It's a Jungle in There: How Competition and Cooperation in the Brain Shape the Mind by David A. Rosenbaum Read Free Book Online Page B

might promote your own future functioning. Biting the neural hand that feeds you won’t help you over the long run.
    Suppose you’re a neuron that happens to be activated whenever the person in whom you live sees a hamburger. You’d be foolish to inhibit a fellow neuron that plays a role in seeing such a meal. You needn’t know that this other neuron helps in this regard. You needn’t know that that neuron is a “vision neuron,” that there is such a thing as an eye, or anything of the sort. You, as a neuron, are simply doing your thing. You needn’t know that you’re a nerve cell, that you’re in a nervous system (whatever that is), that you occupy a person (whatever
that
is), that your survival depends on the sight of hamburgers, and so on. All you need to know or, more precisely, all you need to
do
is excite cells that happen to result in your own activation and inhibit cells that happen to result in your own deactivation. If you do this reliably, and if the other neurons in your network act similarly, your chances of surviving will be good.
The Bell-Magendie Law
    The examples I’ve just given for justifying interneuronal excitation and inhibition are rooted in a feature of neural functioning that is so fundamental to the way scientists view the nervous system that it’s hard to imagine there was ever a time when they didn’t know it. The feature I’m referring to is the distinction between sensing and acting, between perceiving and moving. It turns out that this distinction is built into the structural organization of the nervous system itself. This was shown in the mid-1800s by an English physiologist named Charles Bell and a French physiologist named François Magendie. They discovered a feature of neural organization that proved to be pivotal for neuroscience. According to their Bell-Magendie Law, fibers on the dorsal sideof the spinal cord serve sensory functions, while fibers on the ventral side of the spinal cord serve motor functions. 8 It will help to unpack these terms to show how the Bell-Magendie Law demarcates the neural landscape, and that, in turn, will set the stage for the application of Darwin’s principle to neural organization. 9
    As just stated, one claim of the Bell-Magendie Law is that dorsal fibers serve sensory functions. You’ve encountered the word “dorsal” in connection with fish. The dorsal fin of a fish is the fin jutting up from its torso. Sharks trawling the ocean surface betray their presence to us landlubbers by their dorsal fins. Snorkelers beneath the surface also need to be wary of sharks’
ventral
fins, the fins on the sharks’ bellies.
    Bell and Magendie’s great discovery was that stimulation of
dorsal
nerve fibers elicits feelings of the skin being touched, of the muscles being stretched, of the joints being flexed or extended, and so on. Stimulation of
ventral
nerve fibers, on the other hand, causes muscles to contract.
    These distinct functions of dorsal and ventral fibers become all too familiar when there is neural damage. If dorsal nerve fibers are hurt, sensory loss can follow. If ventral nerve fibers are hurt, motor loss can result. Paralysis or paresis (partial paralysis) can ensue.
    The discovery that sensory and motor functions can be separated, at least for spinal dorsal nerve fibers and for spinal ventral nerve fibers, shows that there are two basic neural niches—one that deals with stimuli and one that deals with responses. Neuroscientists have developed terms for these two sorts of nerve fibers:
afferent
fibers, which carry signals with sensory consequences, and
efferent
fibers, which carry signals with motor consequences. 10
    My reason for focusing on these two kinds of fibers is that they help constrain the Darwinian drama that can unfold in the nervous system. To see what these constraints are, ask yourself what kind of neuron you’d like to be. If your main priority is survival, you’d want to be a neuron that fires often. You’d want