Thinking Beyond Science

The nervous system is divided into two parts. The first part is the central nervous system (the brain and spinal cord), which is the command-and-control center of the system; it was thought to lack plasticity. The second part is the peripheral nervous system, which brings messages from the sense receptors to the spinal cord and brain and carries messages from the brain and spinal cord to the muscles and glands. The peripheral nervous system was long known to be plastic; if you cut a nerve in your hand, it can “regenerate” or heal itself.

Each neuron has three parts. The dendrites are treelike branches that receive input from other neurons. These dendrites lead into the cell body, which sustains the life of the cell and contains its DNA. Finally the axon is a living cable of varying lengths (from microscopic lengths in the brain, to some that can run down to the legs and reach up to six feet long). Axons are often compared to wires because they carry electrical impulses at very high speeds (from 2 to 200 miles per hour) toward the dendrites of neighboring neurons.

A neuron can receive two kinds of signals: those that excite it and those that inhibit it. If a neuron receives enough excitatory signals from other neurons, it will fire off its own signal. When it receives enough inhibitory signals, it becomes less likely to fire. Axons don’t quite touch the neighboring dendrites. They are separated by a microscopic space called a synapse. Once an electrical signal gets to the end of the axon, it triggers the release of a chemical messenger, called a neurotransmitter, into the synapse. The chemical messenger floats over to the dendrite of the adjacent neuron, exciting or inhibiting it.

When we say that neurons “rewire” themselves, we mean that alterations occur at the synapse, strengthening and increasing, or weakening and decreasing, the number of connections between the neurons.

When a large peripheral nerve (which consists of many axons) is cut, sometimes in the process of regeneration the “wires get crossed.” When axons reattach to the axons of the wrong nerve, the person may experience “false localization,” so that a touch on the index finger is felt in the thumb. Scientists assumed that this false localization occurred because the regeneration process “shuffled” the nerves, sending the signal from the index finger to the brain map for the thumb.

A cochlear implant is not a hearing aid. A hearing aid amplifies sound for those who have partial hearing loss due to a partially functioning cochlea that works well enough to detect some sound.

Cochlear implants are for those who are deaf because of a profoundly damaged cochlea. The implant replaces the cochlea, transforming speech sounds into bursts of electrical impulses, which it sends to the brain.

The competitive nature of plasticity affects us all. There is an endless war of nerves going on inside each of our brains. If we stop exercising our mental skills, we do not just forget them: the brain map space for those skills is turned over to the skills we practice instead. If you ever ask yourself, “How often must I practice French, or guitar, or math to keep on top of it?” you are asking a question about competitive plasticity. You are asking how frequently you must practice one activity to make sure its brain map space is not lost to another.

When we say a brain map is organized topographically, we mean that the map is ordered as the body itself is ordered. For instance, our middle finger sits between our index finger and our ring finger. The same is true for our brain map: the map for the middle finger sits between the map for our index finger and that of our ring finger.

Topographical organization is efficient, because it means that parts of the brain that often work together are close together in the brain map, so signals don’t have to travel far in the brain itself.

When a child learns to play piano scales for the first time, he tends to use his whole upper body wrist, arm, shoulder–to play each note. Even the facial muscles tighten into a grimace. With practice the budding pianist stops using irrelevant muscles and soon uses only the correct finger to play the note. He develops a “lighter touch,” and if he becomes skillful, he develops “grace” and relaxes when he plays. This is because the child goes from using a massive number of neurons to an appropriate few, well matched to the task. This more efficient use of neurons occurs whenever we become proficient at a skill, and it explains why we don’t quickly run out of map space as we practice or add skills to our repertoire.

A major reason memory loss occurs as we age is that we have trouble registering new events in our nervous systems, because processing speed slows down, so that the accuracy, strength, and sharpness with which we perceive declines. If you can’t register something clearly, you won’t be able to remember it well.

Psychologically, middle age is often an appealing time because, all else being equal, it can be a relatively placid period compared with what has come before. Our bodies aren’t changing as they did in adolescence; we’re more likely to have a solid sense of who we are and be skilled at a career. We still regard ourselves as active, but we have a tendency to deceive ourselves into thinking that we are learning as we were before. We rarely engage in tasks in which we must focus our attention as closely as we did when we were younger, trying to learn a new vocabulary or master new skills. Such activities as reading the newspaper, practicing a profession of many years, and speaking our own language are mostly the replay of mastered skills, not learning. By the time we hit our seventies, we may not have systematically engaged the systems in the brain that regulate plasticity for fifty years.

Learning a new language in old age is so good for improving and maintaining the memory generally. Because it requires intense focus, studying a new language turns on the control system for plasticity and keeps it in good shape for laying down sharp memories of all kinds.

Source : The Brain that Changes Itself: Stories of Personal Triumph from the Frontiers of Brain Science by Norman Doidge

Goodreads : https://www.goodreads.com/book/show/570172.The_Brain_that_Changes_Itself

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