femme Neuroscience

Some Topics

Hypothalamus

The automatic processes in the brain represent the most innate features of the human brain, programmed in DNA and expressed in individual humans without their knowledge or permission. The vital nature of brain stem, medulla, basal ganglia, hypothalamus and limbic system nuclei only becomes apparent when dysfunction or damage occurs. The hypothalamus has been intensely studied and much is known about its anatomy and physiology. A brief summary follows:

The hypothalamus is composed of several small nuclei with diverse functions, including synthesizing and secreting neurohormones. The hypothalamus connects the brain to endocrine systems in the body via the pituitary gland , regulating homeostatic functions such as hunger, thirst, body temperature, and circadian rhythms.

There are lateral and medial nuclei in three subdivisions:
1. Anterior (or chiasmatic) region, which extends between the lamina terminalis and the anterior infundibular recess
2. Median (or tuberal) region, which proceeds to the anterior column of the fornix
3. Posterior (or mammillary) region, which stretches to the caudal mammillary bodies

The lateral region is largest, but the medial and periventricular regions contain the majority of hypothalamic nuclei. The lateral region contains the medial forebrain bundle which connects the hypothalamus to the limbic system and brainstem autonomic centers. Through the MFB, signals from the brainstem, amygdala, hippocampus, retina, and olfactory system are conveyed to hypothalamic nuclei, underlying the role of the hypothalamus in homeostasis. The medial preoptic nucleus generates gonadotropin-releasing hormone (GnRH); also known as the sexually dimorphic nucleus; its growth is regulated by testosterone exposure in utero and its volume is increased 2-fold in males compared with females. Its role in both sexes relates to sexual behavior and partner preference. The supraoptic nucleus is composed of neurosecretory cells, which produce vasopressin and oxytocin which are released from the posterior pituitary gland.

The many nuclei of the hypothalamus have diverse connections with other parts of the brain. Input from the limbic system arrive via the MFB, stria terminalis, and fornix. The stria terminalis originates at the amygdala and the fornix at the hippocampus. In addition, the brainstem reticular formation sends afferents via the dorsal longitudinal fasciculus, and afferents from the basal nuclei (formerly, basal ganglia) arise in the nucleus accumbens and travel via the substantia innominata. Collectively, these fiber bundles enter the rostromedial hypothalamus near the third ventricular surface, and then they course diagonally through the length of the lateral hypothalamus and deposit axon terminals among its nuclei.

The mammillothalamic tract is an important efferent that links the hypothalamus and thalamus to the limbic system. The ventromedial nucleus of the hypothalamus regulates food intake. Stimulation increases food intake. Bilateral lesion of the medial part of the ventromedial nucleus causes hyperphagia and obesity of the animal. Further lesions of the lateral part of the ventromedial nucleus produce complete cessation of food intake.

Zink et al reviewed the role of hypothalamic orexin producing neurons: "The hypothalamus plays a central role in the control of physical activity, which is executed through coordination of multiple signaling systems, including the orexin neuropeptides. Orexin producing neurons integrate physiological and metabolic information to coordinate multiple behavioral states and modulate physical activity in response to the environment. In the mammalian brain, orexin neurons are concentrated in the lateral hypothalamus (LH), perifornical area, and dorsomedial hypothalamus.Orexin neuron activity is affected by variety of metabolic signaling molecules (i.e., glucose, leptin, amino acids) and environmental factors. Orexin neurons regulate physiological and behavioral processes that have major impacts on energy balance XE "Energy balance" and metabolic state, notably physical activity, blood glucose, levels, and food intake. Obese humans have lower circulating levels of orexin and impaired orexin receptor activity in adipose tissue. Human narcoleptic patients suffer from extreme episodes of daytime sleepiness; post mortem tissue analysis showed a loss of orexinin the hypothalamus. Diminished physical activity , blunted circadian rhythms, and cognitive deficits associated with aging could be explained by compromised orexin signaling in the aged brain."[i]

The pituitary gland is a functional extension of the hypothalamus. The pituitary gland has 2 parts: The anterior pituitary receives hormones from the hypothalamus that either stimulate or inhibit secretion of pituitary hormones. These hormones are secreted directly into the systemic circulation, where they act on specific organs. The hypothalamus secretes its signaling hormones into blood vessels that go to anterior pituitary where the hormone producing cells reside.

Hormones secreted by the anterior pituitary include the following:

Thyrotropin, or thyroid-stimulating hormone (TSH)

Gonadotropins, or follicle-stimulating hormone (FSH)

Lluteinizing hormone (LH)

Somatotropin or growth hormone (GH)

Corticotropin, or adrenocorticotropic hormone (ACTH)

Prolactin

Abnormalities in pituitary function have been well described in the medical literature and provide insight in to the glandular-hormonal identity of the brain. Hormones are slow-acting, whole body signals that represent one of the oldest signaling life-systems. Ever more complicated interactions and feedback loops that connect the hypothalamus, pituitary, endocrine glands, metabolism and behavior are being revealed by researchers who usually specialize in narrowly focused study of a growing list of neuropeptides, neurotransmitters and circuits within the brain.

[i] Anastasia N Zink, et al. The orexin neuropeptide system: Physical activity and hypothalamic function throughout the aging process. Frontiers in Systems Neuroscience. 07 Oct 2014