Module 11 Section 1
 

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Module 11:

 The Visual Pathways and Visual Field Defects
 

Section 1:

 The Visual Pathways
 

 

  
 

When light strikes the retina of the eye, the image is perceived in the brain. The anatomical structures that connect the retina to the visual cortex of the brain make up what is called the visual pathway. Knowledge of the visual pathway is important because damage to particular anatomical structures along the visual pathway produce characteristic changes in a map of the sensitivity of the patient’s visual field. The ophthalmologist can diagnose and treat some neurological diseases based upon what the patient does or does not see.

The visual pathway is made up of:

  • the retina

  • the optic nerve

  • the optic chiasm

  • the optic tract

  • the lateral geniculate body

  • the optic radiations

  • the visual cortex

The Retina

The retina is the first stop along the visual pathway. It is interesting to note that the light sensitive cells of the retina, the rods and cones, are located in the outer layer (away from the vitreous) of the retina. Thus light has to pass through the innermost nerve fiber layer (next to the vitreous) and the middle retinal layers (bipolar and ganglion cells) before reaching the light sensitive layer (rods and cones). As you might imagine, these other layers of the retina are clear in the normal eye.
   
 
   
  The retina is given it’s orange-red color (Caucasians) or it’s brown-green color (Blacks) by the underlying pigment layer and the blood vessel rich choroid.
 
   
  Below: Retina of a Caucasian person   Above: Retina of a Black person
   
 
   
   
 

The nerve fiber layer and the optic disc or optic nerve head

The axons of the ganglion cell layer make up the nerve fiber layer. These fibers move toward the optic nerve head in a particular pattern as shown below.
   
 
   
  Nerve fibers arching over and under the macula comprise the arcuate bundles (A). Fibers traveling from the disc directly to the macula make up the papillo-macular bundle (B). Notice that nerve fibers do not cross over an artificial line drawn at C. This is called the horizontal raphe. There are no photoreceptors over the optic disc, thus creating a physiological blind spot.

The pathway from the eye to the brain

All of the nerve fibers serving the retina pass through the optic nerve head and form the central portion of the optic nerve as it leaves the eye. 
   
 
   
  The optic nerves from both eyes join at a kind of nerve "junction box" called the optic chiasm (5, Fig. B, below). It is here that the wiring gets particularly interesting and useful in terms of diagnostic value.
   
 
  Figure B:  1 visual field,  2,3 retina,  4 optic nerve,  5 optic chiasm,  6 optic tract,  7 lateral geniculate body,  8 optic radiations and occipital cortex.
   
  Look at Fig. B. First of all, notice that the images from the right half of the visual field (1) fall on the left half of the retina in each eye (2,3). Thus, a defect in the visual field originates from a problem with the retina or with the wiring in the opposite half of the eye. The same is true of superior and inferior defects. For instance, the superior arcuate scotoma in the following field diagram is caused by damaged nerve fibers in the inferior nerve arcade of the retina.
   
 
   
 

In Figure B above, notice that the nerve fibers from the temporal half of the left eye travel through the chiasm to the left half of the brain. The nerve fibers from the nasal half of the right eye cross over at the chiasm and also travel to the left side of the brain. Thus, the left side of the brain perceives images from the right half of the visual field and the right side of the brain perceives images from left half of the visual field.

From the optic chiasm (5), nerve fibers travel through the optic tract (6), the lateral geniculate body (7), and on to the optic radiations (8) and the occipital cortex.

We will explore in Section 2 how damage to points along the visual pathway lead to characteristic visual field defects.  Thus, the intricate design of the visual pathway aids the ophthalmologist in diagnosis of diseases of the visual system.
   
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