The Excitable Membrane

As the first chapter starts winding down, Cajal starts unspooling his full philosophy of nervous system function and development. “From a dynamic standpoint,” he says, “the progressive differentiation of the nervous system appears to respond to the essential goal of improving and amplifying the reflex act.” Cajal sees early life forms as something like robotic critters controlled by switches that the world flips. The world holds all the candidate “causes” for them, and these trigger — relatively straightforwardly — the variety of “effects” we observe when animals behave. Even though the coupling of environment to behavior is fairly direct in these cases, that’s not to say those behaviors aren’t complex. It’s not like a photon absorbed by a photoreceptor causes one muscle fiber to twitch. The way to think about it instead, Cajal says, is like “a music box in which the pressure of a spring puts into action a mechanisms that plays one or several melodies.” A complex, self-sustaining behavior stands at the ready, waiting to be released by the right tickle from the environment. This stock of “unconscious mechanisms”, that great set of things that are just kind of taking care of themselves, form “the great background of adaptations of the nervous system” Cajal tell us.

He continues, giving us a kind of great neuronal chain of being argument. At bottom we have simple and compound reflexes. Sitting one rung above this are those more ‘psychic’ reflexes that don’t connect up directly with the outside world, but rather are “under the dependence of excitations originated in the command spheres of the organism itself.”

And one rung above that, giving the most psychologically impactful “refinement” of the reflex act are those memory-based processes that thicken our perception of the present moment, and unyoke us from what’s just immediately available in the environment. Sensory inputs don’t just immediately throw behavioral switches, in other words. They are stored as sensory impressions, being both “absorbed and retained” so they can serve, much later on, as sources and modifiers of behavior in their own right. This is a big idea from James too, you’ll recall. Memory allows us to be influenced by things that are no longer actually there. This is the germ of James’ much larger philosophical idea about “the reality of the unseen.”

From a biological perspective, a key question about the storage of sensory impressions (and memories more generally) is whether it happens in some special dedicated place, or whether it’s diffusely distributed across the brain. Cajal seems to believe the former, and points to work by Flechsig, who argued for special “association zones” that contained an abundance of the association neurons that mediate between sensory and motor function. Cajal then extends this idea into the motor system, and suggests there are three kinds of pyramidal cells, or “psychic neurons.” Two of these are the usual sensory and motor flavors, and the third is what Cajal called an “association or representational neuron.” He clearly attributes special importance to these, and says that “all our knowledge of the external world would be just the assembly of dynamic connections established between representational cells of different categories.” Going even further, and pushing a bit more speculatively, he wonders out loud whether the number of representational neurons is related to the number of our possible ideas.