Pathways are a collection of nerve endings and fibers that pass through specific areas of the brain and spinal cord. The pathways of the central nervous system provide a direct two-way connection between the brain and spinal cord. Studying them, you can understand how all the main organs of the body and the external environment are interconnected and how to manage all this. At the same time, afferent, efferent and associative pathways are distinguished.
Centripetal fibers
Afferent neural pathways are classified on the path of unconscious and conscious sensitivity. It is with the help of them that a connection is ensured between all integration centers located in the brain. For example, they provide a direct connection between the cerebellum and the cerebral cortex.
The main afferent pathways of the central nervous system of conscious general sensitivity are fibers of pain, temperature and tactile sensitivity, as well as conscious proprioceptive. The main unconscious pathways of general sensitivity are the anterior and posterior spinal cord. Special conductive substances include vestibular, auditory, gustatory, olfactory, and visual.
Tactile, temperature and pain fibers
This pathway originates from receptors in the epithelium, the impulses from which enter the cells of the spinal ganglion, and then into the spinal cord, to the nuclei of the visual tubercle. Then to the cortex of the postcentral gyrus, in which they undergo a complete analysis. Three paths are involved in this pathway:
- Talamo-cortical.
- Ganglionospinal.
- The lateral spinal thalamic tract, which runs in the lateral cord of the spinal cord and the lining of the brain stem.
The trigeminal nerve is responsible for obtaining tactile sensations in the front of the head and changes in body temperature. When it is damaged in a person, severe pains begin in the face area, which either disappear or reappear. The trigeminal nerve passes through the cervical spine, where the motor fibers of the corticospinal tract intersect. Axons of sensory trigeminal neurons pass through one of the parts of the medulla oblongata. Through these axons, information about pain in the oral cavity, teeth, and also in the upper and lower jaws enters the brain.
Fibers of Conscious General Sensitivity
This path leads through itself all kinds of general sensitivity from head to neck. Receptors begin their journey in the muscles and skin, conduct impulses to the sensitive ganglia and pass into the nuclei of the trigeminal nerve. Further, the path goes to the visual tubercles, and then extends to the cells of the postcentral gyrus. This includes three main paths:
- thalamocortical;
- ganglionuclear;
- nuclear thalamic.
Fibers of Conscious Proprioceptive Sensitivity
This pathway originates with its receptors in the tendons, periosteum, muscles and ligaments, as well as in the articular bags. In this case, complete information about vibrations, body position, degree of relaxation and contraction of muscles, pressure and weight is carried out. Neurons of this pathway are located in the spinal nodes, the nuclei of the sphenoid and thin tubercles of the medulla oblongata, the optic tubercle of the diencephalon, in which the switching of impulses then begins. Information is analyzed and ends its journey in the central gyrus of the cerebral cortex. This path includes three paths:
- Talamocortical, which ends in the projection center, that is, in the central gyrus of the brain.
- Thin and wedge-shaped bundles passing in the posterior cord of the spinal cord.
- Bulbar-thalamic tract, passing in the tire of the brain stem.
Cerebrospinal fiber
Afferent pathways of the spinal cord are formed using axons, or, as they are also called in other words, the termination of neurons. Axons are located only in the spinal cord and do not go beyond it, and also create a connection between all segments of the organ. The atomic structure of these fibers is that the length of the axons is quite large and connects to other nerve endings. From receptors to the central nervous system, due to the afferent pathways of the spinal cord and brain, nerve signals are conducted. All nerve fibers located along the entire length of the spinal cord participate in this process. The signal to the organs is carried out from different parts of the central nervous system and between neurons. The unhindered passage of the signal from the periphery to the central nervous system is achieved through the pathways of the spinal cord.
Anterior and posterior spinal tracts
The afferent pathways of the cerebellum belong to the unconscious and originate in the lateral cord of the spinal cord, and from there they already provide information about the state of the organs of the musculoskeletal system. The anterior cerebrospinal tract enters the cerebellum through the upper pedicle, and therefore it passes through the tire of the medulla oblongata, midbrain, and bridge. The posterior spinal cord passes through the medulla oblongata and penetrates the lower leg.
These two tracts transmit information from the ligaments, articular bags, muscle receptors, tendons, periosteum to the cerebellum. They are responsible for maintaining equilibrium and coordination of human movements, so their role in the body is very important.
Hearing fibers
This path carries information from the receptors of the organ of Corti, which is located in the inner ear. Nerve impulses enter the bridge, which contains the auditory nuclei along the fibers of the vestibulo-cochlear nerve. Through the auditory nuclei, information is transmitted to the nuclei of the trapezius. After that, the impulses arrive at the subcortical centers of hearing, which include the visual tubercle, lower mounds and articulated medial bodies.
In the middle brain, backward reactions occur to these auditory stimuli, while the afferent auditory pathways switch to the nuclei of the thalamus, in which the auditory stimuli are evaluated - they are responsible for movements that occur involuntarily: walking, running. Auditory radiance begins to emanate from the cranked bodies - this tract conducts impulses from the inner capsule to the projection center of the hearing. Only here is an assessment of sounds already beginning to take place. In the back of the temporal gyrus is the associative auditory center. It is in it that all sounds begin to be perceived as words.
Taste analyzers
Impulses of the afferent pathway of taste analyzers develop from receptors of the root of the tongue, which are part of the glossopharyngeal nerves and located on the tongue, which are part of the facial nerve. Impulses from them enter the medulla oblongata, and then to the nuclei of the facial and glossopharyngeal nerve. The smallest part of all the information received from these impulses is delivered to the cerebellum, thereby forming the nuclear-cerebellar pathway, and provides reflex regulation of muscle tone of the tongue, head, and pharynx. Most of the information enters the visual tubercles, after which the impulses reach the hook of the temporal lobe, in which they are consciously analyzed.
Visual analyzers
The afferent CNS pathways of the visual analyzer begin from the cones and rods of the retina of the eyeball. Impulses enter the optic intersection as part of the optic nerves, and then go along the tract to the subcortical centers of the brain, that is, to the optic tubercle, cranked lateral bodies and upper mounds located in the middle part of the brain.
In the midbrain, a response to these stimuli occurs, while in the nuclei of the thalamus an unconscious assessment of impulses begins, which provide involuntary movements reproduced by a person. The main such unconscious movements are running and walking. In the projection center of vision or in the spur furrow of the occipital lobe of the brain, the impulses arrive at the visual radiance from the cranked bodies that are part of the internal capsule, after which a complete analysis of the incoming data begins. In the cortex, which is adjacent to the spur furrow, the central part responsible for visual memory, which is also called the associative visual center, finds its location.
Olfactory analyzer
The afferent pathway of the olfactory analyzer originates from the receptors of the mucous membrane located in the upper part of the nasal passage. After this, the impulses are directed to the axons of the olfactory bulbs, and they flow along the fibers of the olfactory nerves. Then the pulses are directed to the projection center of smell, which is located in the area of the para-hippocampal gyrus and hook. These impulses follow the path to the cortex of the temporal lobe of the brain. To the subcortical centers, which are located in the middle and intermediate parts of the brain, most of the information received from the olfactory receptors is sent. Subcortical centers of the brain in response to olfactory stimuli provide reflex regulation of muscle tone.
Based on this, it can be determined that the main feature of olfactory receptors is that nerve impulses initially enter the cortex of the cerebral hemispheres, and not to the subcortical centers of smell. In this regard, a person first smells, then begins to evaluate it, and only after that an unconscious coloring of the stimulus on the emotional level forms in the brain. In this case, only a fraction of a second is spent on the entire process.
Vestibular tract
The vestibular afferent pathway begins from the receptors of the semicircular canal of the inner ear, uterus and the receptors that make up this organ. This path in the central nervous system corresponds to the coordination of movements and the maintenance of balance during physical and vestibular loads.
Afferent centripetal pathways and the peculiarity of their structure indicate that a person needs to make a lot of effort to maintain the health and integrity of each organ individually and together. Each component of this path provides the body with all the necessary information, helps to immediately process it and to carry out the implementation of all vital processes. This is important in the work of the whole organism as a whole and of individual organs.