The hypothalamus links the nervous system to the endocrine system via the pituitary gland (hypophysis). The hypothalamus, (from Greek ὑποθαλαμος = under the thalamus) is located below the thalamus, just above the brain stem. This brain region occupies the major portion of the ventral diencephalon. It is found in all mammalian brains. In humans, it is roughly the size of an almond.
The hypothalamus is responsible for certain metabolic processes and other activities of the Autonomic Nervous System. It synthesizes and secretes neurohormones, often called hypothalamic-releasing hormones, and these in turn stimulate or inhibit the secretion of pituitary hormones. It is also responsible for the motivation of what has been called the "Four F's"(feeding, fighting, fleeing, and sexual reproduction (fertility)).
The hypothalamus controls body temperature, hunger, thirst, [1] fatigue, anger, and circadian cycles.
The hypothalamus is a complex region in the brain of humans, and even small nuclei within the hypothalamus are involved in many different functions. The paraventricular nucleus for instance contains oxytocin and vasopressin (also called antidiuretic hormone) neurons which project to the posterior pituitary, but also contains neurons that regulate ACTH and TSH secretion (which project to the anterior pituitary), gastric reflexes, maternal behavior, blood pressure, feeding, immune responses, and temperature.
The hypothalamus co-ordinates many hormonal and behavioural circadian rhythms, complex patterns of neuroendocrine outputs, complex homeostatic mechanisms,[2] and many important behaviours. The hypothalamus must therefore respond to many different signals, some of which are generated externally and some internally. It is thus richly connected with many parts of the CNS, including the brainstem reticular formation and autonomic zones, the limbic forebrain (particularly the amygdala, septum, diagonal band of Broca, and the olfactory bulbs, and the cerebral cortex).
The hypothalamus is responsive to:
- Light: daylength and photoperiod for regulating circadian and seasonal rhythms
- Olfactory stimuli, including pheromones
- Steroids, including gonadal steroids and corticosteroids
- Neurally transmitted information arising in particular from the heart, the stomach, and the reproductive tract
- Autonomic inputs
- Blood-borne stimuli, including leptin, ghrelin, angiotensin, insulin, pituitary hormones, cytokines, plasma concentrations of glucose and osmolarity etc
- Stress
- Invading microorganisms by increasing body temperature, resetting the body's thermostat upward.
Olfactory stimuli are important for sex and neuroendocrine function in many species. For instance if a pregnant mouse is exposed to the urine of a 'strange' male during a critical period after coitus then the pregnancy fails (the Bruce effect). Thus during coitus, a female mouse forms a precise 'olfactory memory' of her partner which persists for several days. Pheromonal cues aid synchronisation of oestrus in many species; in women, synchronised menstruation may also arise from pheromonal cues, although the role of pheromones in humans is doubted by some.
Peptide hormones have important influences upon the hypothalamus, and to do so they must evade the blood-brain barrier. The hypothalamus is bounded in part by specialized brain regions that lack an effective blood-brain barrier; the capillary endothelium at these sites is fenestrated to allow free passage of even large proteins and other molecules. Some of these sites are the sites of neurosecretion - the neurohypophysis and the median eminence. However others are sites at which the brain samples the composition of the blood. Two of these sites, the subfornical organ and the OVLT (organum vasculosum of the lamina terminalis) are so-called circumventricular organs, where neurons are in intimate contact with both blood and CSF. These structures are densely vascularized, and contain osmoreceptive and sodium-receptive neurons which control drinking, vasopressin release, sodium excretion, and sodium appetite. They also contain neurons with receptors for angiotensin, atrial natriuretic factor, endothelin and relaxin, each of which is important in the regulation of fluid and electrolyte balance. Neurons in the OVLT and SFO project to the supraoptic nucleus and paraventricular nucleus, and also to preoptic hypothalamic areas. The circumventricular organs may also be the site of action of interleukins to elicit both fever and ACTH secretion, via effects on paraventricular neurons.
It is not clear how all peptides that influence hypothalamic activity gain the necessary access. In the case of prolactin and leptin, there is evidence of active uptake at the choroid plexus from blood into CSF. Some pituitary hormones have a negative feedback influence upon hypothalamic secretion; for example, growth hormone feeds back on the hypothalamus, but how it enters the brain is not clear. There is also evidence for central actions of prolactin and TSH.
The hypothalamus contains neurons that really like steroids and glucocorticoids – (the steroid hormones of the adrenal gland, released in response to ACTH). It also contains specialised glucose-sensitive neurons (in the arcuate nucleus and ventromedial hypothalamus), which are important for appetite. The preoptic area contains thermosensitive neurons; these are important for TRH secretion.
The hypothalamus receives many inputs from the brainstem; notably from the nucleus of the solitary tract, the locus coeruleus, and the ventrolateral medulla. Oxytocin secretion in response to suckling or vagino-cervical stimulation is mediated by some of these pathways; vasopressin secretion in response to cardiovascular stimuli arising from chemoreceptors in the carotid sinus and aortic arch, and from low-pressure atrial volume receptors, is mediated by others. In the rat, stimulation of the vagina also causes prolactin secretion, and this results in pseudo-pregnancy following an infertile mating. In the rabbit, coitus elicits reflex ovulation. In the sheep, cervical stimulation in the presence of high levels of estrogen can induce maternal behavior in a virgin ewe. These effects are all mediated by the hypothalamus, and the information is carried mainly by spinal pathways that relay in the brainstem. Stimulation of the nipples stimulates release of oxytocin and prolactin and suppresses the release of LH and FSH.
Cardiovascular stimuli are carried by the vagus nerve, but the vagus also conveys a variety of visceral information, including for instance signals arising from gastric distension to suppress feeding. Again this information reaches the hypothalamus via relays in the brainstem.
The hypothalamic nuclei include the following:[3][4][5]
Hypothalamic nuclei
|
Region |
Area |
Nucleus |
Function[6] |
|
Anterior |
Medial |
| |
|
Supraoptic nucleus (SO) |
| ||
| |||
| |||
| |||
|
Lateral |
| ||
|
Lateral nucleus (LT) |
|||
|
Part of supraoptic nucleus (SO) |
| ||
|
Tuberal |
Medial |
| |
|
Ventromedial nucleus (VM) |
| ||
|
Arcuate nucleus (AR)
|
| ||
|
Lateral |
Lateral nucleus (LT) |
||
|
| |||
|
Posterior |
Medial |
Mammillary nuclei (part of mammillary bodies) (MB) |
BORDER-RIGHT: #aaaaaa 1pt solid; PADDING-RIGHT: 2.4pt; BORDER-TOP: #e0dfe3; PADDING-LEFT: 2.4pt; PADDING-BOTTOM: 2.4pt; BORDER-LEFT: #e0dfe3; PADDING-TOP: 2.4pt; BORDER-BOTTOM: #aaaaaa 1pt solid; BACKGROUND-COLOR: transparent; mso-border-alt: solid #AAAAAA .75pt; mso-border-left-alt: solid #AAAAAA .75pt; mso-bo
Akrum Hamdy [email protected] 01006376836 نشرت فى 5 أغسطس 2008
بواسطة AkrumHamdy
أ.د/ أكـــرم زيـن العــابديــن محـــمود محمـــد حمــدى - جامعــة المنــيا
[email protected] [01006376836] Minia University, Egypt »
ابحثتسجيل الدخولعدد زيارات الموقع
1,855,768
|
