It's a Jungle in There: How Competition and Cooperation in the Brain Shape the Mind (6 page)

To pursue this idea, it will help to say more about the elements of the nervous system. The nervous system, as you probably already know, is a complex web made of the central nervous system (including the brain, retina, and spinal cord) and peripheral nervous system (sensory receptors and muscle-activating fibers).

The main building blocks of the nervous system are
neurons
. These come in a variety of shapes and sizes depending on where they’re situated. Neurons specialized for detecting mechanical pressure occupy the skin, neurons specialized for detecting airborne chemicals occupy the nose, and so on. Within the brain, there are also different kinds of neurons.
²

Despite structural differences among neurons, prototypical neurons have three main sections.
Dendrites
receive incoming signals. The
cell body
integrates incoming signals and metabolically sustains the cell as a whole.
Axons
serve as transoms for the release of signals to other neurons or muscles.

Prototypical neurons integrate incoming signals over space and time. The space over which the integration occurs covers the dendritic inputs to the neurons. The time over which the integration occurs is the period over which the inputs sum.

If the integrated inputs to a prototypical neuron exceed a threshold, the neuron can generate a burst of activity called an
action potential
. An action potential races down the axon at a speed that is higher if the axon is coated with myelin (a fatty material) than if it is not.

Myelin takes time to form over the course of development. Not until adolescence is its formation complete in human beings, but by the time of adolescence, myelination isn’t the only neural process that gets completed. Unnecessary connections between neurons are pruned away. Pruning begins in infancy and is completed in the teen years. Pruning can be seen as straightforward example of natural selection.
³

Until the late 1800s, it was not known whether the nervous system is a kind of diffuse cotton ball or a honeycomb of distinct cells with gaps between them. Thanks to the work of the late nineteenth- and early-twentieth-century Spanish physiologist, Santiago Ramón y Cajal, we now know that neurons are separated by tiny spaces called
synapses
, a term coined by a British physiologist, Charles Sherrington, who, like Ramón y Cajal, won a Nobel Prize for his work on neurophysiology.
⁴

Synapses are important because of what goes on in and around them. At synaptic junctions, chemicals are emitted by presynaptic neurons and are picked up by postsynaptic neurons or muscles. The chemicals that are released into the synapse—
neurotransmitters
—are molecules that can affect the postsynaptic membranes. Postsynaptic neurons that have suitable receptors for a neurotransmitter may become more or less excited when they take in the neurotransmitter coming to them. Muscles may likewise contract if they are in a state of readiness to do so and if the neurotransmitter knocking on their doors is one the muscles accept.

Neurons that take up neurotransmitters can respond in either of two ways—by becoming
more
likely to fire or
less
likely to fire. If the neuron is
more
likely to fire upon receipt of a neurotransmitter, the neurotransmitter is said to have an
excitatory
effect on the neuron. If the neuron is
less
likely to fire upon receipt of a neurotransmitter, the neurotransmitter is said to have an
inhibitory
effect on the neuron.
⁵

Neurotransmitters produced by any given neuron have different effects depending on the neurons to which they project. Which neurons produce which neurotransmitters, which neurons accept which neurotransmitters, and whether a neuron gets more or less excited when it takes in a neurotransmitter are matters are of no concern here. More relevant are the effects of neurons being activated or deactivated.

Active neurons suck up more oxygen and glucose than do less active neurons. Active neurons are also more likely than less active neurons to form liaisons with other neurons. The more often two nearby neurons fire in close temporal proximity, the tighter the link between them. Neuroscientists express this principle with a catchy phrase: “Neurons that fire together wire together.”
⁶

Because neurons that fire together wire together, it’s good for neurons to team up with other neurons. Teaming up is important because the neural
ecosystem has a limited supply of oxygen, glucose, and other needed materials. Individual neurons benefit from joining up with other “like-minded” neurons—that is, neurons tuned to similar functional events—just as we humans tend to do better if we join with others than if we go it alone.
⁷

I have just emphasized cooperation among neurons. In response, you might say, “Now wait a minute. If it’s a jungle in there, shouldn’t neurons just
inhibit
their neighbors as much as possible? Shouldn’t neurons be as aggressive as possible all the time, suppressing their neural neighbors as fiercely as they can? Why excite your neural neighbors if it’s a dog-eat-dog world in there?”

Neurons need to be on good terms with other neurons with whom they have connections. If you’re a neuron, you need input to get activated. It won’t serve you well over the long run to deflate all the neurons with whom you have ties because, through their activity, they 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 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 side
of 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 excitement that occurs fairly regularly but not nonstop, for that could exhaust you. And you would not want to be endlessly isolated or interminably inhibited, for then your draw on metabolic resources could get dangerously low.

Being a sensory neuron or a motor neuron would give you a good chance of being called on regularly. Your neighbors would want to be in contact with you because sensing and moving are primary tasks. Afferent and efferent fibers are like essential personnel. They serve critical functions. Everyone relies on them.

Other neurons, so-called interneurons—the neurons lying between the afferent and efferent neurons—are important too because they allow for
communication between the fibers that directly contact sensory receptors and muscle effectors. Interneurons that are removed from direct contact with afferent and efferent fibers tend to support functions that are less directly tied to specific afferent or efferent functions. What they do is more abstract or intellectual. The farther interneurons are from the sensory and motor edges of the nervous system (measured in number of synapses), the more abstract or intellectual their functions tend to be.

Just as neural fibers entering the back of the spinal cord serve sensory functions, whereas neural fibers exiting the front of the spinal cord serve motor functions, fibers in the back of the brain (opposite the face) also tend to support perception. Brain fibers in the front of the brain (toward the face) tend to support action. This generalization helps make sense of the quote from Geoffrey Hinton, “The brain is locally global and globally local.” Globally speaking, action-related functions are represented toward the front of the brain while perception-related functions are represented toward the back. Between these two poles, the functions are more graded, shading roughly from less to more action-based the farther frontward you go.

The second generalization concerns side-to-side organization. Considering the left versus the right side of the brain, a different division emerges. The left cerebral hemisphere of the human brain is thought be specialized for language, at least in most people. Meanwhile, the right side of the brain is thought to be specialized for spatial processing and artistic or intuitive thinking, again in most people.

These differences were made famous by Michael Gazzaniga and Roger Sperry in the 1960s. Gazzaniga and Sperry studied neurological patients who underwent split-brain surgery to alleviate severe epilepsy. Cutting the major neural tract separating the left and right cerebral hemispheres—the
corpus callosum
—created a kind of “fire lane” that stopped the spread of the neural storm producing epileptic symptoms.

Separating the two cerebral hemispheres led to a surprising result. Visual stimuli shown briefly to the visual field that projected to the
left
cerebral hemisphere could be named by the split-brain patients, but visual stimuli shown briefly to the visual field that projected to the
right
cerebral hemisphere could not. This outcome suggested that the left cerebral hemisphere had access to language while the right cerebral hemisphere did not.

It was not that the right cerebral hemisphere was simply dumb, however, as shown in another test where the same visual stimuli were shown to the left or right cerebral hemispheres of the same patients. This time, the patients were instructed to reach out and grasp the visually pictured object. The reaching was done without visual feedback. When the visual stimulus was projected to the
right
cerebral hemisphere, the correct object could be identified through touch, but it could not be named. When the visual stimulus was projected to the
left
cerebral hemisphere, the correct object could be named but could not be identified through touch. Thus, the right hemisphere could display haptic recognition of the seen object, but the left hemisphere could not. Meanwhile, the left hemisphere could display verbal recognition of the seen object, but the right hemisphere could not.

These results and others led Gazzaniga and Sperry to propose that different “mega-functions” are served by the left and right cerebral hemispheres. The left cerebral hemisphere supports verbal-analytic thinking, while the right cerebral hemisphere supports nonverbal-holistic thinking. In terms of the broader message of this chapter, just as macroscopic differences can be found between ecosystems in the outer world, macroscopic functional differences can be found between neural systems in the brain.