Thalamus

The thalamus lies deep in the brain, close to the third ventricle. It’s part of the diencephalon, which also includes the epithalamus, subthalamus, and hypothalamus. Functionally, the thalamus acts as a relay station for sensory and motor information before it reaches the cerebral cortex. It receives all sensory modalities except smell (olfaction).

The thalamus contains more than 100 nuclei. The main nuclear groups are anterior, lateral, medial, and ventrolateral.

• 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 projects to the cingulate gyrus
  • Lateral posterior, which connects to the parietal cortex
• Ventrolateral nuclei are the largest group and are further divided into:
  • Ventral anterior (VA)
  • Ventral lateral (VL)
  • Ventral posterolateral (VPL)
  • Ventral posteromedial (VPM)

VA and VL receive inputs from the basal nuclei and project to the motor cortex. VPL and VPM act as sensory relays for the body and face, respectively. The medial geniculate body processes auditory information, while the lateral geniculate body processes visual information.

Epithalamus

The epithalamus consists of the pineal gland, habenula, habenular commissure, and posterior commissure.

The 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.

The pineal gland (also called the epiphysis) is a pea shaped gland that secretes melatonin. Melatonin helps regulate sleep patterns and circadian rhythm. Melatonin inhibits the secretion of LH and FSH by inhibiting GnRH.

Subthalamus

The major anatomical components of the subthalamus are the subthalamic nucleus, zona incerta, fields of Forel, and ansa lenticularis. The subthalamic nucleus is a component of the basal ganglia. Damage to the subthalamic nucleus may cause movement disorders like hemiballismus.

Hypothalamus

The hypothalamus lies in the wall of the third ventricle, above the midbrain and below the thalamus. It contains anterior, posterior, and middle nuclei.

The 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 consist of supramammillary, mammillary, intercalate, and posterior nuclei.

The middle/ tuberal group consists of infundibular, tuberal, dorsomedial, ventromedial, and lateral nuclei.

• The satiety center is located in the medial nuclei.
• 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. These axons transport ADH and oxytocin, which are released from the nerve terminals.

Pituitary gland

The pituitary gland (hypophysis cerebri) is often called the “master gland” of the endocrine system. It lies within the sella turcica of the sphenoid bone. It is divided into anterior (adenohypophysis) and posterior (neurohypophysis) lobes. The anterior lobe can be further divided into pars distalis, pars tuberalis, and pars intermedia. Because of its unique function, the pituitary gland has a rich vascular supply.

The adenohypophysis contains three distinct cell types:

• 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 acidophils or basophils that have emptied their hormones, or they may be stem cells.

The neurohypophysis is made of the median eminence (technically in the hypothalamus), the infundibular stalk (connects hypothalamus and posterior pituitary), and the infundibular process. It consists chiefly of unmyelinated axons from 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. These hormones are released in response to their respective releasing hormones, which are produced in the hypothalamus and reach the anterior pituitary via the pituitary portal blood vessels. Prolactin is an exception because it is under control of dopamine.

No hormones are synthesized in the posterior pituitary. Instead, oxytocin and ADH/vasopressin are made in hypothalamic nuclei and stored in Herring bodies in the pituitary.

Hypothalamo Hypophyseal Portal system

This blood vessel system transports hormones from the median eminence of the hypothalamus to the anterior pituitary. The superior hypophyseal arteries (branches of the internal carotid artery) form the primary plexus in the median eminence. This plexus 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.