As I mentioned during my post on the anatomy of the outer and middle ears, I would be following up with a post on the anatomy of the inner ear. So today I will just try to give a bit of a summary on how the cochlea converts the vibrations of sound, to a message that can be interpreted by the brain.
The cochlea is like a long tube folded in half, and then coiled up with the folded end being the apex of the coil. This tube is filled with a fluid called perilymph. Now, due to the fold, it looks like there are two tubes running the whole way (but remember they are connected). Between the two tubes is another compartment called the scala media and this is where the real work is done.
Recall that the middle ear cavity contained a number of bones with the stapes resting against the side of the cochlea. The bit where it connects is called the oval window of the cochlea. As the stapes footplate presses on the cochlea, it causes a little ripple in the fluid inside. The middle compartment (the scala media), contains something called the basilar membrane. Attached to the basilar membrane are a number of tiny hair cells. There are two types of hair cells (inner and outer), and these both have slight differences in their anatomy and function. Inner hair cells (IHCs), are about 35 micrometres long, and are pear shaped with a large central nucleus. Outer hair cells (OHCs), are longer (about 25-45 micrometres), and have their nucleus at the basal pole of the cell. IHCs and OHCs also differ in their configuration, seen under a scanning electron microscope. The IHCs look like little toothbrushes, whilst the OHCs are aligned in a horseshoe shape. Another way in which IHCs and OHCs differ is in their innervation. There is one IHC per afferent nerve fibre, and the majority of afferent fibres from the cochlea are leading from an IHC. In contrast, many OHCs will contact a single afferent fibre. In terms of the efferent innervation that comes from the brain, IHC efferent nerves terminate on the afferent terminals at the base of the IHC. The efferent fibres leading to OHCs terminate directly on the cell.
So after a sound wave causes a vibration of the ossicles and that little wave to travel through the fluid in the cochlea what happens? The fluid disturbance causes a wave to travel through the basilar membrane as well. Remember that the hair cells sit on top of the basilar membrane. On top of the hair cells are stereocilia (sort of like mini hairs), and these are connected to one other by tip links. Basically what happens, is with a movement of the basilar membrane, the tip links either open or close, depending on which way the basilar membrane is moving. This is a completely mechanical process and requires the tectorial membrane to provide the pressure to do this. The tectorial membrane is a jelly like substance that sits on top of the hair cells. Anyway, once the hair cells open, potassium is able to enter the cell, which then causes depolarisation and release of a neurotransmitter to the afferent neuron, causing an action potential. This all happens so fast that it seems instantaneous and is going on literally every second of the day!
The final aspect of the inner ear I wanted to discuss was how its anatomy helps us to distinguish between sounds and how it amplifies softer sounds to be loud enough for us to perceive them. Different hair cells at different points along the basilar membrane are sensitive to sounds of particular pitches (or frequencies). A sound of a particular frequency will cause the basilar membrane to move the most at that point, and hence more hair cells are stimulated and there is a bigger response. As a general rule, high frequencies maximally stimulate the basilar membrane at the base of the cochlea, and low frequencies maximally stimulate the apex. At the base, the basilar membrane is thinner, and more taut than it is at the apex. The amplification of sound happens largely due to OHCs. They contain a contractile protein called prestin, and this helps move the basilar membrane higher, hence more shearing force on the sterocilia from the tectorial membrane, and so the sound is amplified by causing an increased response to the same stimulus.
The inner ear is an incredible sensory organ that does so much work, with so little recognition. I know I have mentioned this a couple of times now, but how often do you ever appreciate the fact that the tympanic membrane is constantly vibrating, as are the ossicles, and within the inner ear, the tiniest movements enable you to hear sounds. The main reason for a sensorineural hearing loss, which is hearing loss due to a problem in the cochlea, is as a result of loss of hair cell function.
I hope you have enjoyed reading about the cochlea and how it works, and I hope that all I have said makes sense. Feel free to comment with any questions you may have!