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Key Issues part one: • Almost every subdivision of the CNS communicates with the Hypothalamus. • The hypothalamus communicates with virtually all peripheral organs. • The hypothalamus responds to input from the peripheral organs that it regulates. • The median eminence (at the floor of the third ventricle) is the gateway through which the hypothalamus exerts control. Key Issues:part 2 • Most hypothalamic neurons exert regulatory control by releasing peptides and DA into the vasculature running through the infundibular stalk. • Axons of the hypothalamic neurons can also pass through this stalk to the vascular beds of the posterior pituitary. The vascular beds there can transport peptides to the anterior pituitary. • These peptides can be made within the PVH and arcuate nuclei and released into the anterior pituitary. • So, neural signals coming into the hypothalamus can be integrated, translated into a “humoral” signal, and directly affect peripheral endocrine systems. Key Issues:Part 3 • Neurons in the PVN and supraoptic nuclei project give rise to axons that run through the median eminence and the stalk and synapse in the posterior pituitary. This provides a direct neural connection between the hypothalamus and the pituitary. Development of the Hypophysial-portal system: The Transition between the Hypothalamus and the Pituitary: Alternative labels for the same system: Relevant Cell Types: Adenohypophesis or Anterior Pituitary Acidophils Cells that contain the glycoprotein hormones: •Somatotropes which produce growth hormone •Lactotropes which produce prolactin Basophils Cells that contain the glycoprotein hormones: •Thyrotropes which produce thyroid stimulating hormone •Gonadotropes which produce luteinizing hormone or follicle-stimulating hormone •Corticotropes which produce adrenocorticotrophic hormone Due the high carbohydrate content of the hormones within acidophils, they also stain bright purple with PAS stains. chromophobes These are cells that have minimal or no hormonal content. Many of the chromophobes may be acidophils or basophils that have degranulated and thereby are depleted of hormone. Some chromophobes may also represent stem cells that have not yet differentiated into hormone-producing cells. The Neurohypophysis has herring bodies that contain neurosecratory granules that produce oxytocin and antidiuretic hormone. FIGURE 2 Depiction of a coronal section through the median eminence cell showing its three major zones and the typical projections of neuroendocrine cells to the external zone containing the portal capillaries leading to the anterior pituitary gland. iii, third ventricle; E, ependymal zone; ZI, internal zone; ZE, external zone. Anter ior Pituit ary Poste rior Pituit ary Growth hormone Liver, adipose tissue Promotes growth (indirectly), control of protein, lipid and carbohydrate metabolism Thyroid-stimulating hormone Thyroid gland Stimulates secretion of thyroid hormones Adrenocorticotropic hormone Adrenal gland (cortex) Stimulates secretion of glucocorticoids Prolactin Mammary gland Milk production Luteinizing hormone Ovary and testis Control of reproductive function Follicle-stimulating hormone Ovary and testis Control of reproductive function Antidiuretic hormone Kidney Conservation of body water Oxytocin Ovary and testis Stimulates milk ejection and uterine contractions Key Issues part one: • Almost every subdivision of the CNS communicates with the Hypothalamus. • The hypothalamus communicates with virtually all peripheral organs. • The hypothalamus responds to input from the peripheral organs that it regulates. • The median eminence is the gateway through which the hypothalamus exerts control. Key Issues:part 2 • Most hypothalamic neurons exert regulatory control by releasing peptides and DA into the vasculature running through the infundibular stalk. • Axons of the hypothalamic neurons can also pass through this stalk to the vascular beds of the posterior pituitary. The vascular beds there can transport peptides to the anterior pituitary. • These peptides can be made within the PVH and arcuate nuclei and released into the anterior pituitary. • So, neural signals coming into the hypothalamus can be integrated, translated into a “humoral” signal, and directly affect peripheral endocrine systems. Key Issues:Part 3 • Neurons in the PVN and supraoptic nuclei project give rise to axons that run through the median eminence and the stalk and synapse in the posterior pituitary. This provides a direct neural connection between the hypothalamus and the pituitary. Structures that directly influence the hypothalamus… - nucleus of the solitary tract - this nucleus collects all of the visceral sensory information from the vagus and relays it to the hypothalamus and other targets. Information includes blood pressure and gut distension. - reticular formation - this catchall nucleus in the brainstem receives a variety of inputs from the spinal cord. Among them is information about skin temperature, which is relayed to the hypothalamus. - retina - some fibers from the optic nerve go directly to a small nucleus within the hypothalamus called the suprachiasmatic nucleus. This nucleus regulates circadian rhythms, and couples the rhythms to the light/dark cycles. (MORE) Structures that directly influence the hypothalamus - circumventricular organs - these nuclei are located along the ventricles, and are unique in the brain in that they lack a blood-brain barrier. This allows them to monitor substances in the blood that would normally be shielded from neural tissue. Examples are the OVLT, which is sensitive to changes in osmolarity, and the area postrema, which is sensitive to toxins in the blood and can induce vomiting. Both of these project to the hypothalamus. - limbic and olfactory systems - structures such as the amygdala, the hippocampus, and the olfactory cortex project to the hypothalamus, and probably help to regulate behaviors such as eating and reproduction. How does the hypothalamus maintain a particular set point?? In addition to the feed-back loops, the hypothalamus also has some intrinsic receptors, including thermoreceptors and osmoreceptors to monitor temperature and ionic balance, respectively. Once the hypothalamus is “aware” of a problem, how does it fix it? Essentially, there are two main outputs: 1. Neural signals to the autonomic system - the (lateral) hypothalamus projects to the (lateral) medulla, where the cells that drive the autonomic systems are located. These include the parasympathetic vagal nuclei and a group of cells that descend to the sympathetic system in the spinal cord. With access to these systems, the hypothalamus can control heart rate, vasoconstriction, digestion, sweating, etc. 2. Endocrine signals are sent to/through the pituitary (recall that an endocrine signal is a chemical signal sent via the bloodstream). Ultimately the hypothalamus can control every endocrine gland in the body, and alter blood pressure (through vasopressin, etc.), body temperature, metabolism (through TSH), and adrenaline levels (through ACTH). COMING NEXT – IS IT ALWAYS FUNCTIONAL TO MAINTAIN A SET POINT???
