Blok 1.3 Jaras Penglihatan

Jaras penglihatan Kuswati Departemen Anatomi FK UII

NEURAL RETINA The neural retina is a multilayered

structure. These layers contain supporting glia (Müller cells) and astrocytes, as well as six types of neurons that process visual input prior to its transmission to the diencephalon. They are the photoreceptors (100–120 million rods and 6–7 million cones), the conducting neurons (bipolar cells and ganglion cells

The Cones The cones are activated by high

intensity light (photopic vision). They mediate sharp vision and color vision. Three types of cones are present in the retina: (1) blue cones, which are sensitive to blue light, (2) green cones, which are sensitive to green light, and (3) red cones, which are sensitive to red light.

The Rods Rods are of a single type only They are stimulated by low intensity

light; thus they transmit visual input in dim illumination (scotopic vision) but they cannot detect colors. They are more concentrated in peripheral regions of the retina. Because rods do not provide color vision, in dim light we can see only black, white, and all shades of gray in between.

Visual field The visual field of each eye is divided into two

regions: the nasal or central half and the temporal or peripheral half. For each eye, light rays from an object in the nasal half of the visual field fall on the temporal half of the retina, and light rays from an object in the temporal half of the visual field fall on the nasal half of the retina. Visual information from the right half of each visual field is conveyed to the left side of the brain, and visual information from the left half of each visual field is conveyed to the right side of the brain

Visual pathway The visual pathway consists of photoreceptors,

first order and second order neurons residing in the retina, and third order neurons in the lateral geniculate nucleus of the thalamus Transduction of light energy into a receptor potential occurs in the outer segment of both rods and cones The first step in visual transduction is absorption of light by a photopigment. Light absorption initiates the events that lead to the production of a receptor potential.

From photoreceptors, information flows through

the outer synaptic layer to bipolar cells and then from bipolar cells through the inner synaptic layer to ganglion cells.  The axons of ganglion cells extend posteriorly to the optic disc and exit the eyeball as the optic (II) nerve. The optic nerve is enveloped in a meningeal cover. The optic nerves of the right and left sides form an intersection of fibers, the optic chiasma

All ganglion cell axons arising from the temporal

half of the retina course in the lateral aspect of the optic chiasma without decussating, to join the optic tract of the same side. All ganglion cell axons arising from the nasal half of the retina decussate at the optic chiasma, and enter the optic tract of the opposite side, to join the temporal fibers.  Thus, each optic tract consists of ganglion cell axons arising from both eyes (the ipsilateral temporal half and the contralateral nasal half of the retina).

The fibers retain a retinotopic organization in

the optic tract as it courses around the cerebral peduncle to end and relay visual information primarily in the lateral geniculate nucleus (LGN) of the thalamus, which processes visual input. The optic nerve also ends and relays visual information in: (i) the superior colliculus, a mesencephalic relay nucleus for vision having an important function which control the extrinsic eye muscles.

(ii) the pretectal area, which mediates autonomic reflexes such as the control of pupillary constriction and lens accommodation (iii) the hypothalamus, which has an important function in circadian rhythms (day–night) and the reproductive cycle

The superior colliculus The superior colliculus has important functions in

the control of reflex movements that orient the eyes, head, and neck in response to visual, auditory, and somatic stimuli (tracking moving objects) via its outputs (efferent projections). Efferents from the superior colliculus include projections to the reticular formation, the inferior colliculus, the LGN and the pulvinar of the thalamus, the oculomotor, trochlear, and abducens nuclei via the medial longitudinal fasciculus (MLF), the pontine nuclei and the cerebellum via the tectopontocerebellar tract, and the cervical levels of the spinal cord via the crossed tectospinal tract.

Geniculocalcarine tract (optic radiations, thalamocortical projections) Axons of third order neurons originating

from the LGN form the geniculocalcarine tract (optic radiations, thalamocortical projections), which terminate in the primary visual cortex The geniculocalcarine tract fibers maintain retinotopic organization and form an upper, an intermediate, and a lower division. The upper division consists of fibers conveying information from the superior retinal quadrants.

The lower division consists of fibers

conveying information from the inferior retinal quadrants. The intermediate division of the geniculocalcarine tract consists of fibers relaying visual information to the primary visual cortex from the macular region of the retina

Visual cortex The neurons of the visual cortex respond to different visual

stimuli transmitted by neurons conveying color, motion, three-dimensional vision, or a combination of various visual stimuli. The LGN projects visual information to the primary visual cortex  The primary visual cortex projects to the secondary visual cortex where information is processed and subsequently relayed to the tertiary visual of the cortex (the middle temporal area) The tertiary visual areas function in identifying an object as well as in determining its location and color. The middle temporal area has an important function in detecting moving objects.

Visual reflex Pupillary light reflex Pupillary dilatation refleks Convergence accomodation reflex Corneal blink reflex

Pupillary light reflex The pupillary light reflex is an

autonomic response; it is mediated independently of cortical input. The pupillary light reflex enables the eye to adapt to varying light intensity, which protects the eye and also facilitates vision

Pupillary dilatation reflex Pupillary dilation is mediated by the

sympathetic division of the autonomic nervous system. Pupillary dilation occurs when sympathetic activity is dominant Neurons of the posterior aspect of the hypothalamus project to the ciliospinal center of the spinal cord located in the intermediolateral cell column of cord levels (C8) T1–T2

Here they synapse with preganglionic

sympathetic neurons. Preganglionic sympathetic fibers exit the spinal cord, enter the sympathetic trunk and ascend to the superior cervical ganglion where they synapse with postganglionic sympathetic neurons. Postganglionic sympathetic terminate in the dilatator pupillae muscle. Sympathetic innervation of this muscle causes it to contract, increasing the pupillary diameter, resulting in mydriasis

Convergence accomodation reflex The convergence accommodation reflex

alters the thickness of the lens, which facilitates the projection of a focused image on the retina convergence accommodation reflex, consisting of three reflex changes

1. Convergence. .The term convergence refers to this medial movement of

the two eyeballs so that both are directed toward the object being viewed, for example, tracking a pencil moving toward your eyes. The nearer the object, the greater the degree of convergence needed to maintain binocular vision. .This is mediated by bilateral contraction of the medial recti muscles, which are innervated by the oculomotor nerve (CN III). .This convergence permits the visual image to be projected and focused on the foveae of both retinas. .If this is not accomplished, the individual will experience diplopia (double vision).

2. Accommodation. Parasympathetic stimulation causes contraction of the ciliary muscle of the iris release the tension of suspensory ligaments of the lens it thickens (lens ccommodation), focusing the image on the retina. 3. Pupillary constriction

Corneal blink reflex When a foreign object contacts the eye, the

corneal blink reflex elicits a forceful blinking of both eyes to protect them from possible injury The corneal blink reflex consists of :  receptors (at the peripheral terminals of

pseudounipolar neurons in the ophthalmic division of the trigeminal nerve);  an afferent limb (peripheral processes of pseudounipolar neurons in the ophthalmic division of the trigeminal nerve);

 an integrator (spinal nucleus and main

sensory nucleus of the trigeminal nerve);  an efferent limb (facial nerve branches to the orbicularis oculi muscle);  an effector (orbicularis oculi muscle)