Very broadly auditory physiology can be described in terms of two problems:

1. The transduction of sound into nerve impulses

2. The extraction of information from sound stimuli

The cochlea transduces mechanical sound energy into neural impulses; structurally it resembles a snail shell and is divided longitudinally into three parts, Scala Tympani, Scala Vestibuli & the smaller scala media in which resides the Corti where the transduction occurs; it consists of two membranes, basilar (BM) and tectorial(TM), that are anchored on slightly different axes. The BM has variable mechanical properties such that the apical end vibrates in response to lower frequencies while the basal end vibrates most in response to higher frequencies. The receptors (hair cells) are arranged along the BM. The movement of the BM results in a back and forth movement (shearing) between the BM and the TM which is detected by hair cells. Thus, the differential pattern of movement of the BM in response to complex sounds is encoded in the pattern of activation of the hair cells and transmitted into auditory nerve. This pattern is preserved as a place code of frequency up to auditory cortex.

The inner and outer hair cells are the specialised receptors that actively transduce sound energy into nerve impulses (inner hair cells) and act to increase the sensitivity of the BM to stimulation (outer hair cells). The stereocilia which project from the apical surface of the hair cell are extremely sensitive, moving the tip of one stereocilia by the width of an atom is enough to make the cell respond. The receptor potential of the hair cells can match the frequency of the sound up to a frequency of around 4 kHz. The inner hair cells innervate the afferent fibres of the auditory nerve and the subsequent synchronization of action potentials generated in the auditory nerve is another of the methods used to encode the frequency of a sound. The outer hair cells are motile and increase or decrease their length in response to variation in their receptor potential. The outer hair cells act as positive feedback amplifiers injecting mechanical energy into the BM at the site of activation, sharpening the response of the BM to specific spectra in a complex sound.

Stimuli encoded by the cochlea are carried by 8^th cranial nerve to several targets. The first is the Cochlea nucleus in the brain stem. Cells in the brainstem are sensitive to several aspects of the auditory stimuli. Bushy cells respond to the onset and offset of sounds, chopper cells respond to the frequency of the sound and fusiform cells assist in localising a sound's source. Cells in the next relay nuclei in the superior olive are sensitive to differences in intensity and timing between the two ears, and probably play a role in determining the location of a sound's source.

The inferior colliculus receives synaptic connections from all the major auditory areas in the brainstem and provides nearly all the inputs to higher auditory areas, i.e. it is anatomically well positioned to play a key role in auditory processing. There is some indication that it may be very important in establishing sound source location but the methods by which it achieves this are still unknown (See Davis, 2005 for a recent review).

Auditory cortex is organised on the basis of frequency, this type of tonotopic organisation reflects the place coding of frequency at the cochlea. It is suggested that primary auditory cortex identifies the fundamentals of a sound, i.e. frequency, volume, onset & offset; secondary cortex processes sounds for rhythm, melody and harmonicity, while tertiary auditory cortex may integrate everything into a whole. There are also two specialised areas for speech production and comprehension, Broca's and Wernicke's areas, the two areas are connected by the arcuate fasciculus and receive connections from auditory cortex.

References

Davis, K.A. (2005). Spectral Processing in the Inferior Colliculus. International Review of Neurobiology, 70, 169-205.

Recommended

Pickles, O. (1982). An Introduction to the Physiology of Hearing. London: Academic Press Inc. Ltd.

Joel Cooper

Auditory Neuroscience Laboratory

Discipline of Physiology

School of Medical Sciences

University of Sydney

26 July 2006