What is the physiology of hearing?

Physiology of Hearing

The physiology of hearing involves a complex process where sound waves are transformed into mechanical energy, then into electrical signals that are transmitted to the brain, with the cochlear hair cells serving as the primary auditory receptors that convert mechanical energy into neural impulses.1

Anatomical Pathway of Sound

• Sound enters through the external ear and causes vibration of the tympanic membrane (eardrum), which is then transmitted through the three ossicles of the middle ear (malleus, incus, and stapes) 1
• The stapes acts as a piston against the oval window, initiating oscillatory pressure changes within the fluid-filled chambers of the cochlea 2
• These pressure changes create traveling waves along the basilar membrane, which is graded in mass and stiffness along its length 2
• Each frequency component creates a largely independent traveling wave that peaks at a specific position: low frequencies at the cochlear apex and high frequencies at the base 2

Sensory Transduction

• The basilar membrane's oscillations deflect the hair bundles of sensory hair cells in the organ of Corti 1, 2
• Hair cells serve as auditory receptors, converting the mechanical energy of sound waves into electrical neural impulses 1
• This process involves two key stages:
  • Mechano-electrical transduction: Sound-induced mechanical displacement directly opens ion channels in the stereocilia of hair cells, inducing changes in their electrical potential 3
  • Electro-mechanical transduction: Outer hair cells respond to electrical changes with alterations in their length ("electromotility"), providing mechanical feedback to the basilar membrane 3

The Cochlear Amplifier

• The outer hair cells' electromotility creates a "cochlear amplifier" that:
  • Augments the mechanical displacement of the basilar membrane 3
  • Improves hearing sensitivity and frequency discrimination 3
  • Provides a compressive nonlinearity to responsiveness 2
• This active process operates near a Hopf bifurcation, explaining several key features of hearing 2
• In extremely quiet conditions, the active process can cause a normal ear to actually emit sound 2

Neural Pathway

• Hair cells transmit signals to spiral ganglion neurons, which represent the sole input of auditory information to the brain 4
• Spiral ganglion neurons establish precise connections that link cochlear hair cells to target neurons in the auditory brainstem 4
• The auditory nerve fibers branch into two main pathways: 5
  • A ventral sound-localizing stream
  • A dorsal pattern recognition stream
• Both pathways innervate different divisions of the cochlear nucleus 5
• The outputs from these streams are progressively combined in the inferior colliculus and beyond 5
• The central auditory pathway continues through the brainstem, midbrain, and thalamus before reaching the auditory cortex in the temporal lobe 1, 5

Types of Hearing Loss

• Conductive hearing loss: Results from abnormalities of the external ear, tympanic membrane, middle ear space, or ossicles 1
• Sensorineural hearing loss: Results from abnormalities of the cochlea, auditory nerve, or central auditory pathways 1
• Mixed hearing loss: A combination of both conductive and sensorineural components 1

Clinical Assessment of Hearing

• Audiologic testing is the preferred method for hearing assessment 1
• Physiologic measures used to assess hearing include: 1
  • Otoacoustic emissions (OAEs): Measure cochlear responses to acoustic stimuli, reflecting the status of the peripheral auditory system extending to the cochlear outer hair cells
  • Auditory brainstem response (ABR): Records neural activity generated in the cochlea, auditory nerve, and brainstem in response to acoustic stimuli
• Tuning fork tests (Weber and Rinne) can help differentiate between conductive and sensorineural hearing loss 1

Recent Advances in Understanding Hearing Physiology

• Recent animal studies have shown that moderate noise exposure can trigger irreversible degeneration of auditory nerve fibers without affecting outer hair cells or permanent hearing thresholds 1
• This phenomenon, called "hidden hearing loss," may explain why some patients with normal hearing thresholds still experience difficulty understanding speech in noisy environments 1
• Understanding the genetic pathways that regulate auditory function has revealed new targets for potential pharmacological treatments for hearing loss 6