Hypothalamicpituitary Axis

The human pituitary is composed of two morphologically and functionally distinct glands connected to the hypothalamus. The pituitary gland or hypophysis is located at the base of the brain and is connected to the hypothalamus by a stalk. It sits in a depression in the sphenoid bone of the skull called the sella turcica. The two morphologically and functionally distinct glands comprising the human pituitary are the adenohypophysis and the neurohypophysis (Fig. 32.1). The adenohypophysis consists of the pars tu-beralis, which forms the outer covering of the pituitary

Median Eminence Definition
A midsagittal section of the human pituitary gland.

stalk, and the pars distalis or anterior lobe. The neurohypophysis is composed of the median eminence of the hypothalamus, the infundibular stem, which forms the inner part of the stalk, and the infundibular process or posterior lobe. In most vertebrates, the pituitary contains a third anatomically distinct lobe, the pars intermedia or intermediate lobe. In adult humans, only a vestige of the intermediate lobe is found as a thin diffuse region of cells between the anterior and posterior lobes.

The adenohypophysis and neurohypophysis have different embryological origins. The adenohypophysis is formed from an evagination of the oral ectoderm called Rathke's pouch. The neurohypophysis forms as an extension of the developing hypothalamus, which fuses with Rathke's pouch as development proceeds. The posterior lobe is, therefore, composed of neural tissue and is a functional part of the hypothalamus.

Posterior Pituitary Hormones Are Synthesized by Hypothalamic Neurons Whose Axons Terminate in the Posterior Lobe

The infundibular stem of the pituitary gland contains bundles of nonmyelinated nerve fibers, which terminate on the capillary bed in the posterior lobe. These fibers are the axons of neurons that originate in the supraoptic nuclei and par-aventricular nuclei of the hypothalamus. The cell bodies of these neurons are large compared to those of other hypo-thalamic neurons,- hence, they are called magnocellular neurons. The hormones arginine vasopressin (AVP) and oxytocin are synthesized as parts of larger precursor proteins (prohormones) in the cell bodies of these neurons. Prohormones are then packaged into granules and enzymatically processed to produce AVP and oxytocin. The granules are transported down the axons by axoplasmic flow,- they accumulate at the axon terminals in the posterior lobe.

Stimuli for the secretion of posterior lobe hormones may be generated by events occurring within or outside the body. These stimuli are processed by the central nervous system (CNS), and the signal for the secretion of AVP or oxytocin is then transmitted to neurosecretory neurons in the hypothalamus. Secretory granules containing the hor mone are then released into the nearby capillary circulation, from which they are carried into the systemic circulation.

Anterior Pituitary Hormones Are Synthesized and Secreted in Response to Hypothalamic Releasing Hormones Carried in the Hypophyseal Portal Circulation

The anterior lobe contains clusters of histologically distinct types of cells closely associated with blood sinusoids that drain into the venous circulation. These cells produce anterior pituitary hormones and secrete them into the blood sinusoids. The six well-known anterior pituitary hormones are produced by separate kinds of cells. Adrenocorti-cotropic hormone (ACTH), also known as corticotropin, is secreted by corticotrophs, thyroid-stimulating hormone (TSH) by thyrotrophs, growth hormone (GH) by somatotrophs, prolactin (PRL) by lactotrophs, and follicle-stimulating hormone (FSH) and luteinizing hormone (LH) by gonadotrophs.

The cells that produce anterior pituitary hormones are not innervated and, therefore, are not under direct neural control. Rather, their secretory activity is regulated by releasing hormones, also called hypophysiotropic hormones, synthesized by neural cell bodies in the hypothalamus. Granules containing releasing hormones are stored in the axon terminals of these neurons, located in capillary networks in the median eminence of the hypothalamus and lower infundibular stem. These capillary networks give rise to the principal blood supply to the anterior lobe of the pituitary.

The blood supply to the anterior pituitary is shown in Figure 32.2. Arterial blood is brought to the hypothalamic-pituitary region by the superior and inferior hypophyseal arteries. The superior hypophyseal arteries give rise to a rich capillary network in the median eminence. The capillaries converge into long veins that run down the pituitary stalk and empty into the blood sinusoids in the anterior lobe. They are considered to be portal veins because they deliver blood to the anterior pituitary rather than joining the venous circulation that carries blood back to the heart, therefore, they are called long hypophyseal portal vessels. The inferior hypophyseal arteries provide arterial blood to the posterior lobe. They also penetrate into the lower infundibular stem, where they form another important capillary network. The capillaries of this network converge into short hypophyseal portal vessels, which also deliver blood into the sinusoids of the anterior pituitary. The special blood supply to the anterior lobe of the pituitary gland is known as the hypophyseal portal circulation.

When a neurosecretory neuron is stimulated to secrete, the releasing hormone is discharged into the hypophyseal portal circulation (see Fig. 32.2). Releasing hormones travel only a short distance before they come in contact with their target cells in the anterior lobe. Only the amount of releasing hormone needed to control anterior pituitary hormone secretion is delivered to the hypophyseal portal circulation by neurosecretory neurons. Consequently, releasing hormones are almost undetectable in systemic blood.

A releasing hormone either stimulates or inhibits the synthesis and secretion of a particular anterior pituitary

Trh Neurons

^gfPPHHWHI^ The blood supply to the anterior pituitary.

^ttmmtl^Êm^ This illustration shows the relationship of the hypothalamic magnocellular neurons and hypothalamic neurosecretory cells that produce releasing hormones to the pituitary blood vessels. M represents a magnocellular neuron releasing AVP or oxytocin at its axon terminals into capillaries that give rise to the venous drainage of the posterior lobe. Neurons 1 and 2 are secreting releasing factors into capillary networks that give rise to the long and short hypophyseal portal vessels, respectively. Releasing hormones are shown reaching the hormone-secreting cells of the anterior lobe via the portal vessels.

^gfPPHHWHI^ The blood supply to the anterior pituitary.

^ttmmtl^Êm^ This illustration shows the relationship of the hypothalamic magnocellular neurons and hypothalamic neurosecretory cells that produce releasing hormones to the pituitary blood vessels. M represents a magnocellular neuron releasing AVP or oxytocin at its axon terminals into capillaries that give rise to the venous drainage of the posterior lobe. Neurons 1 and 2 are secreting releasing factors into capillary networks that give rise to the long and short hypophyseal portal vessels, respectively. Releasing hormones are shown reaching the hormone-secreting cells of the anterior lobe via the portal vessels.

hormone. Corticotropin-releasing hormone (CRH), thy-rotropin-releasing hormone (TRH), and growth hormone-releasing hormone (GHRH) stimulate the secretion and synthesis of ACTH, TSH, and GH, respectively (Table 32.1). Luteinizing hormone-releasing hormone (LHRH), also known as gonadotropin-releasing hormone (GnRH), stimulates the synthesis and release of FSH and LH. In contrast, somatostatin, also called somatotropin release inhibiting factor (SRIF), inhibits GH secretion. All of the releasing hormones are peptides, with the exception of dopamine, which is a catecholamine that inhibits the synthesis and secretion of PRL. Releasing hormones can be produced synthetically, and several are currently under study for use in the diagnosis and treatment of diseases of the endocrine system. For example, synthetic GnRH is now used for treating infertility in women.

Releasing hormones are secreted in response to neural inputs from other areas of the CNS. These signals are generated by external events that affect the body or by changes occurring within the body itself. For example, sensory nerve excitation, emotional or physical stress, biological rhythms, changes in sleep patterns or in the sleep-wake cycle, and changes in circulating levels of certain hormones or metabolites all affect the secretion of particular anterior pituitary hormones. Signals generated in the CNS by such events are transmitted to the neurosecretory neurons in the hypothalamus. Depending on the nature of the event and the signal generated, the secretion of a particular releasing hormone may be either stimulated or inhibited. In turn, this response affects the rate of secretion of the appropriate anterior pituitary hormone. The neural pathways involved in transmitting these signals to the neurosecretory neurons in the hypothalamus are not well defined.

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Responses

  • selina
    What consists of axons and axon terminals from neurons that originate in the hypothalamus?
    7 years ago

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