This condition is called bulbar paralysis from the fact that the part affected is the medulla oblongata, which is frequently designated the bulb.
There is progressive atrophy and paralysis of muscles supplied from the medulla oblongata. The muscles are mainly those of the tongue, lips, arches of palate, pharynx, and larynx, and in consequence there is progressive interference with articulation, chewing, swallowing, and even with the production of the voice. The disease may pass on later to affect the more vital functions of the medulla oblongata, those concerned in respiration, etc.
We have already seen in connection with the normal structure of the medulla oblongata that, on passing from the cord, the grey substance is dislocated backwards, and that in the posterior region of the medulla a set of grey nuclei appear, which, as the spinal canal opens up in the fourth ventricle, present themselves in the floor of that ventricle. These nuclei are mainly motor, corresponding with the anterior cornua, and they form the immediate centres for cerebral nerves (see Fig. 318).
In the lower half of the fourth ventricle we find the hypoglossal nucleus (xii') occupying the part next the middle line. Outside it there is the spinal accessory nucleus which begins in the cord, and does not extend far up in the floor of the ventricle; it is not shown in the figure, but gives place to the nucleus of the pneumogastric or vagus (x). Outside the vagus appears the glosso-pharyngeal, which partially divides the vagus nucleus into. two. As we pass upwards the vagus nucleus gets smaller and the hypoglossal and glosso-pharyngeal approximate to each other. Above that again come in the nucleus of the sixth (the abducens) in the middle line, and outside that the motor nucleus of the fifth and that of the facial. Outside these again are sensory nuclei, those of the acoustic and of the fifth.
In bulbar paralysis there is atrophy of these grey nuclei. It is seldom that opportunity is afforded of examining the medulla in recent stages, but according to Benedikt, who had such an opportunity, there are definite signs of inflammation, hyperemia, thickening of the walls of the vessels, and numerous round cells. In later periods increase of the connective tissue, with round cells and amyloid bodies, have been observed. These changes centre in the nucleus of the hypoglossal, and this nucleus is usually most seriously damaged, but they pass soon to the accessory and vagus, while the glosso-pharyngeal sometimes, but not usually, escapes. The nucleus of the facial is often attacked, and sometimes that of the motor branch of the fifth. The disease seldom extends to the abducens, and never attacks the sensory nuclei of the acoustic and fifth nerves.
Fig. 318. - Medulla oblongata at the level of the fourth ventricle, showing the position of the grey nuclei. (Quain).
There is in many cases degeneration of the nerve fibres in the anterior pyramids similar to that in the pyramidal tracts in progressive muscular atrophy. There is reason to believe that in some cases the lesion is supra-nuclear, that is, in the fibres which connect these with the motor region of the cerebral cortex, or at least in the terminations of these fibres in the nuclei, so that the affection is in such cases really one of the upper segment of the cerebro- spinal axis, or of the upper motor neuron; in this latter case there is no obvious wasting of muscles, and the disease would be analogous to lateral sclerosis of the cord rather than to progressive muscular atrophy.
The corresponding nerves usually show considerable atrophy, especially the hypoglossal, and next to it the spinal accessory, vagus, and glossopharyngeal. In them may be found a fatty degeneration involving destruction of the medullary sheath and subsequent overgrowth of connective tissue. In this way the nerve-root may come to be almost nothing but a connective-tissue strand.
In the affected muscles the change is exactly parallel to that in progressive muscular atrophy, with which sooner or later this disease is so frequently associated. There is increase of the muscle-nuclei and connective tissue, with destruction of the proper contractile substance, and consequent atrophy of the muscle as a whole. Thus the tongue, palatine arches, lips, pharynx, and larynx may have their muscles intensely atrophied. Sometimes also the muscles of the neck, especially the trapezius (supplied by the spinal accessory), are affected. In some cases the atrophy is obscured, as in progressive muscular atrophy, by the interstitial tissue becoming adipose.