The Nervous System, The Sense of Hearing

The Tympanic Membrane and the Ossicular System

1. Sound from the tympanic Membrane to the Cochlea
1. Tympanic Membrane, Malleus, Incus, Stapes, Oval Window - the ossicular chain.
2. Malleus is also attached to the tensor tympani muscle.
3. 
4. 
1. Impedance matching between sound waves in air and sound waves in the cochlear fluid is mediated by the ossicular chain.
1. Sound is not amplified by increasing movement at the stapes, but increasing force of movement  at the stapes. Increasing pressure, increases the vibration of the fluid in the membranous labyrinth - via the oval window. 
2. Impedance matching between air waves and fluid waves. 
2. Contraction of the stapedius and the tensor tympani muscles attenuates sound conduction
1. This is the same mechanism used to diminish the sensitivity to ones own speech
2. Increases the rigidity of the stapes, and therefore the ossicular chain.
2. Sound Transmission Through Bone Conduction
1. The cochlea is embedded in bone, vibration of the skull can directly stimulate the cochlea. 
2. Tuning fork is used to test conductive hearing loss 

The Coclea

1. Functional Anatomy
1. 
2. 
3. There are three tubes coiled side by side. The Scala Vestibuli, Scala Media (Cochlear Duct) and the Scala Typani. 
4. The vestibular membrane (Reissner's membrane) separates the  Vestibular Duct  and the Scala media. 
5.  Helicotrema - opposite to the oval and round windows, the Scala Vastibuli is continuous with the Scala Tympani. 
6. The oval window is most stiff and is sensitive to high frequency sounds. Therefore the HELICOTREMA "least stiff area" and is sensitive to low frequency vibrations (sounds). 
2. Transmission of Sound Waves
1. Sound waves travel via the ossicular chain, oval window, initiating a wave that travels on the basilar membrane towards the helicotema.
2. 
1. Vibration patterns  are induced by different sound frequencies
1. Each sound frequency resonates with a different part of the basilar membrane  - the basilar membrane has a resonant frequency equal to the sound wave.
2. The velocity is obviously highest at the oval window and least at the helicotrema.
2. Vibration patterns are induced by different sound amplitudes.  
1. vibration for higher frequency at oval window - 8000Hz
2. vibration for a lower frequency at the HELICOTREMA - 200 Hz
3. 
3. Organ of Corti
1. Nerve impulses in response to Vibration of the Basilar Membrane
1. There are 2 types of hair cells:
1. The inner hair cells - 95% of CN VIII - 
2. The outer hair cells
1. Vibration of the basilar membrane excites the hair cells
1. Stereocilia: The apical surface of the hair cells and 
2. one kinocilium that project upwards into the the overlying tectorial membrane.
3. When the basilar membrane vibrates, the hair cell cilia being embedded in the tectorial membrane are bent to one side and then the other - and the movement that mechanically opens ion channels that depolarises the cell.
2. Hair cell receptor potentials activate nerve fibers
1. The modiolus is a conical shaped central axis in the cochlea. The Modiolus consists of spongy bone and the cochlea turns approximately 2.75 times around the modiolus The spiral ganglion is situated inside the bony modiolus.
2. The ciliated hair cells progressively increase in length from the basal membrane closest to the modiolus to the kinocilium. When the sterocilia are bent towards the kinocilium - cilia membrane potassium channels open and hyperpolarise the cell.
3. The potassium rich and sodium poor endolymph - and differs from the perilymph of the scala vestibuli and scala tympani that is high in sodium and low in potassium - 
4. The electrical potential across the endolymph is +80mV - and increases to +150mV at  the apeical surface - greatly increasing its sensitivity.  
3. 
2. Sound frequency and the "place" principle
3. Loudness of Sound
1. Louder sound increase the amplitude of vibration and the hair cells are activated more rapidly
2. Increased volume increases, more hair cells are activated, and the spacial summation enhances the signal.
3. Outer hair cells when activated by large amplitude sound - signal the nervous system that delimits the loud sound.
4. Sound intensity is:
1. calibrated at a logarithm of the actual intensity
2. measured in 0.1 bel or 1 decibel
3. Threshold for hearing is different at different intensities
4. Range 20 to 20000 Hz - but at an intensity of 60 decibels between 500 to 5000 Hz.
4. 

Central Auditory Mechanisms

1. Anatomy of Central Auditory Pathways
1. 
2. Sensory fibers from the spiral ganglion enter the brain stem and terminate in the dorsal and ventral cochlear nucleus - (trapizoid body and fibers cross over)
3. Superior Olivary Nucleus - fibers enter the lateral lemniscus - terminating in the
4. Inferior Colliculus. Cells in the Inferior Colliculus project via the Brachium of the Inferior Colliculus to the 
5. Medial Geniculate nucleus of the Thelamus, and from here signals are transmitted via the auditory radiations to the lateral fissure 
6. Primary auditory Cortex - the transverse gyrus of Heschel - area 41 and 42 
1. 
2. 



Heschl's gyrus is the blue area.
1.   
2. Role of the Primary Auditory Cortex in Hearing
1. The primary auditory cortex, corresponds to Brodmann's areas 41 and 42 - localising sounds. Surrounding this area is area 22, the secondary auditory cortex - interprets and gives meaning to sound. Receptive aphasia. 
2. 
3. Mechanism for Sound Localisation
1. Olivary nucleus - 
1. Medially - differences in TIME of the 2 ears
2. Laterally - differences in Intensity of the 2 ears
4. Centrifugal Projections In the Auditory System
1. Descending or retrograde fibers - projects back to the cochlea - selectively attenuate sound and select sounds. 
5. Common Hearing Abnormalities
1. Tested with audiometer
2. Nerve deafness: both air and bone conduction
3. Only air conduction - damage to the ossicular chain - Chronic middle ear infections.
6. 
7. Annimation: http://www.sumanasinc.com/webcontent/anisamples/neurobiology/soundtransduction.html http://www.sumanasinc.com/webcontent/animations/content/soundtransduction.html

http://www.colorado.edu/intphys/Class/IPHY3430-200/008sensory.htm