Table of Contents
Hormones are a heterogenous group of substances secreted by internal structures (endocrine glands) directly into the bloodstream. Thus, hormones are disseminated by the circulation to all the cells, but they act only on certain cells which have specific receptors for their action.
They convey a message broadcast to every cell of the body, but only the target cells can decode such message. Hormones exert a prolonged effect upon cell metabolism, although they also contribute to the regulation of signal transmitted between neurons. Hormones may not generate postsynaptic spike discharges, but rather modify release of the primary neurotransmitter or adjust the receptivity or sensitivity of the postsynaptic effects of primary transmitters. So the action of the primary neurotransmitter would be slower or and more prolonged, for example, cholecystokinin (CCK) and dopamine work together in some neurons. CCK may have an enhancing effect on dopamine in these neurons. A substance can also act both as a neurotransmitter and hormone , for example, dopamine acts as a neurotransmitter in the caudate nucleus, while it acts as a hormone in the pituitary. Some nerve cells secrete hormones such as those in the pituitary and the hypothalamus.
The hypothalamus plays an important role in human behaviour. It has a direct control on eating and sleep and other body functions. It also controls the autonomic nervous system. Anterior pituitary hormones are under the control of hypothalamic hormones and posterior pituitary hormones are released by the hypothalamus.
1- Thyrotropin releasing hormone (TRH) TRH is a tripeptide released mainly by the dorsomedial, ventromedial, and arcuate nuclei of the hypothalamus. It stimulates release of the pituitary thyroid-stimulating hormone (TSH) and prolactin.
2-Gonadotrophin releasing hormone (GRH) GRH is the decapeptide (peptide was at 10 molecule chain) released mainly by the arcuate and pre-optic areas of the hypothalamus. GRH principally stimulate release of the Leutinising hormone (LH) and to a lesser extent follicle stimulating hormone (FSH). The release of LH and FSH depends also on differential feedback and inhibition by the level of LH and FSH in blood.
3-Growth Hormone Releasing Hormone (GHRH) is released from the arcuate and ventromedial nuclei. It has much large structure than other hormones.
4- Somatostatin: This hormone is released from the periventricular region and it has a role in inhibition of the growth hormone and a wide range of other inhibiting activities.
5-Corticotropin Releasing Factor: CRF is released from the paraventricular nuclei to stimulate the release of propiocortin peptides.
6-Prolactin Inhibiting Factor: Dopamine is released from the arcuate nuclei to inhibit the secretion of prolactin. Dopamine is a single chain polypeptide, similar structure to GH which is produced by lactotroph cells in the anterior pituitary.
7-Vasopressin And Oxytocin: These neuro hypophyseal peptides are merely formed in supra-optic and paraventricular nuclei.