The hypothalamus is present in all vertebrates and part of the diencephalon which is located inferior to the thalamus. Discussions about its role are thought to date back to the 2nd century AD. Since then, research has highlighted the structure of the hypothalamus and its interactions with cortical regions of the brain, as well as its function.
Anatomy of the Hypothalamus
The hypothalamus is located in the ventral area of the brain between the pituitary gland and the third ventricle. It is characterized as a bilateral group of nuclei which surround mammillary bodies. Each group of nuclei is comprised of distinctive neuronal populations which formulate several neurotransmitters and neuropeptides which work to regulate the bodily functions referred to as homeostasis.
Afferent pathways feed into the hypothalamus stem from several areas including the thalamus, cerebral cortex, brainstem, olfactory areas, and basal ganglia. Furthermore, one of the most significant efferent pathways consists of the network that links the supraoptic and paraventricular nuclei of the hypothalamus to the median eminence, which verge to the posterior and anterior pituitary glands.
Function of the Hypothalamus
The hypothalamus is vital for the maintenance of homeostasis through the coordination of autonomic, endocrine, and somatic behavior. To achieve this, a series of processes occur.
Firstly, the hypothalamus plays a role in the autonomic nervous system. It processes sensory impulses received from the smooth muscle within the body which lines organs such as the intestines and stomach, as well as blood vessels. By processing the impulses, the hypothalamus is able to regulate the movement of food, heart rate, and the contraction of the bladder.
Furthermore, the hypothalamus is considered by some to be the vital interaction between the body’s central systems: the endocrine system which is responsible for regulating bodily states by releasing hormones; and the nervous system which coordinates and sends synaptic information. The hypothalamus detects changes within the body and coordinates responses by stimulating the release of hormones from organs and glands. These pathways enable the hypothalamus to regulate a series of processes within the body.
Regulation of Satiety
Through the coordination of stimuli and processes, the hypothalamus can regulate eating behavior by promoting feelings of satiety. During the consumption of food, an area of the hypothalamus is activated when the stomach is full. Upon activation, the hypothalamus sends signals to promote feelings of satiety to reduce the need for further consumption. Research has found that damage to this particular area of the brain is linked to cases of excessive eating resulting in obesity.
Regulation of Body Temperature
The optimum internal body temperature for bodily processes to occur is approximately 37°C (98.6°F). To regulate an individual’s temperature, the hypothalamus regularly makes comparisons between the exact and ideal body temperature and signals for adjustments to be made to either raise or lower the temperature. For example, if the body temperature is too high and needs lowering, then signals can be transmitted to produce heat, which can have a cooling effect on the body. Conversely, if the body temperature is too low, then the body is stimulated to generate and help maintain heat.
Regulation of Emotions
One of the functions of the hypothalamus is to convert emotion into a physical response. When intense emotional feelings such as fear, anger, or excitement are generated in reaction to either stimulus within the surroundings or by an individual’s thought processes, synaptic impulses are transmitted to the hypothalamus. Based on the type of emotion experienced, the hypothalamus may send a series of signals via the autonomic nervous system and pituitary system to induce a physiological response. For example, in cases of fear, individuals may experience quickened heart rate and shallow breathing.
Regulation of Sleep
Within the hypothalamus is the suprachiasmatic nucleus (SCN), commonly referred to as the circadian pacemaker, which is responsible for the regulation of sleep/wake cycles. During the light cycle, a series of mechanisms deriving from the retinal ganglionic cells to the SCN prevent the secretion of melatonin. By preventing the release of the hormone, individuals are motivated to stay away during daylight hours. As the day progresses, and light signals to the retinal ganglion cells subside due to lack of sunlight, the SCN is inhibited and instead activates the paraventricular nucleus. As a result of this, sleepiness is induced due to the stimulation of the sympathetic nervous system, and the pineal gland begins to release melatonin.
Other functions controlled by the hypothalamus include sexual behavior, reproduction, and water intake.
Disorders of the Hypothalamus
Typical development of the hypothalamus is crucial to ensure that normal function can occur. Research has found that atypical development may lead to a plethora of issues such as depression and sleep disorders caused by abnormalities in the hypothalamic circuits.
Research has highlighted links between the prevalence of depression and the activity of the hypothalamic circuits. It has been noted that hyperactivity or dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis occurs in those diagnosed with major depressive disorder. As the HPA is responsible for acute cortisol responses to stress and the secretion of the hormone, dysfunctions can lead to a range of disorders such as depression. Specifically, the following trends have been found in those with depression which may indicate dysfunction of the hypothalamus: lowered expression of the glucocorticoid receptor; over secretion of cortisol; and reduced inhibitory feedback to the HPA induced by glucocorticoid.
The hypothalamus plays an integral role in the coordination and regulation of many bodily systems. Potential dysfunction of the area can result in a wide range of issues such as insomnia, mood disorders, and obesity.
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