The neuroanatomy is a science that the structure of the nervous system studied. It is a branch of neurobiology as well as a specialty of systematic anatomy and has sister disciplines in neurophysiology and neurochemistry . Neuropathology , which studies the structural changes in the nervous system during pathological processes, is also closely linked to neuroanatomy .
Neuroanatomy deals with the size, location, naming and structure of the nervous system of humans and animals. The sub-discipline of comparative neuroanatomy establishes relationships between brain or spinal cord structures of different groups of animals and can thus make statements about the evolution of the nervous system .
History of Neuroanatomy
The role of the nervous system in the transmission of information was more anticipated than known by ancient Greek philosophers and physicians . In ancient concepts, nerves were mostly mistaken for hollow ducts for pneuma , a type of life-giving substance that was supposed to be made from blood in the brain . The brain itself was rated very differently: sometimes it was considered the center of the mind and seat of the soul , sometimes it was only intended to cool the blood or to produce phlegm . Not until the 3rd century BC With the Alexandrian School, anatomists such as Herophilos of Chalkedon and Erasistratos came into play, who through numerous sections also laid the foundations of a scientifically observational neuroanatomy on humans. Quite specific knowledge - also about physiological relationships - was obtained primarily from Erasistratos, who relied on the vivisection of animals. He already recognized the differences between motor and sensory nerves and divided the brain into its macro-anatomical parts. Galenos , a Greek doctor of the high Roman imperial period, collected the knowledge of his time and also carried out sections himself, but only on animals, which led to many false conclusions in his writings being lost. Since he was considered the undisputed authority in the field of anatomy for almost 1,300 years, these errors went undetected for so long. It was not until the Renaissance that ancient knowledge was checked and partially corrected.
The subject treated is the nervous system, its tissue and cellular structures. A morphological peculiarity of this system is the connectivity , i.e. the principle that the individual cells enter into multiple connections with one another, which enable and define their interaction. The smallest functional unit, the nerve cell (also called neuron ), forms offshoots ( dendrites and axons ) through which they can establish contact with other nerve cells via synapses and pass on information as electrical potentials. Neurons have a membrane potential that is particularly easy to modulate and can therefore be stimulated quickly and repeatedly. The excitation is transmitted via the synapse itself through messenger substances, so-called neurotransmitters .
A distinction is made between the central ( brain and spinal cord ) and the peripheral part ( nerves ) of the nervous system. Furthermore, a distinction is made between the somatic (voluntary) nervous system and the autonomic nervous system . These classifications are more for systematic purposes, since the subsystems described do not really represent units that are independent of one another. The nerve tissue of higher animals is divided into cell-rich gray matter and fiber-rich white matter . The cellular composition, the so-called cytoarchitecture , differs considerably in different parts of the nervous system and reflects the respective function.
All components of the nervous system arise in the embryo from the outer germ layer , the ectoderm . Only some of the progenitor cells differentiate into nerve cells, others become specialized supporting tissue, the glia .
The spatial (topical) relationships between individual sections are particularly important for understanding neuroanatomical relationships. This means the defined course (the projection ) of the trajectories from one cell population to another. These do not run randomly, but are subject to a clear structure in which nerve pathways ( tractus , fasciculi , lemnisci, etc.) formed from the processes of many nerve cells can be distinguished. In addition to the genetically predetermined basic structure, the nervous system has a high degree of plasticity , which means that the formation of the fine anatomical structures is determined not least by imprinting and learning .
Methods of anatomical data acquisition
To record the course of connections within the central nervous system (CNS), a variety of methods are used, the most important of which are listed below:
Even today, many fundamental insights into the brain and its peculiarities are derived from clinical experience. Hippocrates already noticed that a left-sided wound on the head leads to a right-sided loss of motor skills. That the contralateral relationship between body and brain is a real characteristic of the human organism was first recognized by Antonio Maria Valsalva (1666–1723) and his student Giovanni Battista Morgagni (1682–1771) and later with the less common term “Valsalva Doctrine “Occupied. The hippocratic / galenic concepts that had prevailed until then, which saw no connection whatsoever between the brain and motor skills or sensory functions , explain this rather late point in time of the conclusion in human history. Another well-known example is Brown-Séquard syndrome , which occurs after the spinal cord has been severed on one side .
The preparation of the fiber tracts within the CNS is enormously beneficial for a better understanding of the course of large tracts. However, smaller connections can hardly be represented in this way. François Pourfour du Petit and Domenico Mistichelli were able to determine the location of the intersection of the corticospinal tract as early as 1710 by dissecting the lower part of the medulla oblongata , the pyramids , and Franz Joseph Gall (1758-1828) and Johann Spurzheim (1776-1832) managed one hundred Years later, a representation of the fiber pathways from the pyramid to the cortex (cerebral cortex).
Axonal tracing makes use of the transport mechanisms within the neurons , which usually ensure a lively exchange of substances between the synapse and the cell body . The microtubules as components of the cytoskeleton, as well as the transport proteins dynein and kynesin , are particularly involved .
Diffusion tensor imaging
The Diffusion MRI is probably the most detailed noninvasive imaging of fiber tracts in the brain. By measuring the water diffusion along the nerve tracts, these can be shown in great detail. However, there are still some problems with the realistic representation. For example, small tracts can sometimes be poorly delimited from larger tracts, which is particularly the case with small intersecting tracts and large ipsilateral tracts. With the help of this procedure, the Human Connector Project is currently trying to map the course of all nerve fibers in the brain.
Until the 1970s, the course and destination of orbits was mainly determined using degeneration methods. An example to be mentioned here is the so-called Waller's degeneration , which describes the destruction of a nerve portion distal to damage to the nerve. In this way, the course of the damaged axons can be followed.
Transcranial magnetic stimulation
The tracts are stimulated by brief magnetic stimulation of the corresponding brain region. The signals generated can be measured on the innervated muscles.
Central Nervous System Anatomy
The central nervous system is discriminated in the spinal cord ( spinal medulla ) and the brain ( encephalon ) which developmental biology , three primary brain vesicles is classified according to the main sections of the forebrain ( forebrain ), midbrain ( mesencephalon ) and hindbrain ( Rhombencephalon ).
The forebrain consists of the Cerebrum ( telencephalon ), also belonging to the cerebral cortex ( cerebral cortex is one), and the midbrain ( diencephalon ), to which, among others, the thalamus counts.
The midbrain in the roof of the midbrain contains the four- hill plate to which the paired upper hills ( colliculi superiores ) and lower hills ( colliculi inferiores ) belong.
The hindbrain is subdivided into the hindbrain ( Metencephalon ), which includes not only the bridge ( Pons ) and the cerebellum ( cerebellum ) is one, and the Mark Brain ( Myelencephalon ) and medulla ( medulla oblongata called).
The brain stem includes the midbrain and hindbrain without the cerebellum, i.e. mes-, met- and myelencephalons with the exception of the cerebellum.
The Expressing brain stem near the brain stem called term however, includes the diencephalon and beyond sometimes the cerebellum and parts of the telencephalon as the basal ganglia ( basal ganglia ).
The spinal cord is divided into segments , each of which is assigned to the cervical marrow , the breast marrow , the lumbar marrow , the cruciate marrow or the coccyx or tail marrow .
The main motor pathways of non- mammalian vertebrates are the reticulo-, vestibulo- and rubrospinal tracts.
The main part of the pyramidal system is the pyramidal tract ( corticospinal tract ). It is visible on both sides on the underside of the medulla oblongata (myelencephalon) as a shallow longitudinal bulge (pyramis, pyramid). In the pyramidal junction ( Decussatio pyramidum ), at the junction between the posterior brain and the spinal cord , 70 to 90 percent of the neurites cross as the lateral corticospinal tract to the other side ( contralateral ), the remainder run as the anterior paramedian corticospinal tract in the anterior cord of the spinal cord and cross segmentally into the anterior horn on the contralateral side of the spinal cord. Some orbits do not cross at all, but remain ipsilateral . The extent of crossbreeding varies between mammals. The majority of fibers cross in humans and dogs . In ungulates, only about half of the tracks cross.
Medial & lateral reticulospinal tract
The tractus reticulospinalis medialis & laterales can be found in all vertebrate species examined up to 2007 and thus probably represent the first motor fiber tracts. In particular, muscle groups close to the body are contracted.
All mammals and birds examined to date in 2007 have a rubrospinal tract, whereas it occurs in some, but not all, species among fish , amphibians and reptiles . Its emergence goes hand in hand with the beginning development of jaw-owning vertebrates . It is particularly found in animals with extremities (or pseudo- extremities such as rays ). In humans the rubrospinal tract is only developed as an appendix. In terms of quality, it lies between the vestibulo- / reticulospinalis and the corticospinal tract.
The tract has so far only been found in mammals. All mammals studied so far have this tract and its formation coincides with the spread of mammals and the development of the neocortex. However, it can only be traced back within one mammal species, which suggests that it developed independently of one another in each mammal species. This would also explain the variability in the course and the sometimes strong interpersonal variations. It is believed that the corticospinal fibers developed from the corticobulbars. As a rule, almost all fibers cross in all mammals, but the lamina of the interconnection in the spinal cord varies greatly between mammal species. In hedgehogs and snakes , the tract is generally ipsilateral. The crossing point can also be, for example, in the Pons, as with the elephant . Phylogenetically , the fibers from the parietal cortex developed first. Precise control of the extremities.
The basal ganglia as well as the substantia nigra are found in all vertebrate classes and in the trunk of the jawless.
From evolutionary considerations it will not be possible to deduce the sequence of the development of the sensory pathways with certainty. Because all still living animal species have all the senses (to varying degrees, but this is interpreted as a sign of the subsequent differentiation). The events are also too far in the past for fossils to shed light on. Alternatively, embryology could be used : the order in which the senses develop embryo corresponds to the phylogenetic order. This procedure can be justified on the basis of the motor pathways (see above). The ability to nociception has been demonstrated in all phyla in the animal kingdom , even in cnidarians that do not have a trained brain.
The analysis of diseases often allows conclusions to be drawn about the usefulness of the affected structures. The mouse knock-out model works exactly on this principle. There are a number of diseases in which, for example, the orbit of the pyramid has either an abnormal crossing or even no crossing at all. The problem is that many of the diseases are usually associated with further profound changes in the CNS (e.g. corpus callosum agenesis ).
In the event of traumatic effects of violence on the brain, bodily functions of the contralateral extremity are usually affected by possible failures. The few cases in the literature where ipsilateral paralysis occurred after violence have been analyzed in more detail. In such patients, too, the paths crossed, except that ipsilateral motor and sensory deficits were caused by certain circumstances (for example through bleeding-related pressure on the opposite side of the brain). Unilateral damage to the motor or sensory tracts, the ipsilateral (not damaged) cortex is often able to compensate for the loss, especially in young children. For this purpose, ipsilateral efferents and complex polysynaptic interconnections (corticorecticulospinal) are activated.
Patients with progressive external ophthalmoplegia and scoliosis (HGPPS) show no crossing of the pyramidal tract, the superior cerebellar peduncle, and no crossing of the posterior tract tracts , as recorded by magnetic resonance imaging and confirmed by diffusion tensor imaging, transcranial magnetic stimulation and intraoperative stimulation. Fibers of the corpus callosum, as well as corticopontine fibers. those of the spinothalamic tract and the optic chiasm , however, remain unaffected by the mutations. In the corticospinal tract, the lack of crossing is even noticeable as a more pronounced butterfly shape of the spinal cord.A mutation in the Robo3Q gene is responsible for the symptoms. Deletion of the coding gene in mice has already shown that the Robo3 protein is essential for the crossing of the tracts: it found there is no longer any crossing of the tracts in the entire spinal cord or brain stem. The mice died shortly after birth, so a complete lack of the mutated protein is lethal.
The following observation is groundbreaking for an understanding of the crossing fiber tracts in the CNS: In none of the patients affected by HGPPS were sensory or motor impairments such as numbness , muscle weakness or poor coordination found, although the pathways run ipsilateral as described above. This means that an ipsilateral structure of the CNS does not have any significant disadvantages in terms of motor or sensory functions. Changes in the nucleus abducens or the paramedian pontine formatio reticulatio are discussed as the basis of the eye paralysis . Others see the poor coordination of the autochthonous back muscles as a possible origin of the scoliosis .
In this context, two studies on the trochlear nerve , which is known to cross, emerge dorsally from the brain stem and innervate the contralateral superior oblique muscle , are very informative . By suppressing the netrin receptors in mice , which are important for the crossing of axons , a partial or complete ipsilateral innervation of the M. obliquus superior by the N. trochlearis could be achieved. No functional disadvantages of ipsilateral innervation were noticed here. In another experiment, the trochlear nerve was severed in frog embryos. The nerve regenerated, but now supplied the ipsilateral superior oblique muscle without any behavioral problems. These studies also confirm that functionality does not have to be in the foreground at an intersection.
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