The thirst for knowledge, the pursuit of higher ideals, phenomenal mental abilities ... We are, of course, talking about man. It is these qualities that distinguish us from the animal world. The material carrier, in other words, the hard disk on which the psychosomatic programs we have written, are the cerebral hemispheres. This article will be devoted to the study of their structure and functions.
Big brain
Organogenesis - the formation of a system of axial organs and other parts of the body in a human embryo - includes the stage of neurula. The chord, intestine and neural tube appear immediately behind the formation of the third germinal leaf, the mesoderm. The nerve ridges closing on the dorsal side of the embryo form the neural tube. Subsequently, it is completely separated from the rest of the ectoderm zone. The front end of the neural tube is inflated and is divided into five parts - the primary brain bubbles. Now they will form the main sections of the central nervous system.
The cerebral hemispheres and the cortex that covers them are phylogenetically the youngest brain structures, since they arose later than other departments.
Architectonics of the cerebral cortex
Both hemispheres - the right and left - are interconnected by the corpus callosum. It is not only a physical carrier of nerve endings - axons, performing the function of a multicore conductive organ containing a huge number of nerve endings.
The structure also bears the centers of motor and behavioral acts, and its pathology is expressed, for example, in the appearance of symptoms of a severe mental disorder - epilepsy.
The large hemispheres on the outside consist of clusters of neuron bodies - highly specialized cells of the nervous tissue. Visually, the upper brain structure has a gray color, which is why it is called: the gray matter of the brain. Numerous processes, dendrites, branch inside it. Together with very long axon fibers that invade the cortical tissue, dendrites form a white matter located under the cortical zones of the cerebral hemispheres. In it, like a mosaic, scattered clots of bodies of neurons, called nuclei. In anatomy, it is customary to define this part of the brain as a subcortex. It is considered an ancient formation that arose already in the first representatives of vertebrate animals.
The structure of the cerebral hemispheres
To increase the total area of the brain, while maintaining a small amount of the cranial box, almost two-thirds of the surface is hidden in the form of folds in it. They are called convolutions. In anatomical atlases, three main ones are distinguished:
- side furrow
- occipital-parietal,
- central.
On them it is easy to distinguish four lobes of the cerebral cortex. This is the temporal, occipital, frontal, parietal lobes, they anatomically correspond to parts of the skull.
The unique internal structure of the bark, similar to a six-story house. Each floor - layer - consists of completely different in appearance, density and shape of neurons. Let's list these layers:
- internal pyramidal
- polymorphic,
- internal granular
- pyramidal
- external granular
- molecular.
Of interest is the postembryonic period of cortical development. It was established that the greatest changes occur in the first, and then in the six-month and one and a half year interval of a child’s life.
Sensory and motor parts of the brain
The zones of the cerebral hemispheres are responsible for the many-sided and complex life activity of the human body. A huge number of newly arising reflex arcs, performing the role of material carriers of conditioned reflexes, are constantly created in the cerebral cortex. The five main sensory complexes - the olfactory system, visual, tactile, taste and auditory - are the channels through which we receive the greatest amount of various information. In addition to them, we are able to differentiate sensations of thirst, pain, temperature, spatial location of the body, hunger.
Science clearly defines the boundaries of each of these zones, their characteristics are studied when considering the structure of each type of analyzer. In them, sections of the cerebral hemispheres in which sensation is distinguished are called the central or cortical part of any analyzer. For example, the visual sensory system includes, in addition to the receptors of the retina and two optic nerves, the visual zone of the cortex located in the occipital lobe.
How is the control of motor reactions
The main area that controls muscle work is located in the precentral gyrus of the cerebral hemispheres. Axons of efferent neurons emerge from this site and are sent to the skeletal muscle, causing contractions of actin and myosin myofibrils. The innervation of the main motor zone occurs according to the collateral principle: the muscles of the body part opposite the hemisphere of the brain are excited. The exception is the facial area, which is innervated directly.
Additionally, in the brain there is another motor area located below the precentral gyrus. Reductions in skeletal muscles can also occur in the case of excitation of sensory zones, especially visual and auditory. For example, a sudden sudden sound can cause the trembling of the hands or head.
Associative zones
The most important functions of integrating various sensations arising under the influence of signals from the surrounding world are performed by several sections of the right and left lobes of the cerebral hemispheres. Anatomically, they are located in the prefrontal associative region, as well as in areas of the parieto-occipital-temporal part of the cortex. Associative zones are receivers of pulses coming from several analyzers at once.
Next, the nerve cells analyze the information received and direct their excitation to certain parts of the body using their centrifugal axons, causing its mixed visual-auditory and motor reactions. For example, the zone of speech understanding (Wernicke region) is leading not only in the process of formation of speech functions, but also provides the development of higher properties of intelligence. In the upper occipital and posterior parietal lobes is an associative zone that analyzes the position of the body in space.
Zones for object naming and primary processing of reading
There is another area in the cerebral cortex called the primary reading processing area. This zone can perceive impulses coming from the visual and auditory sensory systems. The site of the name of the objects is located in the temporal lobe and in the lateral part of the anterior zone of the occipital lobe, receives information from the auditory analyzer. At the same time, part of the impulses is connected from the visual zone located in the occipital region of the cerebral cortex. Both zones are the basis for the development of higher mental processes: abstract thinking, analysis and synthesis of the received visual and auditory information, which are the basis of human intellectual activity.
The main processes of the cortex
Excitation and inhibition are the most important phenomena inherent in nervous tissue. Neurons of the cerebral hemispheres, forming certain zones, spread (irradiate) electrical impulses to other structures of the brain. For example, the deterioration in falling asleep of a person sitting for a long time in front of a computer monitor is explained by the irradiation of excitation of the visual center of the brain to its neighboring areas. The very process of falling asleep will serve as an example of the irradiation of inhibition. Concentration of nervous processes leads to opposite results: the zone of excitation or inhibition, on the contrary, reduces its area. The concentration of excitation is observed, for example, at the air traffic controller during work associated with providing takeoff or landing of the aircraft.
Induction is the guidance in a certain section of the cerebral hemispheres of the opposite nervous process.
Thus, positive induction stimulates the amplification of excited brain regions near the inhibition site. Negative induction is characterized by the opposite course of nervous processes. In a unit of time, the brain receives a huge amount of signals from the receptors of all organs and systems. All of the above processes occurring in the cerebral cortex are the root cause of the behavioral reactions of both higher mammalian animals and humans.
In our article, we examined the structure and functions of the cortex covering the cerebral hemispheres, and also determined the most important functions of the brain zones.