THE NERVOUS SYSTEM 1 of 14 © Boardworks Ltd 2011 What are reflexes? Reflexes are fast, automatic, protective biological control systems that link a stimulus to a response. A stimulus is a change in the internal or external environment of an organism. Reflex reactions happen without you having to think about them – they are involuntary. This is because the central nervous system (CNS) sends electrical signals to the muscles before the brain can pick up and process the message. Many reflexes, such as sneezing and focusing your eyes, occur naturally, but other reflexes can be learned, i.e. conditioned responses. 2 of 14 © Boardworks Ltd 2011 Reflexes and animals Reflex reactions govern much of how the simplest animals behave and help to ensure their survival. Reflex actions in simple animals help them to: shelter from predators find food. Humans also demonstrate simple reflexes. These include newborn reflexes, such as grasping or sucking, pupil reflexes and knee-jerk reflexes. 3 of 14 © Boardworks Ltd 2011 Why are reflexes so fast? Reflexes need to be fast in order to protect the body. If you pick up a hot plate, the reflex action will make you drop it almost immediately. This is to protect your hand from burning. The quicker you drop the plate, the less damage will be done to your hand. However, we can consciously override reflexes. If the hot plate had your dinner on it, you might try to resist the reflex to drop it so that you could put it down safely. Reflexes happen so quickly because they often only involve three neurones – sensory, relay and motor neurones. 4 of 14 © Boardworks Ltd 2011 5 of 14 © Boardworks Ltd 2011 Responses Neurones are specialized cells that convey information in the form of nerve impulses. The nervous system uses electrical impulses to ensure a swift response to any internal or external stimuli. Hormones (chemicals secreted by glands) travel in the blood and produce slower and longer-lasting responses; examples include testosterone and insulin. In single-cell organisms, hormones and a nervous system are not necessary. These systems of communication have developed with the evolution of multi-cellular organisms. 6 of 14 © Boardworks Ltd 2011 What are receptors and effectors? 7 of 14 © Boardworks Ltd 2011 SYNAPSES Where two neurones meet, there is a tiny gap called a synapse. Signals cross this gap using chemicals. One neurone releases the chemical into the gap. The chemical diffuses across the gap and makes the next neurone transmit an electrical signal. http://www.bbc.co.uk/schools/gcsebitesize/scien ce/aqa/human/thenervoussystemrev2.shtml 8 of 14 © Boardworks Ltd 2011 What is a synapse? A synapse is a junction between two neurones across which electrical signals pass. The human body contains up to 500 trillion synapses. presynaptic cell postsynaptic cell 9 of 14 © Boardworks Ltd 2011 The release of neurotransmitters When a nerve impulse arrives at the end of one neurone it triggers the release of neurotransmitter molecules from synaptic vesicles. synaptic vesicle neurotransmitter molecules 10 of 14 © Boardworks Ltd 2011 Continuing the impulse The neurotransmitters diffuse across the synaptic cleft and bind with receptors on the next neurone, triggering another impulse. synaptic cleft nerve impulse receptor 11 of 14 © Boardworks Ltd 2011 Stimulus to response 12 of 14 © Boardworks Ltd 2011 Key terms 13 of 14 © Boardworks Ltd 2011 14 of 14 © Boardworks Ltd 2011 Glossary 15 of 14 © Boardworks Ltd 2011 Multiple-choice quiz 16 of 14 © Boardworks Ltd 2011
