The endocrine system is composed of several ductless glands that secrete chemical messengers known as hormones directly into the bloodstream. At target tissues, hormones interact with specific high-affinity receptors located on the plasma membrane, as in the case of protein (peptide) hormones or amines, or intracellularly (nuclear, cytosolic) as with thyroid and steroid hormones. Hormones exert their functions in four broad physiological areas: maintenance of internal environment, energy balance, growth and development, and reproduction.
Regulation of endocrine secretion
This involves feedback loop control systems that are tuned to set-points, which may be modified by circadian rhythm, seasonal cycles, age and other influences such as stress. Many hormones function as part of a cascade (endocrine axis, Figure 45.1), where secretions of the hypothalamus-pituitary-target gland permit signal amplification, flexibility of response to a variety of physiological stimuli and fine regulation of the end hormone product.
The core of the neuroendocrine system is represented by the hypothalamic–pituitary complex. Within the hypothalamus, reside important collections of neuronal cell bodies (termed nuclei) involved in endocrine function (Figure 45.2). A funnel-shaped stalk, the infundibulum, extends from the floor of the hypothalamus to connect with the pituitary gland (hypophysis), which lies in a bony cavity (sella turcica) in the sphenoid bone. An important anatomical relation to the pituitary gland is the optic chiasm (Figure 45.2); therefore any expanding lesion of the pituitary or hypothalamus can present with visual field defects.
The pituitary gland is composed of two main lobes: the anterior lobe (adenohypophysis) and the posterior lobe (neurohypophysis). Developmentally, the anterior gland derives from the ectodermal Rathke’s pouch, an extension of the roof of the primitive oral cavity. The posterior gland forms from a neural down-growth of the di-encephalon. The two tissues migrate and fuse to give rise to the pituitary gland.
Hormones of the hypothalamus and pituitary gland
Hypothalamic releasing hormones
Peptide hormones synthesised in the cell bodies of small parvocellular neurones are released from axonal endings into capillary loops at the median eminence (Figure 45.2). They are transported via the hypothalamic-hypophyseal portal vessels to the capillary plexus within the anterior pituitary, where they act to stimulate or inhibit the release of hormones of specific cell types (Figure 45.3). Damage to the pituitary stalk (physical trauma, surgery, or disease) impairing blood flow to the pituitary can lead to disrupted control of anterior pituitary hormone release.
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