Retinal pigment epithelium :

THE AUDITORY SYSTEM

• Describe the organisation of the Organ of Corti
• Describe the physiology of hearing
• Describe the auditory pathways
• Explain auditory reflexes
• Define the term decibel and describe the normal auditory range
• Describe conduction & sensorineural deafness, their causes & how to clinically test for them

The Auditory system is a specialised sensory system that detects changes in sound wave pressure. It comprises the cochlea, its nerve and the pathway to the auditory cortex in the temporal lobe. It can detect frequencies from 20Hz-20Khz. The auditory pathway is more elaborate than the visual or somatosensory pathway. Sounds are detected by both ears and in order to signal the location of the sound complex inhibitory circuitry is used between the 2 sides to magnify differences in the timing and intensity of sounds that occur during normal hearing.
The ear apparatus is divided into 3 main regions: the outer, middle and inner ear.

• The outer ear collects sound waves and channels them to the middle ear.
• The middle ear converts the sound pressure waves to vibration of the fluid in the inner ear by movement of the oval window. This is accomplished by 3 small bones.
• The inner ear houses the hearing and balance receptors that are contained within the bony labyrinth of the temporal bone. The hearing receptors are located within the cochlea while the balance receptors are located in the semicircular canals and vestibule. A delicate continuous membrane is suspended within the bony labyrinth, creating a second chamber within the first in the cochlear. This is the membranous labyrinth.

Anatomy of the auditory system:
The cochlea is shaped like a snail’s shell & consists of 3 fluid filled spaces:

1. scala vestibuli
2. scala tympaniThe scala vestibuli and tympani are enclosed in bone, contain perilymph, and are continuous with each other at the apex ( also known as the helicotrema).
3. cochlea duct (scala media) contains endolymph & is part of the membranous labyrinth

• 
• A delicate Vestibular (Reissner) membrane separates scala vestibuli from the cochlea duct
• Basilar membrane separates scala tympani from the cochlea duct.

Auditory receptors.

• Spiral organ (Organ of Corti) consists of inner & outer hair cells & supporting cells.
• Inner hair cells are arranged in a single row; have a 1:1 relationship with auditory neurons & play a major role in auditory discrimination
• Outer hair cells increase from 3 rows at the base to 5 rows at the apex; receive synapses from a larger number of auditory afferents. It is thought that they help to sharpen the frequency tuning of the cochlea.
• Hair cells have stereocilia whose tips are embedded in the overlying tectorial membrane, when the basilar membrane is in motion, the stereocilia bend causing changes in the membrane potential and evoking action potential firing.

Sound transduction:
1. Sound waves are collected by the outer ear and channelled to the tympanic membrane 2. When sound waves are transmitted from the tympanic membrane along the middle ear ossicles (3) it causes the stapes to vibrate the oval window (4). The stapes vibrates in response to vibrations of the eardrum, setting the perilymph fluid of the inner ear in motion (5). The round window serves as a pressure valve, bulging outward as pressure rises in the inner ear. When this moves inwards, the round window moves outwards & vice versa (9). Because of the size (area) differential between the tympanic memnbrane and oval window, sound is amplified 4 fold in the middle ear. Soundwaves are converted into perilymphatic pressure, which is transmitted through the vestibular membrane to the basilar membrane (6,7). Because the cochlear duct sits on the basilar membrane, it too is set into motion, stimulating auditory receptors located on this membrane (8).
A tonotopic map of sound frequency occurs along the basilar membrane. The width of the membrane increases in size in the direction from the base (near oval window) to the apex. Also, the narrow end is taught while the wider end is more flexible. Consequently, highest frequencies (& pitch) set the narrow (base) end resonating while lower frequencies set the apex in motion. Loudness of the sound is determined by the intensity of the sound waves. Greater intensity causes greater vibration of the Basilar membrane and hence greater activation of spiral ganglion cells.
Auditory pathways
The cell bodies of the cochlea nerve are found in the spiral ganglion.
Peripheral process synapse with the hair cells and their central process travel in the cochlea nerve, which terminates in both the dorsal & ventral cochlea nuclei in the medulla.
Ventral cochlea nucleus encodes intensity information; dorsal cochlea nucleus encodes pitch information and analyzes the quality of sound. The dorsal cochlear nucleus picks apart the tiny frequency differences which make words like "bet" sound different from "bat", "but" and "bit".
Axons from the dorsal & ventral cochlear nuclei cross the midline to form the lateral lemniscus (LL) which ascends through the pons to the inferior colliculus in the midbrain.

Ventral nucleus fibres enter LL via the superior olivary nucleus (SON) & trapezoid body whereas dorsal nucleus goes directly to contralateral LL. From the IC auditory fibres travel to synapse in the main auditory nucleus of the thalamus – the medial geniculate nucleus (MGN) (via inferior brachium). From there thalamocortical axons travel to the primary auditory cortex (AI) in the temporal lobe (Heschl’s gyrus) via auditory radiation of internal capsule. Auditory cortical cells receive input from both ears but are preferetntially response to input from the contralateral ear.
IC, MGN & AI are all tonotopically organised; (in AI, high tones posteromedially, low tones anterolaterally).
Central auditory pathways are unlike other ascending pathways due to

1. Presence of accessory nuclei that modulate the input.
2. Bilateral representation of auditory impulses on each side.

3 groups of nuclei are involved in the pathway between the cochlea nuclei & the inferior colliculus:
Superior Olivary Nucleus .

• It receives input from both ears & gives rise to fibres in both the ipsilateral & contralateral LL.
• Ipsilateral inputs are excitatory, contralateral ones inhibitory (effect mediated by TB).
• Plays a key role in localisation of sounds in space because it is responsive to differences in the intensity & timing between sounds entering the ears simultaneously.

Trapezoid body.

• Helps locate the spatial direction of the sound by exaggerating differences through crossed inhibition of SON cells.

Lateral lemniscus nuclei.

• Located in & adjacent to LL. Send axons to both the ipsilateral and contralateral LL.
• Participate in acoustic reflexes.

Inferior colliculus

• Integrates spatial information from SON, intensity information from Ventral cochlea nucleus & pitch information from dorsal cochlea nucleus
• Projects to MGN & also to contralateral IC (inhibitory).

Auditory modulation

• Reciprocal connections between AI & MGN & IC.
• Also IC, SON & LL send fibres back to the dorsal and ventral cochlear nuclei.
• Fibres (from SON & TB) & RF fibres also terminate on hair cells of Organ of Corti.
• Provides feedback mechanism for regulating selective attention to certain sounds.

Brainstem acoustic reflexes:
Fibres emerge from the LL and connect to interneurons to produce reflex arcs:

1. Fibres enter the V & VII motor nuclei to link with motoneurons supplying tensor tympani (V) & stapedius (VII) muscles in the middle ear. They muscles exert a damping action on the middle ear ossicles. Tensor tympani is activated by ones own voice and stapedius by external sounds.
2. Startle reflex: Loud unexpected sounds (e.g. awakening to an alarm clock) cause flinching; reflex is mediated by reticulospinal fibres and fibres to the VII motor nucleus.

Blood supply to the cochlea & auditory pathway.

• Auditory tube: middle ear branches of external carotid artery – ascending pharyngeal a. & branches of the maxillary artery - middle meningeal & pterygoid canal arteries.
• Brainstem pathway from Cochlear nucleus to the Inferior Colliculus - supplied by the posterior inferior cerebellar artery PICA.
• Veins: drain into pterygoid venous plexus
• Medial geniculate nucleus MGN ( in the thalamus) – supplied by the posterior cerebral artery
• Auditory cortex – supplied by middle cerebral artery

Sound intensity is measured in decibels. A 10db increase in sound represents a ten-fold increase in intensity. Sound becomes uncomfortable to the ear at around 120db and frankly painful above 140dB. Prolonged exposure to loud sounds kills the hair cells and causes deafness or a hearing deficit and the louder the sound, the quicker the loss, the higher frequencies being the first to go. Most people fail to notice the deficit until they cannot hear someone else’s speech.
Relationship between hearing and speaking
The language areas in the brain are located in the dominant (usually left) hemisphere. When the language areas are damaged the resultant clinical syndrome is calledaphasia.
Simple pathway for understanding spoken language

1. When a person hears a sentence this is transmitted via the auditory apparatus to the primary auditory cortex.
2. This then connects to Wernicke’s area (in the temporal lobe), which decodes the language into meaning.
3. If the sentence is to be repeated the information has to be transmitted forward to Broca’s area (expressive speech) in the posterior inferior part of the frontal lobe (via the arcuate tract).
4. This area then produces speech via the motor programs of the motor cortex that activates the tongue and laryngeal muscles.

DAMAGE TO THE AUDITORY SYSTEM

• The clinical signs of damage to the auditory nerve are deafness and tinnitus (ringing in the ears). Deafness can be of two types:

1. Conduction deafness
   • Results from any interference with the passage of sound waves through the external or middle ear e.g. wax build up in outer ear or otis media (inflammation).
   • Bone conduction through cranial bones can still occur.
   • Conduction deafness is never complete or total.
2. Nerve deafness (perception deafness)
   • Results from damage to the receptor cells of the organ of Corti or the cochlear nerve.
   • The defect is in the portion of the auditory mechanism common to both air & bone conduction & thus hearing failure by both routes occurs. Amount of loss depends on degree of damage to organ or nerve. 

Clinical Tests
There are 2 simple tests that together can distinguish conduction from perception deafness. Both depend on the differences between air conduction and bone conduction of sound. Sound transmitted by air conduction depends on the integrity of the middle ear, while bone conduction can bypass the middle ear and activate the basilar membrane of the inner ear directly.
Rinne test

• Compares conduction by air & bone and determines their relative sensitivity.
• Vibrating tuning fork is held near ear & moved away until it is no longer heard.
• Then the stem of the fork is placed in contact with the mastoid bone.
• Normally the sound is heard louder & longer by air conduction.
• In perception deafness, the sound is heard better in the unaffected ear
• In conduction deafness, sound appears to arise from the deaf side because of the improved efficiency of bone conduction in the presence of middle ear damage.

Weber tuning fork test (for sound lateralisation).

• Place tuning fork in middle of forehead & ask patient in which ear tone is heard. If hearing is equal then the sound seems to come from inside the head.
• Patient with unilateral nerve deafness hears tone in unaffected ear because it is more sensitive.
• Patient with unilateral conduction deafness hears the tone louder in the affected ear.

Common causes of deafness

• Old people progressively lose hearing (Presbyacusis). High frequencies are lost first. Starts relatively early in life (during the twenties)
• Otosclerosis – most common adult cause of hearing loss. Autosomal dominant genetic disorder Characteristed by fusion of stapes to oval window causing difficulty of movement followed by eventual cessation of ear ossicle movement. It is amenable to surgery.
• Persistent loud noises can cause cells to die (frequency dependent).
• Infection
• Viruses e.g. mumps. German measles during pregnancy can cause complete destruction of cochlear nerve in fetus.
• Middle ear infections e.g. Otitis media – bacterial infection causing swelling and outward bulging of tympanic membrain, pain and pus collection in middle ear. Most common in children as eustachian tubes are not fully formed and thus don’t drain well into nasopharynx.

THE VISUAL SYSTEM

• Describe the anatomy of the eyeball
• Describe the anatomy and physiology of the retina to include photoreceptors, bipolar cells and ganglion cells.
• Describe the blood supply to the retina, and explain the consequences to vision of any damage to this supply.
• Describe the visual pathways from retina to visual cortex
• Explain the anatomical basis for visual reflexes in particular the pupillary light reflexes.
• Explain visual field defects caused by lesions interrupting the visual pathways.


Anatomy of the Eyeball
Anatomical layers of the eye
The human eye is about 24 mm in diameter. It has three layers. The outer layer is the corneosclera layer. When it reaches the eye, the dura mater covering the optic nerve becomes the sclera that covers the back of the eyeball. At the front of the eye the sclera becomes the conjuctiva (white of the eye) and in front of the pupil of the eye the conjunctiva merges with the transparent cornea. Inside the sclera is the uvea. It has three parts:


1. The choroid, which is a dense capillary network all around the back of the eye.
2. At the front of the eye the choroid fuses with the ciliary body, which contains the ciliary muscle that controls the shape of the lens.
3. The third part of the uvea is the iris, projecting inwards from the ciliary body.


Uveitis is an inflammation of the uvea, often secondary to eye injury.
The innermost layer of the eye is the retina, the light-sensitive layer. The retina is very thin, about 0.2 mm in man.

Cornea
The cornea provides a transparent protective coating for the optical structures of the eye. It covers the pupil (black central part) and the iris (coloured margin) of the central part of the eye. Its lateral margin fuses with the conjunctiva, a specialized epithelium covering the "white" (sclera) of the eye. Although the conjunctiva and sclera have a blood supply, the cornea is normally not vascularized. In certain pathological conditions it may become vascularised and this interferes with vision.
Pupil and Iris
Just behind the cornea is the fluid-filled anterior chamber, which is bounded posteriorly by the iris and the its central aperture, the pupil. The muscles in the iris vary the size of the pupil. The stroma of the iris contains melanocytes that reflect or absorb light to give the iris its characteristic color. Also embedded in the stroma are the circumferentially organized sphincter muscle of the iris and the radially arranged dilator muscle. The iris sphincter is innervated by cholinergic parasympathetic axons. Contraction of the sphincter muscle results in a decrease in diameter of the pupil (miosis). The action of the iris sphincter muscle is opposed by the dilator muscle. Within the iris, sympathetic nerve endings release norepinephrine onto the radially arranged muscles, and their contraction results in pupillary enlargement (mydriasis). Normally an increase in light level produces a decrease in pupil diameter and a decrease in light causes an increase in pupil diameter. This is the pupillary light reflex. However, excitment, pain, or fear usually results in an enlargement of the pupil in the absence of a change in lighting conditions.
The circumference of the pupillary margin changes by a factor of six, a change in muscle length greater than any other in the human body. To accomplish this change, acetylcholine is released onto both the sphincter and dilator muscles. The effect is to activate muscarinic receptors that depolarize sphincter muscle cells and cause contraction. Additionally, acetylcholine released onto the dilator muscle mediates presynaptic inhibition of noradrenaline release and blocks dilator contraction. Thus, as the sphincter contracts, the dilator relaxes, strengthening the pupillary response to light.
Lens
Like the cornea, the lens is transparent and composed of collagen fibres. It acts as a fine control for the focusing of light on the retina. This process of focussing is calledaccommodation. Note that most of the focussing of light by the eye is done by the cornea. This is because the bending of light at an interface depends on the change in refractive index at the interface.The difference in refractive index at the air/corneal junction is much greater than the difference at the lens/ aqueous humour interface.
Opacities in the lens, known as cataracts, are relatively common and can be seen as a cloudiness of the lens. Cataracts may be caused by congenital defects, persistent exposure to ultraviolet light, or other poorly understood mechanisms. Most cataracts however occur as part of the aging process. Current therapy consists of replacement of the lens with an inert plastic prosthesis, restoring sight but with a concomitant loss of accommodation.
Aqueous humour
The second fluid-filled space, the posterior chamber, is bounded anteriorly by the iris and posteriorly by the lens and its encircling suspensory ligament (zonule fibers). The anterior and posterior chambers are filled with a clear fluid , the aqueous humour. Fluid is continuously produced by the epithelium over the ciliary body and flows through the pupillary opening into the anterior chamber. It then drains into a set of modified veins, the canals of Schlemm, that are located around the rim of the anterior chamber in theangle of the eye where the iris meets the cornea. The aqueous humour is replaced every two to three hours. Regulation of the production and reabsorption of the aqueous humour determines the intraocular pressure, normally between 15 and 20 mmHg. This pressure maintains the shape of the eye. If the reabsorption is compromised, intraocular pressure increases. Because the suspensory ligament encircling the lens consists of discrete strands, the fluid in the posterior chamber is in contact with the vitreous body, the gelatinous mass that fills the main space of the eyeball between the lens and the retina. If untreated, increased intraocular pressure can cause blindness by restricting blood flow to the retina, a condition known as glaucoma.
The posterior aspect of the eyeball contains the vitreous body, a jelly-like substance that helps preserve the shape of the eyeball and thus maintain its focussing precision. Between the vitreous body and the choroid is the retina.


Anatomy and Physiology of the Retina
General
The inner surface of the posterior aspect of the eye is covered by the retina, which is composed of the neural retina and the retinal pigment epithelium. When describing the layers and cells of the retina, it is common to use the tenns inner and outer. Inner refers to structures located toward the vitreous and the center of the eyeball, whereas outer is used in reference to structures located toward the pigment epithelium and choroid.


The retinal pigment epithelium is a continuous sheet of pigmented cuboidal cells bound together by tight junctions that block the flow of plasma or ions. It (a) supplies the neural retina with nutrition in the form of glucose and essential ions, (b) protects retinal photoreceptors from potentially damaging levels of light, and (c) plays a key role in the maintenance of photoreceptor anatomy.
The neural retina contains the photoreceptors and associated neurons of the eye and is specialized for sensing light and processing the resultant information. Thephotoreceptors absorb quanta of light (photons) and convert this input to an electrical signal. The signal is then processed by retinal neurons as discussed below. Finally, the retinal neurons called retinal ganglion cells send the processed signal to the brain via axons that travel in the optic nerve.
The contact between the neural retina and the pigment epithelium is mechanically unstable. This instability is clearly demonstrated in a detached retina when the neural retina tears away from the pigment epithelium. Because photoreceptors are metabolically dependent on their contact with pigment epithelial cells, a detached retina must be repaired to avoid further damage. The detached part of the neural retina is welded to the pigment epithelium using surgical procedures. Although this repair prevents an increase in the area of detachment, the detached portion of the retina does not regain function.
The photoreceptor outer segments interdigitate with the melanin-filled processes of pigment epithelial cells. These processes are mobile, and they elongate into the pigmented layer when the light entering the eye is bright (photopic conditions) and retract when the light is dim (scotopic conditions). This mechanism combines with contractions of the iris to protect the retina from light conditions, which would otherwise damage the photoreceptors.
The part of the retina with the best acuity (ability to see the fine detail in an image) is the fovea centralis, which is in the centre of the macula lutea (yellow spot).
In the visual practical, you will use, or have used, an ophthalmoscope to peer into someone's eye to observe that person's
retina. You will have seen the blood vessels and made a sketch of the arrangement. The retinal surface is the only place in the body where blood vessels can be viewed directly as assessed for pathological change such as those that occur in hypertension or diabetes (see later).
Several landmarks are visible:

Normal retina as viewed through an ophthaloscope.
The margin of the optic disc is well delineated (except nasally/medially) and the optic cup, which is paler in colour, is located
within the disc. The macula is the exact centre of the eye in the visual axis and is located temporally to the optic disc. Its pigment is darker than the rest of the retina and it contains the fovea in its middle. The fovea is the area of highest visual acuity. There are usually no deposits in the retina.
Blood supply to the retina
Many clinical problems associated with the eye are due to deficits or derangement of its blood supply. Diabetics often have severe visual problems in middle age due to changes in the choroid capillaries.
The eye is copiously supplied with blood. The ophthalmic artery is a branch of the internal carotid, which enters the orbit via the optic foramen along with the optic nerve. A branch enters the optic nerve and forms the central artery of the retina, which supplies the inner layers of the retina. These vessels can be seen with an ophthalmoscope. The ophthalmic artery also forms ciliary arteries which enter the back of the eye, and then branch profusely in the choroid layer of the eye (between the sclera and the pigment epithelium) to form a dense capillary plexus behind the receptors. Anteriorly, the choroid merges into the ciliary body, which contains the ciliary muscle.
Receptor function
Rod cells are named for the shape of their outer segment, which is a membrane-hound cylinder containing hundreds of tightly stacked membranous discs.The rod outer segment is a site of transduction. Photons travel through cells of the neural retina before striking the membranous discs of the rod outer segment. Molecules of rhodopsin within these membranes undergo a conformational change and along with transducin and phosphodiesterase (PDE) induce biochemical changes in the rod outer segment, which reduce levels of cyclic GMP (cGMP).
In the dark, cGMP levels in the rod outer segment are high. This cyclic GMP keeps sodium channels in the membranes open and allows a constant sodium current to flow into the membranes. this is called the dark current. The high resting level of sodium permeability results in a relatively depolarised positive resting potential for rod cells, about - 40 mv. Light reduces the cGMP and therefore the sodium channels of the outer segment membrane close. The rod cell is hyperpolarized (i.e. brought towards a more normal neuronal resting potential of –70 mv) in response to a light stimulus.
The dark current does not fill the cells with sodium as this ion is constantly pumped out of the cell in the region of the cell body. Thus a constant circulation of sodium occurs in the cells,a s shown in the diagram.
The hyperpolarization of the rod outer segment propagates passively (i.e., without firing an action potential) through the perikaryon to the rod presynaptic ending. In the absence of light, the photoreceptor terminals constantly release the transmitter glutamate at these synapses. The arrival of a light-induced wave of hyperpolarization causes a transientreduction in this tonic release of glutamate. A similar sequence of events happens in cones, but each cone contains not rhodopsin but one of three isoforms of the cone receptor protein, iodopsin.
Rhodopsin molecules are capable of a large but finite number of photoisomerization events. Rather than replace individual rhodopsin molecules, the distal one-tenth of the outer segment is broken off and phagocytosed by the pigment epithelium. Through this process of rod shedding, the outer segment is constantly renewed. New discs are formed at the base of the outer segment and move outward so that the shed discs are replaced. In this way, the rod remains a constant length, and the outer segment is renewed about every 10 days.
The constant need for manufacture of new receptor protein and membrane plus the constant demand for sodium pumping to sustain the dark current means that photoreceptors have one of the highest metabolic rates of any cell in the body.
The receptors need so much oxygen that they have to be very close to a dense capillary plexus. Light cannot get through such a plexus, due to the amount of opaque blood present. Therefore, the blood supply must be on the opposite side to the receptors as the light input. This is why the receptors are at the outside of the retina; so that they can be close to the choroid. Light has to pass through the ganglion and bipolar cells of the retina to reach the receptors. However, the bipolar and ganglion cell layer is very thin , and these cells have much the same refractive index as the vitreous humour, so the light passes through them without distortion. The ganglion cells need much less oxygen that the receptors, so that the vessels that supply them (the vessels visible with an ophthalmoscope) are relatively small, and can be kept away from the point of maximum acuity at the fovea.

The pigment epithelial cells have a very important function in removing the membrane debris that is constantly being shedded from the outer parts of the rods and cones. If these cells do not function properly, old membrane fragments build up and push the receptors away from the choroid. This reduces the diffusion of oxygen and the receptors gradually die from lack of oxygen. This illness is called retinitis pigmentosa, as the retina can be seen to have coloured clumps of pigment (old receptor debris) when examined with an ophthalmoscope.
The presynaptic ending of the photoreceptor is very large, and the synapse between the receptor and bipolar cell is called a ribbon synapse. Processes of horizontal cells interdigitate between the presynaptic receptor terminal and the bipolar cell dendrite. (see pictures of retina above) This enables the horizontal cells to modulate transmissionfrom the receptor to bipolar cell.
Some of the bipolar cells are depolarised by the glutamate from the receptors (i.e. work normally) and others are hyperpolarised. {The glutamate produces different effects in the two kinds of bipolar cell by gating different ion channels. Bipolars hyperpolarised by glutamate may have potassium channels opened by the glutamate.} Thus, in the dark, some bipolar cells are tonically depolarised by the steady efflux of glutamate, some are hyperpolarised. When light reaches the receptor membrane, the receptor becomes more polarised, i.e. its membrane potential moves towards a more ‘normal’ value of -70 mv. This stops the steady efflux of glutamate, and so the bipolar cells that were depolarised become less so, while those that were hyperpolarised become less so. (One way to understand this is to think of dark as the stimulus, and light as the non-stimulus condition.)
Receptive fields of bipolar and ganglion cells
As mentioned above, All receptors are hyperpolarised by light; Some bipolars are depolarised and some hyperpolarised by light. The bipolar cells have no action potentials. Changes in light intensity produce continuously variable changes in bipolar transmitter release. The complex ribbon synapse between the receptor and bipolar cell not only produces an inversion of signal polarity for some bipolars, it also provides bipolar cells with an inhibitory surround. Bipolar cells have a central receptive field consisting of at least one, and normally several rods or cones, even at the fovea. In the peripheral retina they may receive input from many hundreds of cones. Consider a non-inverting bipolar cell. Light falling on any of the cones which make up the centre of its receptive field will hyperpolarise the cell. But, via the horizontal cells, light falling on cones outside the centre field will depolarise it. This is like surround inhibition in the dorsal column nuclei, but without action potentials.
Changes in the polarisation of the bipolar cell spreads across the entire cell, including its output end, and modulates the efflux of transmitter at this output end. Bipolar cells are excitatory, and release glutamate onto the retinal ganglion cells. This synapse is modified by the action of amacrine cells.
Retinal ganglion cells do have action potentials. Their axons project in the optic nerve to the lateral geniculate nucleus of the thalamus. The modification of the inputs from the receptors by the successive layers of horizontal cells and then amacrine cells means that the retinal ganglion cells have complex "centre-surround" receptive fields.
Colour vision
There are three types of cones, each tuned to a different wavelength. L-cones (red cones) are sensitive to long wavelengths, M-cones (green cones) to medium wavelengths, and S-cones (blue cones) to short wavelengths. Because any pure color represents a particular wavelength of light, each color will be represented by a unique combination of responses in the L-, M-, and S-cones.
If one of these cone types is absent because of a genetic defect in the corresponding opsin, the individual will confuse certain colors that look different to normal individuals and is said to be "color blind." Because the genes for the L-cone (red-absorbing) and M-cone (green-absorbing) opsins are located on the X chromosome, color blindness is more common in men. Alteration of the gene for the S-cone (blue-sensitive) pigment, which is located on an autosome, is much rarer. The inability to detect a pure red is known asprotanopia, and inability to detect green is known as deuteranopia.

The visual pathways from retina to visual cortex
Lateral Geniculate Nucleus (LGN).
The cells in the lateral geniculate nucleus (in the thalamus) receive synapses from the retinal ganglion cells and relay this information to the visual or "striate" cortex (area 17). There is a very specific overlay of projections from the two eyes, so that the left lateral geniculate receives information from the right visual field, whereas the right lateral geniculate receives information from the left visual field.
The LGN receives a retinotopic map, or in other words there is a point-to point projection from the retina to the LGN. The LGN is arranged in 6 layers. Each eye projects to an alternate layer. The ipsilateral eye projects to layers 2, 3, and 5, whereas the contralateral eye projects to layers 1, 4 and 6.
When you are looking at a distant object with your eyes parallel, then the projections (maps) from the two eyes to adjacent layers of the LGN should exactly match and the two images fuse to form a single stereoscopic image in your visual cortex.

There is also a pathway from the retina to the superior colliculus, which mediates control of involuntary eye movements like tracking a moving object, or fixation of gaze on a fixed object.

The pupillary light reflex

Information goes in on cranial nerve II (optic nerve)and comes out on cranial nerve III (oculomotor nerve). This is a consensual (both eyes are involved) parasympathetic reflex. If only the eye illuminated constricts then there is damage to the crossing fibres, i.e. damage in the midbrain.
Effect of various lesions in the visual pathway on visual fields

Possible sites of injury to the visual pathways are shown in the figure above. The effects produced correspond to the numbers in the following list.
Lesions Field defects
1 Partial optic nerve Ipsilateral scotoma
2 Complete optic nerve Blindness in that eye
3 Optic chiasm Bitemporal hemianopia
4 Optic tract Homonymous hemianopia
5 Meyer’s loop Homonymous upper quadrant anopia
6 Optic radiation Homonymous hemianopia
7 Visual cortex Homonymous hemianopia
8 Macular cortex Central scotomas (bilateral)
(A scotoma is a patch of blindness.)
Notes on visual field defects


1. Eccentric lesions of the optic nerve produce scotomas in the nasal or temporal field of the affected eye. When a young adult presents with a scotoma, multiple sclerosis must always be suspected.
2. Compression of the middle of the chiasm is most often caused by an adenoma (benign tumor) of the pituitary gland.
3. Lesions of the optic tract are rare. Although homonymous (matching) visual fields are affected, the outer, exposed half of the tract tends to be more affected than the inner half, and the hemianopia is then described as incongruous. Meyer’s loop may be selectively caught by a tumor in the temporal lobe.
4. Lesions involving the optic radiation include tumors arising in the temporal, parietal, or occipital lobe. The visual fields of both eyes tend to be affected to an equal extent (congruously). Tumors impinging on the radiation from below produce an upper quadrant defect at first whereas tumors impinging from above produce a lower quadrant defect. The stem of the radiation occupies the retrolentiform part of the internal capsule and is often compromised for some days by oedema, following hemorrhage from a branch of the middle cerebral artery
5. Thrombosis of the posterior cerebral artery produces a homonymous hemianopia. The notches in chart no. 7 represent macular sparing. Sparing of the macular hemifields is inconstant.
6. Bilateral central scotomas are most often caused by a backward fall with occipital concussion.

Pathological conditions that affect the optic disk
1. Glaucoma. Image shows optic disk atrophy and cupping in glaucoma.

A larger (> 50%) and deeper optic cup than normal suggests glaucoma. Glaucoma is the most common cause of blindness. It is a result of a build up of aqueous humour in the anterior eye chamber due to decreased drainage into the scleral venous sinuses
that encircles the anterior chamber. This causes intraocular eye pressure to become abnormally high. The fluid compresses the lens into the vitreous humour thus compressing the retinal neurons and blood supply, producing an ischaemia of the eye.
Persistent pressure results in a progressive visual impairment from mild to irreversible. Intraocular pressure can be relieved by topical administration of b adrenergic antagonists that reduce the production of aqueous humour by the ciliary processes in theciliary body.
2. Papilloedema Image showing a swollen optic disk and blood vessel constriction.

This is swelling of the optic disk (papilla) due to an increase in CSF pressure, raised intracranial pressure, which slows the venous return from the retina by compressing the optic nerve. It is the most important clinical sign of raised ICP that can be seen during routine neurological examination of the eye, and is often bilateral. The increase pressure also constricts the retinal ganglion cell axons like a tourniquet, which can lead to blindness, if left untreated. A swelling is produced at the point of constriction so that the disk margin becomes blurred and the optic cup is lost. As the pressure increases the papilla may
protrude into the eye. It can also be due to inflammation of the optic nerve head (papillitis) in which case the papilloedema will be unilateral.