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THE ROLE OF HYPOTHALAMUS

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THE ROLE OF HYPOTHALAMUS

Post by pmmutiti on Sat Jul 19, 2008 3:33 pm

What is a
Hypothalamic
Hamartoma?


The tiny hypothalamus serves as the Health Maintenance Organization of the body, regulating its homeostasis, or stable state of equilibrium. The hypothalamus also generates behaviors involved in eating, drinking, general arousal, rage, aggression, embarrassment, escape from danger, pleasure and copulation. It does an amazing number of housekeeping chores for such a small piece of tissue. Its lateral and anterior parts seem to support activation of the parasympathetic nervous system: drop in blood pressure; slowing of pulse; and regulation of digestion, defecation, assimilation, and reproduction in such a way as to contribute on the whole to rest and recovery. The medial and posterior hypothalamus regulate activation: acceleration of pulse and breathing rates, high blood pressure, arousal, fear and anger. Stimulation of specific groups of cells in these areas can elicit pure behaviors. For example, rats placed in an experimental situation where they can press a lever to stimulate a pleasure center will do so to the exclusion of eating and drinking. Stimulation of another area can produce rage.


1. Hypothalamus = Homeostasis

The main function of the hypothalamus is homeostasis, or maintaining the body's status quo. Factors such as blood pressure, body temperature, fluid and electrolyte balance, and body weight are held to a precise value called the set-point. Although this set-point can migrate over time, from day to day it is remarkably fixed.

To achieve this task, the hypothalamus must receive inputs about the state of the body, and must be able to initiate compensatory changes if anything drifts out of whack. The inputs include:

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.
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.
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:

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.
endocrine signals to/through the pituitary - recall that an endocrine signal is a chemical signal sent via the bloodstream. Large hypothalamic cells around the third ventricle send their axons directly to the posterior pituitary, where the axon terminals release oxytocin and vasopressin into the bloodstream. Smaller cells in the same area send their axons only as far as the base of the pituitary, where they empty releasing factors into the capillary system of the anterior pituitary. These releasing factors induce the anterior pituitary to secrete any one of at least six hormones, including ACTH and thyroid-stimulating hormone (TSH).
Ultimately the hypothalamus can control every endocrine gland in the body, and alter blood pressure (through vasopressin and vasoconstriction), body temperature, metabolism (through TSH), and adrenaline levels (through ACTH).

In the news lately: The hypothalamus controls body weight and appetite, but it is not entirely clear how. Sensory inputs, including taste, smell, and gut distension, all tell the hypothalamus if we are hungry, full, or smelling a steak. Yet it is mysterious how we are able to vary our eating habits day to day and yet maintain about the same weight (sometimes despite all efforts to the contrary!) . The "set-point" theory is an old one in diet science, but until recently the mechanics of maintaining that set point were unknown. It appears that there is an endocrine component to the appetite system. Recent studies in mice have shown that the fat cells of normal overfed mice will release a protein called leptin (or OB, after the gene name), which reduces appetite and perks up metabolism. Leptin is presumably acting on the hypothalamus. Underfed mice, on the other hand, produce little or no leptin, and they experience an increase in appetite and a decrease in metabolism. In both of these mice, the result is a return to normal weight. But what would happen if a mouse (or human) had a defective OB gene? Weight gain would never trigger fat cells to release leptin, the hypothalamus would never slow the appetite or increase metabolism, and the mouse would slowly but surely become obese (how the gene got its name). Sure enough, shortly after these experiments hit the news, the human OB gene was discovered and a few obese patients were found to have the mutation. Many more obese patients had normal OB genes, however, indicating that there is much more to the story yet to be discovered.


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Peter Mwaura Mutiti : Teaching old blood cells new tricks:
When you hear someone mention circulation you probably think of the heart and major arteries—and for good reason. Circulatory disorders such as hypertension (high blood pressure) and atherosclerosis (hardening of the arteries) are major risk factors for heart disease, heart attacks, and stroke.

But there’s more to it than that. With all the attention on the heart and arteries, it’s easy to overlook serious health problems affecting the smallest components of the circulatory system—microscopic blood vessels called microcapillaries, where the critical exchange of oxygen and nutrients actually takes place. If blood isn’t flowing through this web properly, it can trigger all sorts of health problems, many of which may not seem related to circulation at all.

A number of factors contribute to poor circulation as we age. Arteries and veins become stiff and congested as cholesterol and calcium plaques accumulate and restrict blood flow. Spasms in the smooth muscles surrounding the circulatory arteries and veins can also choke off circulation. These same processes also occur in our microcapillaries, reducing microcirculation and impairing the critical exchange of nutrients and gases in tissues and major organs.

This problem only gets worse as we get older because of changes in the composition and structure of blood cells. As you reach middle age, the blood starts to thicken and congeal as platelets and blood proteins make cells sticky. Plus, the spleen—the organ that removes old, damaged blood cells from circulation—begins to slow down with age, which means new, healthy blood cells are replaced at a sharply reduced rate. And to make matters even worse, as blood cells age, they become stiff and no longer appear round and evenly shaped. This makes it harder for them to pass smoothly through the capillaries. In fact, the angular, jagged shape of the old cells can damage the fragile microcapillaries even further.

Eventually, these age-related changes take their toll on the microcapillaries, reducing circulation to the tissues and blocking the flow of nutrients and oxygen. Removal of carbon dioxide and other metabolic waste products is also hindered. This leads to a slow buildup of metabolic garbage that can gradually bury the cells in their own waste products. In time, the cells, poisoned by their own metabolic byproducts, begin to waste away and ultimately cease to function altogether.

The combined effect of poor circulation and old blood contributes to a host of symptoms, including deep fatigue, fuzzy thinking, frequent infections, and lowered sex drive—all conditions usually considered just “normal parts of aging.”

If circulation doesn’t improve, it can lead to more serious conditions, such as high blood pressure, heart attack, stroke, diabetes, and arthritis. But giving your body a fresh supply of healthy blood may target all of these problems and more.
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