Pyramidal tracts

The pyramidal system (PS) is a system of movement control in mammals. It describes a collection of central motor neurons and their in the pyramidal tract extending along axons. It owes its name to a special protrusion of the pyramid in the medulla oblongata. About the medulla oblongata, the pyramidal tract runs as a stopover, see chap. Structure and function. The pyramidal system is particularly well developed in primates and especially in humans. Together with the extrapyramidal system controls all voluntary and involuntary part of the running movement ( motor function ).

Design and function

The pyramidal system is responsible for fine motor skills and motor skills arbitrary. It has its origin in the primary motor cortex ( precentral gyrus ), so in a defined part of the cerebral cortex. There sit the cell bodies of central motor neurons, which are histologically is pyramidal cells. Some strikingly large motor neurons are called Betz giant cells. Most of the cells forming the pyramidal system, but are small pyramidal cells of the motor cortex. The axonal fibers of the motor neurons running 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, whereas it plays only a minor role in animals.

It is a mistake to suppose that the name of the pyramidal tract on in their area of ​​origin, the motor cortex, encountered typical pyramidal cells ( Betz giant cells ) were due, see the already mentioned names mentioned derivation of the pyramidal tract. Betz giant cells are found in motor cortex of the isocortex in layer V (lamina V), see also the general cytoarchitecture of the isocortex. Although called giant cells send all their axons in the pyramidal tract, but their proportion of these fibers is less than 5 %. Over 90 % of the fibers is provided by the smaller pyramid cells. Such small pyramidal cells, however, are everywhere in the isocortex and therefore everywhere represented on the cerebral cortex, see in particular layer III ( layer III ). 70% of the neurons in the cortex pyramidal neurons. Of them is the main part of the information processing is carried. Their occurrence is by no means restricted to the motor cortex. Betz giant cells are an exception in this regard.

Pyramidal tract

The main part of the PS is the pyramidal tract ( corticospinal tract ). It is on both sides at the bottom of the medulla oblongata ( Myelencephalon ) as shallow longitudinal ridge ( Pyramis, Pyramid ) visible. In the pyramidal decussation ( decussation pyramidum ), at the transition between hindbrain and spinal cord, cruising 70 to 90 percent of the neurites as corticospinal tract lateral to the other side ( contralateral), the remaining run as corticospinal tract anterior midline in the anterior column of the spinal cord and cross segmental into the anterior horn of the contralateral side of the spinal cord. Some tracks do not intersect but remain ipsilateral. The extent of the intersection, but is different in the individual mammal. In humans and in dogs crosses the majority of the fibers. In ungulates crosses only about half of the sheets.

The PS attracts mainly to the interneurons of the spinal cord and controls the motor root of these cells, the motor anterior horn cells in the spinal cord. Some fibers go a direct ( monosynaptic ) connections.

Damage of the pyramidal system

A unilateral damage to the pyramidal system (such as a stroke ) results in humans and primates as a result of the pyramidal decussation usually to paralysis ( paresis ) of the opposite side of the body. The paralysis is not complete (ie, no plegia ), as a control extrapyramidal usually persists and can take over some functions. Typically, however, the so-called pyramidal tract signs, loss of fine motor skills, associated movements of other muscle groups or the opposite side, and a general awkwardness. In fact, however, these symptoms are always the result of a lesion of several kortikofugaler pathways that affect not only the pyramidal tract, but also about the rubrospinal and the (lateral ) reticulospinale web. In the case of ( extremely rare ) isolated damage to the pyramidal tract other motor pathways take over largely the function, so that only minor disturbances of fine motor skills are expected.

In many mammals, the failures are far less dramatic because the pyramidal system is not as important with them. Here the damage limited to entertainment disorders of the neck and the failure of the position reactions, even if you remove the entire motor cortex of a page. The species-specific movement patterns are not changed, since they are based primarily on the extrapyramidal system and thus of other brain parts.

The crossing of the pyramidal tract was first described in 1709 by Domenico Mistichelli ( 1675-1715 ). A year later, François pour du Petit Four showed the contralaterality of the motor system.