Nerve fiber
As a nerve fiber is called the wrapped glial extension of a nerve cell, for example, the axon of a nerve cell along with its myelin sheath. In the peripheral nervous system, multiple nerve fibers are usually summarized by additional connective tissue covers to nerves.
- 3.1 Sensitive nerve fibers
- 3.2 Motor Nerve fibers
- 3.3 Autonomic nerve fibers
- 3.4 Classification according to line speed according to Erlanger / Gasser
Myelination ( myelination )
History of development
Myelination (Mark ripening) means the equipment of nerve fibers with myelin or medullary substance. This process has evolutionary significance. Paul Flechsig (1847-1929) has pointed out the ontogenetic sequence of myelination of the brain in humans. The nerve fibers obtained in the course of its development, an insulating layer is formed within the CNS of oligodendroglia and partly of astroglia. The course of the axon following myelin sheaths - also known as myelin sheaths - are formed in the peripheral nerves by Schwann cells. The mother cells of nerve cells - - From the fifth week of pregnancy, the cells of the neural tube neuroblasts differentiate and in glioblasts - the mother cells of the glial cells of the nervous system. Glioblasts are also called spongioblasts. Only with the formation of a myelin sheath nerve cells are to be regarded as functional.
Histology
The Schwann cells grow during the development of the myelin sheaths ( myelogenesis or marrow maturation ) depending on the type of nerve cell to about 50 times around the nerve cell process and wrap it around it several times with double layers of their cell membrane. A single Schwann cell wrapped it always only one to 2 mm long section of an axon, the internode, many Schwann cells are therefore lined up next to each other along the common axis cylinder around the axon around.
As the number of windings, the diameter of a nerve fiber, at the same axon diameter increases. Depending on the thickness of the myelin sheath or the number of myelin sheath lamellae differentiates the myelinated nerve fibers in marrow rich (highly myelinated ) and mark poor nerve fibers. So-called non-myelinated nerve fibers (better: myelinated loosely ) have no myelin sheath, but also, in their gussets extend PNS glial cells through a cladding them. However, these single- shell isolated any more than that action potentials must be passed continuously, so the conduction is much slower than in axons in a myelin sheath.
For somatic nerve fibers, the afferent or efferent the body wall relate - and so can provide rapid responses to changes in the environment - represent the myelinated nerve fibers the largest share. The axon diameter makes only about half of each of the nerve fiber cross-section because of the thick insulating layer. This made possible the so-called saltatory conduction with a longer -fold increased nerve. For between two Schwann cells remains a small gap, a Ranvierscher constriction or nodes, and only in this ( nodal ) region, the axon is so clear that here, an action potential can be established, which takes some time. The heavily insulated ( internodal ) spread between two constriction rings is, however, bridged by rapid electrotonic conduction, so that the action potentials are forwarded nurmehr abruptly occurring. In demyelinating diseases such as multiple sclerosis sections is the myelin sheath around axons of central neurons lost, which can manifest itself among other things in numbness or paralysis.
Topography
As shown in Figure 1, allows the knowledge of the stages of maturation of the brain through puberty a topographical description and systematization of the brain in terms of functional anatomy. Such systematization concerns the division into centers of projection and association centers in the brain. Reference is made to the relevant article.
From the cyto - and myeloarchitektonischen structure of the cortex is used when you want to describe the relationship between cells and cord fibers in a given brain section (eg, in the cerebral cortex, see Figure 3). The development of appropriate maps of the cerebral cortex, in the structures and functions were set with each other, going to Karl Kleist (1879-1960), Korbinian Brodmann (1868-1918), Cécile Vogt (1875-1962) and Oskar Vogt (1870-1959 ) and on Constantin von Economo (1876-1931) back.
Anthropology
The neuroscientist Manfred Spitzer ( born 1958 ) has referred to as defect beings and the concept of man as altricial related to the myelination of the brain to anthropological concepts of Arnold Gehlen ( 1904-1976 ) from the people. The concept of the nest stool is taken for example by the psychoanalyst Alexander Mitscherlich ( 1908-1982 ). Here, the immediate innate instinct bondage animal organisms is weighed on one side against the question of the long-term acquisition of intellectual qualities in humans. Spitzer believes by reference to the duration of human myelination to contribute to this question.
Nerve fiber bundles (peripheral nerves)
The individual ummarkten nerve fibers are separated by very fine Bindegewebslamellen, called the endoneurium. It consists of reticular fibers and a basement membrane.
A plurality of such fibers are combined with the so-called perineurium into fiber bundles ( fascicles ) consisting of collagenous connective tissue.
The epineurium eventually enveloped the entire peripheral nerve ( ie several nerve fiber bundles ) and fixes in the surrounding tissue.
These are the nerves Bindegewebsumhüllung a higher elasticity ( by elastic fibers), protects against pressure and it serves the Schwann cells to diet since run the blood vessels in these Bindegewebslamellen.
Fiber qualities
The supply of organs or body parts with nerve fibers and their transmission of impulses is called innervation. Nerve fibers or entire nerve can be classified according to various criteria:
- According to the direction of conduction of excitation: afferent ( to the CNS through ) and efferent ( from the CNS away)
- On the degree of myelination, and thus the line speed
- Based on their impact
After their primary effect, there are three different fiber qualities: sensitive, motor, vegetative.
Sensitive nerve fibers
Sensory fibers conduct sensations that were registered by receptors from the body to the central nervous system. Following the direction of the excitation line is also called afferent fibers. In the German -speaking area are separated from the sensitive frequent sensory fibers. Sensory fibers relay impulses of specific sense organs ( eye, ear, taste). The distinction is, however, rather artificial, because sensory fibers have specialized nerve endings. In English, this differentiation is not performed, both sensitive and sensitive sensory by the term are classified.
A special form are derived viszerosensible ( viscera, " entrails " ) fibers, the sensations of internal organs. However, they are usually counted among the vegetative fibers.
Motor nerve fibers
Motor fibers innervate the skeletal muscles. Where the excitation is transmitted through a motor end plate on the muscle fiber and these stimulated to contraction. The branchomotorischen nerves are separated occasionally by the motor nerves. This innervate skeletal muscles, which arise from the branchial arches. Following the direction of the excitation line is efferent fibers. Pure motor nerve, there is not always run in " motor " nerves also classified as afferent fibers from the receptors in the muscles (muscle spindle, Golgi tendon organ ) who register the current tone of the muscle.
Visceromotor nerves innervate the smooth muscles of the viscera and blood vessels and are among the vegetative fibers.
Autonomic nerve fibers
As vegetative fibers are called the nerves of the autonomic ( vegetative ) nervous system. They can be classified to belong to the respective system as parasympathetic or sympathetic and as enteric. After their primary function or line direction, a distinction is
- Viszeroafferent or viszerosensibel
- Visceral efferent: viszeromotorisch ( on smooth muscle) or secretory ( at surgery)