Biology of Hearing

Study Notes

Hearing

Hearing commences with the capture of sound waves by the ear
Sound waves - small areas of high and low pressure propagating outward from the source
Sound waves are converted into neural signals which are then processed by the brain.
Sound waves are captured by the outer ear and ultimately converted into an electrical signal by hair cells in the cochlea.

The Ear

Composed of outer ear, middle ear and inner ear.

Outer Ear

Pinna - supported by cartilage (captures sound and focuses it into the auditory canal)
Auditory canal - ends at the eardrum

Middle Ear

Eardrum = tympanic membrane
The middle ear contains three bones – Malleus, Incus and Stapes.
Sound vibrations cause the eardrum to vibrate
The vibrating eardrum causes the bones of the middle ear to vibrate
Vibrations are transmitted to fluid in the inner ear via vibration of the membrane at the oval window

Inner Ear

Cochlea - spiral, fluid filled (contains hair cells that serves as receptors for audition)
Vibrations in the cochlea fluid cause hair cells to move
Hair cells convert mechanical signal > electrical signal
Hair cells synapse on spiral ganglion cells

Tuning Curve

Spiral ganglion cells are tuned to specific frequencies
Firing rates dropping of rapidly for lower and higher frequency sounds

Tinnitus

Hearing noise in the absence of any sound stimulus
Subjective sensation (buzzing, humming, whistling)
Can be caused by a variety of factors, including exposure to loud noise, ear infections, and certain medications.
Temporary tinnitus after loud music
Caused by a disease processes affecting the cochlea or auditory nerve (can be cause by spontaneous activity)
Transient - short lived, spontaneous, related to exposure and environment
Chronic - highly disrupted, everyday, disease processes

Inner Ear to CNS

Axons of spiral ganglion cell exit the cochlea and converge with the axons of vestibular neurons to form the vestibulocochlear nerve
Auditory information - travels via vestibulocochlear nerve > brain stem

Vestibulocochlear Nerve

Carries signal from the cochlea > brain stem
Spiral ganglion cells synapse on neurons in the cochlear nuclei (located at the lower pons-upper medulla)

Thalamus

Top of brain stem
Many nuclei on thalamus
Medial geniculate nucleus - receives information from the inferior colliculi (the midbrain)
Sends auditory information to the primary auditory cortex.

Auditory Pathways

Cochlear nuclei (auditory information ascends bilaterally) > inferior colliculi (in the midbrain)
Neurons in the inferior colliculi > synapse onto medial geniculate nucleus (thalamus)
Medial geniculate nucleus > synapse onto primary auditory cortex

Auditory Cortex

First region to process sound
In the superior temporal lobe in the lateral sulcus
A1 (Heschl Gyri)
Frequency tuning properties of the cells define a tonotopic map

Sound Localization

Sounds can be localised based on slight asymmetrics in the arrival time of two stimuli (two cars arriving)
Interaural time - difference inn arrival time of a sound at each ear
Sound localisation along the vertical plane isn't as good
To determine elevation of sound sources, humans depend on bumps and ridges of the outer ear (produce reflections of the entering sound)
- Elevation of a sound sensory is very impaired if the pinnae are covered
Not all information crosses the midline, each of our ears can project information to both hemispheres of the brain
Information may be transferred unilaterally or bilaterally. If information is transferred unilaterally, it helps to determine damage to the sensory system.
This is why both ears can project information to both hemisphere of the brain.
- Damage to one of the sensory pathways could be identified in this manner.

Cochlear Implants

Severely reduced cochlear function
Internal
Electrode stimulates remaining spiral ganglion cells

Description

Explore the fascinating process of hearing, from the capture of sound waves by the outer ear to the conversion of those waves into neural signals. This quiz delves into the anatomy of the ear, including the outer, middle, and inner ear structures, and their functions in the auditory process.