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11.6C: Hypothalamus

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    The hypothalamus serves as a gateway between the nervous system and endocrine system.

     

    LEARNING OBJECTIVES

     

    Describe the functions of the hypothalamus of the brain.

     

    KEY TAKEAWAYS

    Key Points

     

    • The hypothalamus produces and secretes a wide variety of neurohormones that lead to the release or inhibition of pituitary gland
      hormone.
    • The posterior pituitary is composed of tissue derived from the hypothalamus, whereas the anterior pituitary is derived from epithelial tissue.
    • The hypothalamus can pass signals back and forth between the anterior and posterior pituitary by the neurohypophysis and median eminence.
    • The hypothalamus can sample the blood composition at two sites, the subfornical organ and the organum vasculosum of the lamina terminalis. This is important for the uptake of circulating hormones and to determine concentration of substances in the blood.
    • The outputs of the hypothalamus can be divided into neural projections and endocrine hormones. The neural projections tend to run bidirectionally.

     

    Key Terms

     

    • neurohypophysis: The posterior lobe of the pituitary gland, responsible for the release of oxytocin and antidiuretic hormone (ADH), also called vasopressin.
    • median eminence: Part of the inferior boundary for the hypothalamus and one of the seven areas of the brain devoid of a blood-brain barrier.
    • anterior pituitary: Also called the adenohypophysis, the glandular anterior lobe of the pituitary gland. The anterior pituitary regulates several physiological processes including stress, growth, reproduction, and lactation.

    The hypothalamus (derived from the Greek for “under chamber”) is a portion of the brain that contains a number of small, distinct nuclei with various functions and less anatomically distinct areas. The hypothalamus is located below the thalamus just above the brain stem. In the terminology of neuroanatomy, it forms the ventral part of the diencephalon. All vertebrate brains contain a hypothalamus. In humans, it is roughly the size of an almond.

    image

    The location of hypothalamus and pituitary: At center, hypothalamus is located just superior to the pituitary, with which it closely interacts.

    General Functions

    image

    Location of the hypothalamus: This is the location of the human hypothalamus in relation to the thalamus, pituitary gland, sella turcica, and optic chiasm.

    One of the most important functions of the hypothalamus is linking the nervous system to the endocrine system via the pituitary gland (hypophysis).

    The hypothalamus contains thyrotropin-releasing hormone, gonadotropin-releasing hormone, growth hormone-releasing hormone, corticotropin-releasing hormone, somatostatin, and dopamine, as well as vasopressin and oxytocin. These hormones are released into the bloodstream and target other organ systems, most notably the pituitary. The hypothalamus affects the endocrine system and governs emotional behavior such as anger and sexual activity. Most of the hypothalamic hormones generated are distributed to the pituitary via the hypophyseal portal system. The hypothalamus maintains homeostasis, including the regulation of blood pressure, heart rate, and temperature.

    The hypothalamus coordinates hormonal and behavioral circadian rhythms, complex patterns of neuroendocrine outputs, complex homeostatic mechanisms, and important behaviors. It must therefore respond to many different signals, some of which are generated externally and some internally. The hypothalamus is thus richly connected with many parts of the central nervous system, including the brainstem, reticular formation and autonomic zones, and the limbic forebrain (particularly the amygdala, septum, diagonal band of Broca, olfactory bulbs, and cerebral cortex).

    The hypothalamus can sample the blood composition at the subfornical organ and the organum vasculosum of the lamina terminalis. This is important for the uptake of circulating hormones and to determine concentration of substances in the blood.

    Temperature Regulation

    The hypothalamus functions as a type of thermostat for the body. It sets a desired body temperature and stimulates either heat production and retention to raise the blood temperature to a higher level, or sweating and vasodilation to cool the blood to a lower temperature. All fevers result from a raised setting in the hypothalamus; elevated body temperatures due to any other cause are classified as hyperthermia. Rarely, direct damage to the hypothalamus such as from a stroke causes a fever; this is sometimes called a hypothalamic fever. However, such damage more commonly causes abnormally low body temperatures.

    Appetite

    The extreme lateral part of the ventromedial nucleus of the hypothalamus is responsible for control of food intake. Stimulation of this area causes increased food intake. Bilateral lesion in this area causes complete cessation of food intake. Medial parts of the nucleus have a controlling effect on the lateral part. Bilateral lesion of the medial part of the ventromedial nucleus causes hyperphagia and obesity. Further lesion of the lateral part of the ventromedial nucleus in the same animal produces complete cessation of food intake.

    Fear Processing

    The medial zone of hypothalamus is part of a circuitry that controls motivated behaviors, such as defensive behaviors and social defeat. Analyses of Fos-labeling showed that a series of nuclei in the behavioral control column is important in regulating the expression of innate and conditioned defensive behaviors. Nuclei in the medial zone are also mobilized during an encounter with an aggressor. The defeated animal has an increase in Fos levels in sexually dimorphic structures.

    Sexual Dimorphism

    Several hypothalamic nuclei are sexually dimorphic, with clear differences in both structure and function between males and females. Some differences are apparent even in gross neuroanatomy, most notably is the sexually dimorphic nucleus within the preoptic area. However, most are subtle changes in the connectivity and chemical sensitivity of particular sets of neurons. In neonatal life, gonadal steroids influence the development of the neuroendocrine hypothalamus. For instance, they determine the ability of females to exhibit a normal reproductive cycle and of males and females to display appropriate reproductive behaviors in adult life.

    In 2004 and 2006, two studies by Berglund, Lindström, and Savic used Positron Emission Tomography (PET) to observe how the hypothalamus responds to smelling common odors, the scent of testosterone found in male sweat, and the scent of estrogen found in female urine. These studies showed that the hypothalamus of heterosexual men and homosexual women both respond to estrogen. Also, the hypothalamus of homosexual men and heterosexual women both respond to testosterone. In all four groups, common odors were processed similarly involving only the olfactory brain.