Lecture 23: The inner ear and auditory pathways Flashcards
Q: What is the Organ of Corti and its role in hearing?
A: The Organ of Corti is the sensory epithelium in the cochlea, where hair cells convert mechanical sound vibrations into electrical signals by bending in response to fluid movement, which stimulates the auditory nerve.
Q: What is the function of the tectorial membrane in the cochlea?
A: The tectorial membrane is a structure in the cochlea that moves in response to sound waves, interacting with hair cells. This movement bends the hair cells, initiating the transduction of sound into nerve impulses.
Q: What is afferent innervation of hair cells?
A: Afferent innervation involves Type I and Type II spiral ganglion neurons (SGNs).
Type I SGNs innervate inner hair cells (IHCs) and are responsible for conveying most sound information to the brain (90-95%).
Type II SGNs innervate outer hair cells (OHCs) and have a less understood role, possibly related to contralateral suppression.
Q: What is efferent innervation of hair cells?
A: Efferent innervation involves neurons from the superior olivary complex.
Lateral olivocochlear neurons target inner hair cells (IHCs) and modulate the activity of Type I spiral ganglion neurons (SGNs) to protect the ear from loud sounds and improve frequency discrimination.
Medial olivocochlear neurons project to OHCs to inhibit their activity, likely protecting the ear and refining sound sensitivity.
Q: What are the differences between inner and outer hair cells in the cochlea?
A:
Inner hair cells (IHCs): Primarily responsible for sending auditory information to the brain through Type I SGNs.
Outer hair cells (OHCs): Act as amplifiers, fine-tuning sound by changing their length to enhance vibrations in the basilar membrane, aiding hearing sensitivity.
Q: How are hair cells in the cochlea innervated by auditory nerve fibers?
A: Inner hair cells are primarily innervated by Type I afferent nerve fibers, which send sound information to the brain. Outer hair cells are innervated by Type II fibers, with their role believed to involve sound modulation and cochlear protection.
Q: What role does the basilar membrane play in hearing?
A: The basilar membrane vibrates in response to sound waves, which leads to the movement of hair cells in the cochlea, allowing them to convert these mechanical vibrations into electrical signals.
Q: How do hair cells transduce mechanical vibrations into neural signals?
A: Sound waves create fluid movement in the cochlea, bending the stereocilia of hair cells. This bending opens ion channels, leading to neurotransmitter release and activation of auditory nerve fibers, sending signals to the brain.
Q: What creates the endocochlear potential in the cochlea?
A: The stria vascularis pumps potassium (K⁺) into the endolymph (150 mM K⁺, +80 mV), while the hair cells have a resting potential of -60 mV. The perilymph, on the other side, has 5 mM K⁺ and 0 mV. This difference in potential drives potassium into hair cells during depolarization.
Q: How do hair cells convert mechanical vibrations into electrical signals?
A: Hair cells use tip links and mechanotransduction (MET) channels. When sound waves move the basilar membrane, tip links stretch and open MET channels, allowing K⁺ influx from the endolymph, leading to depolarization and neurotransmitter release.
Q: How does the motion of the basilar membrane influence hair cell activation?
A: The basilar membrane moves in response to sound waves. When it moves upward, hair cell stereocilia bend towards the tallest row, opening MET channels and causing depolarization. When it moves downward, stereocilia bend towards the shortest row, closing the channels and causing hyperpolarisation.
Q: What happens during hair cell depolarization and hyperpolarization?
A: During upward movement, tip links stretch, opening more MET channels and causing depolarization (excitation). In downward movement, tip links relax, closing channels, leading to hyperpolarization (inhibition).
Q: What happens after depolarization of hair cells?
A: Depolarization opens voltage-gated calcium (Ca²⁺) channels. Ca²⁺ enters the hair cells, triggering the release of neurotransmitters into the synapse, which stimulates the auditory nerve.
Q: How do outer hair cells act as cochlear amplifiers?
A: Outer hair cells contain the protein prestin, which shortens the cell body in response to depolarization. This boosts the motion of the basilar membrane, amplifying sound by up to 40 dB.
