Visual Pathways PDF

Visual Pathways  Visual Pathway ▪ There are 4 neurons involved in the conduction of visual impulses to the visual cortex. ▪ 1.Neurons; Rod and Cone cells (specialized receptor neurons in the retina) ▪ 2. Neurons; Bipolar neurons (connect rods and cones to the gangli...

Visual Pathways  Visual Pathway ▪ There are 4 neurons involved in the conduction of visual impulses to the visual cortex. ▪ 1.Neurons; Rod and Cone cells (specialized receptor neurons in the retina) ▪ 2. Neurons; Bipolar neurons (connect rods and cones to the ganglionic cells) ▪ 3. Neurons; Ganglionic cells (the axons of the ganglionic cells pass to the lateral geniculate bodies) ▪ 4. Neurons; Neurons of the lateral geniculate bodies (axons of the lateral geniculate bodies pass to the cerebral cortex)  Object to be seen Peripheral Retina Central Retina (fovea in the macula lutea) 3 4  Ganglion cells axons form the optic nerve Bipolar cells Rods and Cones (Receptors) 5 Retinal layers Optic nerve Optic chiasma Optic tract Lateral geniculate body Optic radiation Visual area The axons of ganglion cells exit the eyes via the optic nerve, partially cross at the optic chiasm, and form two optic tracts, so that the right and left hemifields reach the left and right hemispheres. Each optic tract looks at the opposite hemifield, combining inputs from the ipsilateral temporal hemiretina and the contralateral nasal hemiretina.  Visual Field Representation Each eye sees a part of the visual space that defines its visual field. The visual fields of both eyes overlap extensively to create a binocular visual field. left eye both eyes right eye  Visual Fields: Temporal Field of Nasal Field of Left Eye Left Eye Binocular F F Binocular field combines the two Normal Monocular Visual Normal Monocular Visual Field of Left Eye Field of Right Eye monocular visual fields with the foveas (F) aligned with one another. (i.e. the ‘pink Left Visual Field Right Visual Field area’ in the image) Left Visual Field seen by both the left & Upper Fields right eyes. Monocular Monocular Crescent of Crescent of Right Visual Field seen by both the left & Left Eye F Right Eye right eyes. Lower Fields Monocular crescent for each eye (blue for left eye & green for right eye) is only seen by the nasal retina of the same eye. Normal Binocular Visual Field  Visual Pathway ▪ In a binocular vision each visual field, right and left are projected on portions of both retinae. ▪ The image of an object in the right field of vision is projected on the nasal half of the right retina and temporal half of the left retina. ▪ The fibers from the nasal half of the retina cross at the optic chiasma ▪ By this arrangement the whole right field of vision is projected upon the left hemisphere and vice versa. ▪ The upper field of vision projected on the lower wall of the calcarine sulcus, while the lower field of vision projects on the upper wall of the sulcus.  LGN has 2 types of main layers: ▪ Parvocellular layers ▪ Magnocellular layers V1 LGN Optic Radiation The LGN projections reach the primary visual cortex through the optic radiation. Axons carrying information about the superior visual field sweep around the lateral horn of the ventricle under the temporal lobe (Meyer’s loop). Those carrying information about the inferior retina travel under the cortex of the parietal lobe  The primary visual cortex (V1) has a representation of the contralateral visual hemifield. The foveal region is mapped in its most posterior part, whereas the more peripheral regions are mapped in progressively more anterior parts. The upper visual field is mapped below the calcarine fissure, the lower visual field above. Primary Visual Cortex Because of the high density of ganglion cells in the fovea, the visual cortex has an expanded representation of the fovea.  REFLEXES: Pupillary Light reflex ▪ Direct – light on the eye leads to pupil constriction ▪ Consensual – constriction of the pupil in response to light being shone on the other eye – contralateral constriction ▪ *works on unconscious people so a test for brainstem death*  Afferent limb = Pupillary Light Reflex Right Optic Nerve (SSA) Left Direct Consensual Reflex Reflex Efferent limb = Oculomotor Nerve (GVE) Postganglionic Preganglionic  Pupillary Light Reflex The pretectum bilateral projections to the Edinger-Westphal nuclei ensure that both eyes react to light: shining a light into each eye can elicit a direct and a consensual pupillary reflex. This light reflex tells us about one’s visual pathways status. direct consensual Accommodation Reflex ▪ 1. Contraction of ciliary muscles (Relaxation of suspensory ligament of the lens) ▪ 2. Contraction of constrictor pupillae muscle ▪ 3. Contraction of medial rectus (Convergence) Accommodation Reflex Cornea Reflex  REFLEXES: Corneal Reflex Blinking – elicited by sensory stimulation of cornea A direct and consensual response Sensory: CNV1 (ophthalmic division of trigeminal) Motor: CN VII (facial)  Pupillary Dilation (Mydriasis) Decreased light to pupil Cortex, Hypothalamus ? Thalamus & ? Reticular Severe pain (CNS control center Hippocampus for ANS) Formation Strong emotional stimulus Reticulospinal fibers Dilation Superior Preganglionic of pupil (post-ganglionic Cervical (pre-ganglionic Sympathetic Neurons sympathetic) Ganglion sympathetic) in Thoracic Cord (T1- T2) 30  Pupillary skin reflex ▪Pupilla is dilated if there is a painful stimuli to the skin ▪Afferent sensory fibers - efferent preganglionic sympathetic neurons in lateral gray columns of T1-2 – white rami communicantes pass to sympathetic trunk - preganglionic fibers ascend to superior cervical sympathetic ganglion -postganglionic fibers - internal carotid and long ciliary nerves - dilator pupilla Visual Body Reflex Lesions of the visual pathway  Seeing colour ▪ Each cone contains a different opsin protein ▪ Each opsin protein absorbs a different wave length of light preferentially ▪ Alterations in the photopigments within cones leads to colour blindness: ▪ Affect 8% of the population ▪ Protanopia – missing/abnormal L cone ▪ Deuteranopia – missing/abnormal M cone ▪ Tritanopia – missing/abnormal S cone ▪ Monochromacy – missing/abnormal all cone pigment Tritanopia Color Spectrum Protanopia Color Spectrum Blue-Yellow Color Blindness Red-Green Color Blindness Deuteranopia Color Spectrum Red-Green Color Blindness ▪ Light needs to be refracted in order to focus on retina ▪ The cornea , aqueous humor, lens and vitreous humor are all involved in refraction leading to image Refraction formation Emmetropia= normal eye Myopia: near sightedness ▪ ▪ 1] negative spherical lenses to diverge light so that image is Correcting pushed posteriorly on to retina myopia ▪ 2] flatten cornea to decrease refractive surface Hypermetropia: Far sightedness ▪ Eye lacks optical power Correcting ▪ positive spherical lenses to move image anteriorly onto retina hypermetropia  Astigmatism ▪ One\more axis differentially in focus. ▪ ie. One axis focused correctly on retina but other not [or both incorrect]. ▪ Cylindrical lens and\or Correcting converging\diverging lens for myopia\hypermetropia Astigmatism ▪ NOT classified as a refractory error. ▪ Occurs in those >40yrs ▪ loss of accomodation power of lens [ie. cannot adjust optical power of eye] because lens is stiffer and Presbyopia ciliary muscles don’t work as well ▪ Problems only in near vision because eye has to work harder and use muscles. ▪ Positive convex converging lenses Correcting [‘reading glasses’] presbyopia