The sense of hearing is the function of the ears. The ear has three divisions: the outer, middle and inner ear. The outer ear consists of the external ear or pinna and the external ear canal. The pinna gathers and focuses sound vibrations along the external ear canal, to the tympanic membrane, causing it to vibrate. The tympanic membrane separates the outer ear from the middle ear. The middle ear is an air filled cavity which opens into the pharynx via a narrow tube, the Eustachian Canal. Air can move into and out of the middle ear through this tube, so that the air pressures in the middle and outer ear can be equalized. If the Eustachian tube is blocked (due to inflammation etc.) then the pressures may become unequal. This will produce discomfort and cause the tympanic membrane to become stretched and stiff, so that it will not vibrate properly in response to sound waves. Wax secreted by the epithelium of the external ear canal may also accumulate and impact against the tympanic membrane, reducing its ability to vibrate, and so causing hearing loss.

The vestibular apparatus consists of two interconnected, sac-like structures the utriculus and sacculus, from which three arching tubes, the semicircular canals arise (see diagrams).

The vestibular receptors are important in regulation of posture and balance. Hence, vestibular activity relays from the vestibular nuclei, down to spinal motor nuclei, via the vestibulospinal pathways to control postural reflexes. Activity also relays rostrally in the medial longitudinal fasciculus to motor nuclei in the midbrain, which control the extraocular muscles, so that eye movements may be generated to compensate for head movements. Disturbance of the vestibular apparatus, or of the centres in the medulla to which they project, will result in dizziness, imbalance, disturbed muscle tone, nystagmus and related dysfunctions. Function tests rely on assessing the reflex responses to vestibular stimulation.

The Cochlea

The cochlea is a spirally coiled tube connected to the utriculus by a short duct. All along the basal wall of the cochlea (the basilar membrane) four rows of ciliated sensory cells run: a single inner row (towards the axis of the spiral) and three outer rows, separated from the inner row by the tunnel of Corti. The cilia of these cells are embedded in an overlying gelatinous membrane, the tectorial membrane.

The basal portion of the cochlea (next to the utriculus) fits snugly against a small opening in the otic bone, which forms a window (the oval window) into the middle ear. The bony cavity in which the cochlea sits also has nearby, a membrane-covered window (the round window) into the middle ear.

Vibrations of the tympanic membrane are transmitted to a chain of three, small bony ossicles: the malleus is attached to the tympanic membrane, and to the incus, which is also attached by a hinge-like ligament to the roof of the middle ear. The incus is attached to the stapes which has a small base plate, fitting loosely into the oval window. By this means, vibrations of the tympanic membrane can be transmitted into the cochlea. The tympanic membrane and bony ossicles serve to amplify and focus the sound vibrations on to the cochlea. If the tympanic membrane is damaged, or if the ossicular chain is dislodged, or if the base plate of the stapes becomes calcified in the oval window, vibrations will not be readily transmitted to the cochlea, and hearing will be impaired. Hearing defects of this sort are termed conductive deafness.

Basilar Membrane

The sensory cells in the cochlea are innervated by axons of sensory nerve cells in the spiral ganglion of the eighth cranial nerve (vestibulo-cochlear nerve). By far the greater number of axons innervate the inner hair cells, which are fewer in number, than the outer cells, in a pattern analogous the relationship between the central and the peripheral retina in the eye. The inner cells also receive strong efferent innervation which causes the cells to move. It is believed that this helps to amplify the response of the cochlea to applied vibrations, and to enhance the differential responsiveness of different portions of the cochlea to sounds of different pitch (see below). Efferent innervation of the hair cells also modulates their sensitivity to sounds.

Vibrations entering the cochlea, cause oscillatory waves to travel along the basilar membrane. Oscillation of the basilar membrane cause the hair cells to move relative to the tectorial membrane, to which their cilia are attached. This causes depolarization of the hair cells, which is transmitted to the sensory nerve endings, giving rise to action potentials which are carried to the cochlear nuclei in the medulla. Loudness may be appreciated via the differences in the amplitude of the movements of the basilar membrane. Hearing defects can result from damage to the hair cells in the cochlea (e.g. caused by large doses of some antibiotics) or to the sensory nerve cells of the spiral ganglion. Hearing defects of this sort are more serious, and are termed sensori-neural deafness. Simple hearing tests can be used to distinguish between conductive and sensori-neural deafness. You should familiarize yourself with these tests (e.g. the Rinne, Weber and Schwabach tests).

The entire length of the cochlea is not equally responsive to all frequencies of vibration. Very high frequency vibrations (high-pitched sounds) cause maximal movement of the basal regions of the cochlea (near to the utriculus). Lower frequency vibrations (low pitched sounds) cause most vibration near the apex (tip) of the spiral. The cells at a particular point on the basilar membrane show intrinsic oscillatory potentials which are tuned to respond best to vibratory frequencies which cause maximal movement at that point. It is in this way that we can distinguish between high and low pitched sounds depending on the point on the cochlea which is maximally stimulated (the Place Principle). Changes in the basilar membrane results in progressive loss of hearing sensitivity in the higher frequency range with increasing age (presbycusis). Damage to different portions of the cochlea will cause hearing defects particularly for the tones which would cause maximal movements at the damaged region. Exposure to sound reduces the sensitivity of the ear to other sounds; and exposure to sound of a given frequency reduces sensitivity particularly to sounds of similar pitch. This is referred to as masking.

Auditory Pathway

From the cochlear nuclei in the medulla, axons mostly cross over to the opposite side, and run up to the inferior colliculus in the mid-brain. Some also run on the same side. In the inferior colliculi, auditory reflexes are generated. From here, axons further run to the thalamus to an area (the medial geniculate nucleus) close to where the visual inputs project. From the thalamus, there is a further relay to the superior temporal gyrus in the temporal lobe, where the sense of hearing is mediated.

The temporal lobe on the left side of the brain is best at hearing and interpreting the spoken word and syntax. That on the right is better at hearing and appreciating or recognising melodies and prosody. The two cerebral hemispheres therefore, although looking identical, are functionally dissimilar.

Evoked Potential tests record the cortical response to sound and are less subjective since they do not require that the subject indicate ability to hear. In these tests, one ear is tested at a time, and the other ear is masked using white noise (containing a mixture of random frequencies) to reduce its sensitivity to the sounds being presented to the contralateral ear.