AFFERENT, OR SENSORY PATHS The sensory paths conduct two varieties of impulses, viz., general and special. The impulses originate in the end-organs of the cerebral and spinal nerves and by those nerves are con veyed to the cerebrospinal axis through which they reach the proper cortical area in the cerebrum.
General sensation is the function of the sense of touch. This sense has four important subdivisions—the tactile sense, mus cular sense, pain sense, and temperature sense. Stereognosis is only an associated interpretation of the impulses of the sense of touch and not a subdivision of it. Tactile sensations appear to be most elemental and, in the cord may be conducted by the posterior and lateral columns. Other common sensations seem to require some specialization, as yet not understood, in their conducting media; and pain and temperature impulses pursue a path entirely distinct from that followed by impres sions of the muscular sense. In giving the common sensory tracings, the following classification will be adhered to, though conclusive evidence of certain points in it is still lacking.
I. Paths conducting impulses of the muscular and tactile senses, chiefly, from muscles, tendons, joint surfaces, and the skin. Spinal and cerebral (Fig. 149).
II. Paths conveying pain, temperature, and tactile impulses. Spinal and cerebral (Fig. 15o).
I. Paths Transmitting Impulses of the Muscular and Tactile Senses, chiefly from muscles, tendons, ligaments, joint surfaces and the skin.
Through Posterior Column and Fasciculi Gracilis et Cu neatus (Fig. 149).—Impulses originating in the end-organs of the spinal nerves traverse the dendrites of the spinal ganglion neurones (Cajal), the cell-bodies in the ganglia, and then the axones of the same. They enter the cord through the posterior roots of the spinal nerves and ascend through' the posterior column; entering below the eighth thoracic segment they flow through the fasciculus gracilis, or, entering above the eighth thoracic segment, they ascend through the fasciculus cuneatus. In either case they arrive in one of the nuclei of the posterior col umn, namely, the nucleus funiculi gracilis or the nucleus funiculi cuneati. Thence the impulses may proceed either by a direct
or by an indirect route.
i. The direct route carries the impulses by way of the medial fillet through the sensory decussation of the medulla, the forma tio reticularis of pons and mid-brain, to the lateral nucleus of the thalamus, from which they are conducted by the cortical fillet to the somesthetic area of the cerebral cortex. In their last stage the impulses run from the thalamus through the internal capsule and corona radiata to the posterior central gyrus in the equatorial zone of the hemisphere.
Above the nuclei funiculi gracilis et cuneati, that is in the medial fillet, the impulses of the muscle-sense travel through a fasciculus distinct from that conducting impulses of light and deep touch, and from that conducting impulses of tactile dis crimination; though all three fasciculi are contained within the medial fillet.
2. Indirect Route.—Impulses traveling this route do not ordinarily reach the threshold of consciousness and become sen sations; they merely excite cerebellar reflexes. However, if the impulses are powerful, they may overflow the synaptic re sistance of the reflex centers and continue to the cerebrum. Then by this route impulses from the nucleus funiculi gracilis and nucleus funiculi cuneati run to the cortex of the vermis cerebelli superior through the external arcuate fibers; then on, through the brachium conjunctivum, to the red nucleus and thalamus. They traverse the restiform body of the same side, by way of the posterior external arcuate fibers; or, by way of the anterior external arcuate fibers, they traverse the fillet decussation of the medulla and the opposite restiform body to reach the vermis cerebelli superior. From the cerebellar cortex, the impulses continue through cortical axones to the nucleus dentatus, whose axones conduct them to the red nucleus and thalamus of the opposite side. The greater number, therefore, cross over in the tegmentum of the mid-brain. Their course from the red nucleus and thalamus is through the cortical fillet to the cortex.