Neuron
A neuron, or nerve cell is a cell excitability constitute the basic functional unit of the nervous system. The term 'neuron' was introduced into the medical vocabulary in 1881 by German anatomist Heinrich Wilhelm Waldeyer. Neurons provide the transmission of bioelectric signals called nerve impulses. They are 10 to 50 times less likely than glial cells, components of second nervous tissue has several functions including support and nutrition of neurons.
The total number of neurons in the brain human is estimated at about 100 billion (10^11)
Structure
A neuron observed in scanning electron microscope.
Diagram of a neuron
- The neuron is composed of a body called the soma or cell body or soma, and two types of extensions: the axon, single, who drives the action potential of centrifugal manner, and dendrites, which averages 7 000 per neuron. The morphology, location and number of these extensions, and the shape of the soma, vary and contribute to define different families of morphological neurons. For example, there are unipolar or multipolar neurons.
- The body diameter of neurons varies by type, from 5 to 120 microns. It contains the nucleus, blocked in interphase and thus unable to divide, and the cytoplasm. Found in the cytoplasm the rough endoplasmic reticulum (forming Nissl bodies histologists), the Golgi apparatus, the mitochondria and neurofilaments that are bundled together to form neurofibrillary.
- The extensions are of two types: the axon, single, and dendrites.
- The axon (or nerve fiber) has a diameter of between 1 and 15 um, length varies from one millimeter to over a meter. The emergence cone region extremely rich in microtubules, is the origin of the axon. It is also called trigger point because it participates in the genesis of the action potential. He describes a trip or longer before ending in branching (that the terminal arborization). However, also observed "enfilades" of synaptic swellings on the same segment constituting axoxal synapses en passant. Each branch ends in a bulge, the button terminal or synaptic knob. The plasma membrane of the axon, or axolemme contains the axoplasmeen continuity with the cytoplasm of perikarya. It is composed of neurofilaments, microtubules and microvesicles (these are produced by the rough endoplasmic reticulum and Golgi apparatus). Some axons are covered with a sheath of myelin, formed by glial cells, the Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system. It is estimated that about 1 in three axon covered with myelin (the recovery is in fact discontinuous, separated by nodes of Ranvier) which are isolated by astrocytes. The recovery of the axon by myelin allows greater speed of movement of information nervous.
- The dendrites are numerous, short and highly branched from their origin. They are sometimes covered of dendritic spines. Unlike the axon, they do not contain microvesicles allowing the transmission of information outside of the neuron. The dendrite conducts nerve impulses, led to its end, until perikarya: it is an extension thereon.
The axons are gathered into bundles, themselves linked by connective tissue (endoneurium and perineurium) forming the tract and nerves.
Specific markers are used in biology to detect neurons and stained in preparations. Is used for example antibodies directed against neurofilaments and cons of protein tau, which are found only in neurons.