The neuron, or nerve cell, is the basic element of the nervous system. It is the neurons that are responsible for the fact that we feel pain, can we read this text at the moment, and thanks to them it is possible to move our hand, leg or any other part of the body. The performance of such extremely important functions is possible thanks to the complex structure and physiology of neurons. So how is a nerve cell built and what are its functions?

Neurons( nerve cells ), next to glial cells, are the basic building blocks of the nervous system. The world began to learn about the complex structure and functions of nerve cells mainly after 1937 - it was then that J. Z. Young proposed that work on the properties of neurons be carried out on squid cells (as they are much larger than human cells, all experiments are definitely carried out on them). easier).

Nowadays, it is of course possible to conduct research even on the smallest of human cells, but at that time the animal model significantly contributed to the discovery of the physiology of nerve cells.

The neuron is the basic building block of the nervous system and the complexity of the nervous system depends essentially on how many of these cells are in the body.

For example, nematodes that are tested in different laboratories have only 300 neurons.

The well-known fruit fly has definitely more nerve cells, about one hundred thousand. This number is nothing if you consider how many neurons a person has - it is estimated that there are several billion of them in the human nervous system.

Neuron (nerve cell): development

The process of making nerve cells is known as neurogenesis. In general, in the developing organism (especially during the period of intrauterine life) neurons arise from neural stem cells, and the resulting nerve cells generally do not undergo cell division afterwards.

In the past, it was believed that after development in humans, no new nerve cells were formed at all. Such a conviction showed how dangerous all diseases leading to the loss of nerve cells are (we are talking here, for example, about variousneurodegenerative diseases).

Currently, however, it is already known that in certain regions of the brain it is possible to create new neurons even in adulthood - such regions turned out to be, among others, hippocampus and olfactory bulb.

Neuron (nerve cell): general structure

The neuron can be divided into three parts, which are:

  • nerve cell body (perikaryon)
  • dendrites (multiple, usually small protrusions, protruding from the perikaryon)
  • axon (single, long protrusion extending from the body of the nerve cell)

The body of the nerve cell, like its other parts, is covered with a cell membrane. It contains all the basic cellular organelles such as :

  • cell nucleus
  • ribosomes
  • endoplasmic reticulum (aggregates of the reticulum with ribosomes richly scattered within it are referred to as Nissel granules - they are characteristic of nerve cells and are present in them due to the fact that neurons produce a lot of proteins)

Dendrites are primarily responsible for receiving information flowing to the nerve cell. There are many synapses at their ends. There may be only a few dendrites on one nerve cell, and it may have so many of them that they will ultimately make up to 90% of the entire surface of a given neuron.

The axon, in turn, is a different structure. It is a single appendage that extends from the body of the nerve cell. The length of an axon can be extremely different - just as some of them are only a few millimeters, in the human body you can find axons much more than a meter long.

The role of the axon is to transmit the signal received by the dendrites to other nerve cells. Some of them are covered with a special sheath - it is called the myelin sheath and it enables much faster transmission of nerve impulses.

The bodies of nerve cells can be found in strictly defined structures of the nervous system: they are mainly present in the central nervous system, and in the peripheral nervous system - they are located in the so-called ganglia. Clusters of axons, which come from many different nerve cells, and are covered with appropriate membranes, are called nerves.

Neuron (nerve cell): types

There are at least a few divisions of nerve cells. This is because neurons can be divided, for example, due to their structure, where the following are distinguished:

  • unipolar neurons: so called because they only have one protrusion
  • bipolar neurons: nerve cells thathave one axon and one dendrite
  • multipolar neurons: they have three or many more protrusions

Another division of neurons is based on the length of their axons. In this case, the following are exchanged:

  • Projection neurons: they have extremely long axons that allow them to send impulses to parts of the organism that are even very distant from their perikaryons
  • neurons with short axons: their task is to transmit excitations only between nerve cells located in close proximity to them

Usually, however, the most reasonable division of nerve cells is the division of nerve cells taking into account their function in the body. In this case, there are three types of nerve cells:

  • motor neurons (also known as centrifugal or efferent): these are responsible for sending impulses from the central nervous system to executive structures, e.g. muscles and glands
  • sensory neurons (in other words, afferent, afferent): they perceive various types of sensory stimuli, e.g. thermal, touch or smell and transmit the received information to the structures of the central nervous system
  • associative neurons (also known as interneurons, intermediary neurons): they are intermediaries between sensory and motor neurons, generally their role is to transfer information between different nerve cells

Neurons can also be divided due to the way they secrete neurotransmitters (these substances - which will be discussed later - are responsible for the possibility of transmitting information between neurons).

In this approach, we can list, among others :

  • dopaminergic neurons (secreting dopamine)
  • cholinergic neurons (release acetylcholine)
  • noradrenergic neurons (secreting norepinephrine)
  • serotonergic neurons (releasing serotonin)
  • GABAergic neurons (release GABA)

Neuron (nerve cell): features

Basically, the basic functions of a neuron have been mentioned before: these cells are responsible for receiving and transmitting nerve impulses. However, this is not done as a deaf phone, where the cells talk to each other, but through complicated processes that are simply worth looking at.

The transmission of impulses between neurons is possible thanks to specific connections between them - synapses. There are two types of synapses in the human body: electrical (of which there are relatively few) and chemical (dominant, these are what neurotransmitters are related to).

There are three distinguished within the synapseparts:

  • presynaptic ending
  • synaptic cleft
  • postsynaptic ending

The presynaptic end is where neurotransmitters are released - they go to the synaptic cleft. There they can bind to receptors on the postsynaptic terminal. Ultimately, after stimulation by neurotransmitters, the excitation may be triggered and finally the transmission of information from one nerve cell to another.

Resting and action potential - impulse transmission

Here it is worth mentioning another phenomenon related to the transmission of signals between nerve cells - the action potential.

In fact, when it is generated, it begins to spread along the axon and it may come to the point that its end - which is the presynaptic ending - will release a neurotransmitter, thanks to which the excitation will spread further.

Nerve cells, which currently do not send any impulses, i.e. are in a kind of rest, have the so-called resting potential - depends on the difference in the concentrations of various cations between the inside of the nerve cell and the external environment.

The difference is mainly due to the sodium (Na +), potassium (K +) and chloride (Cl -) cations.

Generally, the inside of a neuron is negatively charged in relation to its outside - when the excitation wave reaches it, the situation changes and it becomes much more positively charged.

When the charge inside the neuron reaches the value defined as the threshold potential, the excitation is triggered - the impulse is "fired" along the entire length of the axon.

It should be emphasized here that nerve cells always send the same type of impulse - no matter how strong the stimulation reaching them is, they always respond with the same force (it is even mentioned that they send impulses according to the principle "all or nothing").

Depolarization and hyperpolarization

It is mentioned all the time that when neurotransmitters reach a nerve cell via synapses, it results in the transmission of a nerve impulse. However, just such a description would be a lie - neurotransmitters can be divided into excitatory and inhibitory in two ways.

The first of these actually lead to depolarization, which results in the transmission of information between nerve cells.

There are also inhibitory neurotransmitters, which - when they reach the neuron - lead tohyperpolarization (i.e. lowering the potential of the nerve cell), which means that the neuron becomes much less capable of transmitting impulses.

Inhibition of nerve cells is, contrary to appearances, extremely important - it is thanks to it that it is possible to regenerate or "rest" nerve cells.

Neural networks

When discussing the functions of nerve cells, it is worth mentioning here that it is not only individual neurons that are important, but their entire networks. In the human body there are exceptionally many so-called neural networks. They may include, for example, a sensory neuron, an interneuron and a motor neuron. To illustrate the operation of such a network, an example situation can be given: accidentally touching the wick of a burning candle with a hand.

The fact that we have done it is informed by the sensory neuron - it is this neuron that perceives sensory stimuli associated with high temperature. It transmits information further - it usually does it with the help of the interneuron, thanks to which the message about the harmful stimulus reaches the structures of the central nervous system. There, it is processed, and finally - thanks to the motor neuron - a signal is sent from the appropriate muscles, leading to the fact that we instinctively withdraw our hand from the lit wick.

A fairly simple example of a neural network is described here, but it probably shows how complicated the relations between individual neurons are and why nerve cells and their function are so important for human functioning.

Category:

Anatomy