Retinohypothalamic Tract

The anatomy of the visual pathway may be divided into six separate parts (Fig. 14.1):

1. Optic nerve: This includes all of the optic nerve fiber bundles of the eye.

2. Optic chiasm: This is where the characteristic crossover of the nerve fibers of both optic nerves occurs. The central and peripheral fibers from the temporal halves of the retinas do not cross the midline but continue into the optic tract of the ipsilateral side. The fibers of the nasal halves cross the midline and there enter the contralateral optic tract. Along the way, the inferior nasal fibers travel in a small arc through the proximal end of the contralateral optic nerve (the anterior arc of Wilbrand). The superior nasal fibers travel in a small arc through the ipsilateral optic tract (the posterior arc of Wilbrand).

3. Optic tract: This includes all of the ipsilateral optic nerve fibers and those that cross the midline.

4. Lateral geniculate body: The optic tract ends here. The third neuron connects to the fourth here, which is why atrophy of the optic nerve does not occur in lesions beyond the lateral geniculate body.

5. Optic radiations (geniculocalcarine tracts): The fibers of the inferior retinal quadrants pass through the temporal lobes; those of the superior quadrants pass through the parietal lobes to the occipital lobe and from there to the visual cortex.

6. Primary visual area (striate cortex or Brodmann's area 17 of the visual cortex): The nerve fibers diverge within the primary visual area; the macula lutea accounts for most of these fibers. The macula is represented on the most posterior portion of the occipital lobe. The central and intermediate peripheral regions of the visual field are represented anteriorly. The temporal crescent of the visual field, only present unilaterally, is represented farthest anteriorly.

Other connections extend from the visual cortex to associated centers and oculomotor areas (parastriate and peristriate areas). Aside from the optic tract there is also another tract known as the retinohypothalamic tract. This

390 14 Visual Pathway — Anatomy of the visual pathway.

Anterior Visual Pathway

Anterior arc of Wilbrand

Layer of optic nerve fibers —[

3rd neuron-

(ganglion cells)

2nd neuron-

(bipolar cells)

1st neuron (cones and rods)

Pigment epithelium a

Fig. 14.1 a Overview of the course of the visual pathway. b Structure of the retina. c Course of the nerve fibers in the optic chiasm.

Light

Anterior arc of Wilbrand

Posterior arc of Wilbrand b

Inferior nasal fibers

Temporal fibers

Superior nasal fibers

Posterior arc of Wilbrand b tract is older in evolutionary terms and diverges from the optic chiasm. It transmits light impulses for metabolic and hormonal stimulation to the dien-cephalon and pituitary gland system and influences the circadian rhythm.

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