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Voiceover: At some point in time, you've seen someone walking their dog, and their dog decides to pee on a fire hydrant. Now, this is my attempt to draw a fire hydrant, but you see the dog, and he pees on this fire hydrant. Here's the pee, it's coming, and it splashed all over the place. Why is the dog peeing on the fire hydrant? You might have heard that dogs pee on fire hydrants in order to mark their territory. How exactly is it that they're able to mark their territory? There are particular molecules that are released in the urine, and these molecules can be scented, can be sensed by other animals through their nose. These molecules are known as pheromones, pheromones. Now, we can think of pheromones as specialized olfactory cues. In a previous video, we spoke about how we're able to smell things. Well, pheromones, not only are we able to smell them, and animals in particular are able to smell them, but they also cause some sort of response in the other animal that's smelling them. Basically, we could think of a pheromone as a chemical signal, chemical signal, that is released by one member of a species, so for example, the dog, and is sensed another member of the species, and it triggers an innate response. Pheromones are really important in animals, particularly insects. In insects, they have been linked to mating, to fighting, and also in chemical communications. Bees use pheromones, dogs use pheromones, [unintelligible] use pheromones to mark their territories and basically communicate with other members of the species. This is what pheromones are in a nutshell. Now we're going to go into a little bit of the anatomy and also how, at a molecular level, pheromones work. In a previous video, we went into the olfactory system, and we talked about the olfactory epithelium. Let me just go ahead, and let's imagine that this is the skull of a mouse. Here is a mouse skull, here are its teeth, and here is its nose. This is just the skull. This is the nose. Normally, there would be a little bit of flesh here, but basically, here is the nose, and air comes in, and it moves through the nasal passage, and there is a part of the nasal passage that is known as the olfactory epithelium, so olfactory epithelium. The olfactory epithelium is sensitive to various molecules. There is a specialized part of the olfactory epithelium which is up here in the mouse, and this is known as an accessory olfactory epithelium, accessory olfactory epithelium. The accessory olfactory epithelium actually sends projections to a accessory olfactory bulb. This is the accessory olfactory bulb. The olfactory epithelium sends information to ... [unintelligible] Normal olfactory epithelium sends its information to the olfactory bulb, which is olfactory bulb, but the accessory olfactory epithelium sends information to the accessory olfactory bulb. Let's look at the particular cells in the accessory olfactory epithelium that are responsive to pheromones. Let's look a little bit more closely at the accessory olfactory epithelium. Within the accessory olfactory epithelium, we have a structure known as a vomeronasal system. We got this one zone over here, and then we have beneath it another zone. Let's imagine that this part up here is the nasal passage, so this is the nasal passage, and then over here are all the axons of the various sensory cells that will eventually go to the brain via the accessory olfactory bulb. Within this vomeronasal system, which I will label, vomeronasal system, there are different cells. There are some cells that are situated down here, and they actually send a projection all the way up to this top zone over here. We got these cells in this, since they're situated underneath this bottom zone, are known as basal cells. There are other cells that are over here, and they just have short little projections, and these are known as apical cells. Apical cells are in yellow. This is apical, and this over here is a basal cell. These cells, similar to regular olfactory neurons, will have little receptors at the very tips of their projections, and these receptors will be sensitive to different molecules or pheromones. Let's imagine that in the urine of that dog that peed on the fire hydrant, there is a little molecule. We'll draw it as a little triangle. That triangle will come in, and it might activate a receptor on this basal cell over here. When it does activate this cell, this basal cell will send a axon through the accessory olfactory epithelium and then eventually to the accessory olfactory bulb, so accessory olfactory bulb, and then from there, it will synapse onto a glomerulus and then to a mitral or tufted cell, and then that mitral or tufted cell will send a axon to the brain. There are many of these vomeronasal systems throughout the accessory olfactory bulb here. I just drew one. But basically, again, what we have is we have specialized cells that are responsible with one particular type of molecule that will send little axons that eventually go to the accessory olfactory bulb. All the axons will synapse into one particular location known as a glomerulus. Once these cells reach the accessory olfactory bulb and synapse onto a glomerulus, they will then synapse onto a mitral or tufted cell, and that mitral or tufted cell will actually send a axon to a part of the brain known as the amygdala, amygdala. The amygdala is responsible for a host of things, but it's particularly known for its involvement in emotion and aggression and mating and things like that. So basically, by having various cells respond to various pheromones in the environment, an animal is able to control its behavior via this pathway to the amygdala based on these extracellular cues in the environment. Based on these pheromones, a pheromone will bind, they will cause some kind of cell to fire, and that cell will eventually reach the amygdala to cause a behavioral response in the animal. Now signal transduction, so this part right here, this is where the receptor, the molecule binds to the receptor. This causes a signal to be transduced in the particular sensory neuron. This signal transduction is exactly the same as what happens in regular olfaction, where there is a receptor. The receptor is a G-protein-coupled receptor. When that receptor is activated, it causes this cell to de-polarize and fire [unintelligible]. Now interestingly, in humans, we have evolved to, to rely very little on pheromones. We do have a vomeronasal organ. However, we do not have an accessory olfactory bulb, so we rely very little on pheromones.