Download NERVOUS SYSTEM PHYSIOLOGY 5 (updated)

Dr. S. Voiculescu
Outer layer of the eye

-
Cornea
No bood vessels
Most powerful eye lens!
Dioptric power is not variable
Transparent
Very well innervated (pain)
trigeminal nerve
 Sclera
- continuation of the substantia propria of the cornea
- tough connective tissue coating, protective layer
 The limit between the cornea and the sclera is called limbus
 Visible part of the sclera is covered with conjunctiva
Middle layer of the eye- uvea
(uveal tract)= iris, cilliary body,
choroid
 Iris
- Pigmented disk shaped structure
- Central opening- pupil- variable size- optical

-
diaphragm
Pupilary light reflex- constriction/dilation controls the
quantity of light reaching the retina
Choroid
Many blood vessels
The blood vessels of the choroid supply the pigment
epithelium of the retina
Pupillary light reflex
Myosis=
parasympathetic
reflex
Mydriasis=
sympathetic reflex
 Light enters the retina and from there travels directly to the
pretectal area. After synapsing, the information is sent to
the Edinger-Westphal nuclei on both sides of the
midbrain - this is the crucial step in ensuring that both eyes
react to light. The Edinger-Westphal nuclei, via the IIIrd
nerve, control the pupillary constrictors that narrow the
pupils.
Middle layer of the eye- uvea
(uveal tract)= iris, cilliary body,
choroid
 Cilliary body
- Wide ring- shaped structure adjoining the iris laterally
- 2 parts:
- Posterior= pars plana- muscle named orbiculus ciliarissphincter- like muscle
- Anterior= pars plicata- 80 fringe like ciliary processes
containing capillaries, from which thin fibers
(suspensory ligament or Zinn zonule) pass to the lens
Ciliary body
 Two main functions:
 (1) It produces and secretes aqueous humor into
the posterior chamber of the eye
 (2) it contains the smooth muscle that acts on the
crystalline lens, via the zonular fibers, to shift the
focus of the eye from far to near (accommodation)
Aqueous humor
 Secreted from the ciliary processes into the posterior
chamber of the eye (between the iris and the lens)
 It passes through the pupil into the anterior chamber
of the eye (between the iris and the cornea)
 Aperture of the iris influences flow- myosis loosens it
Aqueous humor drainage
 Most of the AH- absorbed into a critical element
“filtration angle”
 Filtration angle- complex structure located where the
sclera meets the root of the iris
 Here both the iris and the sclera are loosened
 90 % of AH trabecular meshwork Schlemm’s
canal (dilated vessel which encicles the limbus)
aqueous veins episcleral veins anterior ciliary
veins
Aqueous humor drainage
 A small amount of
the AH follows a
more direct routethrough the ciliary
muscle  scleral
veins (called the
uveoscleral
pathway)
Intraocular fluids
 The eye is filled with intraocular fluid, which maintains
sufficient pressure in the eyeball to keep it distended:
 -aqueous humor: a freely flowing fluid which lies in front
of the lens, continually being formed by epithelium of the
ciliary body (2-3 ml/min) and reabsorbed (2.5 μl/min,
mainly by Schlemm’s canal) to maintain normal
intraocular pressure (10-20 mmHg); contains Na+, Cl-,
bicarbonate, amino acids, ascorbic ac., glucose
 -vitreous humor: a gelatinous mass held together by a fine
fibrillar network composed of elongated proteoglycan
molecules, in which water and dissolved substances can
diffuse slowly.
Aqueous humor
 Balance between secretion and elimination of the AH-
determines the intraocular pressure (IOP)
 Nutrition for avascular eye tissues (aminoacids and
glucose)- posterior face of the cornea, lens, trabecular
meshwork
 Immune function- presence of antibodies
 Refractive index
Glaucoma= usually rise in the IOP
 “Usually” because rarely- normal tension glaucoma
 The average normal intraocular pressure is about 15 mm
Hg, with a range from 10 to 20 mm Hg (measured clinically
by using a tonometer).
 Imbalance in the secretion and reabsorption of aqueous
humor can increase the pressure in the eye (more
frequently decrease in the reabsorbtion)
 this condition threatens the viability of the head of the
optic nerve (mechanic and ischemic lesion)= blindness if
left untreated
 Reabsorption can be increased surgically, and secretion
and reabsorbtion can be reduced by medication therapy.
Normal disk –
ratio between the
diameter of the
cup compared to
the one of the
whole disk<1/2
Glaucomatous
disk- wide
cupping, optic
nerve atrophy
Ciliary body
 Two main functions:
 (1) It produces and secretes aqueous humor into the
posterior chamber of the eye
 (2) it contains the smooth muscle that acts on the
crystalline lens, via the zonular fibers, to shift the
focus of the eye from far to near (accommodation)
Accommodation
 Far point of the normal eye is infinity.
The eye is able to focus parallel rays, that originate
at infinity, on the retina (see objects at infinity
effortlessly)- beyond 6 m.
 Near point = minimum distance at which the eye
can see objects clearly.
By increasing its dioptric power the normal eye can
see objects clearly as close as 25 cm.
Accomodation
 Function of the eye that enables it to focus images situated
between 6m and 25 cm on the retina!
 3 phenomena
 Changes in the dioptric power of the cristalline lens
 Pupillary reflex
 Convergence
 Ciliary muscle- sphincter-like (it also has radially disposed
muscle fibres- low importance)
 Muscle contraction= zonular fibres relaxation= lens becomes
more round= higher convergence power
 Muscle relaxation= zonular fibres stretching= lens becomes
more flat= lower convergence power
Accomodation mechanism
 PS stimulation contracts circular ciliary muscle fibers 
relaxes the lens ligaments  lens to become thicker and
increase its refractive power  the eye focuses on objects
nearer than when the eye has less refractive power.
 As a distant object moves toward the eye, the number of PS
impulses impinging on the ciliary muscle must be
progressively increased for the eye to keep the object
constantly in focus.
 Sympathetic stimulation has a weak effect in relaxing
longitudinal ciliary muscle fibers, but this effect plays
almost no role in the normal accommodation mechanism.
Refraction
 Refraction is the bending of a wave when it enters a
medium where it's speed is different. The refraction of
light when it passes from a fast medium to a slow
medium bends the light ray toward the normal to the
boundary between the two media. The amount of
bending depends on the indices of refraction
Refractive system of the eye
four refractive
interfaces:
(1) interface air - anterior
surface of the cornea,
(2) interface posterior surface
of the cornea - aqueous
humor,
(3) interface aqueous humor anterior surface of the lens
of the eye
(4) interface posterior surface
of the lens - vitreous
humor.
Refractive index of a transparent substance
is the ratio of light velocity in air to light
velocity in the substance.
The refractive index of air itself is 1.00.
Refraction and the eye
 The refractive apparatus of the eye- collectively termed
the dioptric media and consist of the cornea, aqueous
humor, lens, and vitreous body.
 Cornea has the highest dioptric power
- if all the refractive surfaces of the eye are
algebraically added together and then considered to
be one single lens, a single refractive surface is
considered to exist, with a focal length = 17 mm and a
total refractive power of 59 diopters (1/0.017) when
the lens is accommodated for distant vision (lens
flattened).
- the total refractive power of the internal lens = 20
diopters; in response to nervous signals its curvature
can be increased markedly to provide
"accommodation“.
- the amplitude of accommodation: increase of power of
the eye up to a 14D.
Refraction and the eye
 Most of that refraction in the eye takes place at the
first surface, since the transition from the air into the
cornea is the largest change in index of refraction
which the light experiences. About 80% of the
refraction occurs in the cornea and about 20% in the
inner crystalline lens.
 While the inner lens is the smaller portion of the
refraction, it is the total source of the ability to
accommodate the focus of the eye for the viewing of
close objects. For the normal eye, the inner lens can
change the total focal length of the eye by 7-8%.
Internal lens (crystalline)
 The lens, biconvex and 1 cm in diameter, is covered by a




capsule and consists of cellular lens fibers.
The lens capsule is anchored to the ciliary body by its
suspensory ligaments, or ciliary zonule.
Main functions- refraction and accommodation
The lens, in addition to becoming increasingly yellow with
age, also becomes harder and less elastic, as a result of
which the power of accommodation is lessened
(presbyopia) and convex spectacles may be required for
reading.
An opacity of the lens is termed a cataract. It is commonly
age-related and it may interfere with vision.
Cataract- opaque lens
Vitreous body
 transparent, gel-like body (mostly water and
hyaluronan) fills the large vitreous space between the
lens and the retina.
 some peripheral fibrils form its capsule= hyaloid
 it contains a few macrophages and hyalocytes – stellate
cells with oval nuclei that produce the fibrils and
hyaluronan.
Normal refraction= emmetropia
 Parallel light rays from distant objects are in sharp
focus on the retina when the ciliary muscle is
completely relaxed
 the emmetropic eye can see all distant
objects clearly with its ciliary muscle relaxed.
 to focus objects at close range, the eye must
contract its ciliary muscle and thereby provide
appropriate degrees of accommodation.
 Correct ratio between eye refractive power and
eyeball length;
 normal eyeball length ~ 21-24 mm.
Hyperopia- farsightedness
 Usually due to either an eyeball that is too short / a lens
system that is too weak (occasionally)
 parallel light rays are not bent sufficiently by the relaxed
lens system to come to focus by the time they reach the
retina.
 the ciliary muscle must contract to increase the strength of
the lens.
 by accommodation, a farsighted person is capable of
focusing distant objects on the retina
Hyperopia
 If the person has used only a small amount of
strength in the ciliary muscle to accommodate for
the distant objects, he or she still has much
accommodative power left, and objects closer and
closer to the eye can also be focused sharply until
the ciliary muscle has contracted to its limit.
 Correction with convex lens or in young persons
through accommodation
Hyperopia
Myopia= nearsightedness
 When the ciliary muscle is completely relaxed, the
light rays coming from distant objects are focused
in front of the retina, usually due to too long
eyeball, or too much refractive power in the
lens system of the eye.
 As an object moves nearer to a myopic’s eye, it
finally gets close enough that its image can be
focused.
 Correction with concave lens.
Myopia
Astigmatism
The third layer of the eye= RETINA
 anterior, nonsensitive portion, which lies over the
ciliary body
 posterior functional, or optical, portion, the
photoreceptor organ.
 The optical (neural) retina lines the choroid from the
papilla of the optic nerve posteriorly to the ora serata
anteriorly.
Retinal layers

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1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Pigment cell layer- OUTERMOST
Layer of rods and cones
External limiting membrane
External nuclear layer
External plexiform layer
Inner nuclear layer
Inner plexiform layer
Ganglionic layer
Layer of optic nerve fibers
Internal limiting membrane- INNERMOST
Synaptic integration
Light must pass through all layers before reaching rods & cones
Cellular organisation of the retina
Synapses
Light
Retinal pigment epithelium
Essential in
phototransduction
Pigmented cells
 Prevent light waves from reflecting (scattering) at the level of the
retina (albinos don’t have melanin- poor visual acuity)
 Light absorption- after it has sensitized photoR cells (melanin)
(macular protection from photo-oxidative stress)
 Spatial buffering of ions- transepithelial ion transport
 Vitamin A transport and esterification- role in visual cycle
 Phagocytic role for the photoR outer segments membranes-
constant destruction of photoR (photoox stress) and renewal
Pigmented cells
 Secretion of factors (TGF, IGF-1, VEGF, etc) to mediate
relationship between neurons and endothelial
cells/immune cells
 Blood- retinal barrier-immune privilege of the eye-
mechanical (tight junctions along lateral walls) and
communication via secreted factors (silence of
immune response in the healthy eye)
Muller cells
 close association of Müller cells with neural elements
in the sensory retina.
 structurally and functionally equivalent to the
astrocytes of the central nervous system
 envelop and support the neurons and nerve processes
of the retina.
Muller cells
 Supplying end products of anaerobic metabolism
(breakdown of glycogen) to fuel aerobic metabolism in the
nerve cells.
 They mop up neural waste products such as carbon dioxide
and ammonia and recycle spent amino acid transmitters.
 They protect neurons from exposure to excess
neurotransmitters such as glutamate using well developed
uptake mechanisms to recycle this transmitter. They are
particularly characterized by the presence of high
concentrations of glutamine synthase.
Muller cells
 They may be involved in both phagocytosis of
neuronal debris and release of neuroactive substances
such as GABA, taurine and dopamine.
 They are thought to synthesize retinoic acid from
retinol (retinoic acid is known to be important in in
the development of the eye and the nervous system)
(Edwards, 1994)
 They control homeostasis and protect neurons from
deleterious changes in their ionic environment by
taking up extracellular K+ and redistributing it.
Horizontal cells
 Interconnecting neurons in the outer plexiform layer
 Integration and regulation of the input from multiple
photorec cells
 Lateral inhibition- high contrast
 ! Outputs of horizontal cells are always inhibitory
Bipolar cells
 Type of neuron located in the inner nuclear layer
 First order neuron of the optic pathway
 Dendrites receive info from the photoreceptor cells and
horizontal cells and pass it to the ggl and amacrine cells
 Receive input from cones (10 types) and rods (1 type)
 Classification
 ON cell- glutamate metabotropic receptor respond to
light by depolarisation= active in light conditions/inactive in
the dark
 OFF cell- glutamate ionotropic receptors respond to light
by hyperpolarisation= inactive in light conditions
Amacrine cells

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Interneurons- 40types
Location- all of them – inner nuclear layer
Connections linking bipolar cells to ganglion cells
At first- thought they lack axons- dendrites end
presynapticaly in other cells, but they do have long axonlike processes
 Functions- unknown
 Analyze visual signals before leaving the retina
 Detecting motion by integrating sequential activation of
neurons
 Adjusting image brightness
Ganglion cells
 Neurons located near the inner surface of the retina
 Final output neurons of the vertebrate retina SECOND
ORDER NEURON
 Collect visual info in their dendrites from bipolar and
amacrine cells and their axons form the OPTIC NERVE
FIBRES
 1-2% of them are photoreceptors- melanopsin- positive
ggl cells- involved in the cyrcadian rhythm
Photoreceptors- modified unipolar
neurons
 Photoreceptor cells (rods and cones) are
subdivided into three regions:

- the outer segment: contains a stack of
membranous discs that are rich in photopigment;
also, contains Na+ channels,

- the inner segment: connects with the outer
segment by way of a modified cilium and contains
nucleus, mitochondria, other organelles; also contains
K+ channels and a high density of Na-K pumps,

- the synaptic terminal (AXON) that contacts
one or more bipolar cells.
Rods and cones are light transducers
Dark pigment layer
absorbs stray light
& reduces reflection
Disks in rods & cones
are the site of
transduction
Transduction
process mediated by
pigments in the
disks (ex. is rod)
Rods/cones
 At the fovea centralis, highest VA, as
 1. there are just cones, at a high density,
 2. thinner inner retinal layers and fine blood vessels –
assisting an increased transparency,
 3. no convergence of the efferents from its cones (1 cone: 1
ggl. cell)
 Information flow through the retina from
photoreceptors to bipolar cells and then to
ganglion cells. The ganglion cell axons form the optic
nerve and provide the output of the retina to the
methathalamus.
Phototransduction
 Conversion of light into electrical signals
 Biological conversion of the photon into an electrical
signal in the retina
 Photoreceptors contain photopigments
 PIGMENT= opsins (G- protein coupled receptors)
plus a chromophore= 11- cis retinal- substance
sensitive to light
 11- cis retinal undergoes photoisomerization alltrans retinal signal transduction cascade
Rods
 Rods – photopigment rhodopsin
RETINAL + OPSIN (SCOTOPSIN)
In the dark, retinal is bound to opsin in the 11cis-retinal form. Absorption of light causes a
change to the all-trans-retinal form, which no
longer binds to opsin.
Before the photopigment can be regenerated, alltrans-retinal must be transported to the pigment
cell layer, reduced, isomerized, and esterified.
Cones
 Cones contain the pigment IODOPSIN
 three different cone opsins (photopsins) found in the
three different types of cones, each sensitive to a different
part of the visible light spectrum PLUS 11 cis retinal
 ONE CONE TYPE RESPONDS BEST TO BLUE LIGHT
(420 nm), ANOTHER TO GREEN (531 nm), AND THE
THIRD TO RED (558 nm).
 trichromatic color vision
 light produced changes resemble the sequence in rods, but
the reactions and the recovery are quicker.
 Na+/K+ pump in inner
segment
 Na+ channel in the outer
segment
 Rods have a steady state
potential of -40 mV (less
negative than normal)
 Light stimulation Na+ ch
closure= hyperpolarisation!!
 Various degree of
hyperpolarisationmaximum
-70- -80 mV
 Rhodopsin= retinal
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
(retinene) + scotopsin
Retinal= vitamin A
derivative (carotenoid)
Scotopsin= protein
Retinal has 2 forms= 11 cis
and all-trans= only cis
binds to retinal
Metharodopsin II=
activated scotopsin
 Light ....metarhodopsin II= enzyme activates
transducin activates phosphodiesterase
 cGMP GMP (by the action of phosphodiesterase)
 Without light presence=> cGMP is attached to the
sodium channel and keeps it open
 Light makes cGMP to dissapear and Na+ ch to
close hyperpolarisation
In the dark
 photoreceptor cells are depolarized at about -40 mV.
in the dark
 in the dark, cGMP levels are high and keep cGMP-gated
sodium channels open allowing a steady inward current,
called the dark current
 Dark current- depolarisation Ca2+ voltage gated
channels open increased intracellular concentration of
Ca2+ causes TONIC (constant) release of the
neurotransmitter into the synaptic cleft
(GLUTAMATE)
Under light conditions
 Light turns 11 cis retinal into 11 all-trans retinal
phototransduction cascade initiated....
 Progressive closure of Na+ channels in the outer segment,
BUT Na+/K+ pump continues to pump Na+ ions out
increased electronegativity inside the cell
 !! Hyperpolarisation= RECEPTOR POTENTIAL is
proportional to light intensity (logharitmic relation) –
maximum light intensity= -70 -80 mV
 Neurotransmitter release DECREASES proportional to the
ammount of light- no more tonic release of glutamate
excitation of the optic pathway
A. In darkness, photoreceptor cells have open Na+ channels, which
result in a dark current and consequently a tonic release of
neurotransmitter (glutamate) onto bipolar cells and horizontal cells.
Dark current in a photoreceptor is caused by passive influx of Na+, which
is returned to the extracellular space by pumping. Light closes the Na+
channels and thus reduces the dark current.
B. Second-messenger system underlying phototransduction. When light
reacts with rhodopsin (RH), G protein transducin (T) is activated. This
in turn activates phosphodiesterase (PDE), which breaks down cGMP
into GMP. The dark current depends on cGMP, and thus a fall in
cGMP concentration reduces the dark current, which causes
hyperpolarization of the photoreceptor. GC, guanylyl cyclase.
Light and dark adaptation
 Light- photopigments are
consumed a lot of vitA is
formed= light adaptation
 Dark- more photopigments
are formed= dark adaptation
 Image- when switching from
bright light to dark= dark
adaptation- retinal
sensitivity raises with more
pigments being formed!
Photopic/scotopic vision
 Photopic vision relates to human vision at high ambient
light levels (e.g. during daylight conditions) when vision is
mediated by the cones. The photopic vision regime applies to
luminance levels > 3 cd/m2
 Scotopic vision relates to human vision at low ambient light
levels (e.g. at night) when vision is mediated by rods. Rods have a
much higher sensitivity than the cones. However, the sense of
color is essentially lost in the scotopic vision regime. At low light
levels such as in a moonless night, objects lose their colors and
only appear to have different gray levels. The scotopic vision
regime applies to luminance levels < 0.003 cd/m2.
 Mesopic vision relates to light levels between the photopic
and scotopic vision regime (0.003 cd/m2 < mesopic luminance
< 3 cd/m2).
Color vision
There are three types of pigments
in three types of cones
All three pigments are necessary
for correct colour vision.
We need at least two pigments for
color vision by color mixing
 Dyschromatopsia is produced by
the absence of one of the
pigments, the subject still sees
almost all colours using only two
pigments
 Absence of the red pigment
protanopia
 Absence of the green pigment
deuteranopia
 Absence of the blue pigment
 tritanopia.
Color vision
 The visible spectrum includes 300 wavelengths (400-
700 nm), and in some portions we can discern color
differences of 1 wavelength. The ability to see so many
colors depends on:
a. a separate cone for each wavelength.
b. optic nerve fibers for each color.
c. visual cortex neurons sensitive to each
color.
d. difference in stimulation of red, green
and blue sensitive cones.
 human eye can detect almost all gradations of colors
when only red, green, and blue monochromatic lights
are appropriately mixed in different combinations
 Interpretation at the CNS level
 At one moment, a colored object stimulates the 3
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cones differently color code for the brain!
ORANGE= R:G:B= 99:42:0
BLUE= R:G:B=0:0:97
White- equal stimulation of all three types
Black- no stimulation
Color blindness-dyschromatopsia
 Ishihara plates
After the retina
 Once the ganglion cell axons leave the retina, they
travel through the optic nerve to the optic chiasm, a
partial crossing of the axons. At the optic chiasm the
left and right visual worlds are separated. After the
chiasm, the fibers are called the optic tract. The optic
tract wraps around the cerebral peduncles of the
midbrain to get to the lateral geniculate nucleus
(LGN). The LGN is a part of the thalamus. Almost all
of the optic tract axons, therefore, synapse in the LGN.
The remaining few branch off to synapse in nuclei of
the midbrain: the superior colliculi and the
pretectal area.
Ear components
 3 parts:
- external ear
- middle ear
- inner ear
External ear
 The external ear funnels sound waves to the external
auditory meatus. In some animals, the ears can be
moved like radar antennas to seek out sound. From the
meatus, the external auditory canal passes inward to
the tympanic membrane (eardrum)
Middle ear
 The middle ear is an air-
filled cavity in the
temporal bone that
opens via the auditory
(eustachian) tube into
the nasopharynx and
through the
nasopharynx to the
exterior.
 The tube is usually
closed, but during
swallowing, chewing,
and yawning it opens,
keeping the air pressure
on the two sides of the
eardrum equalized
 The three auditory ossicles, the
malleus, incus, and stapes, are
located in the middle ear.
 The manubrium (handle of the
malleus) is attached to the back of
the tympanic membrane. Its head
is attached to the wall of the
middle ear, and its short process is
attached to the incus, which in
turn articulates with the head of
the stapes.
 Its foot plate is attached by an
annular ligament to the walls of
the oval window
Inner ear
 The inner ear (labyrinth)-
made up of two parts:
 The bony labyrinth is a
series of channels in the
petrous portion of the
temporal bone. Inside these
channels, surrounded by a
fluid called perilymph, is
the membranous
labyrinth.
 This membranous structure
more or less duplicates the
shape of the bony channels
Path of sound
- external canal
- vibrates eardrum
- vibration moves through ossicles
- Malleus
- Incus
- Stapes
- stapes vibrates oval window of cochlea
- creates pressure wave in the fluid inside
Membranous labyrinth
 3 semicircular channels oriented perpendicular to each
other and following the 3D planes of space
 Utricle and saccule= vestibule
 Cochlea
 The semicircular channels, utricle and saccule-
balance- they contain the vestibular receptors
 Cochlea has a hearing function- auditory receptor
(Corti organ)
Inner ear
 The membranous labyrinth is filled with a fluid called
endolymph K+ RICH
 The bony labyrinth is filled with a fluid called perilymph
 There is no communication between the spaces filled with
endolymph and those filled with perilymph.
Cochlea
 The cochlear portion of the
labyrinth is a coiled tube
which in humans is 35 mm
long and makes 2.5 turns.
 The basilar membrane and
Reissner's membrane divide it
into three chambers (scalae)
 The upper scala vestibuli and
the lower scala tympani
contain perilymph and
communicate with each other
at the apex of the cochlea
through a small opening
called the helicotrema
 At the base of the cochlea, the
scala vestibuli ends at the oval
window, which is closed by the
footplate of the stapes.
 The scala tympani ends at the
round window, a foramen on
the medial wall of the middle
ear that is closed by the
flexible secondary tympanic
membrane.
 The scala media is continuous
with the membranous
labyrinth and does not
communicate with the other
two scalae. It contains
endolymph
 The cochlea is made up of three
canals wrapped around a bony
axis, the modiolus. These canals
are: the scala tympani (3), the
scala vestibuli (2) and the scala
media (or cochlear duct) (1). The
scalae tympani and vestibule are
filled with perilymph (in blue)
and are linked by a small opening
at the apex of the cochlea called
the helicotrema. The triangular
scala media, situated between the
scalae vestibuli and tympani is
filled with endolymph (in green).
Corti organ
 Located on the basilar membrane
 It contains the hair cells which are the auditory
receptors.
 The hair cells are arranged in four rows: three rows
of outer hair cells and one row of inner hair
cells medial to the tunnel of Corti
 There are 20,000 outer hair cells and 3500 inner
hair cells in each human cochlea
 The tips of the hair cells go through the reticular
membrane
 Then they inbed in the thin tectorial membrane
 At the basis of the receptor cell- dendrites of the first
order neuron located in the modeolus= spiral ganglion of
Corti 90-95% of the afferent fibres leave from the
inner hair cells (they travel through the Corti tunnel)
 only 5-10% innervate the more numerous outer hair
cells
Inner/outer hair cells
 Inner hair cells- sound perception
 Outer hair cells- sound amplifiers-mechanic response
to stimulation (vibration) otoacoustic emissions
(can be registered from the external meatus with
microphones) hearing loss screening in babies
Efferent system
 The efferent auditory fibers are originated from many
different sites in the central nervous system. From the
superior olivary complex, they are projected to the
cochlea through two different tracts:
 the medial olivocochlear tract, which comprises large
myelinated neurons that innervate predominantly the
outer hair cells
 lateral olivocochlear tract, with unmyelinated neurons,
that synapses with the inner hair cells.
 Function- change sensitivity of the receptor cells
Sound transmission
 The ear converts sound waves in the external
environment into action potentials in the auditory
nerves.
 The waves are transformed by the eardrum and
auditory ossicles into movements of the footplate of
the stapes.
 These movements set up waves in the fluid of the
inner ear.
 The action of the waves on the organ of Corti generates
action potentials in the nerve fibers.
Sound transmission
 Sound wave.... stapes oval window perilymph in the
scala vestibulihelicotrema perilymph in
 Wave in the perilymph transmits to endolymph in the scala
media
 Basilar membrane vibrates- resonant structure- deflected
in response to waves- deformation is a traveling wave
from basis to apex
 APICAL CILIA OF HAIR CELLS- DEFLECTION
DEPOLARISATION
 http://www.youtube.com/watch?v=1JE8WduJKV4
Basilar membrane
 Unique structure
 Differs according to area- stiff fibres that are attached




firmly to the modeolus, but are “free” at the outer end
Fibres become longer as we approach the helicotrema
Fibres at the basis are more rigid, while the ones at the apex
are more flexible
This makes the basilar membrane resonate to different
sound pitches!!
Sound travels along the basilar membrane BUT this
resonates only in a specific RESONANT point for each
of them
Basilar membrane
Frequency coding
Sound frequency- Hz/ Cicles per
second
Intensity coding (loudness)
 Basilar membrane vibrates direcly proportional with
sound intensity more rapid rates of excitation
 Spatial sumation  no of stimulated cells gets higher
 Outer cell stimulation  only when vibration is very
high
Hair cells
 The hair cells in the inner ear
have a common structure
 the kinocilium, is a true but
nonmotile cilium,it is one of
the largest processes and has
a clubbed end.
 The other processes are
called stereocillia-about 5070
 The membrane potential of
the hair cells is about
-70 mV.
Hair cells
 When the stereocilia are
pushed toward the
kinocilium, the
membrane potential is
decreased to about -50
mV.
 When the bundle of
processes is pushed in the
opposite direction, the
cell is hyperpolarized.
Signal transduction
 Very fine processes called tip links tie
the tip of each stereocilium to the side
of its higher neighbor, and at the
junction there appear to be
mechanically sensitive cation channels
in the higher process.
 When the shorter stereocilia are
pushed toward the higher, the open
time of these channels increases. K+—
the most abundant cation in
endolymph receptor potential
triggers Ca2+ enterance via specific
channels  neurotransmitter release
and produce depolarization.
 Neurotransmitter- probably
glutamate, which initiates
depolarization of neighboring afferent
neurons.
Auditory pathway
 Dendrites of the neurons in the Spiral ganglion are
located at the basis of the sensorial hair cells
 Spiral ganglion in the modiolus. Ganglion is formed
by proper bipolar nerve cells. Their myelinated axons
run together to form the acoustic nerve, which unites
with the vestibular nerve to form the VIIIth cranial
nerve. Myelinated dendrites lose their sheaths as they
perforate the bone and pass to the organ of Corti,
terminating hair cells.
 1st order neuron-
Corti ganglion
 2nd order neurondorsal and ventral
cochlear nuclei in the
pons
 3rd order neuroninferior colliculus in
the midbrain
 4th order neurongeniculate medial
nuclei in the
methathalamus