What are Neurohormones? How do these Neurohormones reach the pituitary glands?
The hypothalamus is located inferior to the thalamus of the brain and it secretes neurohormones. Neurohormones are secreted by neurosecretory cells located within supraoptic and paraventricular nuclei of the hypothalamus. These hormones influence the activity of pituitary glands. These neurohormones secreted by the hypothalamus are connected to the pituitary gland by hypophyseal portal vessels. The neurohormones from the hypothalamus reach to the anterior pituitary gland through the portal vein blood after passing through the stalk. These neurohormones control the secretion of hormones by the cells of the pituitary gland. The pituitary gland hangs below the hypothalamus and is attached to the later by the pituitary stalk.
Some neurosecretory cells send their axons to the posterior pituitary through pituitary stalks. These axons synthesize two non-peptide hormones- Vasopressin and Oxytocin at their terminals, which are then transported as neurophysin– protein bond secretory granules down the nerve fibres. These are then transported into the blood inside the posterior pituitary when they are stimulated.
The various neurohormones are secreted by hypothalamus which may be releasing hormone (stimulate secretion of pituitary hormones and inhibiting hormones which inhibit the secretion of pituitary hormones).
Thyrotrophin Releasing Hormone (TRH): It stimulates the anterior pituitary lobe to release thyroid-stimulating hormone (TSH) or thyrotropin.
Adrenocorticotrophin Releasing Hormone (ARH): It stimulates the anterior pituitary lobe to secrete adreno cortico trophin or adreno cortico trophic hormone (ACTH).
Gonadotrophin Releasing Hormone (GRH): It stimulates the anterior pituitary gland for the secretion of gonadotropin i.e. Follicle Stimulating Hormone (FSH) / Luteinising Hormone (LH) in ovaries and Interstitial Cell Stimulating Hormone (ICSH) in testes.
Somatotrophin Releasing Hormone (SRH): It stimulates the production of growth hormones from the anterior lobe of the pituitary gland.
Somatotrophin Inhibiting Hormone (SIH): It inhibits the adenohypophysis to stop the secretion of growth hormone.
Prolactin Releasing Hormone (PRH): It stimulates the adenohypophysis to secrete prolactin hormone.
Prolactin Inhibiting Hormone (PIH): It stops the secretion of prolactin from the anterior pituitary.
Melanocyte Stimulating Hormone: It induces the intermediate pituitary lobe to secrete melanocyte-stimulating hormone.
Melanocyte Inhibiting Hormone: It stops the synthesis of MSH from the middle pituitary gland.