In GCSE Science students will look at how organisms use nerves and hormones. This is the second of five quizzes on that topic and it looks in particular at how nerves and hormones carry messages around the body.
The nervous system enables humans to react to their surroundings and coordinate their behaviour. Receptors detect stimuli outside the body and relay messages to the central nervous system and brain for processing. Certain actions are voluntary, like walking or waving your hand, and are dealt with by the brain. Others are automatic and save the body from harm e.g. pulling your hand away from fire or your pupils closing up when a light is too bright. These are called reflexes and the messages from the sensors are processed in the central nervous system without needing the brain. Other messages are automatic too, like the ones telling the heart muscles to contract and relax. These are autonomous and continue all of the time.
The body also needs to control its temperature, ion content, water content and blood sugar levels. The nervous system plays its part but most of the control is done by hormones. They have a longer lasting effect and act more widely than a nerve or group of nerves. Hormones are secreted by glands and can spread throughout the body in the bloodstream. It is therefore possible for hormones to reach every cell in the body. If a cell has the right receptor chemicals, a hormone molecule will attach itself to the cell and make the cell carry out a specific action.
For example, if the blood sugar (glucose) level is high, the pancreas has cells that secrete the hormone insulin. This mixes with the blood plasma and is distributed around the body. The target organ for insulin is the liver, where glucose is changed into glycogen which can be stored and used to replenish blood sugars when the level is low. So you can see how nerves and hormones are used to carry messages around our bodies.