Hearing is essential for our functioning in daily life. The process by which we achieve this is fascinating on both an anatomical and electrochemical level. 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. This article will include reference to the detailed anatomy of the system, as well as clinical points.
The cord cochlear originates from the Greek word ‘Kochlias’ 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 (near the oval window) to the apex or top of the spiral.
The cochlea has the spiral shaped tunnel within it named Rosenthal’s canal, which contains the spiral ganglion. This cochlea is divided into three chambers, the scala vestibuli, the scala media and the scala tympani. The scala vestibuli contains perilymph, lies above the scala media, and 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 the scala tympani appears semioval and runs to the round window.
Finally we have the scala media (cochlear duct), which lies in between the two others, and contains endolymph (potassium rich liquid). In cross section it appears like a triangle, with the point medial and the base lateral. Within it lies the 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.
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 footplate. It connects with the incus via its articulating facet. The lateral process of the malleus attached to the lateral 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 mallear 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 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 the most important.
The incus is shaped like an anvil. It attached to the malleus via a facet, and to the stapes via its lenticular process. It has a long and short limb, and its body lies mainly in the epitympanic recess. The posterior incunal ligament as well as the anterior malleal ligament gives 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.
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 sensory epithelium that lies on top of a basilar membrane. The hair cells (anatomical term is stereocilia) arise from the sensory epithelium, and their tips lie in a gel like layer called the tectorial membrane. Their function is to transmit impulses to the cochlear nerve. They achieve this by there being a potential difference across the endolymph and perilymph. Once the vibration moving up the scala vestibuli and down the scala tympani causes movement of the basilar membrane (as described below), the hair cells move up and down and are bent, allowing potassium ions to move in, and generate a local current and then an action potential.
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 articulated 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 incunal ligament. The tendon of the tensor tympani muscle also attaches onto the malleus (the footplate to be exact), 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.
Therefore, 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 VIII). The nerve impulses themselves are created by stereocilia or ‘hair cells.’ The tips 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 are bent against the tectorial membrane. This opens ion channels causing influx of potassium and calcium, and generate a local current, then an action potential. This action potential is then sent via the cochlear division of the Vestibulocochlear nerve (cranial nerve VIII), and is sent to the primary auditory cortex (in the temporal lobe).
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’.