SENSE OF HEARING, ITS MECHANISM AND AUDITORY PATHWAY

Components and Functions of the External, Middle, and Inner Ear

The ear is divided into three main sections: the external ear, middle ear, and inner ear. Each section has distinct components and functions that contribute to the overall process of hearing.

1. External Ear:
   • Components: The external ear consists of the pinna (auricle), external auditory canal (ear canal), and tympanic membrane (eardrum).
   • Functions: The pinna collects sound waves and directs them into the external auditory canal. The sound waves then travel through this canal to vibrate the tympanic membrane, which marks the boundary between the external ear and middle ear.
2. Middle Ear:
   • Components: The middle ear contains three small bones known as ossicles (malleus, incus, stapes), the Eustachian tube, and the oval window.
   • Functions: When sound waves hit the tympanic membrane, it vibrates and causes the ossicles to move. The malleus is attached to the tympanic membrane; it transmits vibrations to the incus, which in turn passes them to the stapes. The stapes then pushes against the oval window, leading to fluid movement in the inner ear. The Eustachian tube helps equalize pressure in the middle ear with atmospheric pressure.
3. Inner Ear:
   • Components: The inner ear comprises structures such as the cochlea, vestibule, semicircular canals, and auditory nerve.
   • Functions: The cochlea is responsible for converting mechanical vibrations from sound into electrical signals that can be interpreted by the brain. It contains hair cells that respond to fluid movement caused by sound vibrations. The vestibule and semicircular canals are involved in maintaining balance and equilibrium.

Sound Stimulus

The sound stimulus refers to any audible vibration that travels through a medium (such as air) and can be detected by human ears. Sound stimuli are characterized by their frequency (pitch) measured in Hertz (Hz) and amplitude (loudness) measured in decibels (dB). These stimuli can originate from various sources including musical instruments, voices, environmental sounds, etc., creating waves that propagate through air or other mediums.

Parts of the Inner Ear and Their Roles in Equilibrium and Hearing

1. Cochlea:
   • Role in Hearing: Converts mechanical energy from sound waves into neural signals via hair cells located within its structure.
2. Vestibule:
   • Role in Equilibrium: Contains otolith organs (utricle and saccule) that detect linear acceleration and head position relative to gravity.
3. Semicircular Canals:
   • Role in Equilibrium: Detect rotational movements of the head through fluid movement within their curved structures.
4. Auditory Nerve (Cochlear Nerve):
   • Role in Hearing: Transmits electrical signals generated by hair cells from cochlea to auditory cortex for interpretation as sound.

Auditory Physiology

Auditory physiology encompasses how sound is processed within our auditory system:

1. Sound waves enter through the external ear.
2. Vibrations cause movement of tympanic membrane.
3. Ossicles amplify these vibrations before transmitting them to oval window.
4. Fluid within cochlea moves due to pressure changes at oval window.
5. Hair cells within cochlea convert fluid motion into electrical impulses.
6. These impulses travel along auditory nerve fibers towards brain regions responsible for processing sounds such as primary auditory cortex.

Pathways for Sensations of Hearing

The pathway for sensations of hearing involves several steps:

1. Sound waves enter through pinna → travel down external auditory canal → vibrate tympanic membrane.
2. Tympanic membrane’s vibrations move ossicles → stapes pushes on oval window → creates fluid wave in cochlea.
3. Fluid wave stimulates hair cells → generates action potentials sent via auditory nerve fibers.
4. Auditory signals reach brainstem nuclei → ascend through medial geniculate nucleus of thalamus → finally reach primary auditory cortex where sounds are perceived.

Auditory Pathology

Auditory pathology refers to various disorders affecting hearing function:

1. Conductive Hearing Loss: Caused by issues with outer/middle ear structures preventing sound transmission; often treatable with surgery or hearing aids.
2. Sensorineural Hearing Loss: Results from damage to inner ear structures or auditory nerve; often permanent but may benefit from hearing aids or cochlear implants.
3. Mixed Hearing Loss: A combination of conductive and sensorineural loss requiring a tailored treatment approach based on underlying causes.
4. Common diagnostic tools include audiometry tests, imaging studies like MRI/CT scans for structural abnormalities, or tympanometry for assessing middle ear function.