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
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
