Outer and Middle Ear
The pinna has evolved into a shape that is able to effectively collect sound waves into the auditory canal. Sound is simply a pressure wave. The sound waves travel down the auditory canal and vibrate the tympanic membrane, also called the eardrum. On the other side of the tympanic membrane is the middle ear. The middle ear consists of three ossicles which work together to amplify the pressure coming from the sound wave. This increased pressure is necessary to move fluid inside the cochlea, because fluid has a greater resistance than air.
The three ossicles in the middle ear are the malleus, incus, and the stapes. Pressure is increased to the cochlea fluid in two ways. The three ossicles form a chain of torque mechanisms, increasing the cumulative force applied to the oval window by the stapes, the last oscile. The oval window is also smaller in area than the tympanic membrane. Decreasing the area of applied force also increases pressure.
The ossicles also have an attenuation reflex. Muscles attached to the ossicles regidify when exposed to very high amplitude sounds. This is meant to protect the middle and inner ear from potentially destructive high amplitude pressure waves. However, this reflex takes many milliseconds to engage, and therefore, will not protect the ear from sudden impulse sounds like a gunshot.
The mechanical importance in the ossicles’ ability to amplify pressure waves is noticeable when humans age. The ossicles over time begin to rigidify to the point where they no longer move and provide torque. This is evident in loss of hearing. However, hearing is often not completely lost. Although the rigidified ossicles may not amplify pressure, they can still vibrate and carry an unamplified, weak signal from the tympanic membrane to the oval window.
The inner ear contains the cochlea, involved in hearing, and the labyrinth, involved with the vestibular system, which we will discuss later.
The cochlea is a snail-shaped structure that plays a big role in transforming sounds into neural signals. It has three “chambers”; separated by the Reissner’s membrane and the basilar membrane. Receptors are located on the organ of Corti on top of the basilar membrane.
The basilar membrane widens and stiffens as it reaches the tip of the uncoiled cochlea. Stapes move in and out at the oval window, which causes perilymph to flow into the chamber. This forms a traveling wave in the basilar membrane. The location dissipation of the wave on the basilar membrane depends on the frequency of the wave; higher in the stiffer/narrower part and lower in the flexible/wider part. This organization of sound frequency in the auditory structures is called tonotopy.
The receptors that convert sound into neural signals are hair cells on the organ of Corti. The movement of the basilar membrane causes the hair cells to bend. When this happens, an ion channel on the tip of each cilia on the hair cell is induced to open. Potassium ions flow into the hair cell, depolarizing it, and activating voltage-gated channels to release calcium ions, which induces the release of neurotransmitter glutamate.
Hair cell damage is the most common cause of deafness. Interestingly, hearing can be restored with a cochlear implant, which takes sound in through a microphone and converts it into electric signal. A digital processor sends the code to a receiver implanted in the skin. The receiver transforms this code into electrical impulses which the cochlea can understand. The implant is able to use the tonotopic information of the basilar membrane to stimulate the correct parts of the membrane. I found it very interesting that hearing can be restored by communicating directly with the auditory nerves, instead of simply amplifying already existing sounds.
The vestibular system
The vestibular system is the sensory system that provides the main contribution to the sense of balance and spatial orientation in order to coordinate movement with balance. The vestibular system detects changes in gravity and head tilt/rotation similarly to how the auditory system perceives sound waves, via hair cells in the labyrinth. The vestibular organ synapses with the vestibular nerve axon which connects to cerebellum and medial and lateral vestibular nuclei. From there, axons are projected to numerous locations to control the eye, head or other muscle movements.
What many people might not be aware of is the vestibulo-ocular reflex. It keeps the eyes pointed in a particular direction no matter how much you would shake your head. The vestibular system senses the rotation of the head and commands compensatory movement of eyes in the opposite direction. Interestingly, I think it is actually more difficult to keep your eyes fixed in your head when you’re shaking it.
In the literature, abnormalities in the vestibular system have also been found to be related to hearing loss.
For example, enlarged vestibular aqueduct (EVA) is a common structural deformity of the inner ear. Research suggests that most children with EVA will develop some amount of hearing loss. Additionally, up to 15 % of children with sensorineural hearing loss have EVA. (Macielak et al. 2019) However, there seems to be contradictory information whether EVA is the cause of hearing loss or whether there is an underlying defect that causes both of these abnormalities.
Macielak R, Mattingly J, Findlen U, Moberly A, Malhotra P, Adunka O. Audiometric findings in children with unilateral enlarged vestibular aqueduct. Int J Pediatr Otorhinolaryngol. Vol. 120, 2019, pp. 25-29. Available (accessed on 29.10.20): https://doi.org/10.1016/j.ijporl.2019.01.034