Hearing is an essential process. It enables us to understand and communicate with our fellow human beings using our ears, and also experience the outside world. The auditory pathway is more complex than the visual and the olfactory pathways. It is composed of a number of nuclei and is dependent on a range of functional areas.
This article will explore the anatomy, function and clinical relevance of the auditory pathway.
The outer ear/visible ear is referred to as the pinna. It collects omnidirectional sound waves and transforms them into a unidirectional source of information. By funneling the sound waves in this way, it is able to direct them into the auditory canal and amplify them.
The pinna has a number of features on its surface, which we will now discuss. The external auditory canal is the opening of the ear. The helix is the folded outer edge of the ear. The antihelix is a y-shaped region of ear cartilage. It has an inferior and superior crus that lie either side of the fossa triangularis. The groove between the helix and anti-helix is called the scapha. The tragus and antitragus are the cartilaginous prominences that lie anterior and inferior respectively to the external auditory opening.
The space between the tragus and antitragus is called the incisura anterior auris. The lobe is either attached or free (genetic determination). The concha is the hollow region that lies adjacent to the external ear opening. Finally the auricular sulcus is the depression that lies posterior to the ear.
The malleus, or hammer in Latin, develops from the first pharyngeal arch cartilage, like the mandible and maxilla jawbones. This small bone is connected with the tympanic membrane via its manubrium and with the incus via its articulating facet. The lateral process of the malleus is attached to the upper part of the tympanic membrane. The lower part of the malleus is attached to the tympanic membrane at the umbo, and is a strong connection. The anterior process is attached to the petrotympanic fissure.
There are anterior, lateral and superior malleal ligaments, which maintain the position of the malleus at the level of the head, neck and head of the malleus respectively, dampen the response of the ossicles to excessively loud sounds, and also reduce the displacement of the ossicles when middle ear pressure changes.
The tensor tympani muscle attaches onto the neck of the malleus, and its role is to dampen sounds. It arises from the greater wing of sphenoid and auditory canal and can be voluntarily controlled. However its involuntary function is most important.
The incus is shaped like an anvil. It is attached to the malleus via a facet, and to the stapes via its lenticular process located at the end of the long crus. It also has a short crus and its body lies mainly in the epitympanic recess. The posterior incudal ligament as well as the anterior malleal ligament give the ossicles their axis of rotation.
This is the smallest bone in the human body. It develops from the second pharyngeal arch, and is the last ossicle of the middle ear. Its footplate articulates with the oval window via the annular ligament.
The stapedius is the smallest skeletal muscle in the human body, and is just over a millimeter in length. It stabilizes the stapes, and is innervated by the facial nerve (cranial nerve 7). Hence in facial nerve palsy (usually a lower motor neurone i.e. Bell’s palsy), one of the symptoms is pain on hearing noises (especially loud noises) on the affected side, due to a lack of innervation of the stapedius. It arises from the cone shaped eminence in the posterior part of the tympanic cavity known as pyramidal eminence, and inserts onto the neck of the stapes.
This nerve provides taste to the anterior two thirds of the tongue, and is a branch of the facial nerve (cranial nerve 7). It passes through the middle ear on its way to the tongue.
This region is found within the bony labyrinth. The cochlea (the region responsible for hearing) is a spiral shaped hollow organ. The cochlear duct is the triangular shaped section of the cochlea, which contains the organ of Corti. The oval window is quite simply an oval shaped window that is moved inwards by the movement of the stapes footplate.
The scala vestibuli is the semicircle shaped region above the scala media and contains perilymph. It is separated from the scala media by Reissner’s membrane. It receives the sound waves from the oval window, and sends them up to the apex of the cochlea (the helicotrema). Here the sound wave vibrations continue and head back down the cochlea via the scala tympani. The scala media lies between the scala vestibuli and the scala tympani and contains endolymph.
The organ of Corti lies within the scala media. The scala tympani lies below the scala media, and is separated from the scala media by the basilar membrane. The round window is a circular window that moves out upon sound transmission.
It is essential for sound transmission in the inner ear, as perilymph is a fluid, and fluids are essentially non-compressible. Without the round window, the compression of the stapes footplate would not transmit the vibrations from the tympanic membrane.
The auditory pathway
The external ear/pinna funnels sound waves into a unidirectional wave, and is able to direct it into the auditory canal. This sound then reaches the tympanic membrane, and causes it to vibrate. The louder the sound the bigger the vibration, the lower pitch the sound the slower the vibration.
The handle of the malleus articulates with the tympanic membrane, and the malleus also has an articulating facet for the Incus. The axis of rotation is maintained by two ligaments (the anterior malleal and posterior incudal ligaments). The incus lies in the epitympanic area, and is shaped like an anvil. It articulates with the stapes via its lenticular process.
The stapes is shaped like a stirrup, and impacts onto the oval window. The stapes moves like a piston, and causes the oval window to move in and out with sounds. There is a round window located below the oval window that moves out when the oval window moves in.
Without it, there would be no transmission of the sound waves into vibrations in the inner ear. The sound waves are sent up the scala vestibuli to the apex of the cochlear duct (the helicotrema). Here it continues back down the spiral shaped cochlear organ in the scala tympani. The scala vestibuli and scala media are separated by Reissner’s membrane. Scala media and scala tympani below are separated by the basilar membrane.
When these waves move up and down the perilymph in the scala vestibuli and scala tympani, the vibrations move the basilar membrane. The organ of Corti lies on the basilar membrane, and is the organ responsible for converting these vibrations into electrochemical signals. There are stereocilia that lie on the organ of Corti. Their tips go into a gel like layer called the tectorial membrane. When vibrations move the basilar membrane, these hair cells bend, and potassium channels open.
The influx of potassium causes the generation of a local current and then an action potential that is sent up the cochlear division of the vestibulocochlear nerve (cranial nerve 8). This nerve then sends the signal to nuclei in the brainstem.
These include the cochlear nuclei. The information from the cochlear nerve passes to the ventral and dorsal cochlear nuclei. These nuclei are the first connection with the auditory information. The three major outputs of these nuclei are to the superior olivary complex (via the trapezoid body). The other half of the information is sent to the contralateral superior olivary complex. The second order neurons are sent via the lateral lemniscus to the inferior colliculus, which receives connections from from the superior olivary complex. The majority of these connections will ultimately terminate in the auditory cortex.
The superior olivary complex - This is a cluster of nuclei found in the brainstem. It has a number of roles in the process of hearing. These include detection of the time difference between sound reaching each ear, and hence localization of where the sound is coming from. The lateral superior olive has a role in detecting the differences in sound intensity between both ears. The medial superior olive will locate which angle the sound is coming from.
The inferior colliculus - This is the ultimate end point of many of the brainstem nuclei outputs. Vertical and horizontal sound location information synapses in the inferior colliculus and localizes where the sound is coming from. It functions as the switchboard and as the convergence of many pathways.
The medial geniculate nucleus - This is the nucleus of the thalamus that acts as the relay point between the inferior colliculus and the auditory cortex. The lateral geniculate nucleus (involved in the visual pathway) lies adjacent to it.
The primary auditory cortex - This is located in the temporal lobe and has a role in the processing of auditory information. It lies in the superior temporal gyrus of the lobe, and extends as far as the transverse temporal gyri. The frontal and parietal lobes are responsible for the final elements of sound processing (secondary auditory cortex). The primary auditory cortex is tonotopically organised, meaning that the cells within the cortex, will receive inputs from cells in the inner ear that respond to specific frequencies.
Wernicke’s area - This is a region on the temporal-parietal junction and on the left side of the brain, which is responsible for understanding of speech. The primary auditory cortex will signal next to this area.
Broca’s area - This is a region within the inferior frontal gyrus of the frontal lobe. On the left side it is responsible for generating speech.
Tinnitus - Tinnitus is a ringing sound in the ears without actual sound coming from the environment. It usually occurs after hearing loss, when the inner hair cells become highly sensitised.
Presbyacusis - This is defined as age related hearing loss. It is progressive and irreversible, and mainly affects high-pitched sounds. It is the commonest cause of hearing loss. Sounds appeared muffled, or dull. Causes include damage to the organ of Corti, basilar membrane stiffening, vascular degeneration, and spiral ganglion cell degeneration.
Wernicke’s aphasia - Wernicke's aphasia is a type of aphasia where the patient is unable to understand their usual language in its spoken or written form. Wernicke’s area on the left side enables us to understand speech, and a stroke affecting the area causes word salad or nonsense sentences or random words. The patient will not be aware of this defect when they speak.
Meniere’s disease - This is a disease caused by a build up of endolymph fluid in the inner earcausing dizziness, vertigo, tinnitus and balance issues. It can be caused by infection or scar tissue following surgery.
Vestibular schwannoma - Vestibular schwannoma is a tumor of the schwann cells of the vestibulocochlear nerve. Symptoms include hearing loss, tinnitus, balance issues, a feeling of pressure in the ears, and rarely a headache with larger tumors. If the tumor is large it can also compress the facial nerve (which also leaves the skull via the internal acoustic meatus) or the trigeminal nerve, causing facial weakness or tingling respectively.
Otitis media - This is an infection of the middle ear, most commonly following an upper respiratory tract infection. The Eustachian tube opens into the middle ear and eustachian tube dysfunction promotes viral or bacterial colonisation of the middle ear. Treatment is conservative. If children have this repeatedly, they get a condition called glue ear (otitis media with effusion), which requires a tympanostomy tube (grommet) to perforate the eardrum and ventilate the middle ear. If bacteria cause an infection then the disease may become suppurative, in which case antibiotics are required.
Otosclerosis - This is defined as abnormal growth of bone in the middle ear, which results in the fixation of the footplate of the stapes The patient will experience increasing deafness as the condition worsens. It is an inherited condition, and is an example of conductive hearing loss. There is evidence that the condition can be triggered by a viral infection.