How the Brain Controls Movement and Coordination

When we look at a human brain today, almost all we can see are the two halves of the cerebrum: the part of the brain which we use for thinking, learning, communicating, deciding, imagining, and just about everything else which makes us human. The other, older parts of the brain are still there, but the cerebrum is so large that it has expanded to spread all over the rest. And since it’s the outer skin, or cortex, of the cerebrum which does most of the work, the cerebrum has become folded and wrinkled, so we can fit more surface area into the space.

If we look at a cross-section of the spinal cord, we can see that it really is a tube: it has a hollow in the middle that is filled with a nutrient fluid. That hollow is surrounded by what we call grey matter, which is mostly made up of nerve cell bodies, and the grey matter is surrounded by white matter, which is nerve fibers carrying information to and from the brain.

The spinal cord is the main route for information going between the body and the brain. That’s why people who experience damage to their spinal cord can become paralyzed. Their brains may be trying to get their muscles to move, but they simply cannot get the instructions through.

The spinal cord also controls some of our reflexes – the rapid muscle movements that happen in response to painful stimuli. This is what happens, for example, if you pull your hand away from a hot surface. You do it quickly, without thinking, because the message of heat and pain only needs to go as far as the spinal cord. When it gets there the message is instantly routed to the nerve cells, which tell your arm to move your hand away. It doesn’t need to go all the way up to the brain. That’s known as a reflex and, because it’s such a basic survival mechanism, it’s controlled by the oldest part of the nervous system.

The top of the spinal cord thickens out and begins to become part of the brain itself. The part where it thickens is known as the medulla, and if we think about it as the next part of the nervous system to evolve, we can see how it, too, is concerned with basic functioning. The medulla is the part of the brain that regulates basic bodily functions, such as breathing, swallowing, digestion and heartbeat – essential functions for all animals apart from the very simplest ones.

Moving upwards from the medulla, we find that the brainstem becomes even thicker, turning into what is known as the midbrain, which is really a collection of several different parts. One of them is the reticular activating system (RAS), which regulates different states of alertness: sleep, wakefulness and attention. In humans and complex mammals, the RAS seems to be able to ‘switch on’ large areas of the cerebral cortex, so we are alert and paying attention to what is around us. It has some sensory pathways and many connections with other areas of the brain.

As the brain evolved, animals were also becoming more sophisticated in how they moved. Another part of the midbrain, the pons, is the main route for connections between the body and the cerebellum, which is the co-ordinating centre for smooth movement. The pons is also involved in dreaming sleep, in animals as well as humans, and it is thought that this might have evolved to help the animal to form the neural pathways needed for smooth movement.

When we see dogs dreaming, for example, it’s evident that they are running or chasing something, which could possibly link with practicing physical skills.

When we are first learning a new skill, our movements are often jerky and a bit clumsy, because we have to think about each movement consciously. But as we practise those movements, control of those sequences of actions moves to the cerebellum, and those movements become smooth and automatic, so we don’t have to think about doing them. The cerebellum doesn’t plan deliberate movement – that’s done by the cerebrum – but it makes sure that our actions are coordinated, precise and accurately timed.

The thalamus receives information from the sensory nerves and from our eyes and ears, and does a certain amount of decoding of those signals before passing the information on to the cerebrum. It also receives the instructions about movement passed down from the cerebrum and sends those instructions on to our muscles. Like several other subcortical structures, it’s involved in sleep and wakefulness as well – those states seem to affect large areas of the brain in a general way rather than being tightly controlled by just one area.

A small ‘lump’ immediately below the thalamus, known as the hypothalamus, is especially important to mammals because it regulates body temperature. It’s the ability to keep our own internal body temperature constant that allows us to be active at night or in cold places. It is also the reason why small mammals can live in underground burrows, which is a possible explanation for how they were able to survive the massive impact cataclysm that finished off the dinosaurs.

The hypothalamus does much more than just regulate temperature, though; it maintains homeostasis throughout the body. Maintaining homeostasis means keeping everything in a steady, comfortable condition. So if your body’s fluid levels fall below what is optimal for your survival, the hypothalamus will initiate feelings of thirst leading you to drink; if your body’s blood glucose levels fall below a certain level, it will initiate hunger, leading you to seek food.

The hypothalamus sends its signals partly by nerve cell (neural). connections, but partly also by releasing hormones. Hormones are chemicals that either stimulate body processes or cause other hormones to be released by other glands in the body. Hormones are particularly important for maintaining ‘states’ such as growth, pregnancy, arousal or anxiety. Together, the hormone-releasing glands form the endocrine system of the body, and the hypothalamus is the brain’s main route for connecting the brain with the endocrine system.

Source : Your Brain and You: A Simple Guide to Neuropsychology by Nicky Hayes

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