In this lecture, we're going to begin talking about sensory systems and I just want to remind you about the basic flow of information. Afferent is neuronal projections to, and we think about sensory when we are thinking about afferent. And efferent is neuronal projections from, and we mostly are thinking about motor information when we're talking about efferent. So remember, afferent arrives and efferent exits. So, sensory information is information concerning your external environment. So, with the sound of my voice, the look of these slides, the birds you can hear singing out the window, the feel of the chair underneath of you, all of those things are from your external environment and you have sensory information coming from your internal environment as well. When you're hungry, if you have an ache or a pain, or if something's going wonky with your digestive system, you will have a sensation about that. So, what we're going to talk about is the differences between these systems, some of their commonalities, and how they all get to the central nervous system. In this section of lectures, we're going to mostly talk about the spinal nerves. The sensory modalities that are associated with the cranial nerves, we'll talk about in the cranial nerves lecture. So, sensory information coming in from the periphery is going to be relayed through primary sensory neurons and those primary sensory neurons reside in the dorsal root ganglion. Those can be seen here in the real spinal cord, this is the spinal cord. These are your dorsal rootlets and this is a dorsal root ganglion, full of dorsal root ganglion neurons. If you recall, in the spinal cord lectures, we talked about the spinal nerve. Here, this is an example of that dorsal primary ramus that we talked about and a ventral primary ramus. These are parts of your spinal nerves that are going to the dorsum and part of the spinal nerve going to the ventral. So, you have sensation on both sides. This is to remind you that you have different modalities of sensation. You have hot, cold, gross touch, fine touch, pressure, and all of those kinds of receptors are found in your skin. I'm not going to ask you to memorize which receptor is responsible for what modality, but this is an image from doctor Alsup's lecture on the integumentary system, and it shows you where you have laminated corpuscles, you have little nerve endings associated with hair follicles, all kinds of touch corpuscles, and all of these actually represent nerve endings out here in the skin. All of these are connected to a dorsal root ganglion neuron The viscera also have sensation, but for the most part, because they're not exposed to the outside, they're mostly concerned with stretch. And some are also concerned with things like ischemia or chemical differences. For example, the ischemia associated with a heart attack, unfortunately. But viscera then don't have as many of these different kinds of receptors. In the next section, we're going to talk more about the different types of modalities, and where they travel, and where they synapse within the spinal cord.