Nervous tissue: Neurons

In this note, we are going to discuss the detailed structure and functions of a neuron.

But first, let’s recall what the nervous system is.

It provides communication among all other organ systems, monitors internal and external changes, and develops proper responses.

For ease of understanding, the nervous system is structurally subdivided into the central nervous system (or CNS), which includes the brain and spinal cord, and the peripheral nervous system (or PNS), which includes nerves and ganglia outside the CNS.

Both the central and peripheral parts of the nervous system consist of nervous tissue, which in turn is formed by neurons (or nerve cells) and neuroglia.

Neurons can receive, generate, and transmit electrical impulses, providing communication with other neurons as well as with effector cells, such as muscle cells.

Neurons consist of the cell body and two types of extensions, or processes, that arise from it.

The body contains a nucleus and the standard set of organelles found in a eukaryotic cell.

A cluster of neuron bodies in the central nervous system is called a nucleus, while in the peripheral nervous system, it’s called a ganglion.

These clusters have a gray color, so the so-called “gray matter” is nothing other than the bodies of neurons.

The first type of neuronal processes are dendrites.

They are short, and each neuron may have many such projections. They receive incoming signals from other neurons.

The second type, axons, are long. Each neuron has only one axon. It is used to transmit impulses to other neurons.

The processes have a white color, so the “white matter” is a collection of neuronal processes.

Depending on the location of a neuron within the nervous system, the number of its processes may vary:

Multipolar neurons have several dendrites and a single axon;

Bipolar neurons have one large dendrite and one axon;

Unipolar neurons contain only one process; according to many researchers, this type of neuron is only present during embryogenesis;

Pseudounipolar neurons have one process that arises from the body and immediately divides in a T-shaped manner. One part of this projection acts as an axon, and the other acts as a dendrite;

Axonless neurons contain many processes, but the axon is so short that it cannot be distinguished from dendrites.

Depending on their location in the reflex arc, neurons can be afferent (or sensory), associative (or interneurons), and efferent (or motor neurons).

The initial part of the axon has a small elevation and is called the axon hillock.

This is a sort of launch area where signals from the dendrites converge and are redirected into the axon. The process ends with one or several axon terminals.

Each terminal is part of a synapse – a structure through which nerve cells communicate with each other, i.e., transmit the nerve impulse.

Synapse - the site of contact of the nerve cells

Depending on which parts of the cells are in contact, a synapse can be axoaxonal, axodendritic, or axosomatic. Additionally, a neuron can interact with a muscle (forming a neuromuscular synapse) or a gland (forming a neuroglandular synapse).

A synapse consists of three parts:

Presynaptic membrane

Synaptic cleft

Postsynaptic membrane

There are two types of synapses based on the mechanism of signal transmission: chemical and electrical.

In a chemical synapse, the transmission of nerve impulses is carried out using special chemical substances – neurotransmitters. They are contained in synaptic vesicles and are released into the synaptic cleft under the influence of the received nerve impulse.

Then, receptors on the postsynaptic membrane capture this neurotransmitter, the membrane electric charge changes, and the nerve impulse proceeds further to the next neuron.

Through a chemical synapse, an impulse can only travel in one direction. Depending on the neurotransmitter, synapses can be cholinergic (acetylcholine), adrenergic (norepinephrine), serotonergic (serotonin), and others (dopaminergic, GABAergic).

In an electrical synapse, the impulse is transmitted directly from one neuron to another without the involvement of chemical substances, due to the very close location of membranes. Such synapses are formed in places where the bodies of neurons are located close to each other. Through an electrical synapse, the impulse can be transmitted in both directions.

Some axons are covered with a myelin sheath.

This is a layer of lipids and proteins that speed up the conduction of nerve impulses, preventing it from spreading beyond the axon.

However, as we can see, the process is not completely covered with myelin. The spared segments are called Ranvier’s nodes.

They contain many ion channels. This feature allows the impulse to “jump” from one myelinated segment to another, significantly increasing the speed of signal transmission.

The myelin sheath in the central nervous system is formed by oligodendrocytes, while in the peripheral nervous system it is formed by Schwann cells. Myelination is discussed in greater detail in the video “Neuroglia”.

The cytoplasm of the neuron contains Nissl bodies (or tigroid substance), which represent a highly developed granular endoplasmic reticulum responsible for protein synthesis.

Nissl substance is found in the body and dendrites, but is absent in axons.

The cytoskeleton of neurons is composed of three primary elements: microtubules, neurofilaments, and microfilaments.

These structures mainly carry out a scaffolding function, maintaining the shape of the cell.

In addition to their structural role, microfilaments participate in the regeneration of damaged processes, while microtubules provide axonal transport. There are two types of axonal transport:

Anterograde transport – this involves the movement of substances from the neuron’s body to the axon terminal
Retrograde transport – this process involves the transport of substances back from the axon terminal to the neuron’s body

Furthermore, retrograde transport can also carry viruses such as those causing poliomyelitis, rabies, and herpes, as well as toxins produced by the tetanus bacteria.

Golosary