Neuropsychology

The Memory of Sensations

The organs of sight, hearing, taste, smell and touch act as windows to the outside world as they keep the human body constantly connected with its environment, by picking up sensory signals which are rapidly processed in the human brain. The memory of select sensations sometimes last a lifetime. For example, we live in a world full of different odours ranging form deeply invigorating odours to the most disgusting ones. Can we ever forget the peculiar yet refreshing scent that fills the air just after it has rained on parched soil? The rich aroma and sight of mouth-watering foods is a pure bliss too, and so is the smell of our favourite perfume that instantly peps us up. On the other hand, the stench of putrefying garbage and bad mouth odours is nauseating.

Not just that. The human brain has the innate power to recognize and remember different odours, so much so that strange smells from age-old preserved items like pieces of cloth, books etc.., can curiously bring instant flashes of incidents that occurred long ago, places visited years back, and people who have already died. How do these smells that us down the memory lane, and revive vivid pictures in our mind of moments that were long forgotten? How the brain sorts out different odours and transmits this information into sensory signals that, in turn, affect perception and memory is today known well, thanks to two American scientists—Richard Axel, a professor at Columbia University College of Physicians and Surgeons, New York, and Linda B.Buck at the Fred Hutchinson Cancer Research Centre in Seattle—who won the Noble Prize in Physiology or Medicine for 2004 for unlocking the secrets of the working of olfactory system from the molecular level to the organization of concerned cells.

Actually, as soon as an odourous substance is inhaled through the nasal passage, it is detected by special neurons—the olfactory receptor cells—sitting in a small area in the upper part of the nasal epithelium. The olfactory epithelium contains about five million olfactory neurons, each of which has about 10 hair-like cilia which have on their surface special proteins called the odurant receptors. These cilia protrude into a thin bath of mucus that covers the olfactory neurons. The inhaled odourous chemical first binds to the olfactory receptors that activates the latter and triggers an electric signal which reaches the oldactory bulb in the brain via the nerve fibers. From the olfactory bulb, the odour signals are further relayed to the brain’s higher cortex, which handles conscious thought processes, and to the limbic system, which generates emotional feelings.

Axel and Buck have discovered a large gene family, comprising some 1000 different genes that encode odorant receptors, the presence of which gives us the ability to discriminate about one lakh odours! They unraveled neural pathways that begin with say, a whiff of our favorite dish and end up with an uncontrollable response like salivation. Interestingly, numerous combinations of receptor inputs are generated from the few olfactory receptors expressed in the olfactory neurons that help in recognizing thousands of different odours. So although human beings have only about 350 functional olfactory receptors, we can easily distinguish thousands of odours by this mix-and-match system.

Brain Signals in Hunger Control

Controlling appetite is indeed a wonderful way of reducing body weight. Amphetamines help in doing so as they boost the activity of some neurotransmitters like dopamine and noradrenaline, which have been linked with the intake of food as they lessen the hunger. However, use of amphetamines is highly restricted as they have mood creating properties and people may easily get addicted to them. A new generation of safer amphetamine- like drugs have been developed like dexfenfluramine. These drugs are not addictive and instead of suppressing the appetite, they create a feeling of having eaten well. But sadly, these appetite controlling drugs lead to depression and also pulmonary high blood pressure.

Another neurotransmitter called Neuropeptide Y (NPY) is well known to be important in food intake. It is produced in a part of the brain called the hypothalamus. Administration of NPY is laboratory animals increases their food intake leading to obesity. Scientists are working to develop drugs which could switch off the NPY appetite signals in the brain, thus reducing the food intake in humans. Nevertheless, indiscriminate and over-use of slimming drugs should be strictly avoided. Casual dieters who simply want to slip into clothes they wore in their teens must be particularly cautious as health risks of taking these drugs far outnumber their cosmetic benefits. All that is needed is a readjustment of lifestyle, which means to eat wisely while doing regular exercise and the key to this surely lies in our own hands.

The neuronal wiring that occurs in our brains is unique in us all. Different brain circuits are known to play a specific role that is reflected in the way each one of us think and act in different situations of our daily lives. The hallmark of such intricate neuronal connections is indeed the passage of chemical signals through the wired neurons, signifying vibrant communication among them, which defines all our actions and thoughts that make us what we are.

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