Inventing Iron Man (9 page)

Since this section of the chapter is about detecting some information from the brain that is then relayed to robot Iron Man, we need to
also understand how your own nervous system works to produce and regulate movement. That is, where does the signal for movement come from and what does it look like? I am pretty sure that you would agree with me that there are lots of things going on in your brain at any given moment. You probably also recognize that your brain doesn't exactly work like your computer or even your car engine. So, there isn't a little port just sitting there ready to have something plugged into it so it can directly relay commands to a computer. Is it even possible to get specific and useful information from all the activity in the brain? Let's investigate that “brain-computer interface” concept I mentioned a bit earlier. Get ready and hold on because we are about to dig deep into your gray matter.

Your brain contains about 100 billion neurons, and there are about 1 billion more living in your spinal cord. As I write this sentence there are about 7 billion people on earth. So, the number of neurons in your nervous system is about 15 times more than all the people on earth right now. If we think of activity of the neurons in the brain like individual people trying to talk to each other, we can ask ourselves this question: what—if anything—can we extract from a conversation among 101 billion people? Luckily all our neurons speak basically the same “language” and communicate in the realm of electrical signals. And, they don't all talk at once and aren't literally all connected to each other. Despite the fact that there are so many neurons with different levels of activity, amazingly we can get something consistent and resembling certain patterns.

Why is it that we can get anything to use as a signal to control things? When we make a purposeful movement, the commands start way up in our brains. Literally at the top, because the part of your brain that helps initiate movement really is at the physical top of your brain. (We will come back to this in more detail later in the chapter.) It is a bit oversimplified to say that areas of the brain are set up completely separate from other areas like isolated little kingdoms. However, different areas of the brain have very specialized functions, and it is usually shown as divided into frontal, parietal, occipital, and temporal lobes (also called “cortices”;
figure 3.4
). The labels in the figure within each lobe are meant to generally indicate the functions for those brain areas. When speaking about commands for movement, we are in the “motor system.”

A common story in physiology and neuroscience is that many of the discoveries about function of parts of the brain and nervous system have come from observing what happens when things don't work well or when there are injuries. In other words, much of what we knew before imaging technology came from descriptions of how movement control was disordered after brain or spinal cord injury. The “Edwin Smith Surgical Papyrus” described motor control problems after head injuries in ancient Egypt—over 5,000 years ago. Even though people have known about the connection between brain injuries and motor control for millennia, for quite some time there were many controversies about how the nervous system itself worked. For example, it took a long time to establish that the cells in the nervous system were “excitable tissue.” That is, they convey information using electrical signaling (see
chapter 2
). This is very important for the issues involved with Inventing Iron Man, since many of the things we are discussing in this book have to do with interfacing electrical devices (like computers) with the basic signaling within the nervous system (which is electrical). However, in classical medicine, Galen (AD 129–199) suggested that nerves were hollow and worked in a kind of pump or pipelike system to convey commands in the body. The substance relaying commands to activate muscle would then flow into the muscles and make them go. This idea was also favored by famous French philosopher René Descartes (1596–1650)—he of “I think therefore I am” fame. However, cutting to the heart of the matter (there is a pun intended as you will read), Alexander Monroe (1697–1762) showed that cutting a nerve did not reveal a gushing or outflowing from the nerve. This would have to have occurred if the older ideas of Galen were correct, so Monroe's experiment proved this wrong. Monroe thought maybe electricity might be involved instead.

Figure 3.4. The human brain showing different areas of specialization in the cerebral cortex and the cerebellum. Modified from Mysid's adaptation of the 1918 edition of
Gray's Anatomy
.

This idea was met head on—with lots of controversy about “animal electricity”—by two very important Italian physiologists, Luigi Galvani (1737–98, from whose name we get galvanic current and the word “galvanize”) and Alessandro Volta (1745–1825, from whose name we get volts as a measure of electrical amplitude). Galvani showed that a frog leg could twitch even (shortly) after death if the nerves going to the leg muscles were electrically stimulated. The controversy arose because Galvani thought this electrical stimulation used electricity within the frog's leg (e.g., animal electricity), whereas Volta thought that the frog's leg was merely a conductor of electricity. So, the combined research of the two men was the first real description of the electrical nature of the nervous system. However (this bit is really important, so pay attention please), when the brains of different animals were stimulated with electricity, not much actually happened. This suggested that maybe the brain didn't do anything specific and related to the control of movement.

In fact, Charlotte Taylor and Charles Gross have described how, up until the eighteenth century, the outer surface of the brain (known as the cortex) was actually considered to be a useless “rind.” By the way, this is actually what the root word “cortex” means in Latin. Some scientists correctly disagreed. Thomas Willis (1621–75), a professor at Oxford, and Francois Pourfour du Petit (1664–1741), a surgeon in the French army, both thought the cortex had an important role in movement control. In particular, from observing lesions in injured soldiers and from parallel experiments in dogs, du Petit noted that the outer surface of the brain was indeed very important for movement. These observations from hundreds of years ago helped show that the brain and nervous system were electrical in nature and that there were specialized parts of the brain, including those related to movement.

Clear evidence of specific functions in different parts of the brain had to wait until the excellent work of Paul Broca (1824–80). In 1861 he wrote about several patients who had difficulties in speaking. They all had damage to the left frontal lobes. This showed clearly that certain functions (in this case, speech) could be largely controlled and affected by very specific parts of the brain. You can roughly locate this part of your own brain by running your hand over the corners of your forehead as your skull moves back toward your ear. Anyway, it would take a bit more creative work after Broca's research to convince people that parts of the brain participated in movement control.

It is often said that the human brain is the most complex organ. Measuring activity in such a complex organ is not as simple as you might imagine. Remember, there are 101 billion neurons to listen in on. And they have to communicate together in useful patterns in order to produce all the behaviors we are capable of. Technology has often been a limitation for this kind of measurement and only small numbers of neurons have been recorded. In 2007, MIT neuroscientists Timothy Buschman and Earl Miller conducted a study aimed at looking at attentional focus in monkeys. They recorded from up to five hundred neurons simultaneously in three different brain regions during different tasks of focusing on targets. This represented a huge advance in the ability to record a large numbers of neurons simultaneously.

An important insight into the role of the cortex in movement control came from the work of John Hughlings Jackson (1835–1911). He was a British neurologist who studied patients with epilepsy. His clinical observations suggested to him that certain parts of the brain must be closely related to specific motor commands. He saw that during a seizure there was a consistent and organized spread of muscle contraction across the body. This made him think that certain parts of the brain should have specific actions in causing movement and that the whole system must be organized in a way reminiscent of the layout of the body. However, he had to wait until the work of Gustav Fritsch (1838–1927) and Eduard Hitzig (1838–1907) for confirmation. Fritsch, while working as a military surgeon had noticed that his
efforts to treat a head wound would sometimes (accidentally) cause small contractions on the side of the body opposite to the injury. In 1870 Fritsch and Hitzig used electrical stimulation of the brain to generate detailed maps of the brain of the dog and showed clearly that movements could be created by stimulating certain brain areas. So, at this point it was known that electrical stimulation of certain parts of the brain (but not others) could evoke twitches in muscles of the body and that there was a kind of map of the body muscles represented somehow by the neurons in the brain. These studies also revealed that the control of activity in muscles is generally found on the opposite side of the brain. If you are using your right hand to turn the pages of this book, it is the cells in the motor cortex of the left side of your brain that are sending the commands. Also, if you choose to turn the page with your left hand, it's the command cells in the motor cortex on the right side.

Canadian neurologist Wilder Penfield and his friend Edwin Boldrey followed up on this work of locating the centers for different functions in the human brain. They did a detailed stimulation exercise and found that they could generate a kind of “map” of the muscles of the body from stimulation of the brain. The basic concept is this: if you give electrical pulses of stimulation to the motor areas of the brain, you can trigger the output cells of the brain to relay commands to the cells in the spinal cord that activate muscle. By moving electrodes over the surface of the brain, movements in different muscles can be observed. Through painstaking effort, it is possible to create a kind of input-output map of the surface of the brain, which is weighted differently depending upon how much area (and therefore numbers of cells) on the brain are devoted to a particular part of the body. Think of how a huge city with 15 highway interchanges compares with a small village with no exits off a highway are represented on a road map.

Penfield and Boldrey's work was the basis for the “homunculus” (little man) concept that describes the map of the muscles of the body on the surface of the brain (
figure 3.5
). The surface area of the body on the map is an indication of the number of brain cells controlling those muscles. These cells are found in the “motor execution” part of the brain shown in
figure 3.4
. We also have similar maps related to the sensory areas of the brain. In that case, the maps are created by recording activity of brain cells when different skin areas are activated. Understanding this is important in grasping whether it might
be possible to tap into this system to control computers and robotic devices. To set the stage for that, I think it is probably useful to ensure that we understand how movement commands arise and are relayed.

Figure 3.5. “Map” of the neurons (upper motor neurons) in the brain used for activating muscle. The distorted shapes of the body part represent the relative number of neurons that control muscles in that part of the body. Modified from Penfield and Rasmussen (1950).

Now let's press ahead and look at how the signal for the activation of Tony Stark's muscles actually occurs. To begin, you have activity in the brain (there are a number of places where this occurs, actually, and we will come back to this shortly). We will focus right now on
that part of the brain where we find the motor map we were just discussing. Activity from these cells is sent down to the spinal cord in the form of what are known as “action potentials” and then out to the muscles. Recall in
chapter 2
we learned about the movement of sodium, calcium, and potassium ions in and out of cells and how this was linked to the electrical energy needed to move muscles. An action potential results when this energy rapidly rises and falls.