A general understanding of the auditory system as a whole is imperative for the quality of a clinician’s work, and this website is designed to provide you with an overview of three of its key systems. Not only will you delve deeper into the anatomy, physiology, and pathology of the middle ear, outer ear, and temporal bone, but also the ways in which we measure hearing and the devices required to do so. The most effective method to diagnose and treat patients is first understanding the function of each system, this way a clinician can begin to develop their course of action. Before we get too ahead of ourselves, below you'll find a brief introduction of the entire auditory system!
THE AUDITORY SYSTEM
The auditory system is the key to how sound is processed and directed from the environment around us to our brains, and it's comprised of two major networks, the peripheral and central auditory systems. The peripheral system consists of the outer ear, middle ear, temporal bone, inner ear, and auditory nerve; it plays a major role in sound transition and is responsible for transmitting sound waves from the pinna to the tympanic membrane, or, the eardrum (Telefoglou). When sound waves travel into the ear canal, it then sends vibrations through the eardrum which eventually reach the middle ear and the three bones housed within it. Detailed further on the "middle ear" tab, these bones called ossicles, vibrate and strike one another to amplify sound through a landmark called the oval window, located in the middle ear space and situated on the opening of the cochlea, which is a snail-shaped, fluid-filled organ housed within the inner ear ear that plays a key role in the sense of hearing. When the ossicles transmit this vibration through the oval window to the inner ear, nerve impulses are activated and directed towards the brain via the auditory nerve, signaling that there is a sound. Responsible for the interpretation of auditory information, the central auditory nervous system consists of the brain, nuclear centers, ascending and descending fibers, ipsilateral and contralateral fibers, auditory cortex, and interhemispheric pathways. It communicates with the peripheral auditory system through said fibers, and processes sounds while also being responsibility for their localization (Staecker). The auditory nerve runs from the cochlea in the inner ear to the nucleus of the brain stem, allowing neural impulses to travel to the brain and signal that a sound is being directed in (Hearing). Both systems work hand-in-hand to capture, process, and deliver auditory information to the brain in order for humans to hear effectively.
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THE EIGHTH CRANIAL NERVE
Also referred to as the vestibulocochlear nerve, the eighth cranial nerve houses the vestibular nerve which is primarily responsible for balance and eye movements, as well as the cochlear nerve, which is responsible for hearing (Bordoni). As the two portions serve their own purpose, they originate from different nuclei in the brain, with the vestibular portion innervating the vestibular system of the inner ear to coordinate balance and detect rotational head movements. For example, this system stabilizes our retinas while our heads turn, which is the reason that we're able to keep our eyes on an object while moving our heads in a different direction. The cochlear nerve, however, travels from the inner ear to the brainstem, detecting the magnitude (intensity) and frequency (pitch) of the sound waves that enter the ear. Once the outer ear collects sounds which in turn vibrate the eardrum, the inner ear directs these signals to the cochlear nerve. These mechanical signals are then converted to electrical impulses which are relayed back to the brainstem through the cochlear nerve. Once there, the brain interprets these signals allowing us to hear the incoming auditory message (Colleen).
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THE INNER EAR
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The inner ear rests in a hole like cavity on either side of the skull, and consists of the cochlea, the vestibule, and the semicircular canals. Once sound enters the inner ear and then into the cochlea—a snail-shaped, fluid-filled organ housed within the inner ear—sound vibrations are translated into electrical impulses which can then be interpreted in the brain as we discussed above. The vestibule, which lies between the cochlea and semicircular canals, is responsible for balance and detecting one's location in space. It continuously sends signals to the brain in order to maintain postural equilibrium (Hawkins). Similarly, the semicircular canals are three fluid filled canals containing 3/5 of the vestibular end organs that respond to head movement and maintain balance. They consist of the anterior, posterior and horizontal canals, and they each track angular or rotational acceleration, relaying to our brain where we are in space. Found in all three of the inner ear landmarks we discussed is fluid called perilymph, which has a chemical composition that is low in potassium and high in sodium. When our head rotates, this fluid travels within the canals, moving the tiny hair cells that line them (Iftikhar). The movement of these hairs translate the movement of the perilymph liquid into nerve messages that are then directed to the brain in order to maintain balance. These balance keeping systems are responsible for why you may feel dizzy after a rollercoaster ride, for example, when you're moving around in space for a while and then come to a resting point, the fluid in your semicircular canals may continue to travel, making you feel disoriented or dizzy.