Nerves and Hormones

1. Nerves and Hormones

2. Control Systems The nervous and hormonal systems are critical in maintaining careful control of animal life. The nervous system uses receptors to gather information about the function of the body and the world outside. It then provides fast response to that information, by acting on glands or muscles. The hormonal system is made up of a number of glands throughout the body, which secrete hormones directly into the blood stream. These control a wide range of functions within the body. The action of the system is slower than the nervous system but has a more widespread and longer-lasting action.

3. The body is a complex interconnection of many different systems. Each system operates within a careful balance. If control is not kept, the systems don't work properly, we get sick and we would die. So it is important that we know how these systems work and what happens if it all goes wrong.

4. Nervous System Your nervous system is divided into two parts: the central and the peripheral nervous system. The central nervous system (CNS) is made up of the brain and the spinal cord. The peripheral nervous system is all the other nerve fibres that connect to it. Together, they take in information about the outside world through special receptor cells. Each cell responds to a specific stimulus. They then pass the information on as electrical impulses to the billions of nerve cells (neurones) that make up the nervous system.

5. Receptors Receptors are specialised nerve cells, which are adapted to respond to a stimulus. For example, each of our 'senses' has a particular group of receptors, which respond to a stimulus. Each receptor is specifically adapted to the stimulus to which it is sensitive. For example, the eye (see 'The Eye') is beautifully adapted to receive light stimuli. It might surprise you to learn that your ear is responsible for keeping you in balance! How bizarre!

6. The following table tells you where the receptors for each stimulus aresituated: Receptors pass electrical impulses to other neurones at tiny junctions called synapses. These signals allow the nervous system to co-ordinate a response.

7. What happens at a synapse? Neurones passing electrical impulses on their own isn't much use. It's the same as people not talking. Neurones 'talk' by passing a small amount of a chemical messenger between them across the synapse. Thisneurotransmitter then sets up the electrical impulse in the second neurone, and so it carries on. Drugs, poisons and other chemicals can effect synapses by interfering with how the neurotransmitter is dealt. Neurone structure There are billions of these nerve cells inside you. There are more of them than there are people on the Earth! One estimate is 1,000,000,000,000 neurones.

8. Neurones are beautiful cells which come in a huge variety of shapes and sizes. Their sizes range from about a two hundredth of a millimetre to a tenth of a millimetre. However, whatever shape they are, neurones share basic features in common.

9. Like all cells, neurones have a cell body and a nucleus surrounded by a cell membrane. They have extensions from the cell membrane called dendrons, which have even finer branches called dendrites. The dendrites overlap between neurones and allow communication between themselves at sites called synapses. Each neurone can make between 200 and 10000 synapses. Many neurones have one dendrite called the axon, which is longer than the others. In some neurones, the axon has other cells wrapped around it like sausages threaded onto a string. These cells contain a fatty chemical called myelin. This myelin sheath of cells insulates the axon and speeds up the nerve impulses passing down it.

10. We can put neurones into three groups on the basis of what they do: These different types of neurone fit together to allow a co-ordinated response. One example of this is in a reflex.

11. Reflex action Reflexes follow this series of steps: A reflex is a very fast, pre-programmed response to a stimulus. They are automatic so that you don't need to think about it beforehand. They act to protect the body. The stimulus is picked up by a receptor, which transmits an impulse to a sensory neurone. This neurone passes the impulse to the co-ordinator, the central nervous system. The CNS co-ordinates the signal and transmits back a response via a motor neurone. The response is carried out by the effector organ, which is either a gland or a muscle.

12. Now lets look at a real reflex, like what happens when you put your hand on something hot. Hot flame acts as a stimulus for skin heat receptors Sensory neurone carries impulse to the spinal cord (part of CNS) Relay neurone links the sensory and motor neurones and passes an impulse Motor neurone carries impulse to the effector organ Effector organ contracts and move arm as a response

13. Sort the descriptions of each event into the correct order:

14. The whole mechanism of the reflex is called a reflex arc, as it isn't a complete loop. You've probably also noticed that the brain isn't directly involved here. So how is it that we know when we do something like this? A side branch of the sensory neurone goes up to the brain - so that we shout 'Ow', or something slightly ruder! This also means that the brain can try and override the reflex.

15. Other reflexes Click on the 'yes' or 'no' buttons to say whether or not the statement is an example of a reflex. Then, check your answer:

16. Hormonal System The hormonal system is the second important control system in the body. It is closely connected with the nervous system, but is also distinct. The pituitary gland in the base of the brain has a key role in secreting many hormones. It acts as a controlling gland that influences many others. Hormones have the advantage of having a widespread, powerful and long-lasting effect on the body's cells. The disadvantage is that the effect is often slower than the nervous system. But together, they are perfect!

17. What are hormones? Hormones are proteins that act as chemical messengers. They are secreted - that is they are released from glands into the blood stream. Hormones are then carried around the blood so that they can reach every cell. How do they affect cells? They are an easy way to communicate with every cell in the body. Not every cell has the right receptor molecules on the surface of its cell membrane. The hormone molecules only bind to those cells that do have the receptor molecules. This effects enzymes inside the cells and causes changes to the cell's function.

18. This is similar to the way that enzyme molecules bind to their substrates.

19. Hormones in action Hormones control a wide variety of things in the body including the amounts of water and glucose. Hormones also control important functions including the production of eggs and sperm. They effect our growth, repair our cells, produce heat, and so on. The pituitary glandcontrols the volume of water in the body by secreting anti-diuretic hormone (ADH) as part of homeostasis. The pituitary gland also controls other glands throughout the body. Hormones form an essential part of our body's control system.

20. This diagram shows some of the important glands:

21. The Pancreas: Controlling Glucose The pancreas is a leaf-shaped organ just below the stomach: It has a complex function in the body. It secretes an alkaline solution containing enzymes into the digestive tract. But it also contains cells that secrete hormones into the blood stream. Two of these are important in controlling the level of glucose in the blood stream.

22. Why is glucose important? Glucose is the basic fuel for aerobic respiration and is needed by all cells. The heart and brain use glucose as their sole fuel supply. The blood stream delivers it from the digestive system to all the cells in the body. Without the ability to maintain its levels, we would be in danger of running out of this important fuel.

23. Storing glucose for later If the blood glucose levels fall, we feel light-headed and run the risk of passing out, coma and death! We could only maintain our blood glucose by eating and digesting all the time. But our body has a system to save us from that. It stores extra glucose for later. The liver converts the simple sugar glucose into a long-chained molecule called glycogen. Glycogen is used as it is insoluble and doesn't need so much water to store it, unlike glucose.

24. This reaction can also be reversed: This process controlled by the two hormones secreted by the pancreas.

25. Too little glucose When there is too little glucose in the blood, it is detected. The pancreas secretes the hormone glucagon. This causes the liver to convert glycogen back into glucose. When this gets back into the blood it returns the glucose level to normal.

26. Too much glucose When we have too much glucose in our blood, we secrete insulin to store it for later.

27. Diabetes Diabetes mellitus is a disease in which the pancreas cannot produce enough insulin. As a result, a diabetic cannot store glucose as glycogen for later use. So they use up all the glucose in their blood and then go into a coma. One treatment is to inject insulin after a meal. This stores the extra glucose as glycogen. Therefore they will have enough glucose for later on. Diabetics have to test their blood regularly to monitor their blood glucose. When it gets too high they inject insulin. If it is too low, they eat something.