Both the nervous and endocrine systems are involved with communication among body parts, but they exert their actions by different methods. The nervous system uses electrochemical impulses to convey messages through nervous tissue at high speeds, while the endocrine system uses chemical hormones that are released into the blood and directly interact with their target cells. There is some evidence that the nervous amd endocrine systems are evolutionarily related.
Hormones can be of two major types chemically: protein (including amines and peptides) or steroid. Each type of hormone normally exerts its effects differently. Protein hormones, such as insulin, adrenalin, ADH, and gonadotropin-releasing hormone (GnRH) interact with a membrane bound extracellular receptor on their target cells, which causes changes inside of the cell. This usually leads to the activation or inhibition of enzymes through a second-messenger system utilizing cyclic AMP (cAMP) as the second messenger. Steroid hormones such as testosterone, estrogen, and cortisol can diffuse through cell membranes, and usually act by affecting gene expression, but only after interacting with an intracellular receptor.
McCune-Albright syndrome is a genetic disease in which cAMP is synthesized in large quantities even when a signaling hormone is not present; it results in the over-secretion of many hormones, and increased effects of many hormones on their target cells.
The major connection between the nervous and endocrine systems that allows them to coordinate their activities is represented by: