Motivation

Those processes responsible for the goal-directed quality of behavior are the motivations, or "drives" for that behavior. Motivation can lead to hormonal, autonomic, and behavioral responses. Primary motivated behavior is behavior related directly to homeostasis—that is, the maintenance of a relatively stable internal environment, an example being getting something to drink when one is thirsty. In such homeostatic goal-directed behavior, specific body "needs" are satisfied. Thus, in our example the correlate of need is a drop in body water concentration, and the correlate of need satisfaction is the return of the body water concentration to normal. The neurophysiological integration of much homeostatic goal-directed behavior will be discussed later (thirst and drinking, Chapter 16; food intake and temperature regulation, Chapter 18).

In many kinds of behavior, however, the relation between the behavior and the primary goal is indirect. For example, the selection of a particular drink—water or soda pop, for example—has little if any apparent relation to homeostasis. The motivation in this case is called secondary. Much of human behavior fits into this latter category and is influenced by habit, learning, intellect, and emotions—factors that can be lumped together under the term "incentives." Sometimes the primary homeostatic goals and secondary goals conflict as, for example, during a religious fast.

The concepts of reward and punishment are inseparable from motivation. Rewards are things that organisms work for or things that make the behavior that leads to them occur more often—in other words, positive reinforcers—and punishments are the opposite.

The neural system subserving reward and punishment is part of the reticular activating system, which you will recall arises in the brainstem and comprises several components. The component involved in motivation is known as the mesolimbic dopamine pathway—meso- because it arises in the midbrain (mesencephalon) area of the brainstem; limbic because it goes to areas of the limbic system, such as the pre-frontal cortex, the nucleus accumbens, and the under-surface of the frontal lobe (Figure 13-9); and dopamine because its fibers release the neurotransmitter dopamine. The mesolimbic dopamine pathway is implicated in evaluating the availability of incentives and reinforcers (asking, "Is it worth it?", for example) and translating the evaluation into action. (We shall meet the mesolimbic dopamine pathway again later when we discuss drug dependence.)

Prefrontal cortex

Nucleus accumbens

Brainstem nuclei

Brainstem nuclei

Nucleus Locus Ceruleus

FIGURE 13-9

Schematic drawing of the mesolimbic dopamine pathway.

Midbrain nuclei -Locus ceruleus -

FIGURE 13-9

Schematic drawing of the mesolimbic dopamine pathway.

Much of the available information concerning the neural substrates of motivation has been obtained by studying behavioral responses of animals and, in some cases, people to rewarding or punishing stimuli. One way in which this can be done is by using the technique of brain self-stimulation. In this technique, an unanesthetized experimental animal (or a human undergoing neurosurgery for other reasons) regulates the rate at which electric stimuli are delivered through electrodes implanted in discrete brain areas. The experimental animal is placed in a box containing a lever it can press (Figure 13-10). If no stimulus is delivered to the brain when the bar is pressed, the animal usually presses it occasionally at random.

If, in contrast, a stimulus is delivered to the brain as a result of a bar press, a different behavior occurs, depending on the location of the electrodes. If the animal increases the bar-pressing rate above the level of random presses, the electric stimulus is by definition rewarding. If the animal decreases the press rate below the random level, the stimulus is punishing.

Thus, the rate of bar pressing with the electrode in different brain areas is taken to be a measure of the effectiveness of the reward or punishment. Different pressing rates are found for different brain regions.

Scientists expected the hypothalamus to play a role in motivation because the neural centers for the regulation of eating, drinking, temperature control, and sexual behavior are there (Chapter 8). Indeed, they found that brain self-stimulation of the lateral regions of the hypothalamus serves as a positive reward. Animals with electrodes in these areas have been known to bar-press to stimulate their brains 2000 times per

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Consciousness and Behavior CHAPTER THIRTEEN

Consciousness and Behavior CHAPTER THIRTEEN

Lateral Hypothalamus Reward

FIGURE 13-10

Apparatus for self-stimulation experiments.

Adapted from Olds.

FIGURE 13-10

Apparatus for self-stimulation experiments.

Adapted from Olds.

hour continuously for 24 h until they drop from exhaustion! In fact, electric stimulation of the lateral hypothalamus is more rewarding than external rewards in that hungry rats, for example, often ignore available food for the sake of electrically stimulating their brains at that location.

Although the rewarding sites—particularly those for primary motivated behavior—are more densely packed in the lateral hypothalamus than anywhere else in the brain, self-stimulation can be obtained from a large number of brain areas. Thus, while the hypothalamus coordinates the sequenced hormonal, auto-nomic, and behavioral responses of motivated behaviors, motivated behaviors based on learning involve additional integrative centers, including the cortex and limbic system. Motivated behaviors also use integrating centers in the midbrain, brainstem, and spinal cord—in other words, all levels of the nervous system can be involved.

Chemical Mediators Dopamine is a major neuro-transmitter in the pathway that mediates the brain reward systems and motivation. For this reason, drugs that increase synaptic activity in the dopamine pathways—amphetamine, for example, which increases the presynaptic release of dopamine and other bio-genic amines—increase self-stimulation rates—that is, provide positive reinforcement. Conversely, drugs such as chlorpromazine, an antipsychotic agent that blocks dopamine receptors and lowers activity in the catecholamine pathways, are negatively reinforcing. The catecholamines are also, as we shall see, implicated in the pathways subserving learning. This is not unexpected since rewards and punishments are believed to constitute incentives for learning.

Emotional behavior can be studied more easily than the anatomical systems or inner emotions because it includes responses that can be measured externally (in terms of behavior). For example, stimulation of certain regions of the lateral hypothalamus causes an experimental animal to arch its back, puff out the fur on its tail, hiss, snarl, bare its claws and teeth, flatten its ears, and strike. Simultaneously, its heart rate, blood pressure, respiration, salivation, and concentrations of plasma epinephrine and fatty acids all increase. Clearly, this behavior typifies that of an enraged or threatened animal. Moreover, the animal's behavior can be changed from savage to docile and back again simply by stimulating different areas of the limbic system, such as parts of the amygdala.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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