This is a different process to the nervous system, in the nervous system; the origin of the stimuli is usually from receptor cells, where as in the endocrine system, the origin of the stimuli is the presence of metabolite in the blood. Where as in the endocrine system, a chemical messenger is emitted, in the nervous system an electrical messenger is emitted. The electrical messenger is transmitted through the nerve cells; this process is a lot faster than the endocrine system, which enables the response to be that much faster as well. Another noticeable difference in the two systems is in the type of receptors, in the endocrine system the type of receptors is cells, while in the nervous system there are various types of receptors including heat, pressure. As in the endocrine system, the effects of the messenger generally takes a long time, in the nervous system the effect is immediate. And the response is also very localised sometimes directed at just one muscle.
As I mentioned earlier, in the endocrine system, the function of the endocrine system is to regulate the body functions by the use of hormones. There are two types of hormones. Steroid hormones and Amino-acid based hormones. Steroid hormones are produced by the Adrenal Cortex, the Ovaries, and the Testes. Steroid hormones don’t have to bind with the plasma membrane receptors of the target cells; they enter freely.
Within the cytoplasm, a steroid hormone can bind with a receptor protein; it will then enter the cell nucleus and bind with the DNA, where it will trigger changes in the chromosomes. The Hormone –Receptor unit attaches to the DNA, this attachment activates certain genes in that cell, causing particular proteins to be produced. Some examples of steroid hormones are testosterone and the female sex hormones oestrogen and progesterone.
Amino Acid based hormones are made up of amino acids, hormones made from Amino Acids; because peptide hormones are polar molecules (posses both positive and negative ends) they cannot diffuse through the Plasma Membrane to their target Cells.
Peptide hormones must send their message from outside the target cell (a two messenger system is commonly required for the action of most of these hormones) Peptide hormones attach to receptors on the plasma membrane, the first messenger carrying the message from the endocrine Gland to the Cell Surface, passes the message to another molecule inside the cytoplasm. Meanwhile the second messenger (the hormone-receptor complex) indirectly activates an enzyme that converts molecules of ATP to cyclic AMP inside the target Cell.
Cyclic AMP acts as a second messenger by indirectly activating other enzymes and proteins in the target cell. Therefore the AMP initiates a chain of biochemical events that leads to functional changes within the target cell. A single hormone molecule binding to a receptor in the plasma membrane can result in the formation of many Second Messengers.
