10. Regulation of water balance

Hypothalamus regulates water content of the body by two mechanisms:

1. Thirst mechanism

2. ADH mechanism

1. Thirst center is in the lateral nucleus of hypothalamus. There are some osmoreceptors in the areas adjacent to this nucleus. When the blood circulating volume decreases, the osmolality of blood increases. This stimulates the osmoreceptors which, in turn, activate the thirst center. Now, the person feels thirsty and drinks water. Water intake increases volume and decreases osmolality of blood.

2. Simultaneously, when the volume of blood decreases with increased osmolality, the supraoptic nucleus is stimulated and ADH is released. ADH causes retention of water by reabsorption in the renal tubules. This brings the osmolality back to the normal level. When fluid volume is more, the supraoptic nucleus is not stimulated and ADH is not secreted. In the absence of ADH, more amount of water is excreted through urine and the volume of blood is brought back to normal.

11. Regulation of sleep and wakefulness

Stimulation of anterior hypothalamus or the lesion of mamillary body leads to sleep and the stimulation of mamillary body causes wakefulness. Mamillary body in the posterior hypothalamus is considered as the wakefulness center.

12. Role in behavior and emotional changes

The behavior of animals and human beings is mostly affected by two opposing responding systems, which involves hypothalamus and other structures of limbic system.

These responding systems are concerned with affective nature of sensations, i.e. whether the sensations are pleasant or painful. These two qualities are called the Reward/Satisfaction and Punishment/Aversion or avoidance. The two centers in hypothalamus involved in the behavior and' emotional' changes are:

A. Reward center and

B. Punishment center.

Reward Center

Electrical stimulation of medial forebrain bundle and ventromedial nucleus of hypothalamus in animals, pleases or satisfies the animals. These areas are called reward centers.

Punishment Center

The electrical stimulation of posterior and lateral nuclei of hypothalamus leads to pain, fear, defense, escape reactions and other elements of punishment. These centers are called punishment centers.

Role of Reward and Punishment Centers

The importance of the reward and punishment centers lies in the behavioral pattern of the individuals. Almost all the activities of day-to-day life depend upon reward and punishment. While doing something, if the person is rewarded or feels satisfied, he or she continues to do so. If the person feels punished or unpleasant, he or she stops doing so. Thus, these two centers play an important role in the development of the behavioral pattern of a person.

Rage

When the punishment centers in posterior and lateral hypothalamus are stimulated in animals, a violent and aggressive emotional state is caused. This is called rage. Following are the reactions of rage:

1. Development of a defense posture

2. Extension of limbs

3. Lifting of tail

4. Hissing and spitting

5. Piloerection

6. Wide opening of eyeballs

7. Dilation of pupil

8. Severe savage attack, even by mild provocation.

Sham Rage

In physiological conditions, the animals and human beings maintain a balance between the rage and the opposite state called the calm emotional state. Major irritations make even a normal person to loose the temper. However, the minor irritation are usually overcome or ignored. This is because of inhibitory influence of cerebral cortex on hypothalamus. But in animals or human beings with brain lesions, the balance is altered. In some lesions, even the mild stimulus evokes a violent and angry reactions of rage. This type of rage is called sham rage. This can occur in decorticated animal also.

Sham rage is due to release of hypothalamus from the inhibitory influence of cortical control.