The medulla oblongata is a structure in the brainstem that controls involuntary bodily functions like respiration and heartbeat. Within the medulla, structures called medullary pyramids contain bundles of motor neuron fibers passing from the one side of the brain to the other side of the spinal cord. Injuries to the medullary pyramids may cause hemiplegia, or weakness/paralysis of one side of the body. In the figure above, a section of the medulla taken from an individual with hemiplegia, Cajal observes that the medullary pyramids on one side of the structure (H) have noticeably degenerated, leaving the space empty of neurons, while its counterpart on the other side remains intact. Other structures, such as the nucleus gracilis (A), and the nucleus cuneatus (B), which contain neurons responsible for fine touch and proprioception, appear to be undamaged. Cajal’s research at the time suggested there was little plasticity in the adult nervous system, and that treatment for hemiplegia was not possible; we now know that various types of therapies involving continued use and strengthening of the hemiplegic side of the body can promote at least small degrees of recovery.

3D model created by Jeremy Swan based on an original illustration by Santiago Ramon y Cajal