Lecture 3: Hearing

Hearing

The human auditory system can detect quick alterations in decibels (also referred to as loudness or sound intensity) and frequency (also referred to as pitch). The responsibility of the auditory system is to convert air pressure changes in to activity which the human brain can recognise and assign a meaning to.

Each part of the ear has different responsibilities and performs different sound functions.

External Ear: Capture, Focus, Filter

Middle Ear: Concentrates Energies

Inner Ear: Tranduces sound into neural activity

The process of hearing:

Sound waves are gathered by the outer ear. The outer ear consists of the external ear, alternatively known as the pinna and a canal leading to the eardrum. The external ear's configurations amplifies the sound, particularly in the frequency range of 2000-5000Hz. This range is typically when speech can be recognised. The shape of the external ear can be important for local sound.

Sound waves vibrate the ear drum from the canal, vibrating three bones in the middle ear, known as the malleus, the incus and the stapes. The vibration of the stapes transmits amplified vibrational energy to the fluids of the cochlea, by vibrating a small membrane at its base. A round window separates the middle ear and the canal.

The complexity of the structures of the inner ear are vital to the process of turning sound into neural activity, i.e. making it recognisable. The cochlea is the term used for the auditory section of the inner ear. The top is referred to as the apex, where as the bottom (closest to round window) is the base of the spiral.

There are three parallel canals situated along the length of the cochlea:

• Tympanic Canal
• Middle Canal
• Vestibular Canal

Situated between the middle canal and the tympanic canal is the basiliar membrane a flexible structure containing the necessary elements for converting sound into neural activity. The basiliar membrane is five times as wide at opposite ends of the cochlea. Its at its widest at the apex, narrowing towards the base. It is responsive towards vibrations initiated by sound transmissions to the fluid-filled cochlea deflected by the oval window, started by the bones of the middle ear.

High frequency sounds displace the narrow base of the inner ear, mid frequency sounds traditionally displace the middle of the basiliar membrane and the low frequency sounds tend to displace the apex.

The organ of Corti is the term for used collectively for all the elements central to the transduction of sounds, it has three main structures - the sensory (hair) cells , an elaborate framework of supporting cells and the terminations of the auditory nerve fibres. At the top of the organ of the corti is the tectorial membrane. The outer hair cells extend to indent the base of the tectorial membrane.

Fluid movements in the cochlea produce vibrations of the basiliar membrane. The inserted steroscilia bend, the direction of the bend determines whether the hair cells increase or decrease the rate the auditory nerve fibres fire at.