Water soluble hormones can’t cross the plasma membrane due to their polarity. They bind to membrane receptors on the outside of cells. Secondary messengers then relay the signal from the outside to the inside of the cell. Lipid-soluble hormones are able to cross the plasma membrane and can directly activate genes. Amino acid hormones bind membrane receptor and G proteins are activated as well as adenylate cyclase. This results in the production of cAMP and a cascade of protein kinase.6 For the phospholipid pathway, amino acid hormone first binds to a membrane receptor. G protein is activated and phospholipase C is activated. Membrane phospholipid split into diacylglycerol (DAG) and inositol trisphosphate (IP3). DAG triggers a protein kinase cascade and IP3 releases Ca2+ from the endoplasmic reticulum. For the steroid pathway, steroid hormone (and thyroid hormone even though it is amino acid based) goes inside the cell. The hormone binds to its receptor inside the cell. A hormone-receptor complex (transcription factor) turns certain genes on inside the nucleus. Hormones travel long distances via the blood and lymph. Specificity of hormones depend upon the target cells having the receptors for the hormone, and non-target cells lacking receptors for the hormone. Cells can either upregulate or downregulate the receptors they express.6
The nervous system can modulate and override normal control of hormones based on the status of the body. For example, the body’s blood normal glucose level is set higher when you’re under stress. Hormones can modulate the nervous system. For example, low estrogen levels during menses give you a bad mood. Humoral glands directly respond to chemical levels in the blood (parathyroid respond to low blood calcium) while neural glands release hormones when stimulated by nerves (fight or flight response). Hormonal glands release hormones when stimulated by other hormones (tropic hormones).
1) Dorland’s (2012). Dorland’s Illustrated Medical Dictionary (32nd ed.). Elsevier Saunders.
2) Black, J.A., Sontheimer, H., Oh, Y., and Waxman, S.G. (1995). In The Axon, S. Waxman, J. Kocsis, and P. Stys, eds. Oxford University Press, New York, pp. 116–143.
3) Jessen KR, Mirsky R (August 1980). “Glial cells in the enteric nervous system contain glial fibrillary acidic protein”. Nature. 736–7
4) Hille, Bertil (2001) . Ion Channels of Excitable Membranes (3rd ed.). Sunderland,
Mass: Sinauer Associates, Inc. p. 5.
5) NV, B. (2002). Medical Biochemistry. San Diego: Harcourt/Academic Press. (lipid)
6) Vander, Arthur (2008). Vander’s Human Physiology: the mechanisms of body function. Boston: McGraw-Hill Higher Education. pp. 345-347