Pyramidal system
The pyramidal system ( PS ) is a system of motion control in mammals . It describes a collection of central motor neurons and their nerve cell processes that run together in the pyramidal tract. The pyramidal system is particularly well developed in primates and especially in humans . Together with the extrapyramidal system , it controls all voluntary and part of the involuntary movements ( motor skills ).
Origin of name
The name derives from the anatomical structure Pyramis medullae oblongatae [from Greek pyramis = pyramid] , a protrusion on the front surface of the myelencephalon , which is reminiscent of a slender, upside-down pyramid. It is often wrongly assumed that the name comes from the pyramid-like structure of the perikarya of its original cells ( pyramidal cells ). This is also unrealistic insofar as pyramidal cells do not only occur as the origin of the pyramidal trajectory (see below).
Layout and function
The pyramidal system is responsible for fine motor skills and voluntary motor skills. It has its origin in the primary motor cortex ( gyrus praecentralis ), i.e. in a defined part of the cerebral cortex . This is where the cell bodies of the central motor neurons sit , which are histologically pyramidal cells . Some noticeably large motor neurons are called Betz giant cells . However, most of the cells that make up the pyramidal system are smaller pyramidal cells of the motor cortex. The axonal fibers of the motor neurons run from the cerebral cortex via the internal capsule , the brain stem, and the white matter of the spinal cord to the lower motor neuron (LMN). The pyramidal system is particularly well developed in humans, while it only plays a subordinate role in animals.
Betz giant cells can be found in layer V ( lamina V ) of the motor cortex of the isocortex , see also the general cytoarchitectonics of the isocortex. These giant cells send all their axons into the pyramidal tract, but their share of these fibers is less than 5%. Over 90% of the fibers are made up of smaller pyramidal cells. Such small pyramidal cells are found everywhere in the isocortex and therefore everywhere on the cerebral cortex, see in particular layer III ( lamina III ). 70% of the nerve cells in the cortex are pyramidal cells. The majority of information processing is carried out by them. Their occurrence is by no means restricted to the motor cortex. Betz's giant cells are an exception in this regard.
Pyramidal track
The main part of the PS 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 the crossing is different in the individual mammals. In humans and also in dogs, the majority of the fibers cross. In ungulates, only about half of the tracks cross. See also contralaterality of the forebrain .
The PS pulls mainly to the interneurons of the spinal cord and controls the motor root cells, the motor anterior horn cells in the spinal cord , via these . Some fibers enter into direct (monosynaptic) connections.
Damage to the pyramidal system
One-sided damage to the pyramidal system (e.g. from a stroke ) usually leads to paralysis of the opposite side of the body in humans and other primates as a result of the pyramid crossing . The paralysis is not complete (so no plegia ), as extrapyramidal control usually continues and can take over some functions. Typical, however, are the so-called pyramidal trajectory signs, the loss of fine motor skills, movements of other muscle groups or the opposite side and general clumsiness. In fact, these symptoms are always the result of a lesion of several corticofugal tracts that affect not only the pyramidal tract, but also the rubrospinal and (lateral) reticulospinal tract. In the case of an (extremely rare) isolated damage to the pyramidal tract, other motor tracts largely take over its function, so that only minor disturbances of the fine motor skills are to be expected.
In many mammals, the failures are far less dramatic because the pyramidal system is not as important to them. Here the damage confined to posture disorders of the neck and the failure of the position reactions , even if you removed the entire motor cortex of a page. The movement patterns typical of the species are hardly changed, as they mainly originate from the extrapyramidal system and thus from other parts of the brain.
The crossing of the pyramidal track was first described in 1709 by Domenico Mistichelli (1675-1715). A year later, François Pourfour du Petit demonstrated the contralaterality of the motor system.
literature
- Martin Trepel: Neuroanatomy. 3. Edition. Urban & Fischer, 2003, ISBN 3-437-41297-3 .
- Franz-Viktor Salomon: nervous system, systema nervosum. In: Salomon, Geyer, Gille (ed.): Anatomy for veterinary medicine. Enke, Stuttgart 2004, ISBN 3-8304-1007-7 , pp. 464-577.
Individual evidence
- ^ Hermann Voss , Robert Herrlinger : Taschenbuch der Anatomie. Volume III: nervous system, sensory system, skin system, increment system. Fischer, Jena 1964, p. 20.
- ↑ Pyramid orbit . Spektrum.de, accessed on March 14, 2018 .
- ↑ Michael Schünke; Erik Schulte; Udo Schumacher. Ill. By Markus Voll ...,: Prometheus / Head, Neck and Neuroanatomy: ... 123 tables. 4th, revised. u. exp. Edition Thieme, Stuttgart 2015, ISBN 978-3-13-139544-3 , p. 298 ff .
- ↑ Alfred Benninghoff , Kurt Goerttler : Textbook of Human Anatomy. Shown with preference given to functional relationships. Volume 3: Nervous System, Skin and Sensory Organs. Urban and Schwarzenberg, Munich 1964, pp. 234, 247.
- ↑ Manfred Spitzer : Spirit on the Net. Models for learning, thinking and acting. Spektrum, Heidelberg 1996, ISBN 3-8274-0109-7 , p. 95.
- ↑ S. Silbernagl, F. Lang (Ed.): Pocket Atlas of Pathophysiology . 2nd Edition. Georg Thieme Verlag KG, Stuttgart 2005, ISBN 3-13-102192-6 , p. 310 .