Human motor function is controlled by an interplay between the two pyramidal tracts and the three extrapyramidal nerve tracts in the spinal cord. Within this framework, the extrapyramidal or extrapyramidal motor system is primarily responsible for involuntary and automated movements. In inflammatory diseases of the central nervous system, as well as in trauma, the extrapyramidal pathways can be damaged.
What is the extrapyramidal motor system?
The extrapyramidal motor or extrapyramidal system is made up of three spinal cord motor pathways. These tracts are specifically responsible for involuntary movements of skeletal muscles. The two pyramidal tracts, which also run through the spinal cord, are to be distinguished from these. Unlike the extrapyramidal system, they serve voluntary movement. Both motor systems belong to the somatomotor system and together enable the movements and inhibitions of the skeletal muscles. An extrapyramidal system is almost exclusively exhibited by primates. Vertebrates, for example, do not even have motor pyramidal tracts. In humans, the motor spinal cord system for involuntary movements of skeletal muscles originates from the motor cortex of the brain. These are Brodmann areas six and eight, also known as areae extrapyramidales. Motor pathways also connect to other core areas of the brain, such as the so-called basal ganglia.
Anatomy and structure
Unlike the pyradmoid pathways, the extrapyramidal pathways are not interconnected in a pyramidal fashion. In addition to the tractus rubrospinalis, the extrapyramidal system includes the tractus vestibulospinalis and the tractus reticulospinalis. The latter itself consists of the medial reticulospinal tract and the lateral reticulospinal tract. The tractus vestibulospinalis extends uncrossed from the rhomboid fossa into the spinal cord. The tractus rubrospinalis arises from the nucleus of the brainstem and crosses into the ventral spinal cord, where it runs downward. The lateral reticulospinal tract of the tractus reticulospinalis originates in the brain area between the midbrain and the spinal cord. The lateral and uncrossed medial reticulospinal tract originates from the so-called bridge of the central nervous system. The tracts are each equipped with multiple switching sites in terms of synaptic nerve terminals.
Function and tasks
The task of the extrapyramidal system is to control movement. It realizes unconscious and automated movements, such as the swinging along of the arms during walking. Coarser-appearing movements of the trunk and extremities are also initiated within the structures, such as automated holding and supporting motor movements and mass movements. The extrapyramidal system is also responsible for maintaining unconscious muscle tension. In this context, we can speak of an interconnectedness with the muscle sense. However, these motor pathways are also networked with the visual system, the sense of balance and the sense of one’s spatial position. Especially the connections to the cerebellum let the pathways thus make automatic corrections of the posture and realize harmonious movements. The tractus vestibulospinalis is responsible for activating the motor neurons and inhibiting the flexors. In contrast, the tractus rubrospinalis inhibits the extensors, activates the flexors, and is the only extrapyramidal nerve involved in fine motor control. So, by and large, the motor neurons of the muscles receive a command from the brain via the motor neural pathways of the spinal cord. The motor neurons are efferent nerves that traverse the entire musculature and are indispensable for movement. The connected brain regions thus take over the switching of the three extrapyramidal motor pathways and plan to contact specific motoneurons. In the basal ganglia of the brain, for example, the selection and processing of currently required movements takes place. Here, among other things, it is planned to reach for an object in the visual field. The motor pathways of the spinal cord are also involved in the inhibition of certain motoneurons, especially the first motoneuron. They thus control the motor volition of the pyramidal tracts. The exchange of information between the brain and the extrapyramidal system takes place biochemically, primarily via the neurotransmitter dopamine.
Diseases
One of the best-known diseases of the extrapyramidal system is extrapyramidal syndrome. In this disorder, the first motor neuron is no longer inhibited. Ataxia, tremor, inhibitions to take off, and a tendency to fall down are among the main symptoms of this disorder. Ultimately, both strongly increased and strongly inhibited movements can occur in this context. The extrapyramidal system can also be damaged in the course of the inflammatory nervous system disease multiple sclerosis. In this case, inflammation forms in the three motor spinal cords or the interconnected cerebral spheres, which in extreme cases leaves permanent damage after healing. When inflammation of the three motor pathways occurs, tissue always perishes. Especially if the inflammation persists too long, the body can no longer compensate for the loss of this tissue. Inflammatory damage to the extrapyramidal system usually manifests itself in slowed stimulus transmission and reactivity. A loss or increase in muscle tension can also occur as part of the damage. In some cases, postural and positional reactions are also disturbed. If the pyramidal tracts are affected by damage instead of the extrapyramidal system, so-called pyramidal tract signs appear. Such pyramidal pathway signs correspond in particular to disturbed body reflexes, such as laterally differentiated foot reflexes or non-exhaustible reflexes of the hand. The distinction between extrapyramidal symptoms and pyramidal symptoms may have prognostic significance for neurologists in the context of multiple sclerosis, for example. For example, pyramidal tract signs are said to be a prognostically unfavorable sign in the early years of the disease.
