RONOS RONOS

“The cooperation of the ophthalmologists, neurologists and neurosurgeons is the only way to solve the complex cases none could approach separately.”


MD. Tatiana Roşca

Virtual Course:
Elements of neuro-ophtalmologic examination

 

Vision is by far the most important of the special senses.

Visual information enters the eyes and is transformed into electrical signals in the retina. The signals are carried by the optic nerve to visual centers in the brain where they are interpreted. (Fig. 1)

 

 

Component
Function
Special sensory
(Special afferent)
To convey visual information
from the retina

Table Components of the optic nerve

 

Light entering the pupil travels to the back of the eye and passes through the retina to reach its deep layers (Fig. 2) where light energy is transduced into an electrical signal by rods and cones that form the photoreceptor layer. (Fig. 3)

Rods and cones are specialized cells with all the usual cellular components and, in addition, a light sensitive outer segment composed of stacked layers of membrane (discs) that are associated with visual pigments. Rods have about 700 such layers and are thought to function in the perception of dim light. There are approximately 130 million rods in each human retina. Cons (about 7 million) are considerably less numerous. The number of discs in the outer segments of a cone varies from 1000 in the central part of the retina to a few hundred in the peripheral areas. Cones are found in high densities in the central part of the retina. They are especially important in visual acuity and in color vision (see Fig. 3)

The information received by the rods and cones is passed forward in the retina to the bipolar cells. These are the primary sensory neurons in the visual pathway. They pass the signals further forward to the secondary sensory neurons, i.e., the ganglion cells in the anterior layers of the retina (see Fig. 2).

Ganglion cells axons converge towards the optic disc near the center of the retina.Most axons take the most direct path towards the disc; however, those whose direct routewould take them across the front of the macula (the most highly sensitive part of the retina) divert around it so as not to interfere with central vision. In the optic disc the axons turn posteriorly, pass through the lamina cribriformis of the sclera, and exit the eyeball as the optic nerve. (Fig. 4)

Therefore, the optic nerve (like the olfactory nerve) is composed of secondary sensory axons rather than primary sensory axons, and so forms a central nervous system tract rather than a nerve. Traditionally, however, the part of the tract that runs from the eye ball to the chiasma has been known as a “nerve”. We continue this tradition.

 

At the chiasma approximately one-half of the axons cross the midline. Most axons in each tract continue posteriorly around the cerebral peduncles to terminate in the lateral geniculate body (nucleus) of the talamus (see Fig. 1). A small proportion of them ascend to terminate in the pretectal area of the mid-brain as part of the pupillary reflex pathway (see Fig. 8).

Cells in the lateral geniculate body (nucleus) are the tertiary sensory neurons. Their axons, which form the geniculocalcarine tract (optic radiation) (Fig. 6), enter the cerebral hemispheres through the internal capsule, fan out above and lateral to the inferior hornof the lateral ventricle and course posteriorly to terminate in the primary visual cortex, which surrounds the calcarine fissure in the occipital lobe. A proportion of these axons form Meyer’s loop by coursing anteriorly towards the pole of the temporal lobe before turning posteriorly (see Fig. 6).

From the primary visual cortex integrated visual signals are sent to the adiacent visual association areas for interpretation and to the frontal  eye fields (see Fig. II - 1) in the frontal lobes where the signals direct changes in visual fixation (see Functional Combinations).

 

 

 

 


Transmission of Information from Various Parts of the Visual Field

When the eyes focus on a given object, light from the object and from the area surrounding it enters the eye. The entire area from which light is received (i.e., that is “seen”) constitutes the visual field. (Normally both eyes focus on the same object and so view the same visual field, but from slightly different angles because of the separation of the eyes.) For convenience in description, the visual field is divided into upper and lower halves and also into right and left halves, or four quadrants (Fig. 7). These quadrants are projected onto appropriate quadrants of retina.

Rays of light reach the retina by converging and passing through the relatively small pupil. This results in the image of the visual field being projected onto the retina both upside-down and reversed (see Fig. 7). Ganglion cell axons carrying visual information from the four retinal quadrants converge towards the optic disc in an orderly fashion and maintain approximately the same relationship to each other within the optic nerve (Fig. 10).

Within the chiasma, axons from the nasal halves of the retinas cross the midline. The crossing of the nasal axons results in the information from the right half of the visual field from both eyes being carried in the left optic tract, and that from the left half of the visual field in both eyes being carried in the right optic tract (Fig. 8). Most of the axons in the optic tracts terminate in the lateral geniculate bodies.

From the lateral geniculate bodies (nuclei), information from the upper halves of the retinas (lower visual field) is carried to the upper wall of the calcarine fissure. Information from the lower halves of the retinas  (upper visual field) terminates in the lower wall of the calcarine fissure (Fig. 9).

 

 

 


 

Clinical Comments

Damage to the visual system can be caused by defects of development, trauma, and vascular and metabolic problems.

Errors during development can result in small eyes (microphthalmia), absent eyes (anophthalmia), or both eye primordia can fuse to form one large eye in the midline (cyclopia).

Mythology notwithstanding, cyclopia is not found in adults since it occurs with other serious anomalies that are incompatible with life.

Developmental defects in the light-transmitting part of the eye, for example, congenital cataract (cloudy lens), also interfere with vision.

Although most of the visual system is encased in bone, the anterior part of the eye is protected only by the lids and can be damaged in trauma to the face (a good reason for the use of protective glasses in games such as squash). Severe traumato the head can damage the visual system as well as other parts of the central nervous system.
Since the optic “nerve” is actually a central nervous system (CNS) tract, its axons are subject to CNS diseases such as multiple sclerosis, and CNS tumors.
The visual system can also be damaged by a problem with its blood supply.
For example, diabetes damages blood vessels in the retina.

 
 

Last update: 13.01.2017