There are two main pathways that are related to our perceptions of pain. And this first pathway that I described is a pathway that involves a type of neuron called an alpha, I'm sorry, called a type A delta neuron, or nerve fiber. Does anybody remember anything about type A nerve fibers? Lydia. >> It's the thickest type of nerve fiber. >> By thickest, what do you mean? >> Does it just have, like, a wider diameter? >> Yes. It's the type of nerve fiber with the biggest diameter. And, what else do we know about type A? Nerve fibers, Naomi. >> They're myelinated. >> They are myelinated. Now, what's myelin? Lauren. >> It's a fatty sheet that wraps around the axon of a nerve so that information passes on faster. >> Exactly. So the myelin is a fatty sheet that wraps around the axon, and it insulates the axon, so that current gets passed faster along the axon, right? So type A fibers tend to have a fairly thick Myelin sheath, and they have a large diameter, and so that translates functionally into what? Stephanie. >> The signals will be transmitted faster. >> Exactly, so these signals that are transmitted by type A delta fibers, get passed very rapidly, okay? Into the central nervous system. So these type A delta fibers, carry stimulus information related specifically to mechanical and thermal stimuli. Okay? And they are the fibers which will later be associated with the rapid sharp, sensations of pain. You know, it's like, you know what I mean when I say rapid and sharp sensations, right? so, one other way to think about their function is that they are associated with the rapid responses that we can make to painful stimuli so. Just hold that in mind because that is very different from what happens with the other type of nerve fiber that carries pain stimulus information or pain related stimulus information. The other type of fiber is a type C fiber. What do you remember about type C fibers? Lauren? >> Type C fibers are not myelinated, so. >> Exactly. >> So information travels a lot slower. >> Exactly, so we say they're unmyelinated, and what that means is that they're not wrapped in myelin, right? They have some myelin packed in around them. So, their signal doesn't get passed on to some other unrelated neuron by mistake, right? So they're, it's like having packing material around them but they don't have their own sheath. What else about the type C fibers is important? Stephanie. >> They are much smaller in diameter. >> They have a much smaller diameter, in fact, type C fibers have the smallest diameter of any type of nerve fiber. So small diameter axons, and being unmyelinated, unmyelinated means what about their conduction speed? Andre. >> That means that their conduction speed is much slower. >> Exactly. So they've got in fact the slowest conducting speed of any neuron in the nervous system. [BLANK_AUDIO] The type C fibers carry stimulus information related to all three of the stimuli that I've talked about, mechanical, thermal, and chemical. All right and they're responsible for the pain sensations that we have that we describe as dull, aching, burning, throbbing. That would be the kind of perception that we have of pain that's involved with the type C fibers. Okay, so the nociceptors pass their signal to either type A delta or type C fibers. Those first-order sensory nerve fibers will carry the signal into the dorsal horn of the spinal cord. At the dorsal horn, where does the signal get passed? Naomi. >> The second-order neuron? >> To the second-order sensory neuron, yes. And at the level of that passing of the signal. There is complex interaction between neurons that can cause the second-order neuron to be more excitable than normal, or less excitable, okay? And I'll talk more about those complex interactions later. Okay. So, then, we talked about the first of the tracts that carries Pain, stimulus information up the spinal cord, and that tract is called the spinothalamic tract. You can see the that second-order neuron's of that pathway crossover, or decussate at about the level that the, the communication occurred. In the dorsal her, horn, between the first and second-order neurons, right? And then where does that second-order neuron pass to in the spinothalamic tract? Lydia? >> To the thalamus? >> Exactly. And in the thalamus, what happens? Stephanie? >> The third-order neuron will be activated, it will have the signal sent into it? >> Exactly, and then the third order sensory neurons will pass the signal onto? Andre. >> Would it be to the primary somatosensory cortex? >> Exactly. And it's a primary somatosensory cortex that will allow us to do what specific things with this information? Stephanie. >> Maybe interpret it? >> Well first it will allow us to localize it. That's a big thing. We'll know that the pain is happening in our, in our foot. Versus our hand, for example. And we may even specifically know what part of the foot the pain is the painful stimuli is occurring at, okay? But, then, because of the communication between the primary somatosensory cortex and other parts of the cerebrum, and even other parts of the brain like the hypothalamus. We might be to interpret more about that input. Exactly. So now we have to talk about the information that's being carried on those type C fibers. Type C fibers also extend into the dorsal horn. And the type C fibers will pass their information onto a second-order sensory neuron. And in the case of the type C fibers, they're associated with a tract called the spinoreticular tract. And that name, spinoreticular, indicates that the tract passes up the spinal cord. But it, it passes to the brain stem. It doesn't pass the whole way up to the thalamus, directly. It passes to the brain stem. And and actually to multiple sites in the brain, so this tract is much more diffuse, if you want to think of it that way. Because it's passing information to multiple sites in the brain. It's a spinal reticular tract that's involved in adding emotional overtones to our perceptions of pain. And that process of adding the emotional overtones is much more complicated. So I think that's a lot of information to take in in one session. We'll be talking more about this topic this week. And we're going to get into how pain is modulated. And specific types of pain when we get back together. [BLANK_AUDIO]
