General aspects Sensory receptors ; respond to changes in the environment. External or internal environment. A stimulus is a change in the environmental condition which is detectable by a sensory receptor energy: mechanical, chemical, radiant, electrical, etc. sensory receptor; transduce (convert) the stimulus into an action potential different strengths of response are transduced into different frequencies of action potentials. 1 Major structural layer of the wall of the eye is a thick layer of dense C.T.; that layer has two parts: Sclera; white in color; generally opaque. Cornea; the remaining front part of the eye; colorless and transparent. relatively thin flat, stratified squamous epithelium covers the cornea and sclera continuous with (essentially a part of) the epidermis of the skin corneal epithelium; covers the cornea conjunctiva; covers the sclera and the inside of the eyelids 2
At the interface between two media of different densities, a non-perpendicular light ray bends. Refraction. Light ray path is made more perpendicular to the interface. Corneal epithelium and cornea are stronger refractors than the lens of the eye. Once the light rays leave the posterior surface of the cornea they pass through the aqueous humor that fills the anterior chamber of the eye. Aqueous humor is a CSF-like liquid. 3 Iris and pupil Iris is flat, circular structure with a hole in its center called the pupil. Mostly loose C.T. no epithelium on the front (anterior) surface of the iris; rear surface has a heavily pigmented (with melanin) epithelium absorbs light rays Loose C.T. of the iris may have a scattered melanocytes; coloring the iris when melanocytes are absent or reduced the iris looks blue. 4
Iris contains two smooth muscles: pupillary constrictor; inner circular pupillary dilator ; outer radially-oriented smooth muscle fibers Contraction and relaxation by those two elements, controlled by the brain stem, control the diameter of the pupil this is mainly for regulating the brightness of the image on the retina. 5 Lens and ciliary muscle Light rays passing through the pupil enter the lens. The lens is mainly composed of very long, thin, transparent, colorless enucleated cells called lens fibers the lens fibers are oriented parallel to the light path through the lens. The lens is attached to the outer wall of the eye all around its circumference, by suspensory ligament and the ciliary body 6
In mammals, the sclera usually exerts a tension on the edge of the lens, through the ciliary body and suspensory ligament lens is flatter and has high diameter low additional light refraction, for focusing on distant objects. Ciliary body is composed mainly of a circular band of smooth muscle tissue called the ciliary muscle quickest-acting smooth muscle in the body controlled by the brain stem and cerebrum when a blurry image is perceived, the cerebrum sends a neural signal to the brain stem, which signals the ciliary muscle to change contraction. 7 8
When the ciliary muscle contracts it pulls the sclera inward, all around; this increases the degree of light refraction in two ways: Increases curvature of the cornea. It reduces tension on the suspensory ligaments of the lens capsule. Lens becomes rounded and thicker. Increases the curvature of the lens and increasing refraction. Greater light-refraction the eye can focus the light rays from a closer object. 9 Light rays leaving the lens pass through the vitreous humor of the posterior compartment of the eye, to the retina. The vitreous humor is a colorless, transparent gel. Light rays from the object are focused at the center of the back of the eye, on the part of the retina called the fovea; other part of the retina, where light rays are not focused, is called the non-foveal retina. 10
Retina Pigmented layer of the retina Is a simple cuboidal epithelium with high [melanin]. The function of this layer is to absorb light rays that pass through the inner, sensory layer of the retina, reducing back reflection which would cause perception of a blurry image 11 Sensory layer of the retina (see retina.pdf) Non-foveal retina (structures listed in order of penetration by light rays) Capillary network: inside surface of sensory layer is vascularized, mainly for providing 0 2, nutrients, and waste-molecule removal for inner layers of sensory layer of retina; tends to scatter light rays to some extent. Internal limiting membrane: the inside surface of the retina. 12
Nerve fiber layer Layer of parallel axons of retinal ganglion cells essentially white matter; the axons are oriented toward the point where the optic tract (nerve) begins 13 Ganglion cell layer layer of somas of retinal ganglion cells largest type of neuron in the retina size of the somas is not obvious, but the nucleus is relatively large and light staining nuclei are generally in a single layer and far from each other; these neurons are multipolar neurons. 14
Inner plexiform layer an eosinophilic layer that is mainly composed of the dendrites of the retinal ganglion cells, the axons of the retinal bipolar cells (small bipolar neurons), the synaptic junctions between these two types of neurons, and axons or dendrites of a third type of neuron, the retinal amacrine cell. Each bipolar neuron has synaptic junctions with several retinal ganglion cells. 15 Inner nuclear layer very basophilic layer composed mainly of the somas of the retinal bipolar cells, the retinal amacrine cells, and the retinal horizontal cells (a fourth type of neuron of the sensory layer) somas are very small, and the nuclei are very close together, small, and darkly stained. 16
Outer plexiform layer an eosinophilic layer, thicker than the inner nuclear layer, mainly composed of the dendrites of the bipolar cells, cell processes of the amacrine cells and horizontal cells, cell processes of the rod cells and cone cells, and synaptic junctions between these cell types. Each bipolar neuron has synaptic junctions with a few to several rod cells and/or cone cells. 17 Outer nuclear layer a very basophilic layer composed mainly of the somas of the rod cells and cone cells somas are very small and the nuclei are small, darkly-stained, and very close together; the layer is usually about twice as thick as the inner nuclear layer. 18
Rod cells and cone cells General aspects Rod cells and cone cells are photoreceptor cells; they are photosensory cells Rod cells: structure and function Rod cells are relatively long, narrow cells cell extension from the soma to the synaptic junction with bipolar cells is quite thin in the other direction from the soma is a thin cell extension to the rod part of the cell, which is of similar width as the soma 19 The rod-has two parts Inner segment of rod: contains nucleus, abundant Golgi and mitochondria. Energy (ATP) for action potentials Biosynthetic center (synthesis of rhodopsin) Rhodopsin is transported to outer segment Inner segment Outer segment 20
Outer segment of rod: consists of a cylindrical stack of interconnected disc-shaped photosensitive lamellae. These lamellae are produced at the base of the rod by inward extensions of the cell membrane that production is continuous, with a lamella slowly moving toward the end of the rod after it is produced, and then sloughing off from the end and phagocytized and destroyed by cells of the pigmented layer of the retina. 21 These disc-shaped membranes are filled with the rhodopsin, aka visual purple. Rhodopsin is a complex of a carotenoid called retinal (which is almost identical to Vitamin A) and a specific form of opsin opsins (or scotopsin) are a class of very similar proteins that are parts of the photopigment molecules of the sensory cells of the retina. When a light ray collides with a rhodopsin molecule the energy of the light ray causes a conformational change in the retinal part of the molecule the retinal dissociates from the opsin; as a result, the cell membrane of the entire cell experiences a change in its membrane potential. Later, the retinal and opsin are recombined via the expenditure energy from ATP 22
23 Bipolar cells (connected to rod cells) send impulses via the ganglion cells, the optic nerves and tracts to the lateral geniculate nuclei, then on to the visual cortex of the occipital lobe of the brain. 24
Cone cells: structure and function Cone cells are similar to rod cells in some ways structural differences include having no thin portion between the soma and the cone, having a wider inner segment of the cone, and having a wider, shorter, tapered and pointed outer segment of the cone. 25 Outer segment of the cone is composed of lamellae which contain one of three kinds of iodopsin photopigment (or photopsins) molecules. The three iodopsins have different abilities to absorb light rays of different wavelengths one of the three absorbs most effectively in the wavelength we perceive as blue, another in the wavelength we perceive as green, and the third in the wavelength perceived as red. Rao 2004 26
Thus the brain can receive an impulse rate ratio from the three cone cell types that provides information on the color of the object the perceived color depends, in addition, on additional steps of information-processing in the brain itself, including comparisons with the information received from adjacent parts of the image Rao 2004 27 Layer of rods and cones contains the rods of the rod cells and the cones of the cone cells. External limiting membrane outer surface of the sensory retina Supportive-cells of the sensory layer of the retina A cell type called the amacrine cell is abundant in the sensory layer also other supportivecells may be present. 28
Light-scattering by layers inside the layer of rods and cones Light rays must pass through all of these layers before reaching the layer of rods and cones although these layers are almost perfectly transparent, a small proportion of the light rays are scattered as they pass through the layers, perceived image is not as sharply focused. 29 Foveal retina Fovea is a small part of the retina in the middle of the back of the eye provides an image of high visual acuity due to the high density of photoreceptors in that area. Also, other layers pushed off to the side 30
Fovea is cone cells only. Compared to rod cells, cone cells have a much lower maximum sensitivity to light and whereas rod cells can respond to light at very low levels, cone cells can not. In very dim lighting conditions we are seeing with our rod cells only; fovea is non-functional; perceived scene is colorless. Foveal retina provides much greater visual acuity than non-foveal retina but provides much lower visual sensitivity than non-foveal retina. 31 Choroid layer of the wall of the eye Choroid layer lies immediately adjacent to the retina, and thus between the retina and the sclera. Choroid layer is mainly composed of loose to dense C.T. moderately well vascularized, for providing 0 2, nutrients, etc., for the outer layers of the retina contains melanocytes for absorption of light rays coming from either direction. Sclera: a thick layer of relatively non-cellular dense C.T. 32
Optic disc ("blind spot"): the part of the back of the eye where the optic tract begins. Our slides are sectioned to include this part of the eye. At this location, all of the axons of the ganglion cells of the retina converge and leave the eye, composing the optic tract. 33 This results in a circular area where the other components of the sensory layer of the retina, as well as other layers of the wall of the eye, are absent that circular area has a diameter of about 1.5 mm, and it is called the optic disc, or "blind spot" the portion of the image that is projected onto the optic disc therefore can not be perceived--that part of the back of the eye is "blind", so to speak. Normally one is not aware of not being able to see part of the image with that eye, because in the other eye that part of the image is being projected onto a different part of the back of the eye and therefore onto fully-functional retina. 34