University of Jordan
Faculty of Medicine
Physiology || for Pharmacy
L18 –Dr. Loai
Nervous-System
Note:
1) Make sure you understand everything, exams questions will be based on
understanding NOT memorizing alone
2) Anything between *** was not mentioned during the lecture (only for your
knowledge)
3) make sure you go through the slides for the nervous system by Dr.loai
----------------------------------------------------------------------------------------------------------In this lecture, we’ll start talking about another two sensations that are
located in the ear which are two sensations:
1) For balance (position of the body)
2) Distinguishes sound (mechanical vibration in the ear)
Anatomy of the ear
The ear consists of 3 parts:
1) External ear  which starts at the auricle  its main function is to
collect the sound waves & helps the nervous system to calculate
sound localization especially in the vertical dimension
Sound will reach the tympanic membrane ‫طبلة األذن‬
Sound vibrations will continue as vibrations of the tympanic membrane
2) Middle ear consists of 3 bones of hearing (Malleus, Incus & Stapes)
 Their main function is to transport vibrations of the tympanic
membrane to the inner ear where there is the cochlea.
Presence of these 3 bones is very important  in making amplification
of the vibrations or the signals  better hearing
3) Inner ear consists of two parts
Cochlea (Auditory sensation) 
 a hard shell form the outside & contains a fluid from the inside
 it has two openings 
1- Oval window where vibrations enter the cochlea and they
make pressure on the oval window
This results in vibration of the fluid inside the cochlea
2- Round window where vibrations are released from cochlear
fluid which is located right under the oval window but in order
for the vibration to reach the round window it has to pass
through the whole cochlea due to presence of the cochlear
duct
 The cochlear duct is not actually a membrane it is
more like a tube that is filled with fluid
 As a conclusion  there is two fluids in the cochlea(
Endolymph inside the cochlear duct (high in K+ )and the
other one is called Perilymph in inside the cochlea itself and
its similar to other body extracellular fluids (high in Na+, Cland low in K+)
So, sound vibrations will vibrate the tympanic membrane  it will be
amplified by the middle ear bones  will reach the oval window and
vibrate it  the oval window will vibrate the fluids inside the cochlea 
Until the vibrates pass through the fluid to reach the round window 
But until now the main function of the receptor to the sensation which
is to convert these vibrations into a neuronal signal  IS NOT DONE
YET
Actually there is something inside the cochlear duct which is called
Spiral organ of Corti  its main function to convert sound vibrations
into neuronal signals
This organ consists of:
1) At the bottom  Wall of the cochlear duct (basilar membrane)
2) Cells on the top of the basilar membrane and these cell are called
 hair cells  which are mainly the neuronal receptors of the ear
that will convert the vibration into neuronal signals  these cells
have hair extensions that will reach (touch) the last part of the
organ of corti from the top which is called tectorial membrane ( a
heavy gelatinous membrane)
So when vibration happens in the fluid of the cochlea  this will
vibrate the tectorial membrane  the membrane will start going
up & down  this will affect the hair cells to move with the
membrane  BUT the membrane is tooo heavy to be pushed by
the cells  this will cause bending of the hair extensions
With every vibrations that enters the cochlea  hair extension will
bend  actually on the hair cells there are mechanical gated ion
channels  when bending happens it will activate these channels
to open  and allow entrance of ions  this will lead to
depolarization  which will end up in an action potential
(neurotransmitters release)
By these steps we converted mechanical force & sound vibrations
 graded and action potential
 Higher sound  higher sound vibrations  higher fluid
vibrations  basilar membrane will go higher  ion
channels will have bigger openings  higher amount of ions
will enter the channels  graded potential will be higher 
frequency or number of actions potentials generated will be
Higher.
 The ion channels here are K+ ion channels  the
endolymph (which is high in K+)  so when channels are
open K+ will move inside from High to Low  Depolarization


But the main aim of the sensation is to have the ability to
differentiate between different types of sensation inside the
sound  so we need different types of receptors  BUT there
is only ONE type of receptors (Uracil) so how will we be able
to differentiate between different sounds with one receptor?
Each sound has a different frequency
For example the sound of a bird has a frequency of 10kilo Hz
and the sound of a car has 6 kilo Hz or the sound of the doctor
saying the letter “B” has 5 kilo Hz
So we need a mechanism to be able to differentiate between
these frequencies  depending on the memory of this
frequency in the brain  this is what the basilar membrane do.
The basilar membrane is present along the cochlea and has two
characteristics:
1) At the base  Narrow & stiffer
2) At the apex  wide & flexible (free)
The shape of the cochlea in addition to elasticity  allows the
frequency to move the membrane from only one side
When you hear a frequency of 15 kilo Hz  these vibrations will
vibrate the fluid inside the cochlea  BUT only one point will
vibrate in the basilar membrane (the point of 15 kilo Hz)
This means only the hair cells above the point of 15 kilo Hz will
vibrate  action potential will happen at this point only  the
brain will receive an action potential and will recognize that it is
from the 15 kilo Hz point  will recognize that the 15 kilo Hz
point is from for example the letter B from dr loai

The ability to differentiate different tones from different
frequencies is called tonotopic organization by the
basilar membrane and the cochlea
***tonotopy: is the spatial arrangement of where sounds of
different frequency are processed in the brain. Tones close to each
other in terms of frequency are represented in topologically
neighboring regions in the brain. Tonotopic maps are a particular
case of topographic organization***
In normal humans the shape of the cochlea & the basilar
membrane  allows to recognize frequencies as low as 20 Hz 
as long as 20 kilo Hz
Low frequencies  on the apex
High frequencies  on the base
If someone had somehow a bigger cochlea  he might be able to
hear as long as 21 kilo Hz
 Q: when a normal person is sitting beside a speaker which gives a
frequency of 25 kilo Hz will he be able to hear it?
It will move the tympanic membrane and so on there will be
vibrations reaching the cochlea  the hair cells will move  but it
will leave the round window WITHOUT any action potential 
because the basilar membrane did not react with it
For other animals  they have a different shape of the cochlea 
they might be able to hear the 25 kilo Hz
After generating the action potential  it will reach the brain
through the vestibulocochlear nerve (nerve numb 8)  then to the
brain stem and the thalamus  until it reaches the cortex (primary
auditory cortex)
Primary auditory cortex:
Anatomical name  upper banks of the temporal lobe
Are number  41
Another special thing in hearing is that the left ear goes to  the
right & left brain stem  right & left thalamus  right and left cortex
And same goes to the right ear
And this has a very important reason  to be able to know from
which side the sound is coming from (direction of the sound) 
when the sound reach the left area faster than the right one  this
means that the sound is coming from the left ear (left side)
And this is very important in unconscious reflexes (in the brain stem
& subcortical)  in a superior part of the brain stem
So right cortex will receive from right and left cortex if the left
auditory cortex was damaged will we be able to hear from right
and left ears?
We’ll answer it later
Vestibular Sensation  which is important for the 2nd sensation
(balance and position)
Starts at the inner ear at another hard shell structure (the 1st one was
the cochlea)  that is filled a fluid called Labyrinth  which is
coming from the cochlea and it is a receptor that is responsible for
balance in addition to head and body position
It consists of 5 sensory operators:
1) + 2) + 3)  semi-circular canals  that contain semi-circular ducts
And those canals are able to recognize any simple rotation
movement in any direction
We have 3 canals to be able to recognize movements on the X, Y &
Z axis
Every canal have a gelatinous membrane (looks like a butterfly) 
at the bottom it has hair cells that are similar to those for hearing &
also contains a fluid that is similar to the one in the cochlear duct.
When a fast rotational movement occur  the fluid will rotate fast
 that will bend the gelatinous membrane  and also will bend the
hair cells allowing the Ca+ to enter  Depolarization  action
potential  the brain will understand that there is a rotational
movement (and it will distinguish if it was laterally (X) or
posteriorly(Y) and if in a ratio between X & Y
In addition to the 3 canals there is two main structures  their main
function is to distinguish movement or gravity
4) Utricle
5) Saccule
Utricle and saccule have a gelatinous membrane  when any
movements occurs  gravity will pull it down and will pull with it the
hair cells
If you were in a car  which is increasing its speed by time the
membrane will be pulled by gravity to the back  a sudden break
will take it forward
To make sure that the gelatinous membrane is heavy enough to be
pulled by the gravity  there is an accumulation for crystals of
Calcium  these are called otoliths  so this membrane is called
otolithtic membrane
Baseline fire or resting
frequency
As you can see the hair cells
are not fully closed or fully
open  this allows having a
resting membrane potential 
a certain amount of ions will
enter  certain depolarization
 frequency of action potential
generated (resting frequency)
Baseline firing  which number of action potentials generated at resting.
When there is a stimulus  rate of frequency is higher than baseline firing
When there is inhibition  rate of frequency is lower than baseline firing
And this will help us to know if we are rotating clockwise or counterclockwise.
Both ears give the brain the same rate of frequency  when there is no
movement
When there is rotation  one ear will increase its rate while the other will
slows down its rate  this will lead to different rate of freq of each ear 
If the difference (between resting and after rotation freq) is higher on the
left  counter clockwise rotation
If the difference is higher on the right  clockwise rotation
For example:
If someone is resting  baseline firing (right = left)  if the right was
damaged  this will result in zero in right while the left is still giving a
frequency  difference in frequencies  the brain will understand it as
there is rotation(while there is NO rotation)  will affect hair cells, nerves
 will cause infection of inflammation  problem in vestibular system
Distention of the vestibular system
The sensation will start from the labyrinth  will go through the
vestibulocochlear nerve  to the brain stem  until it reaches different
areas:
1) Sensation from thalamus  cortex (primary vestibular cortex)
Located in the posterior parietal area
Area number  7
2) Cerebrum Subcortical part of the brain that is about balance
3) Spinal cord  vestebulospinal tract As a reflex for quick response
4) Accessory nerve or accessory nucleus(nerve 11)
5) Optic sensory (Oculomotor (III) &Trochlear (IV) &Abducens (VI)) ( 3
cranial nerves) this destination is The most important  controls the
eye movement (as a quick involuntarily reflex)
Ménière Disease
Disease results from a disruption of normal endolymph volume
Symptoms include:
1) Severe vertigo
2) Positional nystagmus (nystagmus when head in a particular position)
3) Nausea

Affected individuals can also experience-unpredictable attacks of auditory
& vestibular
Symptoms:
1) Vomiting
2) Tinnitus (ringing in ears)
3) Inability to make head movement
4) Inability to stand passively
5) Low frequency hearing loss
Benign Paroxysmal Positional Vertigo
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Common clinical disorder.
Condition characterized by brief episodes of vertigo that coincide with
particular changes in body position.
Pathophysiology poorly understood.
Posterior canal abnormalities are implicated.
otoconia crystals in the utricle may separate from the otolith membrane
and become lodged in the cupula, causing abnormal cupula deflections.
AND partial inflammation of cranial nerve VIII
Done by:
Rahaf Mihyar