What is the goal of the nervous system, part 2 (a twitter story)

My question from Monday spawned some good discussion on twitter (none, alas, on the blog). What is the goal of the nervous system? Well, a lot of people thought it was pretty simple:

As evidence for how important movement is for having a brain:

Yup, when a sea squirt decides to stop moving it just sits there… and consumes its own brain (for some definitions of ‘brain’).  The video at the top, as suggested by Chris Maidan, is a great explanation of the reasoning behind the idea that the brain is for movement. Yet, despite Daniel Wolpert explicitly saying that the reason we have a brain is for movement… he actually makes an argument for something else that came up in the twitter discussion:

Another good alternative:

And to throw in a curveball:

But the answer that I like the best:

I have some thoughts that I’m writing up, though that is taking some time. Let me take a step back, though, and reword what my question was originally intended to be (though clearly in my sleep-deprived state I did a poor job of describing it).

As others suggested above, the overall goal of any part of a biological system is to maximize the transmission of genetic material (by one mechanism or another) from one generation to the next. Yet when examining the nervous system on a more local level that can be hard to notice; the visual system, especially the early stages, are there to extract information about visual scenes; the auditory system is there to extract information about sound; the spinal chord is there to (mostly) generate movement. If you don’t believe me, look at the peripheral sensory nerves, at the retina, at the cochlea: all part of the nervous system, and on a local level they are guided by local goals. It is not until you get to more internal structures that general ideas of value or desire are found.

And we can write this out mathematically; sensory neurons are often said to be maximizing the Shannon mutual information between their response and the visual world which we can write in a very simple way I(neural response;visual stimulus) with plenty of experimental support. In other words, the visual neurons represent as much information about the visual scene as is possible.

But given that we know that the ultimate goal of the nervous system is to enhance reproductive success – why isn’t that included in the equation? Where would we need to include something like that to have a good understanding of the function of some area of the visual system? Is a proxy like ‘utility’ good enough?

Worms, nervous systems, and the beginning of neuroscience

Worms can distinguish between light and dark, and they generally stay underground, safe from predators, during daylight hours. They have no ears, but if they are deaf to aerial vibration, they are exceedingly sensitive to vibrations conducted through the earth, as might be generated by the footsteps of approaching animals. All of these sensations, Darwin noted, are transmitted to collections of nerve cells (he called them “the cerebral ganglia”) in the worm’s head.

“When a worm is suddenly illuminated,” Darwin wrote, it “dashes like a rabbit into its burrow.” He noted that he was “at first led to look at the action as a reflex one,” but then observed that this behavior could be modified—for instance, when a worm was otherwise engaged, it showed no withdrawal with sudden exposure to light.

For Darwin, the ability to modulate responses indicated “the presence of a mind of some kind.” He also wrote of the “mental qualities” of worms in relation to their plugging up their burrows, noting that “if worms are able to judge…having drawn an object close to the mouths of their burrows, how best to drag it in, they must acquire some notion of its general shape.” This moved him to argue that worms “deserve to be called intelligent, for they then act in nearly the same manner as a man under similar circumstances.”

Darwin was discussing the cerebral ganglia of worms in 1881. If you are particularly interested in worms or just plain masochistic, you can find a copy of the book here. It is somehow historically poetic that, by twists and turns, worms have become one of the foundational species of neuroscience research.

Yet it made me realize that I had no idea when the term ‘cerebral ganglia’ first began to be used. When did we realize that we had a ‘nervous system’? I will go into this more in a later post, but the concept began to be used in books around the year 1650 (which is consistent with other sources I have found). On the other hand, we didn’t understand that the neuron was a useful and separate unit until almost 1900!

neuroscience ngram