Okay. So now we're going to dive into that receptor level. So, very broadly speaking, sensation requires two things. First, a stimulus has to excite a receptor, and then an action potential has to actually reach our central nervous system. And there are a couple of requirements in generating that signal successfully. So first, the stimulus energy has to match the specificity of the receptor. So you'll see this in an upcoming video, but basically, different types of receptors respond to different types of energy. For example, the photoreceptors in your eye respond to light energy. So our energy has to match our receptor. Next, our stimulus has to be applied within the receptive field, and the receptive field is just the area that that receptor monitors. So basically, our stimulus has to be close enough to our receptor for it to actually detect it. And then finally, a graded potential has to reach threshold. So our old friend, the graded potential, has returned. Very exciting. So receptors can actually produce two types of graded potentials, and we're going to cover those now.
So first, we have generator potentials, and these are pretty straightforward. Here the receptor region for these is part of a sensory neuron, and this is very common in general sense receptors. So in our general senses, and basically, what happens is that it's going to generate an action potential right there in the sensory neuron. So if we look down to our figure here, this one here on the top is depicting our generator potential. And you can see we have our sensory neuron. Let's just say that that's the sensory neuron in my fingertip, and we have our stimulus. Let's say that's pressure from my pen right there, and what is going to happen is that that stimulus is going to trigger a graded potential right there in that sensory neuron and then assuming that that hits the threshold, it will trigger an action potential, then that will get sent up to our central nervous system. So it's the exact same process that we've already learned about. So if you need a refresher on any of that, feel free to go back and watch my video on graded potentials. We cover all of that in lots of detail.
Alright. And next, we have a receptor potential, and this one's a little bit different. So a receptor potential actually involves two cells. So we have a separate receptor cell and our sensory neuron. So here the receptor region is actually in that separate receptor cell, and this is very common in the special senses. And if you're thinking, Hannah, you just told me that we're focusing on general senses. Why are we even learning this? Fantastic question. So you are going to have an entire chapter on special senses coming up, but they are not going to cover these graded potentials in detail. So we're going to kind of get this out of the way for you, and by the time you get there, you will understand all of this perfectly. Alright? So a nice example of this receptor potential is the rods and cones in your eye. So your eye has these specialized cells called rods and cones, and they are going to communicate with sensory neurons and that is what's happening here. So basically, what happens is that this graded potential will change the amount of neurotransmitter that gets released by that receptor cell onto the sensory neuron. So if we look down at our figure here, we have our receptor potential here at the bottom and you can see we have two cells. We have our receptor cell and our sensory neuron. So this receptor cell again could be like a cone in your eye and the stimulus would come in. In our example, it's going to be light stimuli and that's going to create a graded potential right there in that receptor cell and then that will trigger the release of neurotransmitters across the synapse, and then those neurotransmitters will create a graded potential in our sensory neuron. Again, assuming it's big enough, that will trigger an action potential and that signal will get sent up to the brain. Alright. So that is how a receptor potential works. So very broadly, the difference between the two is that in our generator potential, that graded potential will get generated right there in that sensory neuron and in our receptor potential, it's going to be generated in the receptor cell and it will then create an effect on our sensory neuron. Alright, so that is our receptor level and I'll see you in our next video to cover those circuit and perceptual levels. So, I'll see you there.