0:00 Hello, in this video we're going to talk about pain. 0:07 We're going to focus on the ascending pathway of pain, and we're also going to 0:10 talk about 0:10 the descending pathway. 0:12 As well, we're going to focus on an area called the substantia gelatinosa. 0:17 Imagine you had a right hand injury, what happens, and how do we feel pain? 0:22 The pain signal coming from the side of injury will travel up to the brain, and 0:26 this is where 0:27 the perception of pain essentially is felt. 0:30 The signal going up to the brain is referred to as the ascending pathway. 0:35 Injuries to our body is felt in certain areas of our brain called the somatos 0:39 ensory cortex. 0:40 Let's recap the brain briefly. 0:43 Here's the cerebrum, brainstem and cerebellum. 0:47 The somatosensory cortex sits posterior to the central sulcus. 0:52 Let's get a cross section of the brain, like so, and zoom into the somatosens 0:56 ory cortex. 0:57 The somatosensory cortex is an area where sensation is perceived. 1:00 This includes pain. 1:02 The somatosensory cortex has areas which correlate to different parts of our 1:07 body. 1:08 For example, let's look at the left side of the somatosensory cortex. 1:12 The outer region here represents the leg, hand, and face. 1:16 Again, this is your right side and this is your left side of the brain. 1:23 The brain continues on and joins with the brainstem, which is made up of three 1:26 main components, 1:27 the midbrain, pons, and medulla. 1:30 These are cross sections of the brainstem. 1:32 The brainstem then continues on and becomes the spinal cord. 1:37 Here we are only looking at one section of the spinal cord. 1:43 Let us say this section of the spinal cord is a cervical spine area. 1:48 Spinal cord has nerves coming out from the anterior root and nerves coming in 1:54 from the 1:55 posterior root of the spinal cord. 1:58 Here I am only drawing the nerves on the right side of the spinal cord. 2:05 Different tracts exist within the spinal cord. 2:07 One tract important in the pain pathway, or the ascending pathway, is a spinal 2:11 tholamic 2:12 tract made up of the lateral and anterior spinal tholamic tracts. 2:18 Let's go back to the right hand injury now. 2:20 The right hand has an obvious injury on its palm. 2:24 Within the area, there are immune cells reciting. 2:27 When these immune cells are damaged, or should I say when any cells are damaged 2:31 , including 2:32 our skin cells, they start releasing cytokines, chemicals. 2:35 An important one in the ascending pathway are prostaglandins, represented here 2:41 as PG. 2:42 Prostaglandins are produced by nearly all cells, typically as a response to 2:48 inflammation. 2:50 Sensory nerve fibers exist all over our body. 2:53 The sensory nerve fibers will respond to prostaglandins and will carry the 2:57 signal or impulse to the 2:59 back of the spinal cord, or the dorsal horn of the spinal cord. 3:06 This neuron is a first word of neuron. 3:10 Within the dorsal horn of the spinal cord, the first word of neuron will sign 3:13 up and relay 3:14 this signal, this impulse to the second neuron. 3:18 The second neuron, called the second order neuron, will cross over to the 3:22 opposite side 3:23 and will enter the spinal tholamic tract. 3:27 From here, the second order neuron will continue up, ascending through the 3:33 remaining spinal 3:34 cord through the brain stem and terminate in the thalamus of the brain. 3:40 The thalamus is the relay station. 3:47 In the thalamus, the second order neuron will sign up to the third order neuron 3:51 . 3:51 The third order neuron will carry this impulse and relay it to the region of 3:54 the brain, 3:55 which correlates with the injured right hand. 3:58 Thus, the third order neuron helps discern the area of injury, and also the 4:04 cortex here 4:04 is where the perception of the pain is perceived. 4:09 Important to remember that this is the left side of the brain, whereas the 4:14 stimuli, the 4:14 signal, the initial signal was on the right hand. 4:17 Thus, sensation always is on the opposite side of the brain to where the 4:22 stimuli or stimulus 4:24 occurred. 4:25 Another important thing to remember is a chemical released by the first order 4:29 neuron to transmit 4:30 or relay the signal impulse to the second order neuron. 4:34 This chemical is substance P. Thus, substance P and this whole area at the 4:39 dorsal heart 4:39 of the spinal cord is an important area for the ascending pathway. 4:44 Now whenever there is an ascending pathway, there is a descending pathway. 4:49 While the ascending pathway is responsible for transmitting the pain signal up 4:52 to the 4:53 brain, the descending pathway is responsible for controlling and inhibiting the 4:58 ascending 4:58 pathway essentially. 5:01 Important areas of the descending pathway include the peri-aqueductal gray 5:05 matter of 5:05 the midbrain and the nucleus-raf-magnes of the medulla. 5:09 When not inhibited, the neurons arising from the peri-aqueductal gray matter 5:15 will 5:15 go down to the nucleus-raf-magnes and sign up to the second neuron. 5:21 This second neuron here is a serotonergic nor adrenergic neuron and this ser 5:28 otonergic 5:28 nor adrenergic neuron will travel down towards the dorsal horn of the spinal 5:34 cord as well. 5:35 The serotonergic nor adrenergic neurons role in summary is to inhibit or 5:42 control the communication 5:43 between the first order neuron and the second order neuron of the ascending 5:48 pathway and thus 5:49 help control pain signals going up. 5:53 There is another neuron here I am drawing in blue which also plays an important 5:57 role in 5:57 this area and we'll talk about it later. 6:00 So this dorsal horn of the spinal cord is a pretty important area. 6:05 We can call it like a gate, so gate control of pain. 6:09 If we zoom into this area of the dorsal horn of the spinal cord, it can 6:13 actually be referred 6:14 to as the substantial gelatinosa. 6:17 So let's just recap this whole area. 6:19 Here we have the first order neuron coming in and synapsing with the second 6:23 order neuron 6:24 here. 6:26 The first order neuron will bring in an action potential which will eventually 6:30 stimulate 6:30 vesicles to release its content into the synaptic left. 6:34 The content here in this case is substance P. Substance P will stimulate the 6:39 second order 6:39 neuron. 6:41 The second order neuron is stimulated and will propagate an impulse up to the 6:44 thalamus via 6:45 the spinal thalamic tract. 6:47 Coming down from the medulla here in red is a neuron from the descending 6:52 pathway, the serotonin 6:53 noradrenaline neuron, which will release its content, serotonin and noradren 6:58 aline. 6:59 These chemicals or this neuron will do two things. 7:02 Firstly, it will bind onto receptors of the presynaptic neuron and inhibit the 7:08 release of 7:08 substance P. 7:11 Second, it will stimulate a small neuron in an area called an intranuron. 7:17 This intranuron in the substantial gelatinosa is actually an opioid neuron, 7:22 which when stimulated 7:24 will release an endogenous opioid called encephalin. 7:28 Encephalin, like all opioids, will do two things in the substantial gelatinosa. 7:33 One, it will inhibit the presynaptic neuron from releasing substance P. And two 7:39 , it will 7:40 inhibit the postsynaptic neuron from depolarizing and thus stop the 7:46 continuation of the impulse 7:48 up to the thalamus. 7:51 All in all, opioids such as encafilins will inhibit the ascending pathway of 7:58 pain. 7:58 So that was an overview of the ascending pathway and the descending pathway of 8:02 pain. 8:10 You
