The Visual Pathways

The Retina

 
  • Receptors: Rodes and cones

  • First order neurons: bipolar cells

  • Second order neurons: ganglion cells
 
The Optic Nerve and Optic Chiasma
  • Axons of ganglion cells form the optic nerve, optic chiasma and optic tracts

  • Fibres from the nasal side (with information from the temporal visual field) remain medial and decussate at the optic chiasma

  • Fibres from the temporal side (with information from the nasal visual field) remain lateral and ipsilateral.

  • Optic tracts projects predominantly to lateral geniculate body but also to:

    a) Pre-tectal areas
    b) Superior colliculus
    c) Hypothalamus

  • Third order neurons project mainly to the lingual and cuneus gyri around the calcarine sulcus.

  • A few project to the superior temporal gyrus
 

Visual Reflexes

Involuntary Eye Movements
Reflex turning of the eyes
 

This is towards an object without being conscious of it:

  • Afferent arc: Three neurons from the retinal ganglion cells to the superficial layers of superior colliculus, then to the deep layers. Tectobulbar neurons to the nuclei of cranial nerves III, IV, and VI (actually to the medial reticular formation adjacent to cranial nerve nuclei III, IV and VI)
  • Efferent arc: One neuron from the cranial nerve nuclei to the extra-ocular muscles.

In simple eye movements, there is often some accompanying turning of the head and neck, which is accomplished by collicular neurons that project to anterior horn cells of the cervical spinal cord via the tectospinal neurons.

 
The Accommodation - Convergence reflex

This reflex involves three changes:

  • Convergence of the eyes due to the action of the medial recti muscles.
  • Increased convexity of the lens.
  • Pupillary constriction.

Afferent arc: Retino - LGN - striate projection.

Efferent arc: Corticofugal projection to the superior colliculus, and from this visual centre connections to:

  • The oculomotor nuclei (for ocular convergence)
  • The nucleus of Edinger-Westphal (for pupillary constriction and increased lens convexity). This reflex is spared in Argyll-Robertson pupil.
 
Fixation reflex :

This for example, occurs when both eyes follow the trajectory of an object in the smooth rather than jerky fashion thus ensuring that any point on an object is projected onto corresponding points of the two retinas.

  • Afferent arc: Two neurons: from the retinal ganglion cells to the LGN, from LGN to the striate cortex.
  • Efferent arc: From the striate cortex to the superior colliculus and from the superior colliculus to the extra-ocular muscles via the rectobulbar pathways.
 
 
Voluntary eye movements 
  • Afferent arc: From the retinal ganglion cells to the LGN - from the LGN to the striate cortex via the geniculostriate pathway. From the striate cortex to association visual cortex then to the frontal eye field, which is located in the caudal part of the middle frontal gyrus.
  • Efferent arc: Corticobulbar projections, ending within the superior colliculus. The superior colliculus makes connection via tectobulbar pathway with cranial nerve nuclei III, IV and VI. The final projection is to the extra-ocular muscles. This pathway mediates conjugate eye movements of which the individual is aware, such as “movements of command” elicited by instructing a patient to look to the left or to the right.
 
Pupillary light reflex

This is the reflex constriction of the pupil due to increased light intensity:

  • Afferent arc: From the retina to the pretectal area, and from thence bilaterally to the nuclei of Edinger-Westphal.
  • Efferent arc: Two neurons (parasympathetic) from the nucleus of Edinger-Westphal to the ciliary ganglion and from thence to the pupillary muscles of the eye.

Pupillary dilatation

  • Pupillary dilation which can occur in response to a sudden decrease in light intensity, to severe pain or to certain emotional conditions, is due to a different pathway.

  • The pathway leads from the thalamus to the hypothalamus, and then to the intermediolateral cell column of the upper thoracic cord through relay in the reticular formation.

  • Two neurons pass, to pupillary dilator muscles via the superior cervical sympathetic ganglion.
 
The Argyll-Robertson pupil
This is a disorder in which the pupils are non-reactive to light shined to the retina. The commonest cause is neurosyphilic affliction of the pretectal nuclei or periaqueductal region, ciliary ganglion, singly or in combination.
 
 
The Accomodation-Convergence:

This reflex involves three changes:

  • Convergence of the eyes due to the action of the medial recti muscles.
  • Increased convexity of the lens.
  • Pupillary constriction.

Afferent arc: Retino - LGN - striate projection.

Efferent arc: Corticofugal projection to the superior colliculus, and from this visual centre connections to:

  • The oculomotor nuclei (for ocular convergence) and
  • The nucleus of Edinger-Westphal (for pupillary constriction and increased lens convexity). This reflex is spared in Argyll-Robertson pupil.