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Nervous and Hormonal Control
In order to protect itself the body has developed ways react to changes in its environment. For example, if you get too hot you will sweat to cool down. If light is too bright, your pupils will constrict so that your eyes are not damaged.
In order to do this, the body needs to be able to detect internal and external changes (called stimuli) and make the appropriate response.
To function effectively, there needs to be good communication inside the body.
Hormones are just one of the tools used to send messages to the various parts of the body.
(You also need to understand how nervous impulses are used to send messages within the body, and S-cool! has covered this in the nervous system section - see the nervous system topic section).
Hormones are usually small molecules made by a gland. They are secreted following a suitable stimulus and transported in the blood.
Blood carries hormones to a target organ or group of cells which will recognise the hormone (this triggers a specific chemical response when the correct receptor is activated). The behaviour of the target will then change, bringing about the right response.
Hormones need to combine with specific receptor molecules on, or in, a target cell to have an effect.
There are two structural types of hormone - protein and steroid. They have different ways of binding to and affecting a cell, and you will need to understand these for your exams.
Examples of protein hormones are insulin, glucagon, and adrenaline (try and remember these).
Protein hormone molecules bind with receptors on the surface of a cell membrane. This starts off a chain reaction inside the cell.
Lets look at the protein chain reaction, step by step:
The receptor may change shape and then bind to a protein in the membrane called G protein. As a result, an enzyme called adenyl cyclase (also present in the cell membrane) increases in activity.
This triggers the conversion of ATP (adenosine tri-phosphate) into cAMP (cyclic adenosine mono-phosphate on the inner surface of the cell membrane) in the cytoplasm.
This increase in cAMP levels may activate specific enzymes called protein kinases or activate relevant genes so that the appropriate enzymes can be synthesized (see Genetic Code topic).
The particular enzyme will catalyse a reaction, e.g. in the case of glucagon and adrenaline, glycogen is broken down into glucose. In the case of insulin, glycogen is built up from glucose.
In this process the hormone is known as the first messenger, and cAMP is therefore known as a second messenger. One hormone molecule can cause many cAMP molecules to be formed. At each stage, the number of molecules involved increases so the general process is called cascade amplification.
Examples: testosterone, oestrogen.
Steroid hormones are different to protein hormones in that they cross the cell surface membrane and bind to receptors in the cytoplasm. These hormone- receptor complexes then enter the nucleus.
These complexes then bind to specific DNA sequences so that the transcription rate (this is the rate at which messenger RNA for protein synthesis is formed - see the Genetic Code topic) of the appropriate gene is increased. A protein or enzyme will be produced that will alter the cell's behavior and so bring about the desired response.