OPTIC PATHWAY Physiology Lecture Slideshow

Introduction

• Visual processing occurs in
• Retina
• Visual Pathway
• Visual Cortex
• Visual pathway extends from the 2 retinas to the visual
  cortex
• First order neuron of the visual system is a bipolar cell
  entirely within the retina

Optic Nerve

• Visual signals leave through the optic nerves
• Nerve fibers extends from ganglion cells
• Optic nerves pass posteromedially into the cranial cavity
• Meet in the midline, forming the optic chiasma

Optic Chiasma

• Flat mass of partially decussating fibers
• Lies at the junction of anterior wall and floor of the 3rd
  ventricle
• Only nasal retinal fibers cross over
• Join the fibers from the opposite temporal retinas
• These fibers form the optic tracts

Optic Tract

• Extends from optic chiasm to the lateral geniculate nucleus
  of the thalamus
• Contains axons from ganglion cells from both eyes
• Optic tract fibers terminate primarily in the lateral geniculate
  nucleus of the thalamus

Also terminates

1. Superior colliculus (control rapid directional eye
   movement)
2. Pretectal area (elicit reflex movements of eye to focus on
   important objects and activate pupillary light response)
3. Suprachiasmatic nucleus of the hypothalamus (control
   circardian rhythm).
4. Ventral geniculate body of thalamus and basal regions of
   brain– control body behavioral functions

Lateral Geniculate Body

• Two principal functions

1. Relays visual information from the optic tract to the visual cortex via
   optic radiations. Transmits point-to-point transmission with a high
   degree of spatial fidelity.
2. It ‘gates’ the transmission of signals to the visual cortex. Manages
   both excitatory and inhibitory signals(corticofugal fibres and
   reticular area of the mesencephalon)

• Inhibitory signals turn off the transmission through selected
  portions of the lateral geniculate nucleus

• 6 layers
  •Output Fibers
• Layer I and II– Magnocellular layer
• Layer III and IV– Parvocellular layer
  •Input Fibers
• Layer 1, 4 & 6 from contralateral retina
• Layer 2,3 & 5 from ipsilateral retina

Visual Cortex

• Primary Visual cortex (V1)
  •Occipital Lobe (Calcarine fissure area)
• Six layer
• Afferent fibers enter in layer IV
• Secondary Visual cortex (V2)
• Lies around V1
  •Other visual areas V3 to V8

• Primary visual area (area 17)– with the perception of visual
  impulses
• Secondary visual area or visual association area(area 18)–
  concerned with the interpretation of visual impulses
• Occipital eye field (area 19)— with the movement of eyes

Optic Pathway

• First Order Neuron– Bipolar Cells
• Second Order Neurons– Ganglion Cells
• Third Order Neurons– Lateral geniculate body

Visual Defects

• Injury to any part of optic pathway causes visual defect and the
  nature of defect depends upon the location and extent of injury.
• Loss of vision in one visual field is known as anopia
• Loss of vision in one half of visual field is called hemianopia
• Lesion of upper or lower part of visual cortex leads to inferior or
  superior homonymous hemianopia

Optic Pathway Defects

A. Lesion of left optic nerve: Total blindness of left eye
B. Lesion of right optic nerve: Total blindness of right eye
C. Lesion of lateral fibers in left side of optic chiasma: Left nasal hemianopia
D. Lesion of lateral fibers in right side of optic chiasma: Right nasal
hemianopia
C+D. Lesion of lateral fibers in both sides of optic chiasma: Binasal

hemianopia

E. Lesion of medial fibers in optic chiasma: Bitemporal hemianopia
F. Lesion of left optic radiation: Right homonymous hemianopia
G. Lesion of right optic radiation: Left homonymous hemianopia

Macular Sparing

• Total blindness does not occur in hemianopias because macular vision
  is not lost. This phenomenon in which the macular vision is retained
  (unaffected)– macular sparing.
• Macular sparing occurs because of the following reasons:
  i. Fibers from macula project into the visual cortex of both sides
  ii. Fibers from macular region are projected into both anterior and
  posterior parts of each visual cortex.
• Only the bilateral lesion of visual cortex causes total blindness.

Visual Cortex

• On the medial aspect of occipital lobe
• Divided into
• Primary Visual Cortex also called Visual area I/ Striate
  cortex
• Secondary Visual Cortex also called Visual association area

Primary Visual Cortex

• Lies in calcarine fissure area
• Extends upto occipital pole on the medial part of occipital cortex
• Visual signals directly terminate in this area
• Signals from macula terminate near the pole(outer side)
• From the peripheral retina terminate towards the center
• Upper portion of retina is represented superiorly
• Lower portion of retina is represented inferiorly
• Retinal fovea has largest representation on cortex– highest visual
  acuity

Secondary Visual Cortex

• Lies around the primary visual cortex
• Analysis of visual image
• Brodmann’s area 18

Transmission of visual signals from primary visual cortex into secondary visual areas on lateral surfaces of occipital and parietal cortices.

• Superior portions of the occipital
  lobe and posterior portions of
  the parietal lobe
• Represents form, 3D position,
  and motion are transmitted
• Anteroventral portion of the
  occipital lobe and the ventral
  portion of the posterior
  temporal lobe
• Visual detail and color

Visual Cortex

• 6 layers
• Axons from lateral geniculate nucleus form the magnocellular
  pathway end in layer 4 (deepest part, layer IVcα)
• These fibers extends both in deepest layer and outward
• Axons form the parvocellular pathway also end in layer IVa and Ivcβ
• Fibers extends both in deepest layer and towards the cortex
• Point to point vision and color vision
• Axons from the interlaminar region end in layers 2 and 3

Visual Cortex

• Structurally arranged into several million vertical columns of neuronal
  cells
• Each column having a diameter of 30 to 50 micrometers possessing
  1000 or more neurons
• Each column represents a functional unit
• As optic signals terminate in layer IV are processed to decode pieces
  of visual information
• Signals reaching
• Layers I, II, and III transmit signals for short distances
• Layers V and VI transmit signals at greater distances

Color Blobs

• Merged between primary and secondary visual columns are special
  column-like areas called color blobs
• Also excited by adjacent visual columns
• Activated by color signals
• Color blobs are the primary areas for sorting and recognizing color