When it comes to large multisellular organisms like humans, the different parts of our body need some way of communicating with each other so that we can coordinate our behavior and respond to our surroundings. To do this, we rely on two different organ systems. The nervous system, which we'll discuss here, and the endocrine system, which we'll see in another video. The main things you need to know for this topic are the structure of a nerve cell, the structure of the whole nervous system, and how a reflex arc works. Let's start with an individual neuron, which we can also call a nerve cell. There are actually loads of different types of neurons, but they all look something like this because they're adapted to carry electrical impulses from one point to another. The main thing to notice here is that they're long, they're thin, and they have lots of branch connections to either end, which enables them to pass messages onto other nerve cells. Effectively, they're just a biological version of a wire carrying a small current. For one nerve cell to communicate with another, though, we need a sinapse, which is this connection between the cells. Whenever an electrical impulse hits the end of a nerve, it causes the release of some chemicals which diffuse across the gap to the next nerve cell where they can trigger another electrical impulse. This new electrical impulse will then continue along the new neuron all the way to the other end where it will hit another sinapse and the whole thing will start over. Now imagine that you took 100 billion of these and you added them all together. What you could have now is the whole nervous system. One part of this is the central nervous system which is made up of our brain and our spinal cord. This is where all the so-called thinking takes place. It takes in loads of sensory information, decides what needs to be done about it, and then sends out orders to the rest of the body. To do this, it needs information about what's happening in the rest of the body though, which it receives through sensory neurons. These carry information from receptors all over our body to the central nervous system. So this could be information about changes in temperature or about the level of carbon dioxide in our bloodstream. Once our CNS, which just means central nervous system, has decided what to do with all this information, it sends impulses back out to the body via motor neurons, which are another type of nerve cell to aectors, which are generally muscles or glands that could be told to contract or to release hormones. All of these parts work together to make a nervous system that can detect changes both inside and outside of our bodies. sort through all of that information and then respond to it. A special case of all of this in action is a reflex arc, which is just a technical term for the nerve pathway that underlies our unconscious reflexes, such as when you instantly move your hand away from a hot pan or blink when something touches your eye. The benefit of having these reflexes is that they're rapid and automatic. So, we can respond to things really quickly, which helps us to avoid getting hurt. As an example, let's imagine you reach out and accidentally touch the end of a pin. We can call the pressure of this sharp object a stimuli, and it will be detected by receptor cells in your skin. This will then stimulate a sensory neuron that will carry an impulse from your finger up to the spinal cord. which remember is part of the central nervous system. Here inside the spinal cord, it will transfer the impulse to a relay neuron shown here by this yellow arrow. The relay neuron will then pass the impulse along to a motor neuron where it travels back out to the body to an aector such as your bicep, which will contract and move your hand away from the pen. Remember that between each of the different neurons there will be sinapses where the electrical signal is temporarily converted to a chemical signal so that it can be passed between the different nerve cells. If you haven't heard yet, you can find all of our videos on our website cognito.org. You'll also find questions, flashcards, exam style questions, and pass papers. And we track all of your progress so that you always know what to study next. So sign up for free by clicking here or browse our playlist here on YouTube.