Nerve tissue
The nerve tissue is one of the four basic tissues of tissue animals , which includes vertebrates and humans as well . It consists of nerve cells and glial cells , both of which are derived from common neuroectodermal precursor cells, which in chordates arise from the neural tube and the neural ridges.
In the picture above, surrounded by glia, the processes and large cell bodies of a few Purkinje cells in close contact with some black-stained processes of basket cells are stained light brown .
Below that the position of the cerebellar granule cells ; they make up over 50% of all neurons in mammals.
Nerve cells, or neurons, and glial cells together form the nerve tissue and, together, develop the basic structures of the nervous system . The central area with the brain and spinal cord is formed from the neural tube, while nerves , nerve plexuses and ganglia are formed in the peripheral area , together with enteric components in the gastrointestinal tract . Neurons and glia also work together to carry out the basic functions of this system, the conduction and transmission of neuronal excitations .
To do this, neurons form processes that receive excitations from other cells ( dendrites ) or transmit their own excitation to other cells ( neurite ). In this way, neurons are linked at the points of excitation transmission ( synapses ) and thus form interlinked chains, loops or circles. The contacts of neurons through which it is related to the rest of the body and its surroundings are of particular importance for the functional connection of such neural networks in a nervous system. This includes on the one hand afferent contacts of certain neurons to sensory cells such as sensory cells ( sensors ), which can be specifically changed by their environment, and on the other hand efferent contacts of certain neurons to motor cells that specifically change their environment, such as muscle cells or gland cells ( effectors ). Neurons that mediate between sensory or motor parts in a promoting or inhibiting manner are called interneurons .
In the early development, glial cells form basic structures, on which young neurons can orient themselves while migrating or developing extensions, then they stabilize extensions and connections through a cover and later allow a particularly rapid conduction of excitation ( saltatory ) through multiple coverings . In the more mature nervous system, among other things , they ensure low -interference signal transmission and signal transduction , absorb released messenger substances, provide nutrients and are involved in the blood-brain barrier as, as ependymal cells , in the blood-liquor barrier , with which the nervous tissue opposite the intravascular space of the supplying blood capillaries is delimited in a special way.
In the living organism, nerve tissue appears pink or light gray to whitish, with subtle differences due to structure. In the so-called gray matter , the bodies of nerve cells predominate , the accumulations of which in the central nervous system (CNS) are also known as nuclei , and in the peripheral mostly as ganglia . The white matter consists primarily of the appendages of nerve cells, which appear as an axon encased by glial cells in myelin-containing nerve fibers and are often combined to form conduction paths , for example as a central projection path, usually called nerves in the peripheral nervous system (PNS) .
The relationship between glial cells and neurons
While nerve cells selectively pass on impulses as action potentials ( conduction of excitation ) and transfer them to other cells (transmission of excitation ) in a gigantic network of converging and diverging neurons that influence each other in a pioneering or inhibiting manner, the mostly smaller glial cells support them in this.
Glial cells can be differentiated according to their origin, structure, function and location. Astrocytes , oligodendrocytes , Schwann cells , satellite cells and ependymal cells belong to the actual glia, which emerges from the neuroectoderm like the nerve cells . As microglia immigrant cells are referred from other races, including one in the CNS the macrophages take similar task.
Astrocytes have contact points with the bloodstream and with nearby and distant neurons, but unlike neurons, they do not form a global network. In some sources, the glial networks are explained as syncytium and the connections with gap junctions . The function of the glia is only partially understood. At the beginning of neuronatomical research, glial cells were thought to be a pure cement substance ( Greek γλία glia , glue '). Protection and filter functions were later recognized: Glia maintains the biochemical environment required for nerve cells, produces substances necessary for nerve function, and removes disruptive metabolic products. An astrocyte feeds several neurons with its cell extensions and a neuron is supplied by several astrocytes. Many small astrocyte contacts (peripheral astrocytic process, PAP) often form a basket-like covering on and around a synapse.
Regeneration of nerve tissue
The regenerative capacity of nerve tissue is very limited compared to other tissues, especially since nerve cells are no longer able to divide.
During the early embryonic development, the nervous system is for some time the region with the highest rate of cell division, and fetal nerve cells develop a few thousand young nerve cells per second at peak times in humans. But these neurons are then no longer capable of cell division, postmitotic. And not all of them live as long as the organ of the organism in whose tissue they seek their place (see selective apoptosis ).
In the fully grown (adult) brain, undifferentiated neural progenitor cells are only left in a few regions, which can continue to divide and are able to form neuroblasts and young neurons (see adult neurogenesis ). In humans, for example, young nerve cells can be formed in addition to glial cells, for example in regions of the hippocampus or in the subventricular zone to replace neurons in the olfactory bulbs and olfactory mucosa. For this, these young neurons have to immigrate to that brain region (migration) and look for a place (with chemotaxis or haptotaxis ), extend processes (axogenesis), develop transmission points ( synaptogenesis ), make contacts in the existing network of other neurons, receive signals and send out signals, finally also those with which the state of excitation of certain other individual cells can be changed ( excitation or inhibition ).
On the way there and in the process along the way, a neuron differentiates itself - to take up a position in a cellular environment with certain connections. If it does not succeed, the neuron does not survive long. If it succeeds, the neuron takes a special place in the neural network - and can only be replaced in this place by young neurons that undergo a similar differentiation process. But they cannot be formed from mature neurons through cell division. For this would have to round off, the extensions recede, lose their contacts and thus become inoperable. The replacement of differentiated and function-carrying neurons within a neural network is therefore limited by the complexity of the neural connections.
In the peripheral nervous system, on the other hand, after a nerve fiber has been damaged, the extension of a neuron can grow back into the canal of the medullary sheath as an axon - if it is still present - at roughly the same speed as hair grows.