Hypothalamus

Hypothalamus. Dr: Amany Abdelrahman. thalamus hypothalamus frontal lobe olfactory bulb amygdala hippocampus.

. Hypothalamic nuclei. Paraventreular nucleus Anterior mdeus mous mOus Suvæhiasrnatic nudeus chiasrna Pituitary Antero nucleus PcsterO Lateral nucleus nucleus Mano-y (arcuate) PCBterÜ.

. Anterior or Preoptic group I. Preoptic nucleus 2. Paraventricular nucleus 3. Anterior nucleus 4. Supraoptic nucleus 5. Suprachiasmatic nucleus Middle or Tuberal group 1. Dorsomedial nucleus 2. Ventromedial nucleus 3. Lateral nucleus 4. Arcuate (tuberal) nucleus Posterior or Mamillary group 1. Posterior nucleus 2. Mamillary body.

. Limbic cortex Hippo- campus Amygdala Midbrain Frontal cortex Globus Thalamus pallidus Reticular formation Retina Afferents Efferents Anterior nucleus of thalamus HYPOTHALAMUS Reticular formation Dorsomedial nucleus of thalamus Frontal cortex Posterior pituitary FIGURE 149.2: Connections of hypothalamus.

. TABLE 149.2: Functions of hypothalamus Functions 1. Control of anterior pituitary 2. Secretion of psterior pituitary hormones 3. Control of adrenal cortex 4. Control of adrenal medulla 5. Regulation of autonomic nervous system (ANS) 6. Regulation of heart rate 7. Regulation of blood pressure 8. Regulation of body temperature 9. Regulation of hunger and intake 10. Regulation of water intake 11. Regulation of sleep and wakefulness 12. Regulation of E*havior and emotion 13. Regulation of sexual function 14. Regulation of respnse to smell 15. Role in circadian rhythm Action/Center Releasing hormones Inhibiting hormones Oxytocin Antidiuretic hormone (ADH) Corticotropin-releasing hormone (CRH) Catecholamines during emotion Sympathetic Parasympathetic Acceleration Inhibition Pressor effect Depressor effect Heat gain center Heat loss center Feding center Satiety center Thirst center Water retention by ADH Sleep Wakefulness Reward center Punishment center Sexual cycle Autonomic respnses Rhythmic changes Nuclei/Parts involved Discrete areas Paraventricular nucleus Supraoptic nucleus Paraventricular nucleus Posterior and dorsomedial nuclei Posterior and lateral nuclei Anterior nuclei Posterior and lateral nuclei Preoptic and anterior nuclei Posterior and lateral nuclei Preoptic area Posterior hypothalamus Anterior hypothalamus Lateral nucleus Ventromedial nucleus Lateral nucleus Supraoptic nucleus Anterior hypothalamus Mamillary Ventromedial nucleus Posterior and lateral nuclei Arcuate and p%terior nuclei Posterior hypthalamus Suprachiasmatic nucleus.

. 6. REGULATION OF HEART RATE Hypothalamus regulates heart rate through vasomotor center in the medulla oblongata. Stimulation of posterior and lateral nuclei of hypothalamus increases the heart rate. Stimulation of preoptic and anterior nuclei decreases the heart rate 7. REGULATION OF BLOOD PRESSURE Hypothalamus regulates the blood pressure by acting on the vasomotor center. Stimulation of posterior and lateral hypothalamic nuclei increases arterial blood pressure and stimulation of preoptic area decreases the blood pressure.

. 8. REGULATION OF BODY TEMPERATURE Body temperature is regulated by hypothalamus, which sets the normal range of body temperature. The set point, under normal physiological conditions is 37°C. Hypothalamus has two centers which regulate the body temperature: i . Heat loss center that is present in preoptic nucleus of anterior hypothalamus ii. Heat gain center that is situated in posterior hypothalamic nucleus..

. REGULATION OF HUNGER AND FOOD INTAKE Food intake is regulated by two centers present in hypothalamus: i . Feeding center ii. Satiety center. Feeding Center Feeding center is in the lateral hypothalamic nucleus. In experimental conditions, stimulation of this center in animals le ads to uncontrolled hunger and increased food intake (hyperphagia), resulting in obesity. Destruction of feeding center leads to loss of appetite (anorexia) and the animal refuses to take food. Normally, feeding center is always active. That means,it has the tendency to induce food intake always..

. Satiety Center Satiety center is in the ventromedial nucleus of the hypothalamus. Stimulation of this nucleus in animals causes total loss of appetite and cessation of food intake. Destruction of satiety center leads to hyperphagia and the animal becomes obese. This type of obesity is called hypothalamic obesity. Satiety center plays an important role in the regulation of food intake by temporary inhibition of feeding center after food intake..

. 10. REGULATION OF WATER BALANCE Hypothalamus regulates water content of the body by two mechanisms: i . Thirst mechanism ii. Antidiuretic hormone (ADH) mechanism. i . Thirst Mechanism Thirst center is in the lateral nucleus of hypothalamus. There are some osmoreceptors in the areas adjacent to thirst center. When the ECF volume decreases, the osmolality of ECF is increased. If the osmolarity increases by 1% to 2%, the osmoreceptors are stimulated. Osmoreceptors ,in turn, activate the thirst center and thirst sensation is initiated. Now, the person feels thirsty and drinks water. Water intake increases the ECF volume and decreases the osmolality (Fig. 149.4). ii. ADH Mechanism Simultaneously, when the volume of ECF decreases with increased osmolality, the supraoptic nucleus is stimulated and ADH is released. ADH causes retention of water by facultative reabsorption in the renal tubules. It increases the ECF volume and brings the osmolality back to the normal level. On the contrary, when ECF volume is increased, 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 ECF is brought back to normal ..

. abstract. ii. ADH Mechanism when the volume of ECF decreases with increased osmolality, the supraoptic nucleus is stimulated and ADH is released. ADH causes retention of water by facultative reabsorption in the renal tubules. It increases the ECF volume and brings the osmolality back to the normal level. On the contrary, when ECF volume is increased, 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 ECF is brought back to normal ..

. 11. REGULATION OF SLEEP AND WAKEFULNESS Mamillary body in the posterior hypothalamus is considered as the wakefulness center. Stimulation of mamillary body causes wakefulness and its lesion leads to sleep. Stimulation of anterior hypothalamus also leads to sleep..

. 12. ROLE IN BEHAVIOR AND EMOTIONAL CHANGES The behavior of animals and human beings is mostly affected by two responding systems in hypothalamus and other structures of limbic system. These two systems act opposite to one another. The responding systems are concerned with the 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). Hypothalamus has two centers for behavioral and emotional changes. They are: i . Reward center ii. Punishment center. Reward Center Reward center is situated in medial forebrain bundle and ventromedial nucleus of hypothalamus. Electrical stimulation of these areas in animals pleases or satisfies the animals. Punishment Center Punishment center is situated in posterior and lateral nuclei of hypothalamus. Electrical stimulation of these nuclei in animals leads to pain, fear, defense, escape reactions and other elements of punishment..

. 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..

. 13. REGULATION OF SEXUAL FUNCTION In animals, hypothalamus plays an important role in maintaining the sexual functions, especially in females. A decorticate female animal will have regular estrous cycle, provided the hypothalamus is intact. In human beings also, hypothalamus regulates the sexual functions by secreting gonadotropin releasing hormones . Arcuate and posterior hypothalamic nuclei are involved in the regulation of sex ual functions..

. „ 14. ROLE IN RESPONSE TO SMELL Posterior hypothalamus along with other structures like hippocampus and brainstem nuclei are responsible for the autonomic responses of body to olfactory stimuli. The responses include feeding activities and emotional responses like fear, excitement and pleasure..

. 15. ROLE IN CIRCADIAN RHYTHM Circadian rhythm is the regular recurrence of physiological processes or activities, which occur in cycles of 24 hours. It is also called diurnal rhythm. The term circadian is a Latin word, meaning ‘around the day’. Circadian rhythm develops in response to recurring daylight and darkness. The cyclic changes taking place in various physiological processes are set by means of a hypothetical internal clock that is often called biological clock. Suprachiasmatic nucleus of hypothalamus plays an important role in setting the biological clock by its connection with retina via retino -hypothalamic fibers. Through the efferent fibers, it sends circadian signals to different parts and maintains the circadian rhythm of sleep, hormonal secretion, thirst, hunger, appetite, etc. Whenever body is exposed to a new pattern of daylight or darkness rhythm, the biological clock is reset, provided the new pattern is regular. Accordingly, the circadian rhythm also changes..

APPLIED PHYSIOLOGY – DISORDERS OF HYPOTHALAMUS. The lesion of hypothalamus occurs due to tumors, encephalitis and ischemia. Following features develop in hypothalamic lesion: 1. Disturbances in carbohydrate and fat metabolisms, when lateral, arcuate and ventromedial nuclei are involved in lesion 2. Disturbance in sleep due to lesion in mamillary body and anterior hypothalamus 3. Disturbance in sympathetic or parasympathetic function occurs due to lesion in posterior, lateral and anterior nuclei 4. Emotional manifestations, leading to sham rage due to lesion in ventromedial and posterolateral parts 5. Disturbance in sexual functions due to the lesion in midhypothalamus . One or more of the above features can become prominent, resulting in some clinical manifestation.

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