Methods by which the conduction paths have been determined
A purely anatomical examination of a normal adult cord, prepared by whatever means, gives no indication of the fact that the mass of longitudinally coursing fibers of the white substance is composed of more or less definite bundles or fasciculi, each having a definite course, and whose axons form links (conduction paths) in a definite system of neuron chains.
Present information as to the size, position, and connections of the various fascicuH is based , upon evidence obtained by three different lines of investigation: -
(1) Physiological investigation
- (a) Direct stimulation of definite bundles or areas in section and carefully noting the resulting reactions which indicate the function and course of the axons stimulated, (b) 'Wallerian degeneration' and the application of such methods as that of Marchi. When an axon is severed, that portion of it which is separated from its parent cell-body degenerates. Likewise a bundle of axons severed from their cells of origin, whether by accident or design, will degenerate from the point of the lesion on to the locality of their termination in whichever direction thisimay be. This phenomenon was noted by Waller in 1852 and is known as WaUerian degeneration. By the application of a staining technique which is differential for degenerated or degenerating axons and a study of serial sections con- taining the axons in question, their course and distribution may be determined. The locality of their cells of origin, if unknown, may be determined by repeated experiment till a point of lesion is found not followed by degeneration of the axons under investigation, (c) The axonic reaction or 'reaction from a distance.' Cell-bodies whose axons have been severed undergo chemical change and stain differently from those whose axons are intact. Thus cell-bodies giving origin to a bundle of severed axons may be located in correctly stained sections of the region containing them.
(2) Embryological evidence
- In the first stages of their development axons of the cere- bro-spinal nervous system are non-medullated. They acquire their sheaths of myelin later. Axon pathways forming different chains become medullated at different periods. Based upon this fact a method of investigation originated by Flechsig is employed, by which the posi- tion and course of various pathways may be determined. A staining method differential for medullated axons alone is apphed to the nervous systems of foetuses of different ages, and path- ways meduUated at given stages may be followed from the locality of their origin to their termination. In the later stages, when most of the pathways are medullated and therefore stain alike, the less precocious pathways may be followed by their absence of medullation.
(3) Direct anatomical evidence
- (a) Stains differential for axons alone are applied to a given locality to determine the fact that the axons of a given bundle actually arise from the cell-bodies there, or that axons traced to a given locality actually terminate about the cell-bodies of that locality. For example, it may be proved anatomically that the axons of a dorsal root arise from the cells of the corresponding spinal ganglion, and then these axons may be traced into the spinal cord and their terminations noted either by collateral or terminal twigs, or the fasciculus they join in their cephalic course may be determined. (b) The staining properties and the size and distribution of the tigroid masses in the cell-bodies of sensory neurons differ from those in the motor neurons, and recently Malone has claimed that, in the central system, the cell-bodies in the nuclei of sensory neuron chains, those ascending toward the cerebral cortex, may be distinguished from the cell-bodies of the motor or descending chains by the arrangement and size of their tigroid masses. He claims further that in the same way, the cell- bodies of the somatic efferent neurons may be distinguished from those of the visceral efferent neurons. In this way the locality of origin of certain physiologically known paths may be determined.
(4) The so-called pathologico-anatomical method
It is based upon the same general principles as is the physiological (or experimental) method. A pathological lesion, a local infection or a tumor for example, may destroy a nucleus of cell-bodies or sever a bundle of axons, and the resulting degeneration of the axons may be followed through serial sections suitably prepared. The locality of the lesion known, the path may be followed to determine the locality of its termination; its locality of termination known from the symptoms resulting, the path may be followed to its cells of origin, or to determine whatever be the locality of the lesion.