Arbitrary, they are also conscious movements, are those that a person is able to control with the help of the cortex of the cerebral hemispheres. In the implementation of the motor act involved many levels of the peripheral and central nervous system. These levels do not work in isolation, they are in a constant relationship, transmitting nerve impulses to each other . What provides arbitrary human movements? About this in detail in the article.
The value of afferent signals
The main role in the implementation of arbitrary human movements lies with afferent signals. These are impulses that enter the human body from the outside. Before making any movement, the nerve signal is captured by the receptors and through the sensory nerve paths enters the structures of the central nervous system. Through these pathways, the brain learns that the skeletal muscles are ready for movement.
Afferent impulses perform the following functions:
- report to the cerebral cortex that there is a need to perform movement;
- โsayโ whether it is being performed correctly;
- increase or, conversely, reduce the force of contraction of muscle fibers;
- correct the sequence of contraction of muscle tissue;
- inform the bark about whether it is worth stopping motor activity or there is a need for its continuation.
Two zones of the cortex - motor and sensory - form a single whole of the sensorimotor department. It controls the functioning of the underlying structures of the brain and spinal cord while ensuring arbitrary human movements.
Motor centers
The centers of the human movement system in the cerebral cortex are located in the precentral gyrus. It is placed in front of the central groove in the frontal cortex. This department, together with the paracentral lobule and a small area of โโthe 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.
Arbitrary human movements are integrated in the tertiary field, which is located in the front of the frontal lobe. Thanks to the work of this section of the cortex, the motor act exactly corresponds to the received sensory information.
All processes that occur in the human body are integrated by two departments of the nervous system: autonomous and somatic. It is the autonomous nervous system of a person that controls voluntary movements.
Pyramid cells
In the area of โโthe primary and secondary motor fields in the fifth layer of gray matter of the brain are giant pyramidal cells. These formations were discovered by the scientist V. A. Betz, therefore they are also called in his honor - Betz cells. With these cells begins a long pyramidal path. It, interacting with nerve fibers of the peripheral nervous system and striated muscle tissue, gives us the opportunity to move on our own.
Elements of the cortex-muscular tract
Arbitrary human movements are primarily provided by the cortex-muscular or pyramidal path. The composition of this formation includes two neurons. One of them was called central, the second - peripheral.
The central neuron is the body of the Betz 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 the second neuron. A long process also leaves the body of the second nerve cell, which goes to the periphery and transfers information to the skeletal muscles, forcing them to move. This is how the movement of the trunk and limbs.
But what about the muscles of the face? To make their arbitrary contractions possible, part of the axons of the central nerve cells goes not 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:
- the cortical and spinal pathway, which transmits impulses to neurons of the spinal cord;
- cortical-nuclear pathway leading to the medulla oblongata.
The implementation of body movements
The processes of central neurons are first located under the cortex. Here they radially diverge in the form of a radiant crown. Then they come closer to each other and are located on the knee and back leg of the inner capsule. This is a structure in the cerebral hemispheres that is located between the thalamus and the basal ganglia.
Then the fibers come through the legs of the brain to the medulla oblongata. On the front surface of this structure, the pyramidal paths form two bulges - pyramids. In the place where the medulla oblongata passes into the spinal cord, a part of the nerve fibers crosses.
The crossed part then goes as part of the lateral cord, the non-crossed part is part of the anterior cord of the spinal cord. Thus, the lateral and anterior cortical and spinal canals are formed, respectively. The fibers of these paths gradually become thinner and eventually end on the nuclei of the anterior horns of the spinal cord. They transmit impulses to alpha motor neurons located in this area.
In this case, the fibers of the anterior path make a cross in the spinal cord on its anterior commissure. That is, the entire cortical-spinal path ends on the opposite side.
Long processes of alpha-motor neurons exit 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 arbitrary human movements due to the impulse received from the pyramidal cells of the cerebral cortex.
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 forms the cortical-nuclear pathway. Due to it, a nerve impulse is transmitted from the pyramidal cells to the nuclei of the cranial nerves.
These fibers also partially overlap at the level of the medulla oblongata. But there are processes that carry out a complete cross. They go to the lower part of the nucleus of the facial nerve, as well as to the nucleus of the hyoid nerve. Such an incomplete intersection means that the muscle tissue that provides voluntary human movements at the face level 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 motor activity of the upper is completely preserved.
Symptoms of motor damage
Arbitrary human movements provide, first of all, the cortex and the pyramidal path. Therefore, damage to these areas with a deterioration in the blood circulation of the brain (stroke), trauma or tumor leads to a violation of the motor activity of a person.
At whatever level the lesion occurs, the muscles cease to receive an 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, muscle weakness and difficulty in performing movements are observed - 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. For central paralysis, the following features are characteristic:
- increased muscle tone, or hypertension;
- an increase in the amplitude of tendon reflexes, or hyperreflexia;
- decreased activity of abdominal reflexes;
- the appearance of pathological reflexes.
Symptoms of peripheral paralysis are the exact opposite of central manifestations:
- decreased muscle tone, or hypotension;
- decreased tendon reflex activity;
- lack of pathological reflexes.