Arbitrary, they are also conscious movements - these are those that a person is able to control with the help of the cerebral cortex. Many levels of the peripheral and central nervous system are involved in the implementation of a motor act. These levels do not work in isolation, they are in constant relationship, transmitting nerve impulses to each other. What provides voluntary human movements? This is detailed in the article.
Meaning of afferent signals
The main role in the implementation of voluntary movements of a person falls on afferent signals. These are impulses that come to the human body from the outside. Before any movement is made, the nerve signal is picked up by receptors and through sensory nerve pathways.enters the structures of the central nervous system. Through these pathways, the brain knows that the skeletal muscles are ready to move.
Afferent impulses perform the following functions:
- inform the cerebral cortex that there is a need to perform a movement;
- "tell" if it is done correctly;
- increase or, conversely, reduce the force of contraction of muscle fibers;
- correct the sequence of muscle tissue contraction;
- inform the cortex whether to stop the activity or whether to continue it.
Two zones of the cortex - motor and sensitive - make up a single whole of the sensorimotor department. It controls the work of the underlying structures of the brain and spinal cord while providing voluntary human movements.
Motor centers
The centers of the human movement system in the cerebral cortex are located in the precentral gyrus. It is located in front of the central sulcus in the frontal cortex. This department, together with the paracentral lobule and a small area of the frontal lobe, is called the primary motor projection field.
The secondary field is located in the premotor cortex. It is due to the first two fields that the planned motor act is realized.
The voluntary movements of a person are integrated in the tertiary field, which is located in the anterior parts of the frontal lobe. Thanks to the work of this area of the cortex, the motor act exactly corresponds to the incoming sensory information.
All processes that occur in the human body are integrated by two parts of the nervous system: autonomic and somatic. It is the autonomic nervous system of a person that controls voluntary movements.
Pyramid cells
Giant pyramidal cells are located in the area of the primary and secondary motor fields in the fifth layer of the gray matter of the brain. These formations were discovered by the scientist V. A. Betz, therefore they are also called in his honor - Betz cells. From these cells begins a long pyramidal path. It, interacting with the nerve fibers of the peripheral nervous system and striated muscle tissue, gives us the opportunity to move at will.
Elements of the cortico-muscular pathway
Arbitrary human movements are provided primarily by the cortical-muscular, or pyramidal path. This formation consists of two neurons. One of them was named central, the second - peripheral.
The central neuron is the body of Betz's pyramidal cell, from which a long process (axon) departs. This axon descends to the anterior horns of the spinal cord, where it transmits a nerve impulse to a second neuron. A long process also departs from the body of the second nerve cell, which goes to the periphery and transmits information to the skeletal muscles, forcing them to move. This is how the torso and limbs move.
But what about facial muscles? To their arbitrarycontractions were possible, part of the axons of the central nerve cells did not go to the spinal cord, but to the nuclei of the cranial nerves. These formations are located in the medulla oblongata. They are the second motor neurons for the muscles of the face.
Thus, the pyramidal path consists of two parts:
- cortical-spinal tract, which transmits impulses to the neurons of the spinal cord;
- cortico-nuclear pathway leading to the medulla oblongata.
Making movements of the torso
The processes of the central neurons are first placed under the cortex. Here they diverge radially in the form of a radiant crown. Then they come closer to each other and are located on the knee and the back leg of the internal capsule. It is a structure in the cerebral hemispheres that is located between the thalamus and the basal ganglia.
Then the fibers come up through the legs of the brain to the medulla oblongata. On the front surface of this structure, the pyramidal pathways form two bulges - pyramids. In the place where the medulla oblongata passes into the spinal cord, part of the nerve fibers cross over.
The crossed part is further part of the lateral funiculus, the uncrossed part is part of the anterior funiculus of the spinal cord. This is how the lateral and anterior cortical-spinal tracts are formed, respectively. The fibers of these pathways gradually become thinner and eventually terminate at the nuclei of the anterior horns of the spinal cord. They transmit impulses to alpha motor neurons located in this area.
At the same time, the fibers of the anterior pathway make a decussation in the spinal cord on its anteriorspike. That is, the entire cortical-spinal tract ends on the opposite side.
Long processes of alpha motor neurons come out of the spinal cord, being part of the roots. After they are included in the nerve plexuses and peripheral nerves, carrying an impulse to the skeletal muscles. Thus, the muscles provide voluntary human movements due to the impulse received from the pyramidal cells of the cerebral cortex.
Making facial movements
Part of the processes of the first neurons of the pyramidal pathway does not descend to the spinal cord, but ends at the level of the medulla oblongata. This is how the cortical-nuclear pathway is formed. Due to it, the nerve impulse is transmitted from the pyramidal cells to the nuclei of the cranial nerves.
These fibers also partially cross at the level of the medulla oblongata. But there are also processes that carry out a complete crossover. They go to the lower part of the nucleus of the facial nerve, as well as to the nucleus of the hypoglossal nerve. Such an incomplete decussation means that the muscle tissue, which provides voluntary movements of a person at the level of the face, receives innervation from both sides of the cortex at once.
Due to this feature, damage to the cerebral cortex on one side causes immobilization of only the lower part of the face, and the motor activity of the upper is completely preserved.
Symptoms of motor pathway damage
Voluntary human movements are provided, first of all, by the cortex and the pyramidal path. Therefore, damage to these areas with deteriorationblood circulation of the brain (stroke), trauma or tumor leads to a violation of human motor activity.
At whatever level the lesion occurs, the muscles stop receiving impulse from the cortex, which leads to a complete inability to carry out the action. This symptom is called paralysis. If the damage is partial, there is muscle weakness and difficulty in moving - paresis.
Types of paralysis
There are two main types of immobilization of a person:
- central paralysis;
- Peripheral paralysis.
They got their name from the type of affected neurons. With central paralysis, damage to the first neuron occurs. With peripheral immobilization, the peripheral nerve cell is affected, respectively.
It is possible to determine the type of damage already at the first examination of the patient, without additional instrumental methods. Central paralysis is characterized by the following features:
- increased muscle tone, or hypertension;
- increased amplitude of tendon reflexes, or hyperreflexia;
- decrease in activity of abdominal reflexes;
- appearance of pathological reflexes.
Symptoms of peripheral paralysis are the exact opposite of manifestations of the central one:
- decrease in muscle tone, or hypotension;
- reduced activity of tendon reflexes;
- absence of pathological reflexes.
