How Do We Hear?
The Auditory System Explained
To navigate our worlds, we use many specialized sensory systems that help us to interact with our environment, avoid danger, and socialize with other individuals. The auditory system is a specialized system that allows us to sense noise. This system is made of multiple structures, but there are two main categories of these structures: 1) outer structures (our ears), and 2) the machinery in our nervous system that is specialized to process sound.
Anatomy of the Ear
We are able to hear sounds and voices because our ears collect sound waves and channel them into the ear canal (also known as the external auditory meatus), which is responsible for amplifying the sound. The part of the ear that collects sound waves is called the auricle. This is the portion of the auditory system that we can see on the outside of our heads! After the sound is amplified (or made louder) in the ear canal, the sound waves travel toward the eardrum, which is also called the tympanic membrane, in the middle ear. These waves cause the eardrum to vibrate, and the vibrations set the tiny bones in the middle ear (called ossicles) into motion. Ossicles are the smallest bones in the human body and they are responsible for further amplifying sound. The middle ear also contains the Eustachian tube, which helps to normalize the pressure between the air around us and the air in the ear.
Next, sound waves travel to the inner ear, which is responsible for sound detection and balance. Some structures in the inner ear, such as the semicircular canals, are actually part of the vestibular system (which is important for our sense of balance) and not the auditory system. Regardless, one of the most important parts of the auditory system is located in the inner ear -- a spiral-shaped structure called the cochlea. Sound waves in the form of vibrations travel from the base of the cochlea (located near the oval window in the middle ear) to the top of the spiral in the cochlea.
Sensory transduction in the auditory system.
Anatomy of the cochlea
The cochlea contains many specialized structures and cell types, all of which have important roles in turning sound wave vibrations into nerve impulses. The cochlea is filled with two types of fluid – perilymph and endolymph – and is divided into three different chambers – the scala vestibuli, the scala media, and the scala tympani. The scala vestibuli and scala tympani are filled with a type of fluid called perilymph, while the scala media is filled with endolymph. These two types of fluids have different qualities but both are very important for transmitting sound waves to sensory cells in the cochlea. The sensory cells in the cochlea are called hair cells and there are two types -- inner hair cells and outer hair cells. We are born with more than 3,500 inner hair cells and more than 12,000 outer hair cells! Outer hair cells are important for amplifying sound in the form of vibrations which are then translated into nerve impulses by the inner hair cells. Inner hair cells form excitatory connections with sensory neurons in the inner ear called spiral ganglion neurons. These neurons, in turn, form additional connections with neurons in the cochlear nerve. The cochlear nerve is responsible for transmitting these sound signals to the brainstem.
Auditory pathway from the cochlea to the brain.
The first set of neurons in the brainstem that receive this information are called the ventral and dorsal cochlear nuclei. These, in turn, connect with a structure called the superior olivary complex. The superior olivary complex is a very important structure because it is involved in detecting the difference in time that it takes for a sound to reach each of our ears. This time difference in sound detection is how we are able to figure out where a sound is coming from. Next, neurons in the superior olivary complex project to the inferior colliculus via a nerve tract called the lateral lemniscus. After that, signals are sent to a part of the brain called the medial geniculate nucleus which acts as a site for sharing information between the inferior colliculus and the auditory cortex. In the primary auditory cortex (which is located in the temporal lobe) the cells are organized tonotopically, which means that specific cells respond to specific frequencies of sound. The cochlea is also organized tonotopically (see the image below) !
Tonotopic organization in the cochlea
Interestingly, this tonotopic “map” is species-specific because some species of animals are able to hear sounds at different frequencies than other species. For example, mice are able to hear noises that are much higher-pitched than we humans have the capability to hear! Finally, brain areas in the frontal and parietal lobe are responsible for the more complex processes of perceiving and understanding sounds, including identifying speech.
Deafness
You might be wondering why, if we have all this machinery in our ears and our brains that allow us to hear, some people can’t hear sounds very well or are deaf. There are many causes of hearing loss, but there are two main types: congenital hearing loss (or hearing loss that occurs at birth) and acquired hearing loss (which typically occurs later in life). Acquired hearing loss is much more common in the human population. Deafness can have a large range of causes, including hereditary disorders, genetic disorders, exposure to toxins, loud noises, or trauma to the ear. Some diseases can also cause hearing loss. There are also two main families of causes of hearing loss: conductive causes and sensorineural causes. Conductive hearing loss affects the transmission of sound between the outer and inner ear and can be caused by things like impacted wax in the ear canal or failure of the machinery in the ear to work. Sometimes, normal hearing can return after treatment. Treatment can include anything from removing ear wax to having surgery -- it really depends on each individual case! On the other hand, sensorineural hearing loss occurs when there is damage to the cochlea and electrical impulses aren’t able to reach the brain from the cochlea.
In order to hear better and/or hear more sounds, people with hearing loss may wear hearing aids which can amplify the sound for the ear and help the ear to work more efficiently. Other people, typically those with nerve damage that causes hearing loss, may use a cochlear implant. Cochlear implants are able to bypass the parts of the ear that aren’t working in order to send sounds to the auditory nerve. These types of implants work by using a microphone fitted behind the ear. The microphone sends signals to a receiver that has been surgically implanted under the skin behind the ear, which then sends the signals to electrodes that are surgically implanted in the cochlea. The cochlear implant directs the sound signals to the brain, where we then process and perceive the sound. Both of these amazing devices help many people! However, not everyone who has difficulty hearing may use these types of devices. Deaf people can lead perfectly normal lives and often use sign language to communicate without having to use the auditory system! Check out this video to learn more about sign language and the deaf community:
In summary, hearing is just one of the types of sensory cues that we can use to navigate our environments and communicate with others. The auditory system allows us to hear things like music, language, and sounds of nature. This is possible because of the complex organization of our brains and communication between our ears and the auditory cortex!
References:
https://www.hopkinsmedicine.org/health/conditions-and-diseases/how-the-ear-works
https://pubmed.ncbi.nlm.nih.gov/26335749/#&gid=article-figures&pid=figure-1-uid-0
https://link-springer-com.udel.idm.oclc.org/referenceworkentry/10.1007/978-3-540-29678-2_3957
https://www.betterhealth.vic.gov.au/health/conditionsandtreatments/deafness-a-range-of-causes
