Lecture 24 - Anatomy and Physiology of the Eye.pdf

Neurophysiology of Vision The Eye • The cornea is a thin, transporting epithelium that is devoid of blood vessels and has a cell structure to maintain its high transparency • The lens has closely packed columnar cells that are arranged in concentric shells and sheathed by a thin, tough, transpar...

Neurophysiology of Vision The Eye • The cornea is a thin, transporting epithelium that is devoid of blood vessels and has a cell structure to maintain its high transparency • The lens has closely packed columnar cells that are arranged in concentric shells and sheathed by a thin, tough, transparent capsule that is composed of epithelial cells • The eye is a camera • The aqueous humor is a fixed lens • • The Iris acts as an aperture The lens of the eye has a variable focal length as determined by the ciliary muscles • The retina is a photosensitive surface • The optic nerve carries the electrical signal to the brain for interpretation • The tarsal or Meibomian Glands secrete a fluid to facilitate the movement of the eyeball within its orbit The lacrimal apparatus continuously bathes the eye to remove airborne debris from the surface and protect the eye from infection. Two Compartments Aqueous compartment • Bounded by cornea and suspensory ligaments. Vitreous compartment • From suspensory ligaments to contain the vitreous body Anterior & Posterior Chambers Anterior chamber • From behind the cornea to the anterior surface of iris. • Filled with aqueous humor Posterior chamber • From behind the iris to the suspensory ligaments • Filled with aqueous humor anterior posterior Circulation of fluid in the eye The intraocular fluid circulates freely in the anterior compartment The posterior compartment is filled with the vitreous humor, a fine fibrillar meshwork of macropolyglycans through which this intraocular fluid slowly diffuses VISUAL TRANSDUCTION The Electromagnetic Spectrum The vertebrate eye has two major components: (1) an optical part to gather and focus light and form an image (2) a neural part (the retina) to convert the optical image into a neural code Optical Components of the Eye A ray of light entering the eye passes through transparent elements to reach the retina: a thin film of tears >> the cornea >> the aqueous humor>> the lens >> the vitreous humor Tears are made of plasma ultrafiltrate • bathe the cornea, keep it wet, and allow oxygen to diffuse from the air to the corneal cells • contain lysozymes and antibodies to counter infection • a superficial oily layer that slows evaporation and prevents spillage at the lid margins • help flush away foreign substances n The light must be focused to generate a clear optical image on the retina accomplished by the cornea and the lens n Focusing requires that the path of the light is bent, or refracted Refraction can occur when light passes from a medium in which it travels relatively fast into a medium in which it travels relatively slowly, or vice versa In the eye, most of the focusing takes place at the interface between the air and the tear-covered anterior surface of the cornea • To focus on objects the eye needs to increase its focal power (accommodation) • Accommodation is achieved by changing the shape of the lens • At rest, the lens is suspended around its edge by elastic zonal fibers that keep its capsule stretched and relatively flattened • To accommodate, the ciliary muscle fibers contract and release some of the tension in the zonal fibers. Relieved of the radial pull of its fibers, the lens becomes rounder • This increased curvature means increased focal power and a shift of the focal point closer to the eye • With age, the lens becomes stiffer and less able to round up and accommodate • The loss of accommodation with age is called presbyopia • Astigmatism results from uneven curvature of the refractive surfaces of the eye • As a result, a point source of light cannot be brought to a precise focus on the retina >> diffuse focusing causes blurring of the image The retina • Incoming light reaches the photoreceptor cells in the posterior retina (rods and cones) only after passing through several thin, transparent layers of other neurons • The pigment epithelium absorbs the light that is not absorbed by the photoreceptor cells and thus minimizes reflections of stray light. n n Photoreceptor cells and other neurons communicate by graded synaptic potentials that are conducted electrotonically. The ganglion cells (CNII) communicate with the thalamus by sending action potentials down their axons • The retina is a 200 μm thick sheet of tissue that lines the back of the eye and contains the light-sensitive cells, the photoreceptors • Photoreceptors capture photons, convert their light energy into chemical energy, and generate a synaptic signal for relay to other visual neurons in the retina • The thinness of the retina makes signalling distances very short, thus synaptic potentials can spread effectively within its neurons without the help of action potentials • Only the ganglion cells use action potentials to speed visual information towards thalamus Photoreceptors: Rods & Cones • There is one type of rod, which is responsible for monochromatic dark-adapted vision, and three subtypes of cones, which are responsible for the color-sensitive vision • The central area of the retina is a small pit, called the fovea, which collects light from the center of the gaze • Most foveal receptors synapse on only one bipolar cell, which synapses on only one ganglion cell >>> Because each ganglion cell is devoted to a very small portion of the visual field, central vision has high resolution Scanning electron micrographs of the primate fovea centralis (A) and of inner and outer segments of photoreceptors (B) Foveal retina • False-color scanning electron micrograph of the human retina featuring the central fovea, a crater-like depression in the photosensitive layer of the eye. • The foveal retina is the area of greatest visual acuity and contains only cone receptor cells. Rods are more numerous than cones by a ratio of about 20:1. However, rods have relatively poor spatial and temporal resolution of visual stimuli, and they do not detect color. Their main function is for vision in low-level lighting conditions, where they are far more sensitive than cones. In normal daylight the response of most rods is saturated. Cones are less numerous overall, but they are much more highly represented in the fovea, where visual acuity is highest. Cones have relatively high spatial and temporal resolution, and they detect colors The photoreceptors form the outermost layer, farthest from the lens. Therefore, light must traverse the entire thickness of the retina to reach them. In the fovea, however, the other layers of the retina are not present, allowing light to reach the photoreceptors without distortion Comparison of Receptive Fields in the Fovea & Periphery of the Retina Photoreceptors of the Retina The outer segments contain stacks of membranes, where the lightabsorbing molecules are found Pigmented epithelia of retina: •Phagocytose apical parts of cones and digest. •Synthesize melanin •Transport and esterify vitamin A. •Active in ion transport away from cones to assist in potential difference. Note apical invaginations of pigmented epithelia to receive apical portion of cones Signal Transduction1 A. A brief flash of light (photons) hyperpolarizes the photoreceptor cell. The change in Em and duration of the receptor potential increase with the increasing intensity of the flash. At high intensities, the peak response saturates, but the plateau becomes longer. B. In the absence of light, Na+ enters the outer segment of the rod through cGMP-gated channels and depolarizes the cell. The electrical circuit for this dark current is completed by K+ leaving the inner segment. The dark current, which depolarizes the cell, leads to constant transmitter release. C. In the presence of light, Na+ can not enter the cell because cGMP levels are low, and the cGMP-gated channel closes. The photoreceptor cell hyperpolarizes, and transmitter release decreases. Signal Transduction2 The apical portion of photoreceptor cell membranes have high concentrations of a family of integral proteins opsins, the membrane spanning helices of which contain retinal. Signal Transduction3 Each rod contains about 109 rhodopsin molecules >>> This density ensures an optimized capture rate for photons passing through a photoreceptor Rhodopsin has two key components: retinal (500 Da) and opsin (41 kDa) Retinal has an unstable 11-cis retinal form >>> The cis form sits within a pocket of the opsin Due to its instability, the cis form can exist only in the dark. If 11-cis retinal absorbs a photon, it isomerizes to all-trans retinal. This isomerization triggers a series of conformational changes in the opsin that lead to a form called metarhodopsin II Signal Transduction4 photon •The excitation of 1 electron by 1 photon on rhodopsin is sufficient to change isomers, i.e. generate neuronal response. •Excitation of ~ 6 rods for sensation of sight. Signal Transduction5 • After rhodopsin absorbs a photon of light, it activates many transducins • The activated a subunit of transducin (Gat) dissociates from the bg subunit • Gat binds to and activates phosphodiesterase (PDE) • PDE hydrolyzes cGMP • The resultant decrease in [cGMP]i closes cGMP-gated Na+ channels and produces a hyperpolarization (receptor potential). Signal Transduction6 • The activated subunit of transducin (Gat) activates phosphodiesterase, which hydrolyzes cGMP. • The resultant decrease in [cGMP]i closes cGMPgated Na+channels and produces hyperpolarization (receptor potential). • Non-selective cation channels conduct both K+ and Ca2+. The Ca2+ influx acts as negative feed back by • inhibiting guanylyl cyclase • Stimulating PDE • Thus Non-selective cation channels prevents “run away” in light and “primes” the system in dark. Adaptation to a Wide Range of Light Levels • The size of the pupil by the iris can change light sensitivity by about 16-fold • During dark adaptation, the first phase of adaptation is finished within 10 minutes by cones; the second takes ~ 30 min by the rods • A fully dark-adapted retina, relying on rods, can have a light threshold that is as much as 15,000 times lower than a retina relying on cones The Effect of Dark Adaptation on the Visual Threshold In bright sunlight, rods become ineffective because most of their rhodopsin remains inactivated, or bleached • After returning to darkness, the rods slowly regenerate rhodopsin and become sensitive once again • Color vision is possible only at relatively high light intensities, and depends on the different spectral properties of rhodopsins in three types of cones. • The absorbance peaks for the rhodopsins are at ~420 nm for S cones, ~530 nm for M cones, and ~560 nm for L cones Increasing diopters: Ability to focus at short distance increases