Anatomy of Hearing
Hearing is essential for our functioning in daily life. The process by which we achieve this is fascinating from an anatomical and electrochemical point of view. The main apparatus for hearing is the cochlear system, which relies on a number of closely linked and highly sensitive components. Understanding the process of hearing requires a basic knowledge of vibration, fluid pressure and electrochemical stimulation. After the sensory transduction in the inner ear, the brain must process and make sense of the sounds we hear. This article will include reference to the detailed anatomy of the hearing process, as well as its clinical relevance.
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 on either side of the fossa triangularis. The groove between the helix and anti-helix is called the scapha. The Tragus is the cartilaginous prominence that lies anterior to the external auditory opening. The Antitragus is the cartilaginous prominence that lies inferior 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 we have the auricular sulcus, which is the depression that lies posterior to the ear.
The word cochlea originates from the Greek word ‘Kokhlias’ meaning screw or snail. The cochlear system is the organ of the inner ear, which enables us to hear. It is a hollow, spiral shaped organ that curves around the modiolus (its central axis) two and a half times. Sounds propagate from the base (close to the oval window) to the apex or top of the spiral.
The cochlea is hollow, and the spiral shaped tunnel within it is named Rosenthal’s canal. This canal is divided into three chambers, the scala vestibuli, the scala media and the scala tympani. The scala vestibuli contains perilymph, and lies above the scala media. It begins at the oval window. In cross section this chamber appears like a semi circle. Reissner’s membrane separates the scala vestibuli above from the scala media below. The scala tympani lies below the scala media and also contains perilymph. The membrane separating them is the basilar membrane. In cross section it appears like a semioval. This window lies inferior to the scala media and runs towards the round window. Finally we have the scala media, which lies between the two others, and contains endolymph (potassium rich liquid). In cross section it appears like a triangle, with the base lateral and the opposite vertex medial. Within it lies the cochlear duct and organ of Corti. The point at which the scala tympani and scala vestibuli merge at the top (apex) of the cochlea, and where the direction of the sound waves transmitted by the tympanic membrane reverse, is called the helicotrema.
Organ of Corti
One of the essential parts of the cochlear system is the Organ of Corti. This organ is directly responsible for enabling us to hear, and spirals around with the cochlea. It lies within the scala media, and is composed of mechanosensory epithelium that lies on top of a basilar membrane. The hair cells arise from the sensory epithelium, and their stereocilia lie in a gel like layer called the tectorial membrane. Their function is to transduce a mechanical signal into an electrical input to the cochlear nerve. They achieve this as a result of the shearing force between the tectorial and basilar membranes. Once the vibration moving up the scala vestibuli and down the scala tympani causes movement of the basilar membrane (as described below), the stereocilia are bent and the hair cells shorten and elongate, allowing potassium ions to move in, and generate a local current and then an action potential.
Malleus- In mammals, the malleus develops from the lower jawbone, and the word malleus means hammer. It develops from the first pharyngeal arch, like the mandible and maxilla jawbones. This small hammer shaped bone connects to the tympanic membrane via its handle or manubrium. It connects to 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, dampen the response of the ossicles to excessively loud sounds, and also reduce the displacement of the bones 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 the sphenoid bone and auditory canal, and can be voluntarily controlled. However, its involuntary function is most important.
Incus- The incus is shaped like an anvil. It is attached to the malleus via a facet, and to the stapes via its lenticular process. It has a long and 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 an axis of rotation for their pendulum-like movement.
Stapes- 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 neuron 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 the pyramidal eminence, and inserts onto the neck of the stapes.
How do we hear?
The process of hearing is completed through a process known as auditory transduction. The ear is able to convert sound waves from the air, into electrical impulses, which can be interpreted by the brain. As sound enters the ear, it moves through the external auditory canal where it meets the tympanic membrane. The tympanic membrane then vibrates in response to the sound. Quieter sounds produce a smaller vibration, lower pitched sounds cause slower vibrations. The tympanic membrane is cone shaped and articulates with a chain of three auditory ossicles. When the tympanic membrane is vibrating due to sound waves, the three ossicles of the middle ear transmit these waves. The malleus connects to the tympanic membrane, and connects to the incus, which connects to the stapes. The handle of the malleus connects to the tympanic membrane, and the footplate of the stapes lies on the oval window. The axis at which these bones move is supported by the presence of numerous ligaments. These include the anterior malleal ligament and the posterior incudal ligament. The tendon of the tensor tympani muscle is attached to the neck of the malleus , and the chorda tympani (the special branch of the facial nerve for taste) runs perpendicular to this tendon.
The stapes has a piston-like movement, which sends vibrations into a perilymph cavity called the bony labyrinth. The inner ear is filled with perilymph, and fluid as a state of matter does not compress significantly. However the presence of a round window below the oval window (that moves out upon pressure within the bony labyrinth rising), means that this force can be transmitted. This force is transmitted up the spiral-shaped cochlea.
So when the oval window is compressed, the wave is transmitted from the piston-like action of the stapes, through the perilymph up the scala vestibuli. The descending portion of the passage is the scala tympani, which terminates at the round window. The cochlear duct (scala media) is positioned between the scala tympani and scala vestibuli, and is filled with a fluid called endolymph. Separating the scala vestibuli and the scala media is Reissner’s membrane, and between the scala tympani and the scala media is the basilar membrane. These flexible membranes move when sound waves are being transmitted up the scala vestibuli and down the scala tympani.
Just above the basilar membrane is a specialized structure known as the organ of Corti. As the basilar membrane vibrates, the organ of Corti is stimulated, and sends electrical signals to the brain, via the cochlear division of the Vestibulocochlear nerve (cranial nerve 8). The nerve impulses themselves are created by ‘hair cells.’ The stereocilia of these hair cells lie within a gelatinous layer called the tectorial membrane. As the basilar membrane vibrates, the small clusters of hair cells vibrate, and the stereocilia are bent against the tectorial membrane. This opens potassium channels, which allow the ions to enter, and generate a local current, then an action potential. This action potential is then sent to the primary auditory cortex (in the temporal lobe) via the cochlear division of the Vestibulocochlear nerve (cranial nerve 8). The entire basilar membrane does not vibrate at the same time. Specific areas of the basilar membrane move variably when exposed to different frequencies of sound. Lower frequencies cause vibration near the apex, and higher frequencies cause vibration near the cochlear base. This arrangement is known as ‘tonotopic organisation’.
What Happens Next?
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. There are ventral and dorsal cochlear nuclei. The information from the cochlear nerve passes to the ventral and deep part of the 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 ipsilateral superior olivary complex. The second order neurons are sent via the lateral lemniscus to the inferior colliculus 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, including sound localization The medial superior olive detects the time difference between sound reaching each ear (interaural time difference). The lateral superior olive has a role in detecting the differences in sound intensity between both ears arising from a sound shadow created by the head (interaural intensity difference). The superior olivary complex will locate which angle the sound is coming from.
The Lateral Lemniscus - This is the pathway that transmits the information from the cochlear nuclei to other brainstem nuclei.
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 temporoparietal junction and on the left side of the brain, 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.
Arcuate Fasciculus - This is a white matter tract that connects Wernicke’s area to Broca’s area.