EFFERENT, OR MOTOR PATHS The cerebrospinal or pyramidal paths (Fig. 147) are direct, as they do not pass through the cerebellum. Their impulses ultimately run either through the spinal or the cerebral nerves, and are both motor and inhibitory. Hence the increased reflexes and spastic contractions of lateral sclerosis in which these tracts are diseased.
I. Through the Spinal Nerves (Fig. 147).—Starting in the upper three-fourths of the gyrus centralis anterior of the cerebral cortex, motor and inhibitory impulses run down through the corona radiata, the anterior two-thirds of the occipital part of the internal capsule, the middle three-fifths of the basis pedun culi, the basilar longitudinal fibers of the pons, and the pyramid of the medulla oblongata, whence they proceed by the lateral and anterior pyramidal tracts to the gray crescent, partly in the same side but chiefly in the opposite side of the spinal cord. By the former route, the impulses cross over in the medulla, through the decussation of the pyramids, and descend in the lateral column of the spinal cord to the gray substance in the vicinity of the nucleus dorsalis (Clarki), where, the path is relayed, and intrinsic neurones carry the impulses forward into the anterior columna; but by the anterior route, they descend in the anterior column of the cord and decussate, in succession, through the white anterior commissure. Impulses by either route finally reach the anterior gray columna of the spinal cord and, with the exception of a small per cent. of them, they reach the columna opposite to their cortical origin. The few undecussated fibers in the lateral pyramidal tract conduct uncrossed impulses to the anterior columna of the same side. Thus are explained two symptoms of hemiplegia due to cerebral lesion, viz., weakness on the well side and slight motion on the paralyzed side. From the anterior gray columna of the spinal cord the nerve commotions are conducted by the efferent, or motor fibers of the spinal nerves to the muscles.
2. Through the Cerebral Nerves (Fig. 147).—Impulses des tined to the cerebral nerves run chiefly from the lower two fourths of the anterior central gyrus through corona radiata, genu of internal capsule and on, by the same path as the im pulses to spinal nerves, down to .the point where they leave the pyramidal tract to enter the nuclei of the cerebral nerves, which some of them do in the vicinity of the several nuclei. According to Bechterew they run, at least in part, through pyramidal fibers which constitute the accessory lemniscus. These fibers leave the pyramidal tract near the internal capsule, and descend through the medial portion of the fillet to points near the respect ive nuclei in which they end by multiple division. From either hemisphere impulses proceed to the nuclei of both sides. But the greater number enter the nucleus of the fourth nerve on the same side, and the nuclei of the third, fifth, sixth, seventh, ninth, tenth, eleventh and twelfth cerebral nerves of the opposite side. By the above nine nerves innervation is conducted to the mus cles of the orbit; the muscles of mastication and expression; the muscles of the tongue, palate and ear; the digastric and styloid muscles; the muscles of the larynx, trachea and bronchi, and of the pharynx, espohagus, stomach, and the intestines down to the descending colon. And inhibitory impulses are
carried to the heart; also vasodilator, secretory, trophic and inhibito-secretory impulses to alimentary glands, etc.
The Paths, Frontal, Temporal and Inter paths are indirect for spinal nerves since they run through the cerebellum.
(Fig. 148).—The impulses originate in the pre frontal region and descend through corona radiata, the frontal part of the internal capsule, and the medial fifth of the basis pedunculi to the nucleus pontis. rise in the temporal cortex and run through corona radiata, the occipital part and inferior lamina of the internal capsule, and the lateral fifth of the basis pedunculi to the nucleus pontis. Inter mediate impulses of unknown origin are received by the corpus striatum, by way of the thalamus and conveyed by a bundle of strio-fugal axones which form the deep portion of the basis pedunculi, to the substantia nigra and then to the nucleus pontis, chiefly of the same side. The impulses thus traverse the internal capsule and a broad but thin area in the basis pedunculi just ventral to the substantia nigra and dorsal to the pyramidal tract. In all three of these paths the impulses run to the nucleus pontis of the same side and to motor nuclei of the cerebral nerves. Whence they proceed from nucleus pontis: (1) To Spinal Nerves. They run through the brachium pontis to the cerebellar cortex and thence continue (a) down a cerebello-spinal path to the anterior gray columna of the spinal cord. Their course from the cerebellar cortex is through the cortico-nuclear neurones of Purkinje, to nucleus fastigii; the fastigio-bulbar fibers, to nucleus of Deiters in the medulla; and the vestibulo-spinal tract, to the anterior columna of the spinal cord. (b) Again, from the cerebellar cortex these impulses reach the cord via the red nucleus and thalamus. They are conducted in succession by the following neurones Purkinje's cortico-nuclear neurones, to nuclei dentatus and emboliformis; the cerebello-tegmental neurones, to opposite red nucleus and thalamus through the brachium conjunctivum; and the rubro-spinal and thalamo-spinal neurones, to the ante rior columna of the cord. From the gray matter of the spinal cord the impulses are conveyed by the motor fibers of the spinal nerves to the muscles which they supply. (2) To Cerebral Nerves. The impulses run from synapses formed in the cere bral nuclei, by the fibers of the cerebro-pontal tracts, through the motor fibers of these nerves to their distribution. These impulses also reach cranial nerve nuclei by the indirect route through the cerebellum. The cortico-nuclear fibers, and the cerebello-tegmental fibers, in the brachium conjunctivum and restiform body, connect the cerebellar cortex with the motor nuclei of cranial nerves. Certain fibers of rubro-spinal and thalamo-spinal tracts also end in these nuclei.