Inventing Iron Man (5 page)

Neurons are excitable cells that generate and carry electrical signals. (Don't worry. You will get a look at a neuron from the motor system in
figure 2.3
later in the chapter.) The electrical signals result from an unbalanced concentration of ions on either side of the nerve cell membrane. Quite a lot is accomplished with just three household-sounding ions: potassium (K+), sodium (Na+), and chloride (Cl−). By the way, bananas, sweet potatoes, and halibut are all excellent sources of dietary potassium intake. You probably get most of your sodium and chloride in the form of table salt, NaCl. Other trace sources of chloride are olives, tomatoes, and celery, and sodium can be found in barley and beets. In addition to these three main players, you also have some special other ions inside your cells. Actually, these ions are found inside and outside of your neurons, but a dominant concentration is maintained in your cells by cellular pumps.

The cellular pump (or exchanger) is usually referred to by its biochemical name, Na-K ATPase. This cellular pump works in a similar way to a revolving door that helps move people inside and outside of a building. Put the potassium ions in the crowd moving in and sodium in the crowd moving out and you get the basic picture, with one twist. The Na-K ATPase revolving door doesn't create equal opportunity openings. For every two K+ ions that are pumped in three Na+ are pumped out. As a result, at rest, sodium and chloride ions are much more highly concentrated outside your cells, while potassium is more highly concentrated inside. Because ions have either a positive or negative charge and they aren't evenly distributed across the cell membrane, you wind up with an electrical difference between the inside and outside of the neuron. This is called the “membrane potential.”

A way to appreciate this potential difference is to think about a physical example. Imagine a tub of water. Now take a glass and put it into the water upside down so there is air trapped inside it. As you push it down into the tub, the glass will have some water enter it as
the water pressure overcomes the air pressure in the glass. This is similar to the way that the membrane potential increases and decreases across your neurons. This changing level signals information flow in the nervous system. All cells have resting membrane potential differences, but it is only excitable cells in nerve and muscle that can generate changes in membrane potential and transmit those changes as information.

The spinal cord is highly organized. Sensory innervation of—or bringing nerves to—the skin on different parts of the body corresponds with different levels of the spinal cord. The relationship between nerves in the spine and what are called “dermatomes” is shown in
figure 2.2
. The organization of the dermatomes is typically thought of using the four regions of the spine and spinal cord. From head to “tail” these are cervical, thoracic, lumbar, and sacral. This concept of dermatomes is useful for diagnosis in clinical neurology. A good (bad?) example is with back injuries. If you or someone you know has had a disc herniation, they likely had some changes in how sensory information from the skin on the legs was relayed. For example, about 20 years ago I had three herniated lumbar discs at L3-L4, L4-L5, and L5-S1. As a nice reminder, I now have patchy sensation on the skin of my lower legs associated with the dermatomes for those spinal levels.

Activity in the nervous system leads to the activation of muscle. If Tony Stark decides he wants to pick up a laser-guided tool of some kind—maybe an arc welder—to make a modification to the Iron Man armor, a command will arrive at the motor cortex of the brain. This is the main movement-control area of his brain and the place where the commands that are sent down the spinal cord to trigger muscle contraction come from.

There is also the issue of motor innervations to “motoneurons”—short for motor neurons—for different muscles in the body. The motoneurons are also organized at anatomical levels. For example, when the signal descends to the part of the spinal cord where the nerve cells for the muscles involved in reaching are found, the cervical region in this case, activation of those cells commands the muscles to become active. For biological systems like your body, the lowest level of control for generating force is using what is called a “motor unit.” This unit is the nerve cell in the spinal cord and all the muscle cells (or fibers) that it connects with. So, when a muscle contraction occurs, it results from activity in many muscle fibers. The muscle fibers have proteins within them that regulate contraction and also produce the actual contraction forces. In
figure 2.3
, there is an example of a motor unit showing just a few muscle fibers. The motoneuron is at the top of the figure showing the cell body and dendritic tree (the filament-like bits at the top) in the spinal cord with the axon shown extending out to the muscle. The things that look like hot dog buns on the outside of the axon are the sheaths of fatty insulation called “myelin” that help keep signals moving quickly.

Figure 2.2. Dermatomes are areas of skin that are innervated—literally, supplied with nerves—by the sensory fibers from nerve roots in the spinal cord. Note that each dermatome is named according to the spinal nerve supplying it. Although there are seven cervical vertebrae, there are eight cervical dermatomes. Courtesy Ralf Stephan.

Figure 2.3. A motor unit—the basic functional unit for movement—consists of neurons (cell bodies) in the spinal cord, along with the extension of the axon out to the fibers in the muscle. Courtesy Johannes Noth (1992).

Once the electrical signal from a motoneuron arrives at the synapse (called the neuromuscular junction), that signal becomes chemical. The term “synapse” was first used by Sir Charles Sherrington, a Nobel Prize–winning physiologist (and, as readers of
Becoming Batman
already know, one of my superheroes of science). At the synapse for the neuromuscular junction, the chemical neurotransmitter acetylcholine is released, crosses the gap, and leads to depolarization of the muscle fibers. This turns the signal into an electrical one again and causes the release of calcium ions that serve as triggers for muscle contraction.

Muscle fibers are composed of different proteins. Some of these proteins have a direct role in producing muscle force and are called “contractile proteins.” Others have an indirect role in regulating contraction. The contractile proteins are actin and myosin molecules. Motor units come in “sizes,” which means that the number of muscle fibers controlled by each motoneuron differs. But the number is variable in a logical way and can range from about ten muscle fibers for one motoneuron up to thousands of muscle fibers. Keep in mind that when a motoneuron is commanded to be active, all the innervated muscle fibers must also be active. Your motor units (or at least the muscle fibers innervated by them) are real team players! This means that a small motor unit with ten fibers will produce less force than a large unit with a hundred fibers. Not surprisingly then, we find that the motor units controlling the movement of your eye (which has a very small mass) are the smaller units and those controlling your much larger (and heavier) leg muscles are the larger units.

If Tony needs to push more forcefully with a handheld laser—that is, he wants to change force production in his muscles—his motor commands will make more motor units become active (called “recruitment”) or the motor units that are already going will be active at a higher frequency. Both of these things happen more or less at the same time, except motor units are recruited and then activated at a higher rate. Then more units are recruited and made to discharge higher and so on. Meanwhile, the force of contraction will be steadily increased. This is something you wouldn't notice at all, but it is a great example of matching between the output of the nervous system and the mechanical ability of your muscles to produce force.

Tony Stark's motor units come in two basic types depending on how fast they contract (or “twitch”) and how fatigable they are (how long they can keep contracting before having to stop). These two main types are called, rather unimaginatively, type I and type II or “slow twitch” and “fast twitch.” The fastest twitch and most forceful units are the type II. The slightly less strong and slower twitch units are the type I. When Tony is manipulating that laser, he would be bringing into activity more and more units at different frequencies. If he had to hold it for a prolonged time, he would begin to experience fatigue. His muscles would start to ache from the pain detected by the metabolic processes occurring and his arm might begin to shake or appear to vibrate a bit. This is called “physiological tremor” and wouldn't be particularly helpful if accuracy were needed. So, he would have to rest a bit between his efforts.

Your muscles are a really efficient kind of biological motor. If we were to compare them with real technological motors, it would probably be most useful to compare power output based on weight. According to Steven Vogel and his work on human muscle, your skeletal muscle produces about 200 watts per kilogram (90 watts per pound). The steam pump, debuting during the industrial revolution in 1712 in the hands of Thomas Newcomen and refined by James Watt in 1775, produced just over half of that at about 50 watts. That doesn't quite match 200 or 500 watts per pound for a car or motorcycle engine, but it still is pretty good. By the way, a jet aircraft turbine clocks in at approximately 2,500 watts per pound. All in all, for a squishy bit of biological material, our muscles do pretty well.

Probably the main thing you think about concerning your muscles is how much force they produce. You might be surprised to learn that there are a number of oddities that occur during muscle activation and force production that introduce a few wrinkles into what we are able to do and how we (unconsciously) do it. Imagine a forklift tractor with the loader on the front. The hydraulic piston that helps raise and lower the load on the tractor behaves in the same way each time it is used. This is a highly linear system, and it is tempting to think of muscles as working in a similar way. But, muscles operate in a nonlinear system and have some peculiarities. The force that your muscles produce depends on the length of the muscle fibers and the speed at which the muscle is contracting. The details of the relationship can be a bit complex but, generally, the faster a contraction is occurring, the less force that can be produced. During a slower
contraction, more force can be produced. This is called the “force-velocity relation.” You can flip this around into a load-velocity relation and think instead about how fast you could contract arm muscles to move your arm when holding a light weight versus holding a heavy weight. The lower the weight, the faster you can move, since, as we've noted, less force is needed to move a lighter weight. All of this applies to what are called “shortening contractions,” which are when muscles are active and are shortening (also called “concentric actions”).

If you are sitting down right now, you could do a shortening contraction with your knee extensor muscles (your “quads,” or quadriceps, on the front of your upper leg) by raising your leg until your foot is parallel with your hip. If you hold it out at complete knee extension, you are performing a constant length, or “isometric,” contraction. When you slowly bring your foot back down and let your knee extensors relax, you are performing a lengthening, or “eccentric,” contraction. A lengthening contraction produces the highest forces, and this force goes up the faster you move.

During everyday tasks, your muscles are constantly doing both shortening and lengthening contractions. For example, when you go up stairs, the knee extensor muscles are doing a lot of shortening actions. When you come down, they perform lengthening contractions. But your muscles are about 30% more efficient going downstairs than they are going upstairs. The actual force that moves those bones comes from the contractile parts of the muscle fibers as well as the connective tissue that holds the fibers and the muscle as a whole and keeps everything connected to the tendons. All of those pieces together have some elasticity, and it is because muscles use this elasticity when they contract while stretched that makes them more efficient. Elasticity in muscle and tendon has important implications for moving Tony Stark and his Iron Man suit around.