30: The Inner Ear

Introduction

• The Ear Has Three Functional Parts

• Hearing Commences with the Capture of Sound Energy by the Ear

• The Hydrodynamic and Mechanical Apparatus of the Cochlea Delivers Mechanical Stimuli to the Receptor Cells

  • The Basilar Membrane Is a Mechanical Analyzer of Sound Frequency

  • The Organ of Corti Is the Site of Mechanoelectrical Transduction in the Cochlea

• Hair Cells Transform Mechanical Energy into Neural Signals

  • Deflection of the Hair Bundle Initiates Mechanoelectrical Transduction

  • Mechanical Force Directly Opens Transduction Channels

  • Direct Mechanoelectrical Transduction Is Rapid

• The Temporal Responsiveness of Hair Cells Determines Their Sensitivity

  • Hair Cells Adapt to Sustained Stimulation

  • Hair Cells Are Tuned to Specific Stimulus Frequencies

  • Sound Energy Is Mechanically Amplified in the Cochlea

• Hair Cells Use Specialized Ribbon Synapses

• Auditory Information Flows Initially Through the Cochlear Nerve

  • Bipolar Neurons in the Spiral Ganglion Innervate Cochlear Hair Cells

  • Cochlear Nerve Fibers Encode Stimulus Frequency and Intensity

• Sensorineural Hearing Loss Is Common but Treatable

• An Overall View

Human experience is enriched by our ability to distinguish a remarkable range of sounds—from the intimacy of a whisper to the warmth of a conversation, from the complexity of a symphony to the roar of a stadium. Hearing begins when the cochlea, the snail-shaped receptor organ of the inner ear, transduces sound energy into electrical signals and forwards them to the brain. Our ability to recognize small differences in sounds stems from the cochlea's capacity to distinguish among frequency components and to inform us of both the tones present and their amplitudes.

Deafness can be devastating. For the elderly, hearing loss can result in a painful and protracted estrangement from family, friends, and colleagues. Children may lack hearing as a result of pre- or perinatal infections and especially of genetic conditions, which affect one child in a thousand. Such children are often deprived of the normal avenue to the development of speech, and thus of reading and writing as well. It is for this reason that a modern pediatric examination must include an assessment of hearing. Many children thought to be cognitively impaired are found instead to be hard of hearing, and their intellectual development resumes its normal course when this problem is corrected.

Acute hearing loss in the intermediate years exacts an enormous price for two reasons. First, hearing plays an important, but often overlooked, role in our psychological well-being. Daily conversation with family and colleagues helps to establish our social context. The abrupt loss of such social intercourse as a result of sudden deafness leaves a person distressingly lonely and may lead to depression and even suicide. Hearing also serves us in another, more subtle way. Our auditory system is a remarkably efficient early warning system, subconsciously informing us about our environment. For example, when other people enter the room or approach us we often hear them before we see them. More obviously, awareness of fire alarms and the sirens of emergency vehicles can be lifesaving. ...