Auditory System I: The Ear And Cochlea
The auditory system is responsible for sound perception. The receptive end-organ is the cochlea of the inner ear, which converts sound waves into electrical signals by mechanotransduction. The electrical signal generated in response to a sound is passed (together with information from the vestibular system; see Chapter 29), via the eighth cranial nerve (vestibulocochlear nerve) to the brainstem where it synapses in the cochlear nuclear complex (see Chapter 28). Although the auditory system as a whole performs many functions, the primary site responsible for frequency discrimination is at the level of the cochlea.
Properties of sound waves
A sound wave is characterized by:
• Amplitude or loudness (measured in decibels [db]);
• Frequency or pitch (measured in hertz [Hz]);
• Phase and
• Quality or timbre.
The intensity of sound can vary enormously but in general we can discriminate changes in intensity of around 1–2 dB. The arrival of a sound at the head creates phase and intensity differences between the two ears unless the sound originates from the midline. The degree of delay and intensity change between the two ears as a result of their physical separation is useful but probably not necessary for the localization of sounds (see Chapter 28).
External and middle ear
On reaching the ear the sound passes down the external auditory meatus to the tympanic membrane or eardrum, which vibrates at a frequency and strength determined by the impinging sound. This causes the three ear ossicles in the middle ear to move, displacing fluid within the cochlea as the stapedial foot process moves within the oval window of the cochlea. This process is essential in reducing the acoustic impedance of the system and in enhancing the response to sound, because a sound hitting a fluid directly is largely reflected.
There are two small muscles associated with the ear ossicles, which protect them from damage by loud noises as well as modifying the movement of the stapedial foot process in the oval window. Damage to the ear ossicles (e.g. otosclerosis), middle ear (e.g. infection or otitis media) or external auditory meatus (e.g. blockage by wax) all lead to a reduction in hearing or deafness that is conductive in nature.
Inner ear and cochlea
The displacement of the stapedial foot process in the oval window generates waves in the perilymph-filled scala vestibuli and tympani of the cochlea. These two scalae are in communication at the apical end of the cochlea, the helicotrema, but are separated for the rest of their length by the scala media, which contains the transduction apparatus in the organ of Corti.
The organ of Cortisits on the floor of the scala media on a structure known as the basilar membrane (BM), the width of which increases with distance from the stapedial end. This increase in width coupled to a decrease in stiffness of the BM means that sounds of high frequency maximally displace the BM at the stapedial end of the cochlea while low-frequency sounds maximally activate the apical end of the BM. Thus, frequency tuning is, in part, a function of the BM although it is greatly enhanced and made more selective by the hair cells of the organ of Corti that lie on this membrane.
The organ of Corti is a complex structure that contains the cells of auditory transduction, the hair cells (see Chapter 23), which are of two types in this structure:
• a single row of inner hair cells (IHCs) – which provide most of the signal in the eighth cranial nerve;
• 3–4 rows of outer hair cells (OHCs) – which have a role in modulating the response of IHCs to a given sound.
These two types of hair cell are morphologically and electrophysiologically distinct:
• While the IHCs receive little input from the brainstem, the OHCs do so from the superior olivary complex, which has the effect of modifying the shape and response properties of these cells.
• Some of the OHCs make direct contact with the overlying tectorial membrane (TM) in the organ of Corti which may be important in modifying the response of the IHCs to sound, as these cells do not contact the TM but provide 93% of the afferent input of the cochlear nerve.
• One afferent fibre receives from many OHCs, but a single IHC is associated with many afferent fibres.
In addition to these differences between OHCs and IHCs, there are subtle alterations in the hair cells themselves with distance along the scala media. These alterations in shape modify their tuning characteristics, which adds a degree of refinement to frequency tuning beyond that imparted by the resonance properties of the BM.
Damage to the cochlea, hair cells or cochlear part of the vestibulocochlear nerve leads to deafness that is described as being sensorineural in nature. Trauma, ischaemia and tumours of the eighth cranial nerve can lead to this. Certain hereditary causes of deafness have been associated recently with defects in the proteins found in the stereocilia of hair cells (see also Chapter 23).