16 Questions
What type of information do low frequencies provide to the brainstem and cortex?
Timing information
Where are afferent nerve fibers connected to in the ear?
Inner hair cells
What is the characteristic of nerve fibers located at the base of the basilar membrane?
High characteristic frequency
What does the post-stimulus time histogram measure?
Adaptation of firing in nerve fibers
What is the characteristic of nerve fibers located at the apex of the basilar membrane?
Low characteristic frequency
What is the phenomenon seen at low frequencies where the nerve fires on a particular phase of the stimulus?
Phase locking
What is the main job of hair cells in the auditory system?
Convert energy into electrical signals
Which component is responsible for the lateral movement that causes displacement of stereocilia in hair cells?
Actin
What causes the inside of a hair cell to become hyperpolarized?
Potassium gates opening
What is the main function of outer hair cells (OHCs) in the auditory system?
Amplify stereocilia vibration
What is the primary role of prestin in outer hair cells (OHCs)?
Control cell membrane movement
What role do inner hair cells (IHCs) play in the cochlear amplifier?
Cause synaptic vesicle release
What is the primary function of the synaptic ribbon in relation to the auditory nerve?
Interact with vesicles containing neurotransmitters
How are action potentials created in the 8th nerve fiber related to auditory function?
Link sensory input to electrical signals in the brain
What distinguishes high spontaneous rate auditory nerve fibers from low spontaneous rate fibers in terms of firing rate?
High spontaneous rate fibers create more action potentials per second
How are responses from the auditory nerve quantified and studied?
Recording firing rates of nerve fibers attached to hair cells
Study Notes
Hair Cell Transduction
- Hair cells convert energy into electrical signals
- Basilar membrane movement transfers energy through stereocilia bending
- Stereocilia are made up of actin monomers and myosin protein
Movement of Stereocilia
- Lateral movement causes displacement of stereocilia, pulling sodium channels open
- Positively charged ions flow into the endolymphatic space, causing depolarization
- Pivot points are the spiral limbus on the tectorial membrane and the spiral lamina on the basilar membrane
Ion Channels and Gradients
- Voltage gradient: mutual repulsion of like-charged particles
- Concentration gradient: particles distribute homogeneously in a medium
- Endolymphatic potential: +80 mV, measured by putting an electrode in the endolymph and grounding anywhere on the body
- Inner hair cell potential: -45 mV, measured by putting an electrode in the inner hair cell and grounding anywhere on the body
- Total voltage potential: 125 mV (+80 + -45 mV)
Transduction in the Cochlea
- Basilar membrane movement:
- Down (condensation): stereocilia deflect towards the modiolus, potassium gates are closed, and the inside of the hair cell becomes hyperpolarized
- Up (rarefaction): stereocilia deflect up towards the stria, potassium ions flow through channels into the hair cells, and depolarization occurs
- Calcium channels open, and calcium enters the cell, releasing neurotransmitters
Potassium Recycling
- Recycled through hair cells, fibrocytes in the spiral ligament, cells of stria, and back to the endolymph
Key Concepts
- Hair cell transduction is non-linear and has asymmetry in compressive non-linearity
- Membrane voltage is always negative
- Hair cell receptor potentials are measured in both OHCs and IHCs
OHCs and Mechanical Transduction
- OHCs contribute to mechanical transduction through motility
- OHCs shorten when depolarized and lengthen when hyperpolarized
- Prestin is embedded in the cell membrane of OHCs
OHC vs IHC
- OHCs provide mechanical feedback to BM motion
- IHCs cause synaptic vesicle release
- Cochlear amplifier: added mechanical feedback produced by OHCs in concert with the tectorial membrane
Auditory Nerve
- Synaptic ribbon anchors near the presynaptic membrane and interacts with vesicles containing neurotransmitters
- Neurotransmitter is glutamate
- Action potentials are created in the 8th nerve fiber
- Firing of nerve fibers occurs once the threshold of excitation has been met
- High spontaneous rates (>50 spikes per second) attach on the pillar side of the hair cell
- Low spontaneous rate (<10 spikes per second) attach on the modiolar side
Responses from the Auditory Nerve
- Histograms, post-stimulus time period, input-output functions, and tuning curves are used to quantify and study responses
- Nerve fibers have a characteristic frequency
- Afferent nerve fibers are connected to inner hair cells and are located in the basilar membrane
Test your knowledge on hair cell transduction in auditory anatomy and physiology. Learn about the process of converting energy and messages into another form, how hair cells function as sensory cells, and how basilar membrane movement transfers energy.
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