Neurons are nerve cells destined for the production and exchange of signals; therefore they represent the functional unit of the nervous system, that is the smallest structure able to perform all the functions for which it is intended.
Our brain contains about 100 billion neurons, which vary in shape and position but have some characteristics. The main peculiarity regards the long extensions that depart from the cellular body, called dendrites if they receive information and axons if they transmit them.
Most neurons are characterized by three regions: the cellular body (also called pyrenophore, perikarion or soma), dendrites and the axon (or neuritis).
The position of the soma varies from neuron to neuron, often it is central and usually has small dimensions, even if there are exceptions.
The dendrites (from dendrom, tree) are thin branches of tubular form, whose main function is to receive incoming signals (afferent). They are therefore deputies to conduct 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, possess only one dendrite, while others are characterized by highly complex ramifications. Furthermore, 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 is taken into account. In the CNS the function of dendrites can be more complex than described; their spines, in particular, can function as separate compartments, capable of exchanging signals with other neurons; not by chance that many of these thorns possess 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 nerve fibers, as well as to increase the speed of transmission of the impulse (from 1 m / s to 100 m / s, that is 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 guina - 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 Nodes. This interruption forces the electrical impulses to jump from one node to another, accelerating their transfer.
The nerve fiber is constituted by the axon - which is the fundamental structure of the conduction of the impulse - and by the sheath (mileinica or amielinica) that covers it.
The point of somatic origin of the axon is called axonal crest (or monticolo), while at the opposite end most neurons have a swelling, called axonal (or synaptic) button (or terminal), which contains mitochondria and important membranous vesicles for synapse operation. These latter structures are points of connection between the synaptic buttons of the neuron and other cells (nervous and non-nervous), responsible for the transfer of the nervous impulse. Most of the synapses are chemical and as such involve the release, by axonal buttons, of particular substances called neurotransmitters and stored in vesicles.
| MAIN DIFFERENCES BETWEEN | |
| ASSONI | eDENDRITI |
| They carry the information away from the cellular body | They carry the information to the cellular body |
| Their surface is smooth | Rough surface dendritic spines |
| Generally there is only one per cell | There are generally many for each cell |
| They don't have ribosomes | They have ribosomes |
| They can be myelinated | They are not myelinated |
| They branch away from the cell body | They branch near the cell body |
The axon contains numerous mitochondria, neurotubules and neurofilaments. These latter structures support the axon, which is sometimes particularly long, and allow the transport of substances within it. However, while dendrites are rich in ribosomes, an important characteristic of axons is the absence of Nissl bodies, hence of ribosomes and of a rough endoplasmic reticulum. For this reason every protein destined to the axon must be synthesized at the level of the cellular 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 takes place at two different speeds (fast or slow). The slow axonal transport conveys elements from the pirenophore to the axon at a speed of 0.2-2.5 mm per day; as such it mainly involves cytoskeletal constituents and other components that are not consumed quickly by the cell. Fast transport, on the contrary, mainly affects secretory vesicles, enzymes of neurotransmitter metabolism and mitochondria, which proceed towards the synaptic button at speeds between 5 and 40 cm (400 mm) per day.
According to the form, numerous types of neurons can be 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).
Depending on the function, neurons can be classified into:
Sensitive neurons (tactile, visual, gustatory, etc.): deputies to receive sensory signals;
Interneurons: deputies for signal integration;
Motor neurons: deputies for the transmission of signals.
Sensory (or sense) neurons collect sensory information from the outside (somatic sensory neurons) and from inside the body (visceral sensory neurons). Both belong to the category of psuedo-unipolar neurons; their pyrenophore is always placed inside a ganglion (aggregate of cellular bodies) outside the CNS, while the axons of these neurons (afferent fibers) extend from the receptor to the central nervous system (see figure).
The motor neurons (or motoneurons) present axons (efferent fibers) that move away from the central nervous system (in whose gray substance the soma is found) and reach the peripheral organs. They are distinguished in somatic motor neurons (for skeletal muscles) and visceral effector neurons (for smooth muscles, heart and glands).
The associative neurons or interneurons are found in the CNS and are the most numerous. They analyze the input sense stimuli and coordinate the outgoing stimuli, thus allowing MODULATE the nerve responses.
