Hair cells are sensory receptors used by the auditory and vestibular systems, and they'll actually respond to physical stimulation, which will result in the opening of ion channels. Surprisingly, the sensory receptors responsible for your perception of sound, your ability to hear, are actually responding to physical stimulation. Now, the reason they're called hair cells is because they have these hair-like projections called stereocilia that extend from the hair cells and are what will be physically manipulated in order to open those ion channels, which will result in a change in membrane potential. When these stereocilia are bent, there are ion channels located near their base that get pulled open due to the bending. Imagine a stereocilium like this; if it gets bent over, it will pull open this ion channel and allow some ions to go through. If we pull heavily on our stereocilia, depicted with three arrows to represent strong force, it will lead to more ions getting through, causing a greater depolarization. The actual bending, the grading of the bending, will affect the membrane.
Another interesting way these hair cells are used is in something called a statocyst. This is a balance sensory receptor used by, for example, marine invertebrates to get a sense of gravity. The ocean makes it hard to tell up from down without gravity, so this is their way of sensing gravity. The statocyst is a sac-like structure surrounded by hair cells. You can see all these hair cells each having numerous projections off of them, those are their stereocilia. Inside this sac of hair cells are what's called statoliths. These are crystals that will touch the hair cells to stimulate them. As you see here, our statolith will sink due to gravity, and wherever it touches the hair cells, it will cause stimulation, that will be transduced, and that signal will allow the organism to detect the direction of gravity's pull on the statolith and the statocyst.
Yet another fascinating way that organisms use hair cells is in the lateral line system. This system is found in some fish and amphibians and allows them to detect movements in water. The lateral line system has a canal that lets water in, and hair cells are located within. Here we have a zoomed-in version where these hair cells have a large dome over them known as a cupula. You don't need to worry about that detail; the key point is that water moving through these channels can lead to the stimulation of these hair cells, and consequently, these organisms can detect where the water is moving around them and thus have some idea of where, for example, another organism is relative to them. With that, let's actually go ahead and flip the page.
