Prof. Muni Mahendra Kumar
0. Hunger Drive (Āhāra Saṁjña)
We have seen that all living beings from one-sensed organisms to humans are possessed of several primordial drives (Saṁjñās) as essential aids for survival. The first of these is the hunger-drive (āhāra Saṁjña) which urges the organism to go in search for food and prompts it to take nourishment.
The primal drives are subtle impulsive forces which originate in the micro-body (karmana śarīra). Āhāra Saṁjña seems to be produced by the combined fruition of several sub-species of three aghāti karman viz. feeling-producing (vedanīya), body-making (nāma) and life-span-determining (āyuṣya) karman. The hunger-drive is transmitted to and transformed in the gross physical body as desire (need) for food. We shall see how this desire affects the brain and stimulates it to take action that satisfies the need. Each human activity starts from a definite intention, directed at a definite goal and is regulated by a definite program, which requires that constant cortical tone is maintained. Study of the programs that regulate such primitive needs of the body as eating and drinking provide a basis for examining the higher ones. Let us, therefore, begin by examining the systems that ensure the genesis of the basic needs whose fulfillment ensures the continued existence of the body.
1. Nourishment
The intake of food and drink is at the very centre of all activities for survival and the amount that we eat and drink is very precisely regulated. During a lifetime, each of us consumes several tons of food and thousands of liters of liquid, yet, once adult, we remain of about the same size and weight within limits of a few kgs. Where is the reference standards by which the intake is regulated to produce this remarkable result?
Ordinary individual eats and drinks what he needs, with a rhythm of regular times and if his habits are interrupted, he feels hungry or thirsty. The empty stomach gives waves of contraction and we all vaguely feel hunger in our stomachs. Even more definitely the throat is dry when we are thirsty. But these simple visceral feelings are not the only sources of the nerve signals that set us off eating and drinking. It is the function of nerve-cells in certain regions of the brain, in the hypothalamus, which are programmed to send signals either in a regular pattern of habits or when the composition of the blood indicated that there is a need for food or water. The operations of these parts of the brain, together with those of the stomach, are accompanied by feelings of hunger or thirst.
During an experiment on the control of drinking, Dr. Andersson had previously inserted, under anesthetic, a fine tube into the hypothalamus of the brain of a goat. Whenever a tiny drop of strong salt solution was sent down the tube the goat dipped its head into a bowl of water in front of it and began to drink. If a dilute solution was sent down to the brain the drinking stopped at once. The minute injections were imitating the condition in which when the blood becomes more concentrated, the goat is thirsty, and the blood then becomes diluted when the animal drinks. These nerve-cells also regulate the amount of water in the body.
Besides controlling the intake of water, it controls the amount of water that is let out, by increasing or decreasing the amount of a hormone - ADH (anti-diuretic-hormone) - sent from the pituitary gland to the kidney, which stops secretion of the urine. The hypothalamus thus controls both the intake and output of water.
2. Central Regulator
Thus particular nerve-cells in the hypothalamus serve as the central regulator of the amount of water in the body. They do this by actually swelling or shrinking when the salt-concentration of the blood changes. The setting of this "homeostat" has been made by heredity and is modified by the subsequent physiological experience of the individuals. The amount of food intake is also regulated from the same region. The lateral areas provide the drive to start eating and the medial ones to stop it.
The stimulus that normally sets these cells into action is partly the composition of the blood. Some of the nerve-cells here take up sugar from the blood and they send out signals when there is not enough of it there. These cells provide the homeostat's reference standard for sugar. Probably other cells are sensitive to other substances, but many details of the system are still not known. Most of our eating and drinking follows acquired rhythmic programs and control of them is a complicated business, influenced by many higher levels of the brain. One of these centers lying just above the hypothalamus is known as the amygdala, from its shape like a nut.
The influence of the cerebral cortex on hunger are still more complicated. We all form habits as to when to eat, and also how much. People can change their habits according to personal and social demands. We cannot pretend that we fully understand even the simpler controllers of the needs for food. Nevertheless in the hypothalamus there are those lateral centers that become operative. When we are hungry, they give us appetite. The medial centers send messages when appetite has been assuaged and we are satisfied. The two areas are interconnected and inhibit each other, corresponding, we may suppose, with the mutual interplay of appetite and consummation.
3. Artificial Stimulation of the Brain
Feelings of satisfaction can also be produced by artificial stimulation of some of these very areas, injury to which causes error of eating, drinking, and sexual and other behavior. With the electrodes in some positions, the shocks have rewarding effects and are sought for as if they were satisfying.
Similar observations have been made during stimulation of the brain in therapeutic procedures designed to alleviate the pains of dying patients. Stimulation by electrodes in some regions was reported to give great reduction in pains and sometimes pleasurable feeling*, including those of sexual satisfaction.
Experiments by electrical stimulation show that there are some areas of the brain whose activity produces drives to action, others that indicate satiation. Areas where stimulation produces very active responses may be quite close to others that produce the opposite effects. These are intricate networks of millions of distinct nerve-cells, with many different properties and connections. We are able to reproduce the normal actions of the brain very crudely. Thus, the operations of the whole system determine not only how actions are normally related to bodily needs but also, in man, how the rewards are influenced by the activities of certain brain-areas and the chemical substances contained by them. These areas are responsible for the quality of affection, pleasure, satisfaction or the reverse.
These reward - centers are influenced by signals coming both from outside the body and within it, including, for example, those from the nose and from the taste-buds of the tongue. A specially interesting pathway begins in the cells of the locus coeruleus, lying near the taste-centers of the medulla oblongata showing some relationship between the sensation of taste and the drive to search for food.
4. Selection by Instinct
In ancient times, both men and animal had the instinct to guide them to find whatever they needed to keep them healthy and to detect and reject the poisonous and unhealthy. They made special efforts to meet particular deficiencies. Children, for example, deficient in calcium, will eat plaster. Some of these capacities for selection of taking what is needed and rejecting what is harmful are certainly innate while others are learned. Unfortunately, while the animals have retained the capacity, humans, with progress of civilization, gradually lost the instinct to a great extent.
