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

Mydriasis= sympathetic reflex

Myosis= parasympathetic 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 ciliaris- sphincter- 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 route-

through 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 principal 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

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 fibris 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 1. Pigment cell layer- OUTERMOST

2. Layer of rods and cones

3. External limiting membrane

4. External nuclear layer

5. External plexiform layer

6. Inner nuclear layer

7. Inner plexiform layer

8. Ganglionic layer

9. Layer of optic nerve fibers

10. Internal limiting membrane- INNERMOST

Retinal pigment epithelium

Essential in phototransduction

Pigmented cells Prevent light waves from reflecting at the level of the

retina (albinos don’t have melanin- poor visual acuity)

Vitamin A production

Phagocytic role

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.

Horizontal cells Interconnecting neurons in the outer plexiform layer

Integration and regulation of the input from multiple photorec cells

Allow eyes to adjust to see well under both bright and dim conditions

Amacrine cells Interneurons – 40 types

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 axon- like processes

Functions- unknown Detecting motion by integrating sequential activation of

neurons

Adjusting image brightness

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

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/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 all- trans retinal signal transduction cascade

Rods Rods – photopigment rhodopsin RETINAL + OPSIN (SCOTOPSIN) In the dark, retinal is bound to opsin in the 11-

cis-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, all-trans-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+ channel closure = hyperpolarisation!!

Various degree of hyperpolarisation maximum -70 – -80 mV

Rhodopsin= retinal (retinene) + scotopsin

Retinal= vitamin A derivative (caroteniod)

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 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 vision

Color vision A range of wavelengths of light stimulates each of these receptor

types to varying degrees. Yellowish-green light, for example, stimulates both L and M

cones equally strongly, but only stimulates S-cones weakly. Red light stimulates L cones much more than M cones, and S

cones hardly at all; Blue-green light stimulates M cones more than L cones, and S

cones a bit more strongly, and is also the peak stimulant for rod cells;

Blue light stimulates S cones more strongly than red or green light, but L and M cones more weakly.

The brain combines the information from each type of receptor to give rise to different perceptions of different wavelengths of light.

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 pf 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

Corti organ

Efferent system- no known function

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.

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 the scala timpani

Wave in the perilymph transmits to endolymph in the scala media

Basilar membrane vibrates- moves up and down- cilia are trapped between basilar and tectorial membranes cilia bending stimulation

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!!

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 50-70

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 neuron- dorsal and ventral cochlear nuclei in the pons

3rd order neuron- inferior colliculus in the midbrain

4th order neuron- geniculate medial nuclei in the methathalamus