How do neurons work?

It is widely known in popular culture that neurons are cells that act as a sort of messengers, sending information back and forth throughout our system nervous.

How neurons work, which are the basic functional unit of our brain, spinal cord and nerves, is the subject of today's article. Let's find out how these sophisticated works of nature engineering work.

• Related article: "Types of neurons: characteristics and functions"

How do neurons work? An overview

Neurons are cells that are part of the nervous system, being its basic functional unit. These cells have the main function of receiving and transmitting information in the form of electrical impulses along a complex lattice or network made of neurons, which constitutes the system nervous system, both the central (CNS), made up of the spinal cord and brain, and the peripheral (SNP) made up of the nerves.

It is clear that, based on this definition, the nervous system could not function without neurons, along with glia cells. However, to understand more how they work it is necessary to make a series of notes regarding its typology, its structure and its shape, since these directly influence its operation.

Structure

The functions of neurons cannot be understood without understanding how these nerve cells are organized. These are the parts of the neuron.

1. Soma

The soma is the cell body of the neuron, and it is the place where the nucleus is located, in addition to having a great protein synthesis activity, essential for the functioning of the neuron. It is from here that various protrusions or appendages extend: the dendrites and the axon.

2. Dendrites

Dendrites are spiny, tree-shaped protrusions that allow the neuron to receive and process information. Depending on the type of signals it receives, it can induce the excitation or inhibition of the neuron, causing the action potential to occur or not, that is, to trigger a nerve impulse.

3. The axon

The axon consists of a single prolongation in the neuron with a homogeneous thickness. This structure has its origin in the cell body, specifically in the axonal cone. In motor neurons and interneurons, it is in this axonal cone where the action potential is produced.

Axons are coated with a special insulating substance: myelin. This myelin has a fundamental function in the nervous system, since it makes the nervous impulse more efficient and faster.

Coming to the end of the axon are many branches, which form bulbous structures known as axon or nerve terminals. These terminals form connections with target cells, be they motor or interneurons.

Types of neurons according to their function

According to their functions, we can distinguish between three types: sensory, motor and interneurons.

1. Sensory neurons

Sensory neurons They are those that are in charge of capturing the information external to the organism or the sensations, such as pain, light, sound, touch, taste... This information is captured and sent in the form of an electrical impulse, directing it towards the central nervous system, where it will be processed.

2. Motor neurons

Motor neurons receive information from other neurons, taking care of transmitting orders to muscles, organs and glands. In this way, a movement can be carried out or a certain biological function can be carried out, such as the production of hormones.

3. Interneurons

Interneurons are a special type of cell present in the central nervous system that are responsible for connecting one neuron with another, that is, they function as a kind of bridge. They receive information from some neurons, be they sensory or other interneurons, and transmit them to others, which may be motor neurons or other interneurons.

Neurons work by forming networks

Regardless of how healthy a neuron is, if it is isolated from the others, it is useless at all. In order for these cells to carry out their functions, they must be connected with each other, working together. Thus, when these cells connect with each other, they stimulate or inhibit each other, process incoming information and contribute to the emission of a motor or hormonal response. These neural circuits can be very complex, although there are also quite simple ones, especially related to reflexes.

When working as a team, neurons can perform three basic functions, these being to receive nerve signals or information from other neurons; integrate those signals, in order to determine if the information is important or not; and communicating the signals to target cells, which can be muscles, glands, or other neurons.

To further understand these three functions, we are going to describe an example, a situation in which involve the three types of neurons based on their function: sensory neurons, motor neurons and interneurons.

Let's imagine that we are preparing a tea, with the kettle on top of the fire. When we see it, we are activating sensory neurons, specifically those that are responsible for sight, transmitting nervous information captured in the cones and rods of the retina to the brain. Visual information will be processed in the brain and we will be aware that we are seeing the kettle.

As we want to serve ourselves a tea, we get ready to take the kettle. In order to move the arm, we need to use our motor neurons. These neurons have received the signal from the brain to activate the muscles of the arm, stretch it and take the kettle. So, we make that movement: we reach out and take the kettle, whose handle is made of metal.

Turns out we hadn't turned off the heat and the kettle was very hot. This sensation is captured by the thermal sensors of the skin when touching the hot handle. This information, captured by sensory neurons, travels rapidly to the spinal cord that, through an interneuron, sends information to motor neurons without having to send it to the brain. The arm is ordered to move quickly to avoid getting burned. Still, some of the information reaches the brain, which interprets it in the form of pain.

Synapse

Neuron-to-neuron connections are normally formed on the axon and dendrite of two neurons. The meeting place between these two neurons is what is known as the synapse or synaptic space, giving rise to the transmission of information from the first (presynaptic) neuron to the next, the target neuron being (postsynaptic).

The transmission of information is done through chemical messengers, neurotransmitters, having many types (p. g., serotonin, dopamine, acetylcholine, GABA, endorphins ...).

When an action potential travels through the axon of the presynaptic cell and reaches its terminal, this neuron releases a neurotransmitter in the synaptic space which binds to the receptors of the postsynaptic cell membrane and, thus, the transmission of the nerve signal occurs. This signal can be excitatory or inhibitory and, depending on the type of neurotransmitter, a specific function will be performed. another, in addition to depending on which path the nerve impulse follows, going towards the nerve center or target cell correspondent.

• You may be interested: "Synapses: what they are, types and functions"

And what about glial cells?

Although the protagonists are the neurons, we can't forget about her secondary friends, the glial cells, although "secondary" is not synonymous with "expendable." If the neuron is the basic functional unit of the nervous system, glial cells are the majority cell of it. This is why they cannot be left behind when trying to explain how the neurons, especially considering that they have a very supportive role for the nervous system. important.

Broadly speaking, there are four types of glial cells, three of which are astrocytes, oligodendrocytes, and microglia that can only be found in the central nervous system. The fourth type is Schwann cells, which are only found in the peripheral nervous system.

1. Astrocytes

Astrocytes are the most numerous type of glial cells in the brain. Its main functions are to regulate blood flow in the brain, to maintain the composition of the fluid that surrounds neurons, and to regulate communication between neurons in the synaptic space.

During embryonic development, astrocytes help neurons reach their destinations, in addition to contributing to the formation of the blood-brain barrier, the part that isolates the brain from toxic substances that may be dissolved in the blood.

2. Microglia

Microglia are related to macrophages of the immune system, the "scavengers" that remove dead cells and residues that can be toxic if they accumulate.

3. Oligodendrocytes and Schwann cells

Oligodendrocytes and Schwann cells share a similar function, although the former are found in the central nervous system and the latter in the peripheral. Both are glial cells that produce myelin, the insulating substance found in a sheath around neuronal axons.

Bibliographic references:

• Purves, D., Augustine, G. J., Fitzpatrick, D., Katz, L. C., LaMantia, A.-S and McNamara, J. OR. (1997). The organization of the nervous system. In Neuroscience (pp. 1-10). Sunderland, MA: Sinauer Associates.
• Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V and Jackson, R. B. (2011). Nervous systems consist of circuits of neurons and supporting cells. In Campbell biology (10th ed., P. 1080-1084). San Francisco, CA: Pearson.
• Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V and Jackson, R. B. (2011). Neuron structure and organization reflect function in information transfer. In Campbell biology (10th ed., P. 1062-1064). San Francisco, CA: Pearson.
• Sadava, D. E., Hillis, D. M., Heller, H. C and Berenbaum, M. R. (2009). Neurons and nervous systems. In Life: The science of biology (9th ed., Pp. 988-993). Sunderland, MA: Sinauer Associates.