Popular physiology III: Moving around
Animals move; plants (usually) do not. Movements, both internal and external, characterise animal physiology. Movements of all kinds are brought about by muscles; the very act of reading this paragraph involves your eye muscles, both oustide the eye (who obey your every command), and inside the eye (i.e. your pupil, which is not really under your wilful control). You can pick up a samosa with your hands, put it inside your mouth, chew it for a while, and then gulp it down your throat. That’s as far as your will goes. After that things move on their own. At least you hope they do, or else the samosa gets stuck in your esophagus, and only a quick visit to the emergency room can save you. The automatic, involuntary movements of our gut, our heart and our urinary tract (yes, the kidneys, ureters and bladder) keep us up and running. Consider the alternative: what if you had to plan the movement of the said samosa through your gastro-intestinal tract? Most would have given up long ago. In fact, if such vital bodily functions were left to the brain and its ‘free will’ (shivers), evolution wouldn’t have progressed beyond the simplest one celled organisms.
Voluntary movements
‘Voluntary’ as in movements of your own free will — walking, talking, pestering around, running for your life, picking up a samosa, making a commotion and the whatnot. As Newton would have put it, you as a whole can’t move in a straight line unless someone pushes you. You can however, move around parts of your body in rotating arcs. Flex your biceps and see your forearm rotate about your elbow. Turn your head, both sideways and up-down. Such rotations are the only things physics permits.
From the diagram, it is obvious that to flex your elbow (i.e. bring your hand near your mouth) the biceps muscle needs to pull. What is not obvious is that for such a movement to happen, the triceps muscle (the one on the extreme left) has to let go, i.e. relax. If the two muscles are not exactly coordinated, both pull at the same bone and nothing happens except for a painful cramp.
Such ‘voluntary’ muscles are often called ‘skeletal’ muscles, because most of them are attached to bones. Skeletal muscles can sometimes squeeze on their own, without your order — just so you know that they are living things on their own right. If you have waken up at midnight with a cramped ankle, you know one of your muscles is acting up.
On microscopy, skeletal muscles show a breathtakingly stunning geometry of actin (‘rays’) and myosin (‘muscle’) proteins, ordered in overlapping fascicles (bunches or aggregates). These fascicles give them a striated (‘lines’) look.
The way our brain exerts control over these muscle is — wires. Nerves. Nerves come out of the brain and spinal cord, divide into finer and finer wires until each muscle cell is directly under control of the brain. Once you ‘wish’ to make a movement, the impulse originates deep inside your brain, and electrical signals travel to the concerned nerve, which then releases a molecule, ‘acetyl-choline’ into the muscle. The effect is an immediate twitch — contraction — of the muscle. If your ‘will’ is very short lived, so is the contraction. However, if your ‘will’ sustains, the muscle fibers contract one by one until finally the entire thing — the muscle and its attached bone — moves.
When you think you are ‘resting’, i.e. sitting on a chair, your brain is releasing neurotransmitters — ‘commands’ — to each group of muscles in your body, to maintain some degree of tension (‘tone’) even at rest. Your back muscles are holding your head up. If your forearm is on the desk, it is because both biceps and triceps have equal tone. Once you wish to lift your forearm, the tone of the biceps will increase, with a concurrent reduction of tone in triceps. These adjustments are made by the brain by titrating the amount of aceyl-choline received by each muscle group.
Making a movement — any movement — then, requires fine control of several different muscles, learning to pull at one and relax another at the same time. The precise coordination requires a while to learn: the infant can pick up a marble with two fingers (‘pincer grasp’) only after 9 months of birth, and can stand up properly at one year of age. Walking takes still more. Finer movements, like controlling the vocal cord to produce syllables — come only at one and a half years.
The skeletal muscles in your ribs, and a large muscle separating your chest and abdomen (the diaphragm) are your ‘respiratory muscles’. If you pay attention, you can feel them move with every breath. Now, respiration is too important to be left entirely to ‘free will’ (one might forget to breathe!). Thus, these muscles are under dual control, both voluntary (through cerebral cortex) and automatic (from brainstem). You can hold your breath, but not for long — your brainstem reflexes will eventually kick in and force you to breathe.
Another set of muscles, although they look and feel like voluntary muscles, are neither voluntary nor skeletal. It’s your heart, which is made entirely of muscle, and completely out of your wilful control. Together, maintaining the rhythms of the heart and lungs is the core function of the brainstem, and one only dies once and for all when the brainstem ceases to function.
Involuntary movements
We feel involuntary movements all the time, with every burp, every hiccup, every sneeze, every yawn, every rumbling in our bellies (‘peristalsis’), every fart (‘flatus’), every visit to the toilet. In fact, is you ask a random person an innocuous ‘How are your movements today?’ — he might take offense at such a personal question.
The most obvious movements are that of the gut. The food you eat must be digested, absorbed and finally ejected from your aboral end. This requires an complex dance of sorts, between your stomach, intestine, bile duct, colon (and rectum of course). With all this movement inside, it certainly seems at times that the gut has a mind of its own, and sure it does. The enteric nervous system (ENS) is older (in evolutionary scale) than your brain, and just as massive. Its sheer scale and complexity makes it difficult to grasp. We can feel only some of its gross actions, like peristalsis (that rumbling in your belly after a heavy meal), vomiting¹ (reverse perstalsis) and defecation (moving the colon and rectum in consort). Occasionally, the intestine will squeeze at a particular segment, and you get a cramp. When the intestine stops moving entirely, it’s a sign something has gone terribly wrong (i.e. some organ has burst inside, maybe the appendix), and it’s time for the emergency room again.
Similar movements control the flow of urine from kidney to bladder and then finally out of the urethra. Both the rectum and the bladder, the two outlets of the body, have some degree of voluntary control — i.e. you can hold your urine and stool for a while (only for a while though). This voluntary control is exerted through skeletal muscles, ‘sphincters’ around the bladder and rectum, who are under control of the brain. (Interestingly, you can not loosen one sphincter without also loosening the other — because the same nerves cover the two. Bladder and bowel must evacuate together. The only reason we are spared the inconvenience of making a mess everytime we visit the loo is because our rectums are empty.)
Similar movements occur deep inside your lungs, to regulate the amount of air entry; your bronchioles (the finer divisions of the trachea which go deep into your lungs) might contract in response to very cold air, or some dust particle you just inhaled. Sometimes they contract without rhyme or reason, resulting in fits of breathlessness — asthma — that are triggered by harmless particles in air.
The movements of the gut, urinary tract and respiratory tract are brought about by muscle fibers which are incorporated in their very walls. They are not skeletal, neither are they ‘voluntary’ in the usual sense of the word. You can’t move your stomach the way you can move your hand. Unlike skeletal muscles, these muscles in the gut lack ‘striations’, and are thus called (very un-originally) smooth muscles. Again, unlike skeletal muscles, smooth muscles are not direcly overseen by the brain, or by anyone. They respond to a diverse array of stimuli — including nerves, hormones, food, dust, bacteria and a hundred other things. This makes sense. Unlike your ankle, your gut faces a hundred different things coming down from your esophagus, every day. So does your lungs. The responses of the smooth muscles in these organs must be fine tuned to deal with all that stuff coming in, and going out. Smooth muscles keep your gut and your lungs and your bladder running, albeit in a messy — but effective — manner, and that’s more than one can ask of these tiny little fibres.
- Why do we vomit? Vomiting after being poisoned makes evolutionary sense: the stomach is trying to send the poison out the same way it came in. But what about motion sickness? Or vomiting in disgust? Motion sickness is easier to understand. Humans have technology, which means we can move around fast, really fast, in our cars and planes. However, the body was designed millions of years ago, before cars and planes. Our brains are used to walking and running; any faster and objects around us start whooshing by in a swirl of images. The brain, being the millennia old model that it is, can not figure this plethora of moving images and comes up with the only plausible explanation: that you have been poisoned and hallucinating. And thus vomiting.
