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.