Guide on the Pathways of the nervous system
Depending on the type of movement, numerous cortical and subcortical structures are directly or indirectly involved in motor skills. In principle, a distinction is made between two systems:
- The pyramidal (PS) and
- The extrapyramidal system (EPS).
Both terms are historical. The pyramidal tract (corticospinal tract) is not a uniform motor tract, but a collection of different descending systems with a common course. The extrapyramidal system, on the other hand, is a collective term for various structures of the central nervous system.
The pyramidal tract is mainly responsible for voluntary motor skills, while the extrapyramidal structures are mainly involved in non-voluntary movements.
However, since the two systems also influence each other and are therefore involved in both types of movement, a strict subdivision is outdated.
A small core area that is functionally involved in all aspects of motor skills is the nucleus rubber (red nucleus). It is often assigned to the extrapyramidal motor skills but actually represents a separate entity.
With regard to function, a distinction is made between four types of motor activity:
- Support and holding motor skills
- Voluntary motor skills
- spinal reflexes
- cortical and subcortical reflexes
A large number of brain areas, including the higher functions of the cortex, are involved in performing voluntary motor movements. Only the support and holding motor skills, however, do not require a higher function. Spinal and subcortical reflexes are performed by areas located in the telencephalon and mesencephalon as well as the brain stem.
Modulating influences from the structures of the cerebrum are very small. Voluntary, supporting and holding motor skills do not work in isolation from one another; rather, they complement, modulate and influence each other.
Over fifty different core areas or parts of the brain are involved in human motor skills. Since it is not possible to represent them completely and in a single contribution, models are used. These often either represent the overall context in a simplified manner, or they only show a single (or a few linked) chains of neurons in detail. In the following, models are used, each of which particularly emphasizes a single aspect or a connection.
Spinal reflexes do not count as motor skills in the narrower sense, since they are interconnected at the spinal cord level and physiologically take place without the direct involvement of the brain. They are also not part of the motor skills cortical and subcortical reflexes, which include the sneeze and gag reflex, as well as a variety of other reflexes. These are partly protective mechanisms or phylogenetic holdovers.
Movements are only carried out under physiological conditions if they do not lead to an injury to the organism. In the first place is the purely mechanical component, if the movement is not possible, for example, because a bone inhibition is in the way (e.g. touching the back of the head with the big toe), it is not carried out.
Furthermore, movements are not carried out if they lead directly to damage to the organism (e.g. cutting open the arteries on the wrist). Since humans essentially have free will, these natural protective mechanisms can be "overwritten". It is, therefore, possible to voluntarily control the muscle groups to perform the two movements (acrobatic bending of the body and self-harm).
Three systems or areas are significantly involved:
- Amygdala (almond kernel),
- limbic system and
- Nucleus accumbens.
All three are responsible for motivation with different functional focuses. The amygdala is involved in the motivation of actions related to fear, the Ncl. accumbens to those that are influenced by reward mechanisms and the prefrontal cortex controls the conscious planning of actions (voluntary actions).
Amygdala and Nucleus accumbens are in a lively exchange with the transmitter systems, which have a modulating effect on them and the rest of the central nervous system. The four major transmitters are dopamine, serotonin, acetylcholine, and norepinephrine. They are significantly influenced by external influences and complex cognitive functions ("thinking").
Support and holding motor skills
Functionally, the beginning of a movement is the determination of the current position of the body and the extremities in space. This information is used to weigh up whether a movement is feasible and harmless to the organism. The basis for this is proprioception in connection with information from sensory systems (eye, vestibular organ). Information that carries epicritic and protopathic fibers is also incorporated.
The focus is on the following fiber connections:
Peripheral signals reach the cerebellum at the inferior cerebellar peduncle via the spinocerebellar tract and the cuneocerebellar tract (epicritic impulses). In addition, there are fibers from the lower olive, which arrive via the olivocerebellar tract. They contain projections from the cortex and the central tegmental tract (central hood).
Furthermore, fibers from spinothalamic tracts, trigeminal nuclei and the arcuate nucleus are added. Vestibular fibers enter the cerebellum via the vestibulocerebellar tract.
Fibers travel to the pons from the cortex, frontal lobe, and temporal lobe. From there they run into the cerebellum via the pedunculus cerebellaris medius. Projections of proprioceptive fibers in the anterior spinocerebellar tract enter the cerebellum via the superior cerebellar peduncle.
All the fibers and pathways mentioned that go into the cerebellum provide it with various information about the position of the body in space. They are processed in the cerebellum and switched to efferent neurons. Most of the efferents, which mainly originate from the nucleus fastigii and the nucleus globosus, leave the cerebellum via the pedunculus cerebellaris superior.
Your projections end in the reticular formation of the pons, where they are switched and reach the spinal cord via the reticulospinal tract. The second supply of fibers to the spinal cord occurs via the vestibulospinal tract. Projections from the cerebellum itself run via the cerebellovestibular tract to the vestibular nuclei. From there, they reach the spinal cord via the vestibulospinal tract.
Fiber tracts terminate directly at α and γ motor neurons as well as spinal interneurons. Their predominant effect is the activation of inhibiting interneurons, which act on the flexor muscles and the excitation of α- and γ-motor neurons of the extensors. Impulses that are delivered via the rubrospinal or reticulospinal tract have an antagonistic effect. Extensors are inhibited and α- and γ-motor neurons of the flexors are activated. The fibers from the reticular formation come from their medullary part. Both fibers enter the spinal cord with the reticulospinal tract.
The supporting and holding motor skills, therefore, mean that the body can constantly maintain its balance by recording and comparing information from peripheral sensitive and sensory systems and activating the skeletal muscles necessary for balancing or inhibiting their antagonists.
This type of motor skills does not require any conscious perception unless the balancing itself is part of conscious activity (e.g. a wire rope dancer). But even then - in addition to the conscious action - motor corrections are carried out automatically.
It is possible to optimize the motor system through repetitions (motor learning). Example: The ability to hit a ball or to perform complex moves in a martial art without looking at the ball or the opponent is justified by learning processes.
Seen from the outside, it seems to be a matter of "reflexes" that make a person act very quickly and purposefully - but this is not functionally correct.
Rather, it is a question of movement patterns that have been repeated very often and thus learned. As a result, a kind of "copy" in the neuronal circuits of the cerebellum is kept ready for retrieval more quickly. The basis is switching from the pyramidal tract to the rubrospinal tract.
These movements assume that no influences from inhibiting or activating systems hinder processing. The colloquial statement that only a "clear head" is able to do this is very appropriate because influences from other areas of the brain can disrupt the trained movement patterns and strengthening neural connections. Extreme fear, for example, can lead to an interruption of the corresponding pathways due to the strong influence of the amygdala on the motor system.
Voluntary motor skills
The voluntary motor function is a process that takes place in four steps:
- The decision, including cortical and subcortical areas
- Programming, which mainly involves the cerebellum and basal ganglia
- Movement command that is mainly carried out by motor neurons
- the actual movement
The intention to perform a movement occurs primarily in associative areas of the cortex. In order for an action to follow the making of a decision, it needs motivation. Motivational signals come in particular from the limbic system and prefrontal cortex. The amygdala and the Ncl. accumbens are involved when external influences play an increasingly important role. The motivational areas provide the drive to act.
Precentral gyrus / anterior central turn / primary motor cortex - lateral
Somatosensory and somatosensitive areas process the input from sensory and peripheral sensitive systems as well as the sensitivity of the cranial nerves. On this basis, a strategy is developed, the feasibility of which is checked in the next step and corrected if necessary.
To create a program, the following requirements must be met:
- The position of the body in space and the state of tension of the muscles must be known (actual state).
- Muscles involved in the movement, the time sequence and the strength of the contraction must be determined based on the actual state (target state).
The actual condition is provided by the cerebellum, and the basal ganglia are involved in creating the program for the target condition. After the actual and target state has been compared and the program created, it is transmitted to the motor cortex.
The motor cortex transmits the finished program to the spinal cord via descending pathways. There is a switch to executive motor neurons and inhibitory interneurons of the corresponding antagonistic muscle groups. The descending pathways can still be modified by collaterals of other pathway systems in the brain stem and spinal cord.
Motor neurons of the corresponding skeletal muscles are excited and ensure the contraction. From this point on, no further modifications are possible. Once activated, the motor neuron can no longer be inhibited.
Modification due to external circumstances
The described process describes the ideal case in which the body is not changed in its position during steps 1 to 4 by external influences. However, this happens very rarely. Corrections can be made several times during steps 1 and 2.
However, so that a movement can ever be carried out, the corrections are limited to a few times. From step 3 onwards, only small modifications are possible, and the program itself can no longer be corrected. Step 4 can no longer be cancelled.
Nucleus rubber and motor skills
The nucleus rubber is assigned to the extrapyramidal motor system for histological reasons. Functionally, however, it is involved in all motor functions. It receives information from the thalamus, the globus pallidus medialis, the cerebellum and the cortex. The fibers to the lower olive (Tractus rubrospinal) are part of the tegmental tract.
Nucleus rubber / red nucleus - caudal
The cerebellum is the integration center for determining the actual situation of the muscles. The Ncl. rubber is involved in these cycles. In doing so, it forms its own circle of neurons, which extends from the cerebellum via the Ncl. rubber, to the olive and back to the cerebellum. On the one hand, this information contributes to the determination of the actual situation by the cerebellum, and on the other hand, it ensures a modification of the motor impulses of descending pathways. Furthermore, they play a role in the creation of programs in the context of an arbitrary movement.
The rubrospinal tract (efference from the rubber to the spinal cord) primarily, but not exclusively, controls the fine movements of distal extremity sections - especially with automated movements. This is by no means a matter of supporting and holding motor skills, but rather of learned movements.
A beginner in a sport in which fixed movement patterns are exercised over and over again will first imitate the movements of the trainer. Sometimes people look at their own hands or feet. This is done arbitrarily and consciously, while the pyramidal path controls the motor skills.
If the movement sequences with continuous training have been carried out very frequently, the control of the motor skills is changed: the rubrospinal tract now functions as an efferent system for these movements.
The practitioner no longer needs active thinking, but plays "internalized" processes and can meanwhile concentrate mentally on other things (e.g. rehearsing even more complex movements). In the course of motor learning processes, from a certain threshold value of practising the recurring movement patterns, a switch is made from pyramidal to rubrospinal efference. It is not possible to predict when this value will be exceeded.
Author: Vicki Lezama