Lecture #16 Flashcards
what is the unit for frequency?
Hertz
what is the unit of amplitude?
decible
what is the speed of sound propagation?
340 m/s
what receives the mechanical inputs of sound and transforms them into physical inputs via electrical responses?
hair cells
what are the two parts of the external ear?
concha and pinna
after sound are captured by the external ear, where are they transferred to?
the meatus: part of the ear that allows the signal to trance though the short cavity up to the eardrum
what is the the anatomical name for the eardrum?
tympanum
what are the three bones of the inner ear?
malleus (hammer), incus (anvil), stapes (stirrup)
how is the eardrum + oval window serve as an amplifier?
you have a large membrane connected to a very small vibrating membrane
what are the three bones of the inner ear connected with?
small muscles
what is the most important part of the hearing structure?
cochlea (comes from the latin word meaning snail)
describe the cochlea:
liquid-filled structure → first step in auditory perception in which you have something going from an air medium to a fluid medium
name the three chambers of the cochlea:
scala vestibuli, scala tympani, organ of Corti in the scala media
where is the Scala vestibuli located?
dorsal chamber of the cochlea
where is the Scala tympani located?
ventral portion of the cochlea
describe perilymph:
composed of a water solution that has a very specific concentration of sodium and potassium
describe the endolymph:
fluid in the scala media → concentration of potassium is very high, and the potassium contained is the driving force for the transformation of sound into an electrical signal
what are the two external chambers (Scala vestibuli and the Scala tympani) connected by?
helicotrema at the tip of the cochlea
what membrane separates the scala vestibuli from the Scala media?
Reissner’s membrane, also called the vestibular membrane
what is the most important membrane and what does it separate?
basilar membrane - separates the scala tympani and the scala media
what is the equation used to describe loudness?
L= 20log10 (P/Pref)
p= pressure of an incoming signal
pref= pressure of a reference
when the stirrup acts on the oval window, the pressure propagates the liquid in which chamber?
scala vestibuli
describe where the signal travels from the scala vestibuli:
stimulation goes up to the terminal part of the cochlea
here there is the helicotrema (a region of the cochlea in which the scala vestibuli and the scala tympani are connected) and therefore the pressure can start to run into the second chamber
finally at the end of the scala tympani there is another small opening (round window) which is separated from the middle ear with a small membrane and this compression can increase the pressure of this small membrane causing an outwork bowing of the round window
how is the cochlea able to sense different frequencies of sound?
the basilar membrane and the Reissner’s membrane are not uniform in thickness along the cochlea → they are thick in the region that is in close contact with the oval window, and they are small in the lateral dimension, but start to increase in lateral dimension towards the helicotrema and reduce in thickness
the membrane can vibrate in different ways along the length of the cochlea allowing different frequencies o be distinguished
where do sounds with very low frequencies peak?
near the heiocotrema
what is the peak amplitude in front of the oval window?
1600 Hz
on which end do vibrations of high frequencies peak?
in a region not far from the oval window
what organ in the scala media is involved in transforming a physical signal into an electrical signal?
corti organ → composed of inner and outer hair cells
at birth how many hair cells do we have in our inner ear?
3,500
what is connected with the individual hair cells because it enters in contact with the hair bundle?
tectorial membrane
is it possible to regenerate the hair cells in the inner ear?
no
what are the outer hairs innervated by?
we do not know exactly how these cells are involved - in the corti organs there are specific supporting cells (pillar cells and Deiters cells) that can support from a trophic and physical pov the individual sensor cells
what are the inner hair cells innervated by?
pseudo-polar neurons which send an exon to the sensor cells which send an axon inside the dorsal root of the spinal cord allowing a connection between the sensor and the spinal cord
we have a very small but important ganglia behind our eat and these neurons can send an axon to the sensor cells and another one to the spinal cord
how are the basilar membrane and the tectorial membrane connected?
by two independent pivots - when the basal membrane starts to vibrate it stars to move the hard cells up and down, displacing a little bit the corti organs
at the same time we also have the movement of the tectorial membrane, but because the pivot to which the tectorial is connected through the system is different, the vibration is also different
the two pivots allow for the vibration of these two different systems
which nerve do the corti organs communicate with?
nerve number VIII → auditory nerve
when the motion deflects the hair bundles, what occurs?
specific ion channels open and transforms the physical vibration into an electrical signal
what happens to the cilia when there is any upward displacement of the basilar membrane due to the propagation of a wave?
causes the bending of the cilia of the inner and outer hair cells
when we have the downward inflection of the basinal membrane the cilia will go in the opposite direction
what is the position of the two membranes at resting potential?
the cilia are in contact with the tectorial membrane
what is the hair bundle?
cilia that are arising from the dorsal tip of the individual inner hair cell and this tip of cilia is fundamental for the transformation of the physical signal into an electrical signal
what is the specific living cilium in which we have 20-30 other cilia connected to this forming a sort of needle?
kinocilium
what is the difference between true and false cilium?
only the kinocilium are true cilium
all other cilia are false cilia because they re characterized by the presence of actin and cytoplasmic solution
what happens when the cilium are bent to the left?
the hyperpolarization of the membrane
what happens when the cilium are bent to the right?
ion channels are opened
what ion channels are involved?
mechanoreceptor ion channels that work in response to a physical alteration of the cell
they allow potassium to flux inside of the cell → endolymph is displaced by high concentrations of potassium and in this case potassium induces the depolarization
how is the tip link linked with the ion channels?
the tip link is a sort of coil and when we move the cilia the tip link starts to open the external gate of ion channels allowing potassium to enter inside and therefor shifting the membrane potential
at the resting state, what is the status of the mechanoreceptors?
virtually all of the mechanoreceptors are closed(10% are open) and potassium is on the outside
what is the most important aspect of the hair cells?
the ability for adaptation → if we have a very strong bending of the cilia and a huge amount of potassium enters the cell membrane, a very strong depolarization is created
if we apply these frequencies uniformly after a while the sensory of the system starts to go down
how does the system adjust to constant signals?
depends on calcium → if we have prolonged activation of our hair cells, we don’t only have a huge amount of potassium drifting the membrane potential, but also an accumulation of calcium
the calcium enters in contact with the myosin protein and these myosin proteins sitar to move the tip link from the initial position to another position
the new position is less prone to induce the opening of channels so therefore the system starts to response less and less
this allows us to protect out sensory system
what is another form of adaptation?
stereocilia → the system can also emit certain sounds the are due to the vibrations of the membrane and the vibrations of the eardrum
what is a pathology of the ear when you perceive a constant sound in your ear?
tinnitus
what neurotransmitter for this system?
glutamate: usually they are excitatory sensors and the very same hair cells are also receiving innervation from the CNS
acetylcholine: axons are organized in ganglia that is behind the cochlea but also an axon inside the CNS innervating a specific area which is the first auditory part of the pathway (vast majority of cases?)
what do 90% of these ganglionic neurons innervate?
inner hair cells
describe the distribution of ganglion in the inner hair cells:
a single hair cell can receiver innervation from several ganglion cells, while the outer hair cells are receiving innervation from a very small percentage of axons, and sometimes these axon can interact with more than one
often times what does full deafness require?
an artificial cochlea
what is the delay of time from one ear to the other?
10 µs
what is a tonotopic innervation?
ganglia that are innervating inner cells in a specific position → they are not mixed but they interact with specific neurons and the distribution of these neurons is linearly correlated with the distribution of the fibers and inner hair cells in the cochlea
what is the most important relay center in the CNS?
the thalamus
from the thalamus, where do we enter in contact with?
the primary and secondary auditory cortex, which starts from the area of Wenicke
what is the tip link?
a very small but fundamental connection between the tip of the individual cilia with other cilia and with the kinocilium
characterized by specific proteins (cadherin and proteocadherin) - some of the molecular aspects are still under investigation
this system only works when the cilia are connected via a tip link
