Hypothalamus and Pituitary Gland

The hypothalamus is the most inferior portion of the dien-cephalon. Located below the thalamus, it forms the floor and part of the lateral walls of the third ventricle. This small but extremely important brain region contains neural centers for hunger and thirst and for the regulation of body temperature and hormone secretion from the pituitary gland (fig. 8.17). In addition, centers in the hypothalamus contribute to the regulation of sleep, wakefulness, sexual arousal and performance, and such emotions as anger, fear, pain, and pleasure. Acting through its connections with the medulla oblongata of the brain stem, the hypothalamus helps to evoke the visceral responses to various emotional states. In its regulation of emotion, the hypothalamus works together with the limbic system, as was discussed in the previous section.

Experimental stimulation of different areas of the hypothalamus can evoke the autonomic responses characteristic of aggression, sexual behavior, hunger, or satiety. Chronic stimulation of the lateral hypothalamus, for example, can make an animal eat and become obese, whereas stimulation of the medial hypothalamus inhibits eating. Other areas contain osmorecep-tors that stimulate thirst and the release of antidiuretic hormone (ADH) from the posterior pituitary.

The hypothalamus is also where the body's "thermostat" is located. Experimental cooling of the preoptic-anterior hypothalamus causes shivering (a somatic motor response) and nonshiver-ing thermogenesis (a sympathetic motor response). Experimental heating of this hypothalamic area results in hyperventilation (stimulated by somatic motor nerves), vasodilation, salivation, and sweat-gland secretion (regulated by sympathetic nerves). These responses serve to correct the temperature deviations in a negative feedback fashion.

The coordination of sympathetic and parasympathetic reflexes is thus integrated with the control of somatic and endocrine responses by the hypothalamus. The activities of the hypothalamus are in turn influenced by higher brain centers.

The pituitary gland is located immediately inferior to the hypothalamus. Indeed, the posterior pituitary derives embryoni-cally from a downgrowth of the diencephalon, and the entire pituitary remains connected to the diencephalon by means of a stalk (a relationship described in more detail in chapter 11). Neurons within the supraoptic and paraventricular nuclei of the hypothalamus (fig. 8.17) produce two hormones—antidiuretic hormone (ADH), which is also known as vasopressin, and oxytocin. These two hormones are transported in axons of the hypothalamo-hypophyseal tract to the neurohypophysis (posterior pituitary), where they are stored and released in response to hypothalamic stimulation. Oxytocin stimulates contractions of the uterus during labor, and ADH stimulates the kidneys to reabsorb water and thus to excrete a smaller volume of urine. Neurons in the hypothalamus also produce hormones known as releasing hormones and inhibiting hormones that are transported by the blood to the adenohypophysis

Dorsomedial nucleus

Paraventricular nucleus

Anterior nucleus

Preoptic area

Supraoptic nucleus

Dorsomedial nucleus

Anterior nucleus

Preoptic area

Supraoptic nucleus

Posterior nucleus

Ventromedial nucleus

Mammillary body

Median eminence

Optic chiasma

Anterior pituitary Posterior pituitary

(adenohypophysis) (neurohypophysis)

Pituitary gland

Posterior nucleus

Ventromedial nucleus

Mammillary body

Optic chiasma

Anterior pituitary Posterior pituitary

(adenohypophysis) (neurohypophysis)

Pituitary gland

Median eminence

â–  Figure 8.17 A diagram of some of the nuclei within the hypothalamus. The hypothalamic nuclei, composed of neuron cell bodies have different functions.

(anterior pituitary). These hypothalamic releasing and inhibiting hormones regulate the secretions of the anterior pituitary and, by this means, regulate the secretions of other endocrine glands (as described in chapter 11).

Test Yourself Before You Continue

1. Describe the location of the diencephalon relative to the cerebrum and the brain ventricles.

2. List the functions of the hypothalamus and indicate the other brain regions that cooperate with the hypothalamus in the performance of these functions.

3. Explain the structural and functional relationships between the hypothalamus and the pituitary gland.

volved in behavior and reward, and the release of dopamine from these neurons is promoted by abused drugs.

The positive reinforcement elicited by abused drugs (table 8.3) involves the release of dopamine by q axons of the mesolimbic system. These axons arise in the midbrain and terminate in the nucleus accumbens of the forebrain. Nicotine from tobacco stimulates dopaminergic neurons in the midbrain by means of nicotinic ACh receptors. Heroin and morphine activate this pathway by means of opioid receptors in the midbrain, while cocaine and amphetamines act at the nucleus accumbens to inhibit dopamine reuptake into presynaptic axons. As might be predicted, symptoms of the withdrawal from abused drugs are associated with decreased levels of dopamine in the nucleus accumbens.

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