Human Anatomy, Part 2 - K. A. Dyubenko, A. K. Kolomiytsev, Yu. B. Chaykovsky 2008

Special Part
Nervous System (Systema nervosum) — Special Part
Pathways of the Central Nervous System

The Study of the Central Nervous system pathways began intensively In the second half of the 19th century. Significant contributions to this issue were made by prominent scientists, notably V. M. Bekhterev, A. M. Grinshtein, S. A. Sarkisov, S. D. Dzugayeva, P. Flechsig, W. A. von Gudden, M. Clara, and others. They investigated the Location of functionally homogeneous bundles of nerve fibers within the substance of the Brain AND SPINAL cord. The following Methods were used to study the pathways: macroscopic examination of brain and Spinal Cord sections, comparative anatomical method, embryological method, experimental method, anatomical dissection method, and pathological-anatomical examination. At the present stage of research, experimental-morphological Research Methods using neurohistological, histochemical, and other techniques are predominantly utilized.

Pathways are defined as groups of nerve fibers characterized by commonality of Structure, function, and placement within the White matter OF the brain and spinal cord. They interconnect the Gray matter Regions of the brain and spinal cord, thereby ensuring the integrity of the Organism and its interaction with the environment.

Nerve fibers of a single pathway originate from homologous Nerve Cells and terminate in synapses on nerve cells that perform the same function. As noted previously, The activity of The Nervous System is based on the reflex arc, which in The process of phylogenesis became complicated by links of nerve cells whose axons form pathways - /tractus/. Through the chains of nerve cells that form the reflex arcs, excitation is transmitted from the periphery to the central Divisions of the CNS, where analysis and synthesis are carried out. Afterward, impulses travel in the reverse direction back to the periphery to effectors. In this manner, bidirectional connections between the spinal cord and the brain are established.

In the brain and spinal cord, according to their functional characteristics, three main groups of nerve fibers are distinguished: associative, commissural, and projection.

Associative nerve fibers, neurofibrae associationes, provide unidirectional connections between individual PARTS OF THE brain or spinal cord. In the cerebrum, among the fibers connecting different areas of the cortex of the same hemisphere, a distinction is made between intracortical associative nerve fibers, which spread within the cortex of a single hemisphere, and extracortical ones, which extend beyond the cortex and become part of the white matter of the cerebral hemispheres.

Associative nerve fibers form long and short pathways: long ones connect cortical areas belonging to different lobes of the cerebral hemispheres; short ones connect cortical areas within the same lobe. Long associative pathways form:

- superior longitudinal fasciculus, fasciculus longitudinalis superior;

- inferior longitudinal fasciculus, fasciculus longitudinalis inferior;

- uncinate fasciculus, fasciculus uncinatus.

Within the spinal cord, associative fibers form intrinsic intersegmental bundles, fasciculi proprii, which interconnect the segments of the spinal cord. They are also divided into long and short ones.

Commissural nerve fibers, neurofibrae commissurales, link functionally homogeneous opposite regions of various parts of the brain and spinal cord. They form the following commissures: anterior commissure, commissura cerebri anterior, corpus callosum, and hippocampal commissure, commissura fornicis.

Projection nerve fibers, neurofibrae projectiones, connect the Cerebral Cortex with lower-lying structures and the spinal cord, as well as link the spinal cord and Brainstem nuclei with the subcortex and cortex. Projection fibers form the main pathways* that ensure the conduction of information reaching the cortical analyzers, where it is perceived, processed, and analyzed, and where its conscious evaluation is performed.

* There are 168 pathways, tractus, and bundles, fasciculi, in total. N. A. Vorobyova, E. P. Kononova, 1983.

Depending on the direction of the impulse, projection nerve fibers are divided into two main groups:

- ascending, or afferent (sensory);

- descending, or efferent (motor);

Ascending pathways (afferent) are formed by centripetal fibers through which impulses are transmitted from receptors—which perceive information from the external and internal environment—to various regions of the brain.

Depending on the type of sensitivity they conduct, ascending (afferent) pathways are divided into: exteroceptive, proprioceptive, and interoceptive sensitivity.

Exteroceptive sensitivity pathways (from Latin externus, external) transmit impulses arising in receptors that perceive stimuli from the external environment (Temperature, pain, and tactile sensitivity). This group also includes the PATHWAYS OF THE visual, auditory, olfactory, and gustatory systems.

Proprioceptive sensitivity pathways (from Latin proprius, own/one's own) conduct impulses from Organs of motion, Muscles, joints, and tendons (deep sensitivity). They reach the cerebral cortex or cerebellar cortex.

Interoceptive sensitivity pathways (from Latin internus, internal) conduct impulses from Internal Organs and Blood Vessels.

Descending pathways (efferent) are formed by centrifugal fibers through which impulses are transmitted from the CEREBRAL CORTEX AND subcortical structures to the motor nuclei of Cranial Nerves and the nerve Cells of the anterior horns of the spinal cord; that is, they execute appropriate responses to various external and internal stimuli. Descending pathways are grouped into two major systems: the pyramidal and extrapyramidal systems.