Projection Fibers

PROJECTION FIBERS connect the cerebral cortex with the underlying regions, and through them with the periphery; projection fibers can be divided into two main groups: 1) centrifugal, serving to transmit impulses from the central nervous system to the periphery, predominantly to the organs of movement, and 2) centripetal, conducting impulses from the periphery to the central nervous system. Projection fibers, like the cortex itself, belong to two systems: the old brain, or rhinencephalon, and the new brain, or pallium. The latter system reaches its highest development in humans; it connects the cortex with the subcortical ganglia, the brainstem, the medulla oblongata, the spinal cord, and through them with the periphery. The first system connects the cortex of the rhinencephalon with the formations of the diencephalon and mesencephalon. Little developed in humans, it reaches its highest development in lower mammals. With the exception of the fibers of the rhinencephalon system (olfactory fibers), projection fibers, together with association and commissural fibers, form the corona radiata and all pass through the internal capsule (see). Projection fibers are divided into long and short. The long ones include: 1) the motor pathway, which divides into two—the cortico-spinal tract, running from the cortex of the anterior central gyrus to the anterior horns of the spinal cord, and the cortico-bulbar tract, ending at the motor nuclei of the cranial nerves (see Pyramidal system); 2) the frontopontine, temporopontine, and occipitopontine pathways, running from the corresponding lobes of the brain to the pons (see Temporal lobe, Frontal lobe, and Occipital lobe); 3) sensory pathways, running from the spinal ganglia to the cortex of the posterior central gyrus; 4) visual pathways—from the retina of the eye to the cortex of the occipital lobe (see Visual pathways, centers); 5) auditory pathways—from the ganglion of Scarpa to the centers of hearing in the first temporal gyrus (see Auditory pathways, centers); 6) gustatory pathways—from the mucous membrane of the tongue to the centers of taste (see Taste, gustatory conduction pathways); 7) olfactory pathways—from the mucous membrane of the nasal cavity to the olfactory centers (see Olfaction). The short ones include fibers connecting the cortex with the thalamus opticus, with the nuclei of the vegetative system located in the diencephalon—substantia nigra, corpus Luysi, with the nuclei of the floor of the third ventricle, with the nucleus ruber, and with the substantia reticularis. The cerebellum has its own conduction pathways connecting it with the periphery and with the centers (see Cerebellum).