SENSORY SYSTEMS—AUDITORY

SENSORY SYSTEMS—AUDITORY

Auditory System

Auditory System Overview and Signal Transduction

1. Sensory Organ—the ear

   1. Outer ear

      1. Pinna serves to collect sound and vertical sound localization

      2. Sound is funneled down the auditory canal to the eardrum/tympanic membrane

   2. Middle ear

      1. Amplifies sound as it moves from air (outer and middle ear) to fluid (inner ear)

      2. Vibration of tympanic membrane is passed through the ossicles to the oval window—the reduction in size from the tympanic membrane to the oval window provides additional amplification

   3. Inner ear

      1. Begins at oval window, which opens into fluid filled cochlea

         1. Three chambers—scala media, scala tympani, scala vestibuli

         2. Fluids—perilymph and endolymph

            1. Perilymph is in the tympani and vestibuli compartments, and is basically regular extracellular fluid

            2. Endolymph is in the scala media compartment and has low Na+ and high K+ (the opposite of regular extracellular fluid)

      2. Inner ear is the site of sensory transduction

      3. Basilar membrane

         1. Separates the media and tympani

         2. Variably flexible along the length of the membrane

            1. Apex—floppy and wide, responds to low frequency

            2. Base—rigid and narrow, responds to high frequency

         3. Membrane will vibrate with sound frequency (one type of stimulus coding)

         4. Supports the Organ of Corti, where hair cells are found

            1. Inner hair cells—sensory receptors for sound

            2. Outer hair cells—3 rows of cells, modulate auditory response by altering the physical characteristics of the basilar membrane (amplifiers)

2. Sensory Receptors—inner hair cells

3. Transduction Mechanism

   1. Inner hair cells have stereocilia

      1. Bathed in endolymph

      2. Have mechanically gated (tip-link) K+ channels at their tips

      3. These K+ channels are partially open at rest

   2. Sound waves cause deflection of stereocilia, opening/closing the tip-links/K+ channels

      1. Deflection can either cause hyperpolarization or depolarization

   3. High extracellular K+ creates the potential across the hair cell membrane

   4. Depolarization of the hair cell activates voltage-gated Ca2+ channels, depolarizing membrane and releasing NT

4. Role of outer hair cell receptors

   1. Outer hair cells act to amplify sound in the cochlea

   2. Transduction

      1. Stereocilia on the outer hair cells are attached to the tectorial membrane, while the cell body is attached to the basilar membrane

2. K+ influx causes contraction of a voltage-sensitive protein in the outer hair cell that leads to a conformational change in the hair cell—this change produces a downward motion of 

3. the hair cell leading to a larger vibration of the basilar membrane
   

Auditory System

Stimulus Encoding

1. Encoding Frequency

   1. Frequency is coded by the physical position on the basilar membrane at which hair cells respond—remember the rigidity of the basilar membrane varies along its length

      1. Apex (wide and floppy) responds to low frequency

      2. Base (rigid) responds to high frequency

2. To discriminate between frequencies, labelled lines and population coding must be used (remember that hair cells are broadly tuned and that louder sounds will cause a larger portion of the basilar membrane to vibrate)

   1. Population coding—with low and medium frequencies (up to 5000Hz), The population of cells that are active convey information about the stimulus frequency

   2. Labelled lines -work for all frequencies but in particular for very high frequencies

2. Encoding Intensity

   1. Intensity is encoded by the number of auditory fibers that are activated by a stimulus

      1. Louder sounds will activate more auditory fibers

         1. A larger section of the basilar membrane will vibrate

         2. Each individual hair cell with activate more spiral ganglion cells—louder sounds lead to larger depolarization in the hair cell and each spiral ganglion cell contacted by a specific hair cell has a different AP threshold

   2. Intensity is also encoded by the number of AP’s in each nerve fiber … which is known as rate coding

3. Encoding Location (on a horizontal plane)

   1. Interaural time delay is used to encode location for low frequencies (<2000 Hz)

      1. The brain uses the time delay between a sound hitting your left and right ear, to determine where the sound came from

      2. APs generated in the left and right auditory nerve are transmitted to the left and right cochlear nucleus respectively

      3. Neurons in the left and right cochlear nucleus generate APs and synapse onto neurons in the superior olive (SO)

      4. SO neurons utilize “AND” gates and only generate APs when they receive simultaneous

input from both the left and right cochlear nucleus

1. The time delay in sound reaching each ear will result in a difference in which SO neuron fires an AP

2. Neurons in the LEFT superior olive will get active when the sound comes from the right, neurons in the RIGHT superior olive will get active when the sound comes from the left, neurons in the CENTER of the superior olive will get active when the sound comes from the center.

2. Interaural intensity differences are used for high frequencies >2000Hz

3. The brain calculates the difference of sound intensity in each ear. For example, if a sound comes from the left, it will be more intense in the left ear. 

4. Ascending Brain Pathways

   Auditory nerve sends information to the dorsal/ventral cochlear nuclei

   1. These nuclei have a tonotopic map—corresponds to the frequency map on the basilar membrane

2. Cochlear neurons project to the medial geniculate nucleus (MGN) of the thalamus

   1. The tonotopic map is maintained

3. Primary auditory cortex

   1. The ionotopic map is also conserved in primary auditory cortex