Observation of birds or, for example, sharks reveals in their activity a well-coordinated and rapid movement, which, however, is rather crude and instinctive. The motor activity of such animals is controlled by the extrapyramidal system in the brain, while in humans and other mammals this motor system is joined by a more highly developed one. It is located in the cerebral motor cortex and makes it possible for humans to perform extremely fine, precise and very determined movements, for example with their fingers or hand, this is the pyramidal system.
What is the pyramidal system?
The system of control of all movements in humans and mammals is called the pyramidal system. This refers to the pyramidal tract of all converging nerve cell processes and the collection of central motor neurons, which in turn are efferent neurons and form the basis of skeletal muscle. The structure of these cells of origin is striking and arranged like a pyramid by the course of fibers and fiber connections. The pyramidal system also does not function independently of the extrapyramidal system, as was assumed for some time, but together with the latter controls all voluntary and involuntary motor activity.
Anatomy and structure
The pyramidal system is located directly in the cerebral cortex. Motor neurons there form cell bodies called pyramidal cells, which belong to the motor cortex. There are small pyramidal cells as well as conspicuously large ones called Betz giant cells. These, in turn, are a neuronal cell type that is exclusively present in the primary motor cortex. Such giant cells are located in the fifth layer of the cerebral cortex and transmit their information through axons to cranial nerve nuclei and spinal cord. The number of such Betz cells is small. In humans, there are about thirty thousand in the cerebral cortex. Small pyramidal cells, on the other hand, are found throughout the cerebral cortex and especially in the isocortex, which is distinct from the second area of the allocortex. The third layer is populated with about seventy percent of neurons. The main part of all information transmissions and the complete processing of these take place there. In connection with the pyramidal system there is always the pyramidal tract, which is the main part of this region and is a transition from the brain to the spinal cord. It always descends and transmits all impulses as a nerve pathway in these regions. It begins with the cell bodies of the motor cortex, also called the precentral gyrus, which is one brain turn before the central furrow. The nerve fibers coming from it bundle in the area of the internal capsule (capsula interna) and run over the brain legs and bridge to the medulla oblongata. This is where the pyramidal crossing of almost 90 percent of all fibers, which is particularly well developed in humans, occurs. The uncrossed fibers, in turn, continue and do not cross until they reach the spinal cord segment or terminate at the alphamotoneurons in the anterior horn cells in the spinal cord.
Function and tasks
The pyramidal tract is responsible for all voluntary unconscious movements of the body muscles. It also inhibits basic muscle tension or the muscle intrinsic reflex. This originates from the receptors of the muscle spindles, which control the muscle fiber length. The stimulus is identical in location and organ and is transmitted through a reflex arc. The pathways in the extrapyramidal system, in turn, activate the limb and trunk muscles. This allows mass movements, which are the basis of all movements passing through the pyramidal pathway. Again, the movement of the hand serves as an example. To move it, the upper arm must also be moved. The latter is done by the extrapyramidal system.
Diseases
If the pyramidal system is damaged, paralysis occurs. Defects are distinguished according to whether they have arisen in the first or second neuron. Such a paralysis does not have to be complete, it can only affect certain regions, e.g. after a stroke, if circulatory disturbances have occurred within the brain. If processes in the pyramidal system fail due to such a disturbance, the extrapyramidal takes over the control of some of the functions. If the pyramidal pathway within the brain is damaged, flaccid paralysis results.This leads to an impairment of fine motor skills, to an uncontrolled co-movement of other muscles or to an awkwardness in the flow of motor skills. In most cases, not only the pathways in the pyramidal system are blocked in such manifestations, but others are also affected. Flaccid paralysis then changes to spastic paralysis. Neurological symptoms in such circumstances are usually various reflexes, including, for example, in the foot the Babinski reflex. Generally, such neurological symptoms are called pyramidal tract signs, insofar as they are caused by a lesion of the pyramidal tract. Pathologically, this results in very specific occurring reflexes in the upper and lower extremities, which have become known by various names. If, on the other hand, the extrapyramidal system is disturbed, much more serious disorders occur. We always speak of an “extrapyramidal” motor system when motor processes are either not controlled by the pyramidal pathway or take place outside of it. If disturbances occur here, movement disorders can result that are genetically determined or neurological. These include Huntington’s disease and Parkinson’s disease. Such diseases are caused by lesions in the primitive subcortical nuclei, disrupting muscle tone and leading to abnormal or involuntary movements. Parkinson’s in particular is a slow-moving, degenerative disease, usually occurs in old age and leads to hypokinetic movement disorders, which in turn is based on overactivity of all output nuclei. This causes increased inhibition of transmission to the appropriate projection pathways in the thalamus. Under such conditions, not only is facial expression lost and freezes into a mask, but arms and legs also begin to twitch uncontrollably.
Typical and common brain disorders.
- Dementia
- Creutzfeldt-Jakob disease
- Memory gaps
- Brain hemorrhage
- Meningitis