

· adrenergic – the mediator is adrenaline.

Electrical synapses

Electrical synapses are less common, common in the central nervous system. Cells communicate through special protein channels. The presynaptic and postsynaptic membranes in electrical synapses are located close to each other, so the pulse is able to pass directly from cell to cell.

The rate of transmission of an impulse by electrical synapses is much higher than by chemical synapses, so they are located mainly in those departments where a quick reaction is needed, for example, those responsible for protective reflexes.

Another difference between the two types of synapses is in the direction of information transmission: if chemical synapses can transmit an impulse in only one direction, then electrical synapses are universal in this sense.

Conclusion

Neurons are perhaps the most unusual cells in the body. Every action that the human body performs is provided by the work of neurons. A complex neural network forms personality and consciousness. They are responsible for both the most primitive reflexes and the most complex processes associated with thinking.

Aminat Adzhieva, portal "Eternal Youth" http://vechnayamolodost.ru based on Medical News Today: Neurons: The basics.