[Music] hi and welcome back to free sighs lessons cold okay by the end of this video you should be able to describe how the structure of the nervous system is related to its functions you should then be able to describe the structure of the reflex arc and explain why reflex actions are important in the last video we saw that the human body keeps internal conditions as constant as possible and scientists call this homeostasis homeostasis involves automatic control systems and I'm showing you one of those here remember that all automatic control systems have a stimulus a receptor a Coordination Center an effector and a response now a key part of homeostasis involves the nervous system the nervous system consists of two parts we've got the central nervous system or CNS which is the brain and the spinal cord and then we've got other nerves running to and from the central nervous system so going back to a previous diagram receptors detect a stimulus ants and electrical impulses down new loans to the central nervous system and the word new loans is another way of saying nerve cells the central nervous system is the Coordination Center this now sends electrical impulses down other neurons to affect us and the effectors bring about a response effectors are usually a muscle which contracts over gland which secretes a hormone now a key role of the nervous system is that it enables humans to react to their surroundings and coordinate their behavior and one way that this is done is through the reflex arc the reflex arc is a very common examine so you need to make certain that you learn all the stages we're going to look at what happens when you touch a hot object first the stimulus is detected by a receptor and in this case the stimulus is heat and the receptors in the skin electrical impulse has now passed from the receptor along a sensory neuron to the central nervous system remember that sensory neurons are connected to receptors and receptors allow us to sense our environment at the end of the sensory neurone there's a junction caller sign ups other sign ups a chemical is released on this chemical now diffuses across to a relay neurone in the central nervous system where it triggers an electrical impulse the electrical impulse now passes across the relay neurone and reaches another synapse once again a chemical is released this chemical triggers an electrical impulse in a motor neuron the electrical impulse now passes down the motor neuron to an effector in this case the effect was a muscle the muscle now contracts aren't poles are harmed away from the heat and remember that pulling the hand away is called the response now there is one important point that I need to make about reflexes earlier in the video we said that the central nervous system act as a coordinator and in most cases the brain actually makes decisions about what action to take however in the case of reflexes there is no decision-making by the conscious part of the brain this makes reflexes automatic and rapid so because their rapid reflexes help to protect us from danger remember you find plenty of questions on the nervous system on the reflex arc in my vision workbook and you can get that by clicking on the link above [Music] you [Music]