Sound waves travel through the canal of the external ear, and are then amplified by structures of the middle ear: the tympanic membrane (eardrum) and three small bones (malleus, incus, and stapes). Two small muscles may pull on the bones of the middle ear to dampen the amplification system when needed, such as when speaking or chewing. These are the tensor tympani muscle, innervated by the trigeminal nerve, and the stapedius muscle, innervated by the facial nerve. These bones vibrate a membrane called the oval window, which vibrates a fluid called endolymph in the part of the inner ear called the cochlea. In the cochlea, the endolymph vibrates a membrane called the basilar membrane, which vibrates to higher frequencies toward one end and lower frequencies toward the other end. Embedded in the basilar membrane are the hairs of cells called cochlear hair cells, which are bent when it vibrates. Sound information detected by the cochlear hair cells is passed to axons of the vestibulocochlear nerve. These axons enter the lower brainstem to synapse in nuclei called cochlear nuclei.
The cochlear nuclei send axons bilaterally to ascend both sides of the brainstem in a tract called the lateral lemniscus, either directly or indirectly via other brainstem nuclei, to synapse in a nucleus in the upper brainstem called the inferior colliculus, which then projects to the thalamus. Axons from the thalamus form a tract called the auditory radiation that projects to the primary auditory cortex on the superior temporal lobe. Neurons from here project to association auditory cortices that is near the primary auditory cortex on the superior temporal lobe. Because the cochlear nuclei project information bilaterally up the brainstem to both sides of the cerebrum, unilateral lesions of the auditory pathways after the cochlear nuclei usually do not cause hearing loss.
Hearing can be testing by rubbing fingers or whispering close to the ear, or with a formal test called audiometry. Hearing loss may occur with dysfunction of the external or middle ear, which is called conductive hearing loss, or with dysfunction of the inner ear, the vestibulocochlear nerve, or the cochlear nuclei, which is called sensorineural hearing loss. With unilateral hearing loss, using a tuning fork to test air versus bone conduction of sound can help distinguish the location of dysfunction.
Weber’s test consists of placing a vibrating tuning fork on the midline of the head, and having the patient state which side sounds louder. With sensorineural hearing loss, the sound is louder on the normal side. With conductive hearing loss the sound is louder on the abnormal side; the reason for this is unclear, but may be because less ambient sound is being conducted into the cochlea to compete with the bone vibration. Rinne’s test consists of placing a vibrating tuning fork on the mastoid process (the bony bump posterior to the ear), and when sound is no longer heard moving it close to the ear, and having the patient state if they can hear it. Air conduction is usually better than bone conduction, which also usually occurs with sensorineural hearing loss, but with conductive hearing loss the opposite occurs.