Eye - Anatomy & Physiology
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The eye is a paired organ, the organ of vision. The eye is made up of various components, which enable it to receive light stimuli from the environment, and deliver this stimuli to the brain in the form of an electrical signal. Vision involves all components of the eye.
The eye is contained within the bony orbit of the head. The bony orbit is a cavity, comprising parts of the lacrimal bone (includes fossa for nasolacrimal duct) and the maxilla (includes caudal foramen of infraorbital canal). It is continuous with the temporal bone and the pterygopalatine fossa caudally. The bony orbit is located laterally in herbivores, but is set forward in hunting animals, that is carnivores.
Movement of the eyeball is achieved through the contraction and relaxation of six extraocular muscles:
- Dorsal rectus muscle
- Ventral rectus muscle
- Medial rectus muscle
- Lateral rectus muscle
- Dorsal oblique muscle
- Ventral oblique muscle
The Transparent Media (conjunctiva, cornea, lens, and vitreous and aqueous humour)
- Conjunctiva: it is continuous with the skin of the eyelids. The palpebral conjunctiva is the part of the conjunctiva that covers the inner surface of the eyelid, the bulbar conjunctiva covers the surface of the eyeball. It is lined by stratified squamous epithelium, and contains goblet cells, which secrete the deepest, mucus, layer of tear film, which adheres to the surface of the globe. It is highly vascular.
- Cornea: layers of the cornea:
2. Bowman’s membrane (basal lamina)
3. Thick transparent fibrous layer
4. Descemet’s membrane
5. Endothelium (inner lining of the cornea)
The level of hydration of the cornea is critical for transparency. It is avascular, so nutrients and oxygen are obtained from the aqueous humour, and oxygen is also obtained from air. The cornea joins with the sclera at the point of the limbus; the sclera is vascular, but otherwise similar to the cornea.
- Aqueous humour: produced by ciliary processes of ciliary body. It provides nutrients for lens and cornea. It also maintains intraocular pressure (25mm.Hg), and is replaced several times a day (2µl/min). An increase in intraocular pressure can cause glaucoma.
- Lens: a derivative of the optic placode. Lens fibres are live cells, shaped in an onion structure. It is lined rostrally by cuboidal epithleium. It has a capsule, with rostral and caudal sutures, with a softer cortex and a firmer nucleus. It has no blood vessels or nerves, so nutrients are obtained from the aqueous humour, by diffusion.
- Vitreous humour: secreted by the ciliary body up to the time of maturity. It is gelatinous, so has very loose connective tissue: contains water, hyalouronic acid and collagen. Pressure from the vitreous humour prevents retinal detachment. It supports the lens anteriorly and the retina posteriorly. It contains a hyaloid canal, which is a remnant of blood vessels during development.
The Wall (retina, uvea and sclera)
- Retina: the inner layer of the eyeball, it develops from the optic vesicle, which is an outgrowth of the diencephalon. It remains connected to the diencephalon via the optic nerve. It can be divided into two parts:
1. non-visual retina (lines the back of the ciliary body)
2. optic part of the retina
The non-visual part of the retina is lined by inner and outer single-layers of epithelium: the outer layer is pigmented, the inner layer is neural. This neural inner layer contains photoreceptors, interneurons, ganglion cells and associated stromal cells, called MÜLLER cells. Rods and cones are photosensitive receptor cells, and are found in the optic part of the retina: rods are mainly utilised at night, as they are highly sensitive receptors to black and white, while cones are mainly utilised during the day as they are used for colour vision. The fovea is an area of the retina that contains mostly cones, and in high numbers. Here, oxygen is obtained from the choroid (a pigmented layer that makes up part of the uveal tract - mentioned further down) by diffusion. In this region, there is one nerve fibre per cone.
Layers of the retina, from vitreous humour to choroid:
1. Inner limiting membrane
2. Optic nerve fibres and axons of ganglion cells
3. Ganglion cell bodies
4. Inner plexiform layer - synapse of bipolar cells with ganglion cells
5. Inner nuclear layer – nuclei of bipolar cells
6. Outer plexiform layer - synapses of photoreceptors with bipolar cells
7. Outer nuclear layer – photoreceptor cell nuclei
8. Outer limiting membrane
9. Photoreceptor layer – rods and cones
Line of Detachment
10. Pigment cells – retinal pigmented epithelium
- Uveal tract: a three-part physiological and pathological unit, positioned between the sclera and the retina:
1. choroid: pigmented, highly vascular layer, containing:
a. tapetum lucidum (inner layer, nearest retina): reflective and coloured, so increases sensitivity to poor light
b. vascular layer: nutritive
c. black/connective layer (outer layer, nearest sclera)
2. ciliary body: produces aqueous humour and vitreous humour, and is involved in lens accommodation, as its muscle fibres stretch the lens into a flatter shape, allowing distant vision
3. iris: vascular, coloured, and contractile for pupil size
- Sclera: continuous with the cornea at the point of the limbus. It is similar to the cornea, except that it is vascular, and has dense, irregular, fibrous connective tissue.
- Iridocorneal angle: also called the filtration angle, this is the acute angle between the iris and the cornea, which is at the periphery of the anterior chamber of the eye. Its purpose is to drain the aqueous humour. A wider angle allows for better drainage.
Around the Eye
|Dorsal Rectus|| Oculomotor nerve
|Elevates the eyeball|
|Ventral Rectus|| Oculomotor nerve
|Depresses the eyeball|
|Medial Rectus|| Oculomotor nerve
|Adduction of the eyeball|
|Ventral Oblique|| Oculomotor nerve
|Outward rotation of the eyeball|
|Levator Palpebrae Superioris|| Oculomotor nerve
|Elevates the upper eyelid|
|Lateral Rectus|| Abducens nerve
|Abduction of the eyeball|
|Retractor Bulbi|| Abducens nerve
|Retracts the eyeball|
|Dorsal Oblique|| Trochlear nerve
|Inward roation of the eyeball|
All of the extraocular muscles originate at the equator of the eyeball.
- Adnexa: accessory structures of the eye, including the eyelids and the lacrimal apparatus. It contains three layers of the tear film:
1. Deep mucous: from conjunctival goblet cells, adheres tears to the conjunctiva
2. Middle aqueous: from main and third eyelid lacrimal glands; it cleanses, oxygenates and fills optimal defects; it contains IgA (immunoglobulin A)
3. Superficial oily layer: from tarsal glands (modified sebaceous glands); prevents evaporation
- Air = 1
- Cornea, aqueous humour and vitreous humour = 1.33
- Lens = 1.42 (greater in the nucleus)
Accommodation is primarily achieved by the lens, which is stretched by rectus muscle fibres of the ciliary body, causing the optical power of the eye to increase. This allows the eye to maintain a clear focus on an object, especially as the animal moves nearer towards the object.
Autonomic Innervation of the Eye
Parasympathetic innervation to the eye is supplied by the oculomotor nerve (CN III). When parasympathetic innervation is predominant, it acts upon the circular muscles of the iris, causing constriction of the pupil.
Sympathetic innervation to the eye is via the cranial cervical ganglion. When sympathetic innervation is predominant, it acts upon the radial muscles of the iris, causing dilation of the pupil.
Rods and cones are the photosensitive receptor cells of the retina, that is the light-sensitive parts of the retina.
Rod cells are mainly used at night (scotopic vision), as they function at lower light intensity. Rods are so-named due to their cylindrical shape. They are positioned in higher numbers at the edges of the retina, so are also used in peripheral vision. Rod cells are a lot more sensitive to light than cone cells, meaning that they require less light to function than cones, this being the reason that they are the predominant cells providing visual information at night. However, many rod cells converge into one interneuron, meaning that signals are amplified, but visual acuity is not as great, so the visual information that is collected may be less distinct.
Rod cells contain a pigment called rhodopsin, which itself consists of a protein called opsin. Bound to opsin is a molecule called retinene - a derivative of vitamin A. Retinene exists in the form of cis-retinene, an aldehyde, when in the dark. When stimulated by light, the cis-retinene undergoes structural change to form trans-retinene:
Vitamin A (alcohol) → cis-retinene (aldehyde) → trans-retinene
This leads to hyperpolarisation of the cell.
When rods and cones are not being activated, they are in a state of depolarisation, and release a neurotransmitter – rods cells release glutamate, cone cells release acetylcholine. This state of depolarisation is achieved in darkness, because the cells have a high concentration of cGMP, which causes the opening of ion channels. Sodium and calcium ions pass through these ion channels, and its their positive charges that cause the depolarisation of the cell, which leads to the release of neurotransmitters. When light hits the rod and cone cells, the cis-retinene undergoes structural change to become trans-retinene (explained above). This structural change causes the activation of transducin, which is a regulatory protein. Transducin activates cGMP phosphodiesterase, which causes the break-down of cGMP into GMP. This action allows the sodium channels to close, thereby preventing influx of positive ions, which leads to hyperpolarisation of the cell, and therefore also preventing the release of neurotransmitters.
The breakdown of one molecule of rhodopsin leads to signal amplification, so more molecules of rhodopsin are activated, hence rod cells being highly sensitive to small amounts of light. However, when rod cells are exposed to high levels of light intensity for prolonged periods of time, they become desensitised.
Cone cells function better in higher intensities of light (photopic vision). Most cone cells are found concentrated in the fovea. The fovea is the region of the retina where the retinal layers are parted, allowing light to fall directly onto the cone cells. Cone cells provide greater visual acuity, as each cone synapses with a single interneuron, meaning that the visual signal is not amplified, therefore is more distinct.
Cone cells respond to light in the same way as rod cells. The only difference is that the pigment present in cone cells is iodopsin, as opposed to rhodopsin. Retinene that is bound to opsin within the iodopsin is stimulated in the presence of light to undergo structural change to form trans-retinene.
- Saccades: the rapid, involuntary eye movements that occur in both eyes simultaneously when changing the point of fixation. Can be up to 400°/s.
- Smooth pursuit movements, up to 30°/s.
- Vergence, which is limited by lens accommodation.
- Vestibular, up to 30°/s - 'Nystagmus': rhythmic, oscillating motions of the eye.
Determination of Distance
- Parallax: eyes see objects from slightly different aspects.
- Head movement: exaggerates the parallax effect - this is especially the case in cats.
- Vergence: the brain detects the degree to which eyes must cross.
- Overlay: determination of which objects lie in front of which.
Central Visual Pathways
The optic nerve (CN II) is a paired nerve that carries visual information from the retina to the brain. The ganglion cell axons leave the retina and information passes through the optic nerve to the optic chiasm, where some nerve fibres cross over. The optic tract (the optic nerve fibres) wraps around the cerebral peduncles of the midbrain, where it passes into the lateral geniculate nucleus, which is part of the thalamus. Most of the optic tract axons synapse here, and the remaining fibres branch off and synapse in the pretectal nuclei of the superior colliculi. The integrated visual information is then passed via nerve fibres to the cerebral cortex.
Reflexes with Optic Nerve as the Sensory Arm
1. Pupillary light reflex: the constriction of a pupil in response to increased light intensity, and a dilatation of a pupil in response to a decreased light intensity. This reflex also involves parasympathetic fibres of the Oculomotor nerve (CN III). The presence of this reflex shows the efficiency of the retina, the optic and oculomotor nerves, and the musculature of the iris.
2. Pupil dilation: also called mydriasis. It is a sympathetic response.
3. Menace response: involves the facial nerve (CN VII) in the motor arm of the reflex. The response also requires integration from the cerebral cortex, the cerebellum and the rostral colliculi. However, the menace response is a learned response, so it will not be present in the first few weeks of life.
- The cornea has an important role in image formation, it forms a primary refractive element in the eye.
- The anterior epithelium and anterior subepithelial basement membrane of the cornea are lined by non-keratinised stratified squamous epithelium.
- The posterior epithelium (corneal endothelium) and posterior limiting membrane (Descement's membrane) are lined by simple squamous epithelium.
- The corneal limbus is the corneo-scleral junction. Here, the collagen fibres of the corneal stroma become irregular, and blood vessels supplying nutrients to the cornea are seen. The anterior epithelium becomes the conjunctival epithelium.
- The sclera is made up of dense connective tissue, containing collagenous and elastic fibres.
- The iris is the most anterior part of the vascular tunic (uvea), a continuation of the choroid layer.
- The stroma of the iris is made up of vascularised loose connective tissue, surrounded by loose collagen fibres.
- Most of the ciliary body is made up of small amounts of loose connective tissue, which are situated between smooth muscle cells.
- The inner surface of the ciliary body is lined by columnar cells.
- The processes of the ciliary body contain a network of capillaries.
- Zonule fibres extend from the ciliary processes towards the lens, and form the suspensory ligament of the lens.
- The choroid is made up of loose connective tissue, which contains a network of blood vessels.
- Zonule fibres from the ciliary body insert into the lens.
- Lens fibres contain a cytoplasm that is filled with crystalline proteins. It is these proteins that allow for the transparency and refractive properties of the lens.
- Palpebral conjunctiva is lined by stratified squamous epithelium.
- Bulbar conjunctiva is lined by stratified columnar epithelium.
- Contains goblet cells.
- The bony orbit is continuous in herbivores, but is open laterally in carnivores. In the case of carnivores, the orbit is completed by the orbital ligament.
- The shape and size of the eyeball varies between species. In carnivores it is spherical, but in the horse, its width is greater than its height and length. In terms of relative body size, the cat has the largest eye, followed by the dog, the horse, the ox, and the pig has the smallest relative eye.
- The tapetum lucidum (within the choroid) is present in all domestic species except the pig.
- On the upper pupillary margin of the iris in the horse are irregular out growths, negri bodies. It is postualted that they may secrete aqueous humour.
Avians and Reptiles
- The avian and reptile iris is striated muscle.
- Avians and reptiles have scleral ossicles, just caudal to the limbus.
- The avian and lizard retina is avascular.
- The eye glands of many reptiles are excretory organs for salt.
- Some chelonians (for example, tortoise, turtle or terrapin) do not have eye glands.
- Some reptiles have what is termed a parietal eye, sometimes called their third eye. Although it is called an eye, the reptiles cannot actually see out of it, although it can be used to detect light and dark, meaning that is is photoreceptive. It is part of the epithalamus, and is associated with the pineal gland. It can be used to determine circadian rhythms. Circadian rhythms are the cycle of light and dark - these cycles have a big impact on feeding and sleeping habits, and they also have an impact on thermoregulation.
- As water has a refractive index similar to the cornea, aqueous humour and vitreous humour, most refraction is carried out by the lens itself, which often has an extremely high refractive index. The lens is often extremely spherical, and accommodation is effected by moving it rather than distorting it.
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