Why are we moving? The answer would seem obvious. Thanks to the movement of arms and legs, muscle contraction. However, this is only the last step in the implementation of the motor act. The central nervous system directs our limbs. First of all, the brain. Namely, two structures located in parallel, but at the same time in constant interaction: the pyramidal and extrapyramidal systems.
Pyramid system - what is it?
It is a collection of nerve cells (motor neurons), grouped into a complex of pathways that extend from the cerebral cortex (center of higher nervous activity) through the front horns of the spinal cord to the receptors located in the muscles. Thanks to this transmission of a nerve impulse, movement is carried out.
The peculiarity is that the transmission of these nerve impulses is arbitrary, that is, controlled by consciousness.
What is an extrapyramidal system?
This is a system of neurons and pathways of the brain, due to which it is possible to have clear, fast, high-precision movements. Although these paths do not intersect with the paths of the pyramidal system, they constantly interact.
The main difference between the pyramidal and extrapyramidal systems: the involuntary functioning of extrapyramidal pathways. For example, when there is a desire to take a pen, a person arbitrarily, consciously reaches for a certain pen (pyramidal paths). However, in order to reach her, he does not need to aim, calculate the distance and strength, all this happens automatically (extrapyramidal paths).
Structure
The extrapyramidal system consists of the gray matter of the brain, that is, the accumulation of bodies of nerve cells. The same gray matter is present in the cerebral cortex. The basic structural unit is the basal nucleus or basal ganglion. These are kernels such as:
- Striopalidar system, consisting of a lenticular and caudate nucleus. The lenticular ganglion, in turn, consists of a shell and a pale ball.
- The striatum is a more phylogenetically ancient component of this part of the basal nuclei; it includes the caudate nucleus and shell.
- Pallidum is a younger component, represented by a pale ball.
- Subthalamic nucleus.
- Red core.
- A black substance located in the midbrain.
In addition to the basal ganglia, the extrapyramidal system of the brain also includes:
- thalamus;
- cerebellum;
- olive and vestibule nuclei in the medulla oblongata;
- reticular formation;
- associative centers of the cerebral cortex.
In addition, there is a division of the above structures into two departments:
- Neostriatum - includes the cortex, shell and caudate nucleus. This system is evolutionarily younger;
- Paleostriatum - consists of a pale sphere, vestibular nuclei, subthalamic nucleus, structures of the midbrain (tire and substantia nigra).
Striopallid system
Perhaps the most significant part of the basal ganglia, which has the greatest number of relationships with the rest of the central nervous system, is the striopallid system.
The shell and caudate nucleus receive information on nerve fibers from the cerebral cortex, thalamus, midbrain (substantia nigra). From the striatum, the fibers are first sent to the pallidum, and then to the remaining parts of the nervous system: the thalamus, hypothalamus, subthalamic nucleus, trunk (consists of the midbrain, bridge and medulla oblongata). Through the thalamus, the striopallid system also interacts with the cerebellum - the main coordinator of movements and balance.
Pathways
The structure of the brain, including extrapyramidal tracts, is really complex. And for its better memorization it is worth imagining all the structures through which the path passes. And the best method is to sketch them.
The following paths of the extrapyramidal system are distinguished:
- reticular spinal cord;
- spinal cord;
- vestibule-spinal;
- spinal roof;
- olive-spinal.
The reticular and spinal path originates in the reticular formation in the area of the brain stem. Here is the first neuron. The impulse along the nerve fibers extends down to the second neuron. Its localization is the anterior columns of the spinal cord. Then, along the spinal nerves, the impulse reaches the striated muscles, where this path ends.
The spinal cord path begins with the first neuron in the red nuclei located in the midbrain. The processes of this neuron go to the opposite side and then continue on this side to the segments of the spinal cord, where it ends with an intercalary neuron (interneuron) - in its gray matter. On the motor nuclei of the spinal cord, from which the impulse goes further to the skeletal muscles, this path affects indirectly - through the intercalary neuron.
The vestibule-spinal pathway consists of the first neuron located in the vestibular nuclei (lateral - the Deuterium nucleus, lower - the Roller nucleus). Second neurons are also located in the anterior columns of the spinal cord, to which the impulse reaches the medulla oblongata and the anterior cord of the spinal cord. Then the axons of the second neurons reach the skeletal muscles.
The roof-spinal canal is considered the youngest of all paths that make up the anatomy of the extrapyramidal system. Starting in the upper mounds of the midbrain, where visual information enters, it further moves to the opposite side, heading similarly to other paths to the corresponding segments of the spinal cord.
The olive-spinal path is necessary to maintain muscle tone of the neck and ensure balance. It begins with the formation in the medulla oblongata - the olive kernel, reaches the sixth segment of the cervical spinal cord. From there, the processes of motoneurons conduct an impulse to the muscles of the neck.
Main functions
As already noted above, the extrapyramidal system is an important component of the central nervous system, which makes it possible to perform everyday activities. But how does it regulate our movements, making them so precise and accurate?
The main functions of the extrapyramidal system are listed below:
- the ordering of arbitrary movements, originally regulated by the pyramidal system;
- regulation of automatic motor acts of both congenital and acquired nature;
- maintaining balance;
- regulation of muscle tone;
- involuntary contraction of facial muscles;
- regulation of movements that play the role of concomitant (for example, rapid movement of hands when running).
Pathological changes
Violation of the function and structure of the extrapyramidal system is called dyskinesia, literally - violation of movements. It can change both in the direction of increased motor activity - hyperkinesis, and in the direction of decreasing the amount of movement, the appearance of their avarice - hypokinesis. Characteristically, such a violation cannot be altered by consciousness. Hyperkinesis is involuntary, appears suddenly and also suddenly stops.
Hyperkinesis
The following types of hyperkinetic disorders of the physiology of the extrapyramidal system are distinguished:
- Chorea - fast, suddenly occurring, erratic, involuntary movements of the arms, legs, muscles of the face. This is manifested by the appearance of grimaces on the face, strange gestures.
- Athetosis - movements of the fingers, can also be in the muscles of the tongue and face. It is manifested by arching, vermiform finger movements, twisting of the tongue.
- Torsion dystonia - sudden turns of the body in different directions, arching of the whole body. Often have a corkscrew appearance. First of all, the neck muscles are affected.
- Hemibalism - one-sided, sweeping movements, most often, hands resembling the flapping of a bird's wing.
- Tick - fast, simple, stereotypic movements of small muscle groups.
- Myoclonus - short twitching of individual muscle fibers at a very fast pace. Often, limb movements are not observed.
Parkinson's disease
The classic manifestation of hypokinetic syndrome is Parkinson's disease or Parkinson's syndrome. Their difference lies in the fact that Parkinson's disease occurs with direct damage to the structure of the extrapyramidal system, and Parkinson's syndrome is one of the manifestations of any other diseases that are not associated with damage to the basal nuclei.
Parkinson's disease develops with damage to the substantia nigra, which begins to produce less dopamine. Its main function is to reduce the inhibitory effect of the caudate nucleus on motor activity. When this function decreases, the caudate nucleus inhibits motility to a greater extent, which leads to the development of hypokinesis.
Parkinson's disease: symptoms
In addition to stiffness and a decrease in the number of movements, Parkinson's disease also manifests itself:
- increased muscle tone - in neurology, the term "hyperplastic type tone increase" is used;
- stiffness of facial expressions;
- stooped posture;
- the petitioner's position is characteristic - the head bent downward, the arms are bent at the elbows, the body is tilted;
- trembling limbs (tremor);
- difficulties at the beginning of the movement and at its completion (braking).
The structure of the brain is really complex and multifaceted. This is due to the many functions that it performs. What is only one extrapyramidal system worth! So that we can make an elementary movement, it is necessary to simultaneously involve many formations of the brain. It is also interesting that the mass of processes occurs in a split second.
Yes, medicine has gone a long way in recent decades. However, neurosurgeons do not even know how many secrets the brain holds.