Neurons are nerve cells intended for the production and exchange of signals; they therefore represent the functional unit of the nervous system, that is, the smallest structure capable of performing all the functions it is responsible for.
Our brain contains about 100 billion neurons, varying in shape and position but with certain characteristics. The main peculiarity concerns the long extensions that depart from the cell body, called dendrites if they receive information and axons if they transmit it.
Most neurons are characterized by three regions: the cell body (also called pyrenophore, perikarion or soma), the dendrites and the axon (or neurite).
The position of the soma varies from neuron to neuron, it is often central and usually has small dimensions, although there are exceptions.
The dendrites (from dendrom, tree) are thin tubular branches whose main function is to receive incoming (afferent) signals. They are therefore responsible for the conduction of stimuli from the periphery towards the center or soma (centripetal direction). These structures amplify the surface of the neuron, allowing it to communicate with many other nerve cells, sometimes several thousand. Also for this cellular element there is no lack of variables; some neurons, for example, have only one dendrite, while others are characterized by highly complex ramifications. In addition, the surface of a dendrite can be further extended by the so-called dendritic spines (cytoplasmic protrusions), on each of which an axon from another neuron takes synactic contact. In the CNS, the function of the dendrites can be more complex than described; their spines, in particular, can function as separate compartments, capable of exchanging signals with other neurons; it is no coincidence that many of these spines have polyribosomes and as such can synthesize their own proteins.
The axon is often wrapped in a lipid sheath (the myelin sheath or myelin), which helps to isolate and protect the nerve fibers, as well as increase the transmission speed of the impulse (from 1 m / s to 100 m / s , i.e. almost 400 km / h). Myelinated axons are generally found in peripheral nerves (motor and sensory neurons), while non-myelinated neurons are found in the brain and spinal cord.
The myelin sheath - synthesized by Schwann cells in the SNP and by oligodendrocytes in the CNS - does not uniformly cover the entire surface of the axon, but leaves some of its points uncovered, called Ranvier's Nodes. This interruption forces the electrical impulses to jump from one node to another, accelerating their transfer.
The nerve fiber is made up of the axon - which is the fundamental structure of impulse conduction - and the sheath (mileine or unmyelinated) that covers it.
The somatic point of origin of the axon is called axonal crest (or mound), while at the opposite end most neurons have a bulge, called axonal (or synaptic) button (or terminal), which contains important mitochondria and membranous vesicles for the functioning of the synapse. These last structures are points of connection between the synaptic buttons of the neuron and other cells (nerve and not), responsible for the transfer of the nerve impulse. Most of the synapses are of a chemical type and as such require the release, by the axonal buttons, of particular substances called neurotransmitters and stored in vesicles.
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The axon contains numerous mitochondria, neurotubules and neurofilaments. These last structures support the axon, which is sometimes particularly long, and allow the transport of substances inside it. However, while dendrites are rich in ribosomes, an important characteristic of axons is the absence of Nissl bodies, therefore of ribosomes and rough endoplasmic reticulum. For this reason, any protein destined for the "axon" must be synthesized at the level of the cell body. of the neuron and then conveyed towards it.This traffic - called axonal (or axonal) transport (or flow) - is essential to supply the synaptic button with the enzymes necessary for the synthesis of neurotransmitters.
Forward traffic runs at two different speeds (fast or slow). Slow axonal transport carries elements from the pyrenophore to the axon at a rate of 0.2-2.5 mm per day; as such it mainly affects cytoskeletal constituents and other components that are not rapidly consumed by the cell. Fast transport, on the contrary, it mainly affects secretory vesicles, neurotransmitter metabolism enzymes and mitochondria, which proceed towards the synaptic button at speeds between 5 and 40 cm (400 mm) per day.
According to the shape, numerous types of neurons are recognized. The most common are multipolar, that is, they have a single axon and many dendrites (they are typically the neurons that control skeletal muscles).
Based on function, neurons can be classified into:
Sensitive neurons (tactile, visual, gustatory, etc.): deputies to receive sensory signals;
Interneurons: deputies for the integration of signals;
Motor neurons: deputies for the transmission of signals.
Sensory (or sense) neurons collect sensory information from outside (somatic sensory neurons) and from inside the body (visceral sensory neurons). Both belong to the category of psuedounipolar neurons; their pyrenophore is always located inside a ganglion (aggregate of cell bodies) outside the CNS, while the axons of these neurons (afferent fibers) extend from the receptor to the central nervous system (see figure).
Motor neurons (or motor neurons) have axons (efferent fibers) that move away from the central nervous system (in whose gray matter is the soma) and reach the peripheral organs. They are divided into somatic motor neurons (for skeletal muscles) and visceral effector neurons (for smooth muscles, heart and glands).
Associative neurons or interneurons are found in the CNS and are the most numerous. They analyze the incoming sense stimuli and coordinate the outgoing ones, thus allowing to MODULATE the nerve responses.
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