The neuron
The nerve cell [41, 52, 64, 94, 100] or neuron is the structural and functional unit of the nervous system [39, 64]. The latter contains approximately 100 billion neurons [1, 4] (Note: Modern stereological studies (e.g., Herculano-Houzel, 2009) have established the average human brain contains ~86 billion neurons, not 100 billion).
Anatomy of a typical neuron:
A typical nerve cell has three distinct regions [1, 40, 99] : the cell body, dendrites, and the axon.
The cell body :
Also called the soma [5, 75] or perikaryon [57, 101], it contains the nucleus and the cytoplasm. It includes:
A rough endoplasmic reticulum: the site of protein synthesis.
A Golgi apparatus: the site of protein storage and maturation. It is the mandatory passage point and regulator of small vesicle traffic.
Several mitochondria, which provide the energy essential for cellular metabolism.
The diameter of the perikaryon varies according to the type of neuron, ranging from 5 to 120 µm [91, 102].
The dendrites :
These are extensions of the perikaryon [5, 96]; they serve primarily to increase the receptive surface area for nerve impulses [1, 4, 36]. They are often covered with bud-like structures called dendritic spines [41, 74, 103]. A typical neuron contains tens of thousands of dendritic spines [103] , each forming a synapse [74].
The axon :
The axon [5, 91, 96] emerges from the perikaryon, starting at the axon hillock [81, 104], an area extremely rich in microtubules, also called the trigger zone [40, 81] because it is the usual starting point of nerve impulses [40].
The axon follows a path of varying length before ending in a terminal arborization (telodendria [75]). This gives rise to several nerve endings, at the tip of each there is a swelling: the terminal button or synaptic knob [101], which contains several synaptic vesicles filled with neurotransmitters.
Before the terminal arborization, a neuron may give off side branches called axon collaterals to distribute the nerve impulse to other targets. [105].
The axon membrane (axolemma [106]) encloses the axoplasm [91] (an extension of the perikaryon's cytoplasm), which is traversed throughout its length by neurofilaments and microtubules that stabilize the structure of the axon and ensure the bidirectional transfer of substances between the perikaryon and the axonal terminals [74].
Axonal transport :
There are four types of axonal transport [35, 50,107]:
- Two rapid transports (anterograde and retrograde) that carry the vesicles at a speed of 10 to 40 cm/day [108] from the perikaryon to the nerve ending and vice versa.
- Slow anterograde transport with a speed of 0.1 to 2 mm/day [57].
- A mitochondrial transport [104] that continuously renews the mitochondria in the nerve endings at a speed of 1 to 4 cm/day [5].
Features of the nerve cell:
The neuron possesses several characteristics that set it apart from other cells in the body.
It is an excitable cell:
The neuron receives and transmits electrochemical signals thanks to the presence in its membrane of specific proteins that regulate the transfer of ions into or out of the cell [42,110].
It is a secretory cell:
The nerve cell secretes neurotransmitters, sometimes even hormones, at the level of its axonal terminal [109].
It is an amitotic cell:
With the exception of a few nerve zones where low levels of neuronal mitosis occur (notably in the hippocampus) [40], mature neurons are arrested in interphase and [110], as post-mitotic cells, no longer undergo division, making them cells with extreme longevity [110]. This also explains the rarity of brain tumors of neuronal origin.
Every day, tens of thousands of neurons are lost [111], yet this depletion rarely results in a notable disorder because of neural plasticity; surviving neurons compensate by reorganizing their connections and strengthening existing circuits.
It is a polarized cell:
There are two essential poles: the somatodendritic tree, which receives the signal, and the axonal pole, which propagates it [112]. However, the axon can receive a signal directly at the level of an axo-axonal synapse [75].
It is a cell with a very high metabolism:
Indeed, the neuron requires a constant and abundant supply of oxygen and glucose [110], which explains cerebral death within minutes in the event of cerebral anoxia.
Neurons are characterized by extraordinary polymorphism:
More than 150 types are distinguished [Jeanette Norden, Understanding the Brain] according to their size, polarity, structure, function, and location [35]. However, based on their shape, we can distinguish between three main (non-exclusive) categories [40, 49]:
- Unipolar or pseudo-unipolar neurons (often sensory).
- Bipolar neurons (e.g., certain interneurons).
- Multipolar neurons (e.g., motor neurons).
On a functional level, we distinguish between sensory neurons, which conduct impulses toward the CNS, and motor neurons, which conduct impulses away from the CNS. Between the two are other neurons called interneurons.
Nerve fibers are organized into tracts within the CNS and nerves within the PNS. As for the cell bodies: they gather in the cortex and nuclei within the CNS, and in ganglia within the PNS.
Neurons are the longest cells in the body:
They can reach up to 1 meter in length in humans [113]. If the cell body were the size of a grapefruit, the dendrites would be 2 to 5 cm long, and the axon would be 1 km long.