Everything about Hypothalamus totally explained
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's 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.
The hypothalamus controls
body temperature,
hunger,
thirst, fatigue, anger, and
circadian cycles.
Inputs
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, complexity patterns of neuroendocrine outputs, complex homeostatic mechanisms, 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
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.
Blood-borne stimuli
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 isn't clear how all peptides that influence hypothalamic activity gain the necessary access. In the case of
prolactin and
leptin, there's 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 isn't clear. There is also evidence for central actions of
prolactin and
TSH.
Steroids
The hypothalamus contains neurons that are sensitive to gonadal 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.
Neural inputs
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.
Nuclei
The hypothalamic nuclei include the following:
Outputs
The outputs of the hypothalamus can be divided into two categories: neural projections, and endocrine hormones.
Neural projections
Most fiber systems of the hypothalamus run in two ways (bidirectional).
Projections to areas caudal to the hypothalamus go through the medial forebrain bundle, the mammillotegmental tract and the dorsal longitudinal fasciculus.
Projections to areas rostral to the hypothalamus are carried by the mammillothalamic tract, the fornix and terminal stria.
Endocrine hormones
The Hypothalamus affects the endocrine system and governs emotional behavior, such as, anger and sexual activity. Most of the hypothalamic hormones generated are distributed to the pituitary via the hypophyseal portal system. The hypothalamus maintains homeostasis this includes a regulation of blood pressure, heart rate, and temperature.
The primary hypothalamic hormones are:
Further Information
Get more info on 'Hypothalamus'.
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