Human Anatomy - Kotsan I. Y. 2009

Pathways of the Brain and Spinal Cord
Association Pathways

Associative, or connecting, pathways interconnect groups of Neurons (nerve centers) within the same half of the Brain. They represent neural chains of varying complexity that occupy an intermediate position between the terminal neurons of the afferent pathway and the initial section of the efferent pathway, thereby closing the reflex arcs (Y. A. Diskin, N. F. Konkin, 1977).

Associative pathways serve to integrate and correlate neural processes at a qualitatively new, higher level, which is made possible by the establishment of numerous bidirectional connections between the cortical ends of various analyzers. Based on the incoming information, afferent synthesis is constructed and corresponding behavioral responses are formed. According to V. M. Bekhterev, human mental activity is impossible without the associative system.

A distinction is made between short and long associative pathways.

Short associative pathways include fibers that typically run at the bottom of sulci, connecting cortical areas of adjacent lobes without extending beyond the respective lobe of the cerebral hemisphere (intralobular fiber bundles). These are the arcuate fibers of the cerebrum (fibrae arcuatae cerebri). Among the White matter fibers of the cerebral hemispheres, the arcuate fibers are located most superficially, directly beneath the cortex. Short associative pathways also comprise fibers connecting neurons of adjacent Brainstem nuclei with each other and with neurons of the reticular formation.

Long associative pathways interconnect areas of Gray matter that are located at a considerable distance from each other, usually in different lobes (interlobular fiber bundles). These are well-defined fiber bundles that can be dissected on a specimen. Long associative pathways include:

1. Cingulum — a large bundle of olfactory brain fibers located deep within the fornix gyrus and enclosing the corpus callosum as an incomplete ring. It extends from the region of the anterior perforated substance and olfactory trigone, terminating in the uncus of the parahippocampal gyrus. The cingulum connects various cortical areas within the Medial surface of the cerebral hemisphere, thereby providing functional intercommunication among the fields of the frontal, occipital, and temporal lobes. Because the cingulum is associated with the limbic system, it develops phylogenetically and ontogenetically much earlier than other associative pathways.

2. Superior longitudinal fasciculus (fasciculus longitudinalis superior) lies in the upper part of the WHITE MATTER OF the cerebral hemisphere and connects the cortex of the frontal lobe with the parietal and occipital lobes.

3. Inferior longitudinal fasciculus (fasciculus longitudinalis inferior) lies in the lower Regions of the hemisphere, connecting the cortex of the temporal lobe with the occipital lobe.

4. Uncinate fasciculus (fasciculus uncinatus), arching anterior to the insula, connects the cortex in the region of the frontal pole with the anterior part of the temporal lobe.

5. Fronto-occipital fasciculus (fasciculus frontooccipitale) lies lateral to the cingulum, connecting the frontal gyri with the gyri of the lateral surface of the occipital lobe and the gyri of the insular lobe.

In addition, long associative pathways include subcortical fibers running as part of the stria terminalis, stria medullaris thalami, and the dorsal and medial longitudinal fasciculi.

In the Spinal Cord, associative fibers connect neurons located in different spinal segments. They form the anterior, lateral, and posterior proper bundles (fasciculi proprii anteriores, laterales et posteriores) of the spinal cord and are located directly adjacent to the gray matter. Short bundles connect adjacent segments of the spinal cord, whereas long bundles connect more distant ones.