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KEY CONCEPTS

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  • Hormones are signaling molecules that reach their target cells via the bloodstream.

  • The hypothalamus is the key regulator of the endocrine system by releasing hormones directly into the blood and by regulating the anterior pituitary.

  • The hypothalamic–neurohypophyseal (posterior pituitary) system secretes two peptide hormones directly into the blood, vasopressin and oxytocin.

  • To regulate the anterior pituitary, hypothalamic neurons synthesize peptide-releasing and inhibitory factors.

  • The hypothalamic–pituitary–adrenal (HPA) axis comprises corticotropin-releasing factor (CRF), released by the hypothalamus; adrenocorticotropic hormone (ACTH), released by the anterior pituitary; and glucocorticoids, released by the adrenal cortex.

  • The hypothalamic–pituitary–thyroid axis consists of hypothalamic thyrotropin-releasing hormone (TRH), the anterior pituitary hormone thyroid-stimulating hormone (TSH), and the thyroid hormones triiodothyronine (T3) and tetraiodothyronine or thyroxine (T4).

  • The hypothalamic–pituitary–gonadal axis comprises hypothalamic gonadotropin-releasing hormone (GnRH), the anterior pituitary luteinizing hormone (LH) and follicle-stimulating hormone (FSH), and the gonadal steroids.

  • Prolactin is synthesized and released by the anterior pituitary and triggers lactation in females; its release is inhibited by dopamine acting at D2 dopamine receptors.

  • Growth hormone, also released by the anterior pituitary, is under the stimulatory control of growth hormone–releasing hormone (GHRH) and inhibitory control of somatostatin.

  • The hypothalamus contains a complex network of peptide-releasing neurons that regulate feeding and energy expenditure.

  • Leptin, released by adipocytes, regulates the long-term state of the body’s energy reserves largely via its actions on the hypothalamus.

  • The arcuate nucleus of the hypothalamus plays a key role in feeding and energy expenditure. It contains one set of neurons that express proopiomelanocortin (POMC) that suppresses feeding and another set that express neuropeptide Y (NPY) that stimulates feeding.

  • The POMC product, melanocortin (also called α-melanocyte-stimulating hormone) and NPY, in turn, regulate several other feeding peptides (eg, melanin-concentrating hormone) to produce their ultimate effects on food intake and energy metabolism.

  • The hypothalamus responds to inflammatory cytokines by causing fever and sickness behavior.

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INTRODUCTION

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The brain controls bodily function and behavior through the motor system, the autonomic nervous system (Chapter 9), and hormones released by neuroendocrine neurons. Hormones are signaling molecules that reach their target cells by means of the bloodstream. They comprise three major classes: peptides, neurotransmitter-like small molecules such as epinephrine (adrenalin), and steroid-like molecules. Through its neuroendocrine system, the brain contributes to body homeostasis, regulates growth and maturation, and facilitates the body’s response to environmental stressors, illness, and injury.

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NEUROENDOCRINE HYPOTHALAMUS

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Afferent Signals

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The hypothalamus, located at the base of the diencephalon, is in a good position to integrate information about the state of the organism from diverse sources. It receives information about the external environment from forebrain sensory systems and about noxious stimuli from nociceptors ascending in the spinal cord and brainstem (Chapter 11). The hypothalamus is activated by subcortical circuits that process emotional responses such as fear, reward, and disgust (eg, from ingesting noxious food), and receives information ...

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