Hearing Part 2

Tuning Curve

• Sensitivity of a single sensory neuron to a particular frequency of sound

• Two mechanisms for fine tuning of sensory neurons,– Mechanical=location along the basilar

membrane– Electrical=natural oscillations in membrane

voltage

Where are the hairs cells andNeurons located along theBasilar membrane in the Organ of Corti? That dictatesTheir Tuning Curve.

Frequency of cilia movementMatches the innate oscillation In membrane potential

Frequency Tuning in Hair Cells

• Hair cells have oscillating waves of depolarizations that are unique to each hair cell

• If the frequency of sound wave matches the innate oscilliation of membrane depolarization, then the response amplitude is large

• A high frequency tuned hair cell has strong depolarization in response to high frequency sound waves but small depolarization to low frequency sound waves.

Oscillation In Membrane Potential

• Due to voltage sensitive calcium channel and calcium sensitive potassium channel

• Frequency of oscillation depends on time delay between opening of calcium channels and opening of potassium channels

• Density of ion channels and different sensitivity of potassium channel to calcium levels

Innate Characteristics

• High Frequency Hair Cells

• Short delay between opening of calcium and potassium channels

• Low Frequency Hair Cells

• Long Delay between opening of calcium and potassium channels

Efferent Synapses

• Brainstem neurons innervate outer hair cells and reduces sensitivity of cochlea to sound and inhibits sensory output to brain

• Ach acts on muscarinic receptors to open potassium channels in hair cell

• Leads to hyperpolarization, inhibits oscillation of membrane potential & inhibits cilia movement

Attenuation-Efferent Synapse

• Muscles to middle ear bone and tympanic membrane can contract and relax modulating movement of basilar membrane

• Muscles on the ossicles: – tensor tympani attached to maleus and bones of the ear canal– stapedius attached to stapes

• Muscle contraction makes the ossicles more rigid and do not move the oval window as much (50-100 msec delay in response to loud sound)

• Works best with low frequency sound

CN8 Connection to Brain

• Brainstem-medulla, pons, midbrain

• Thalamus=medial geniculate nucleus

• Cortex=primary and secondary auditory cortex

Parallel Connections

• Central process of Spiral Ganglion Neurons innervate cochlear nucleus and then the cochlear neuron branches to innervate 3 additional cochlear nuclei in medulla

Sound Information Transduced

• Frequency

• Intensity

• Point of Origin

Each are transmitted separately to brain

Tonotopy

• Systematic representation of sound frequency along the length of basilar membrane.

• This is maintained at all levels of ascending information until after primary auditory cortex in temporal lobe

• Individual neurons at each brain level respond to a characteristic frequency innervate adjacent neurons so there is a geometric representation of the basilar membrane at each level of the auditory pathway

Tonotopic Map

• In the cochlear nucleus in the medulla, there is a map of the basilar membrane, ie neurons are organized from low to high frequency responses, so neighboring neurons respond to similar frequencies.

Monaural and Binaural

• Information from 1 ear and from 2 ears.

• A neuron can receive monaural or binaural inputs

• Interaural=between ears=measuring the time difference it takes for the sound waves to reach each ear. Can detect 10-700usec = to 1 degree

Cochlear Nucleus

• First synapse in brain is cochlear nucleus in medulla of brainstem

• Tonotopic map of cochlea is maintained so that adjacent areas of cochlea are mapped to adjacent areas of cochlear nucleus-allow frequency to be decoded

• Some cochlear neurons fire at the beginning, end our during sound. Keep track of PHASE

Superior Olivary Nucleus

• Give rise to Efferent neurons to cochlea • Used to detect sound location; have binaural

inputs from each cochlear nucleus• Detects time of the arrival of sound to detect

direction of sound. Sound to each ear reaches there at different times if sound is not directly ahead or behind you.

• Detects difference in intensity between sound reaching each ear

Inferior Colliculus

• Major connection for ascending auditory information in brainstem is the inferior colliculus in the midbrain

• Receives bilateral direct input from cochlear nucleus and indirect input from superior olivary nucleus=parallel processing

Function of IC

• Contains (barn owl) a space diagram

• Neurons will respond only to sound coming from specific areas of space

• Have preferred elevation and horizontal location

• Also process temporal patterns of sound important for behavior

Medial Geniculate Nucleus

• In thalamus, MGN is the gateway to auditory cortex

• Ipsilateral connection but since input was bilateral previously each MGN receives binaural input

Function of MGN

• Responds to selective combination of frequencies

• Responds to specific time intervals of sound

Auditory Cortex

• Located in superior part of temporal lobe

• Input is mapped to cortex in tonotopic fashion

• Cortex has 2 functional columns, 1 for frequency in which neighboring cells in cortex have similar frequencies varying from low to high across the cortex

Primary cortex

• Tonotopic and receives precisely mapped information form MGN

• Process combination of frequencies

• Process modulations of amplitude or frequency

Binaural Columns

• Summation columns, neurons respond strongly if stimulus arrives simultaneously to both ears

• Suppression column, responds best to sound in 1 ear only

Secondary Auditory cortex

• Also referred to as belt areas

• Involved in understanding speech, ie recognizing temporal organization of sound

• Wernicke’s area in secondary cortex when damaged patients cannot understand speech because the sounds are all out of order

Nice diagram-thought it Might help

Sound Intensity

Encoded by rate of firing and # of active neurons

The greater the amplitude of sound wave the greater distance along basilar membrane that moves—activate more hair cells and more SGNs

Phase Locking

• Neurons fire in phase with some point of the sound wave

• either at the peak, valley or in between but it is constant for that neuron

• This firing rate will then tell you the sound frequency

Volley Principal

• As the frequency of sound gets higher 1K-4K Hz then groups of neurons fire in phase

• So one neuron will fire every 4th cycle in phase

• Above 4 K only tonotopy is used to encode frequency information

Sound Localization

• Horizontal Plane

• Vertical Plane: Based on reflection of sound from pinna that delay arrival of sound as compared to sound waves that do not reflect off pinna

Interaural Time Delay

• Difference in time it takes for sound to reach each ear if sound is not coming from directly in front or behind you

• Head size 20 cm, sound delay 0.6 msec

• Use for detecting direction of a sudden sound

• Used for frequencies of 20-2000Hz

Phase Detection

• Used to localize continuous sound

• Sound waves (200Hz) travel 172 cm /second

• So the cycle of the sound wave will be 0.6 msec delayed from ear to ear and will be at a different phase.

Interaural Intensity Differences

• Your head blocks sound so intensity is less in the ear away from the sound

• Your brain detects and computes this differences in intensity to localize sound direction.

• Used at frequencies of 2000-20,000 Hz

Localization

• At frequencies below 3kz, interaural time differences alone are used to localize sound since you have exact representation of frequency with sensory neuron firing.

• At higher frequencies changes in intensity are used as well. Occurs in superior Olive