2: Sensations and Perception [EAR]
Why doesn't the fluid move back to the oval window when it travels back from the tip of the cochlea?
because in the very middle of the cochlea there's a membrane, which separates the two routes. Organ of Corti: => basilar membrane => tectorial membrane Vibrations enter in cochlea via oval window (elliptical window) Vibrations leave cochlea via round window (circular window) But where does transduction occur?
Where and how does auditory transduction occur?
As the vibrations in the fluid of the cochlea travel through the membranous labyrinth, the hair cells inside pick up the vibratio ns and transmits electrical signals via the auditory nerve to the brain.
Tip link
Attached to the gate of a potassium channel. When the the kinocili gets pushed back and forth due to the movement of the fluid in the cochlea, it stretches the tip link, which opens the gate to the potassium channel. Potassium flows into the cell then calcium channels get activated. The influx of potassium and calcium into the cell causes the cell to fire an action potential. This stimulates the spiral ganglion cell, which then activates another cell that's part of the auditory nerve, which then goes to the brain.
Cross Section of Organ of Corti
Cochlea is divided into three parts called scalae. The middle scalae is where transduction of sound waves occur. It is composed of thousands of hair cells, which are bathed in enolymph. The top of the organ of Corti there is a relatively immobile membrane called the tectorial membrane. The other two scalae that surround the hearing apparatus (middle scalae) are filled with perilymph and are continuous with the oval and round windows of the cochlea. Thus sound entering the cochlea through the oval window causes vibrations in the perilymph, which are transmitted to the basilar membrane. Because fluids are incompressible, the round window, a membrane covered hole in the cochlea, permits the perilymph to actually move within the cochlea.
Auditory pathway and how auditory signals are mapped to the brain
From the basilar membrane where transduction of the sound waves occur, the electrical impulse travels through the vestibulocochlear nerve, to the brainstem, ascends to the medial geniculate nucleus (MGN) of the thalamus, and finally reaches the PRIMARY AUDITORY CORTEX. Some information is also sent to the superior olive, which localizes the sound, and to the inferior colliculus, which is involved in the startle reflex and helps keep the eyes fixed on a point while the head is turned (vestibulo-ocular reflex)
How we differentiate between sounds The brain uses the cochlea's basilar tuning
Hair cells at the base of the basilar membrane are activated by high frequency sounds. Hair cells at the apex of the cochlea (end of basilar membrane) are activated by low frequency sounds. **low frequency sounds (high wavelengths) travel further into the cochlea
Inner Ear
Inner ear sits within a bony labyrinth. Semicircular canals (they are all orthogonal to each other): => Anterior => Lateral => Posterior Vestibule (Otolithic organs): => Utricle => Saccule Cochlea => tectorial membrane => 3 scalae (middle scalae containing organ of Corti, which rests on the basilar membrane) **These structures are continuous with each other and form the membranous labyrinth, which is suspended within the bony labyrinth by a thin layer of fluid called perilymph. Perilymph simultaneously transmits vibrations from the outside world and cushions the inner ear structures. Endolymph is the potassium-rich fluid that baths the hair cells of the inner ear, all of which are found within the membranous labyrinth. Parilymph is found in the space between the membranous labyrinth and the bony labyrinth. Both the membranous labyrinth and bony labyrinth contribute to the cochlea and the vestibule. **The membranous labyrinth is filled with potassium-rich fluid called endolymph. Vestibulocochlear nerve => Cochlear nerve => Vestibular nerves
How louder sounds affect the tympanic membrane?
Louder sounds have greater intensity, which corresponds to an increased amplitude of tympanic membrane vibration.
Hair Bundle
Not really hair but a bunch of filaments called kinocilium filaments
Name the three smallest bones in our body.
Ossicles are the three smallest bones in our body. They are: Malleus (hammer) Incus (anvil) Stapes (stirrup) *The ossicles help transmit and amplify the vibrations from the tympanic membrane to the inner ear.
Middle Ear
Ossicles: => Malleus (hammer) => Incus (anvil) => Stapes (stirrup) - the baseplate of stapes rests in the oval window of the cochlea, which is the entrance to the inner ear ***The middle ear is connected to the nasal cavity via the Estachian tube (aka auditory tube). This helps equalize pressure between the middle ear and the environment
Information relating to balance and spatial orientation
Ototlithic organs =< Detect linear acceleration and head position within these structures are calcium carbon crystals called otolith crystals that are attached to hair cells and are bathed in a viscous fluid. When we accelerate in a direction, this causes the crystals to move because they are heavier than the surrounding environment and this will pull on the hair cells, which transduces into an electrical impulse, travels through the vesti
Outer Ear
Pinna External Auditory Canal Tympanic membrane (eardrum) Ear lobe
List the structures in the auditory pathway, from where sound enters the pinna to the auditory projection areas in the brain.
Pinna -> External auditory canal -> tympanic membrane -> malleus -> incus -> stapes -> oval window -> perilymph in cochlea -> basilar membrane -> hair cells -> vestibulocochlear nerve -> brain stem -> medial geniculate nucleus (MGN) of thalamus -> auditory cortex (temporal lobe)
How we sense rotational acceleration
The semicircular canals contains endolymph and when we rotate along a certain plane, it causes the endolymph to shift within that particular semicircular canal, interacting with hair cells. Our brains can tell how much we rotate and how fast we rotate (the strength of the rotation) because it is sensitive to the amount endolymph shifts and how fast the endolymph shifts.
Sound waves first go to the:
The sound waves get funneled into the pinna (aka auricle). Pinna is the outer visible part of the ear.
What causes the oval window to vibrate?
The stapes causes the oval window (aka elliptical window) to vibrate. **Do not confuse oval window with round window. The vibrations are transmitted to the liquid inside the cochlea.
Once in the cochlea, where do the vibrations go then?
The vibrations travel back through the fluid through the pathway it came from but doesn't travel through the oval window, instead now it dissipates through the circular window (aka round window).
Auditory Processing
We can hear between frequencies of 20 Hz and 20 kHz. How can the brain differentiate between these sounds? The brain uses basilar tuning.
Auditory (Vestibular) Nerve
carries bundle of axons from each hair cell inside the cochlea. It carries the signal to the primary auditory cortex where the bundle separates again into separate axons into their respective regions of the primary auditory cortex. ***In the peripheral nervous system a bundle of axons is called a nerve. In the central nervous system a bundle of axons is called a tract.
The main function of the auricle is to:
channel sound waves into the auditory canal (aka external auditory meatus)
Tonotypical mapping
is divided by sections that receive particular frequencies similar to the way the basilar membrane is divided. This mapping of sounds of different frequencies are known as tonotypical mapping
Sound waves travel the auditory canal and to hit the:
tympanic membrane (eardrum). Tympanic membrane separates the outer ear from the middle ear. The eardrum vibrates back and forth in phase with the incoming sound waves and causes the three little bones (ossicles) in the middle ear to vibrate (starting with the malleus [see pic of next card]). **The frequency of the sound waves determine the rate at which the tympanic membrane vibrates.
