Thalamus
It is located deep in the brain, in close proximity to the third ventricle. It is a component of the diencephalon which also consists of the epithalamus, subthalamus and hypothalamus. It acts as a relay center for sensory and motor systems before being projected onto the cerebral cortex. It receives all types of sensory modalities except smell sensation. The thalamus comprises of more than 100 nuclei. The main groups are anterior, lateral, medial and ventrolateral.
The anterior nuclei are associated with the limbic system. Medial nuclei connect to the frontal association cortex and premotor cortex. Lateral nuclei are divided into lateral dorsal which project to the cingulate gyrus and lateral posterior which connect to the parietal cortex. The ventrolateral nuclei are the largest group and are further divided into ventral anterior (VA), ventral lateral (VL), ventral posterolateral (VPL) and ventral posteromedial (VPM). VA and VL receive inputs from the basal nuclei and project to the motor cortex. VPL and VPM are sensory relays for the body and face respectively. Medial geniculate body processes auditory information while lateral geniculate body processes visual information.
Epithalamus
It is made of pineal gland, habenula, habenular commissure and posterior commissure. Habenula is involved in behavioral responses to pain, stress, anxiety, sleep and reward. Dysfunction of the habenula is seen in depression, schizophrenia and drug induced psychosis. Pineal gland is also called epiphysis. It is a pea shaped gland that secretes melatonin which is involved in the regulation of sleep patterns and circadian rhythm. Melatonin inhibits the secretion of LH and FSH by inhibiting GnRH.
Subthalamus
It consists of the subthalamic nucleus, red nucleus and substantia nigra. They are involved with movement.
Hypothalamus
It lies in the wall of the third ventricle, above the midbrain and below the thalamus. It has anterior, posterior and middle nuclei.
Anterior/chiasmatic nuclei are further divided into preoptic, supraoptic and paraventricular groups. The preoptic nuclei secrete GnRH. The lateral preoptic nuclei mediate NREM sleep. The supraoptic and paraventricular nuclei secrete ADH/ vasopressin and oxytocin. The suprachiasmatic nucleus is important in regulation of circadian rhythms.
The posterior nuclei consists of supramammillary, mammillary, intercalate and posterior nuclei. The middle/ tuberal group consists of infundibular, tuberal, dorsomedial, ventromedial and lateral nuclei. Satiety center is located in the medial nuclei while hunger is localized to the lateral nuclei. Parasympathetic control is a function of anterior and medial nuclei. Anterior nuclei play a key role in thermoregulation and circadian rhythms. Sympathetic control is a function of posterior and lateral groups.
Unmyelinated axons pass from the supraoptic and paraventricular nuclei of the hypothalamus to the posterior pituitary. They transport ADH and oxytocin, which are released from the nerve terminals.
Pituitary gland
Also called hypophysis cerebri or “master gland” of the endocrine system. It is located within the sella turcica of the sphenoid bone. It can be divided into anterior (adenohypophysis) and posterior lobes (neurohypophysis). Anterior lobe can be further divided into pars distalis, pars tuberalis and pars intermedia. Due to its unique function, the pituitary gland has a rich vascular supply.
The adenohypophysis shows three distinct types of cells - acidophils, basophils and chromophobes. The acidophils stain red or orange on H and E. They produce polypeptide hormones and have large granules. Examples are somatotrophs and lactotrophs. Basophils stain bluish. They produce glycoprotein hormones. Examples are thyrotrophs, gonadotrophs and corticotrophs. Chromophobes do not stain prominently. These are actually acidophils or basophils which have emptied their hormones or are stem cells.
The neurohypophysis is made of median eminence (technically in the hypothalamus), infundibular stalk (connects hypothalamus and posterior pituitary) and infundibular process. It consists of chiefly unmyelinated axons from the hypothalamic secretory neurons arising in the supraoptic and paraventricular nuclei. In addition, glial cells and pituicytes are seen. It has fenestrated capillaries. The paraventricular nuclei produce oxytocin while the supraoptic nuclei produce ADH.
There are 6 major hormones produced in the anterior pituitary - growth hormone, prolactin, ACTH, TSH, FSH and LH. MSH and beta endorphins are also produced in the anterior pituitary. They are released on stimulation by their respective releasing hormone which is produced in the hypothalamus and reaches the anterior pituitary via the portal blood vessels of the pituitary. Only prolactin is an exception as it is under control of dopamine. No hormones are synthesized in the posterior pituitary but in fact oxytocin and ADH/vasopressin are made in the hypothalamic nuclei and stored in Herring bodies in the pituitary.
Hypothalamo Hypophyseal Portal system
This system of blood vessels transports hormones from the median eminence of the hypothalamus to the anterior pituitary. The superior hypophyseal arteries , which are a branch of the internal carotid artery, form the primary plexus in the median eminence, which then drains via the hypophyseal portal veins into a secondary plexus in the adenohypophysis. This system has fenestrated capillaries.
The pituitary can be surgically accessed by the transsphenoidal approach via the nasal cavity.
