"Anatomical mechanism of ideation, association and attention" [1895] and "Certain points in neurological histophysiology" [1896]: Cajal's conjectures, then and now

Abstract

The purpose of this article is two-fold: first, to preserve, in updated English translations, two theoretical papers written by Santiago Ramón y Cajal (1852-1934) in 1895 and 1896 under the titles, "Conjectures on the anatomical mechanism of ideation, association and attention" and "Conjectural interpretations of certain points in neurological histophysiology"; and second, to set some of the ideas proposed by Cajal in a modern perspective. In his "Conjectures," Cajal ventured to explain the mechanisms of perception, association and attention in cellular terms. He introduced the term "impression unit," which would propagate, leading to conscious act via an "avalanche of conduction." Additionally, he attributed mental repose and sleep to morphological variations of neuroglia; at times of relaxation, astrocytes would grow appendices that penetrated among nerve cell connections and blocked the conduction of the "nervous current"; in energetic contraction, such glial "pseudopodia" would shrink, allowing neuronal processes to come into contact again. In the sequel to the "Conjectures," Cajal presented strong arguments defending the neuron theory against the reticular theory. Moreover, he discussed the functional differentiation of spinal motor neurons and cortical pyramidal cells, which respectively subserve movement and consciousness, despite their morphological similarity.