Central visual pathways

* The retinal image is an inversion of the visual field. The visual field is what each eye sees.

* Partial decussation of optic nerve fibres in the optic chiasm leads to each optic tract consisting of fibres arising in the temporal retina of the ipsilateral eye and fibres arising in the nasal retina of the contralateral eye.

* The pretectal area of the midbrain controls the pupillary reflexes.

* The superior colliculus controls saccadic eye movements.

* The lateral geniculate nucleus in the thalamus possesses visual information.

Neurons in the lateral geniculate nucleus have concentric receptive fields.

* There is point-to-point representation of the retina in the visual cortex, which can be described as being retinotopically mapped.

* The primary visual cortex transforms concentric receptive fields into linear segments and boundaries.

* The primary visual cortex is functionally organised into narrow vertical columns of cells that run from the pial surface to the white matter. These columns subserve a variety of functions: ocular dominance, specific line orientation, direction of movement, spatial frequency and image disparity.

The most effective stimulus shapes are slits, dark bars, and edges. The cortical columns represent cells with the same receptive-field axis of orientation).

* The columnar units are linked by horizontal connections. Pupillary light reflex

Pupil size is determined by the balance between sympathetic and parasympa-

thetic innervation of the iris musculature. Pupillary constriction occurs in response to light. The pathway for the reflex involved includes:

An afferent limb:

Fibres originate in retinal receptor cells and pass through bipolar cells. They synapse with retinal ganglion cells. Axons of these cells run in the optic nerve and tract. Light reflex fibres leave the optic nerve just rostral to the lateral geniculate body and enter the high midbrain, where they synapse in the pretectal nucleus.

Intercalated neurons give rise to pupilloconstrictor (parasympathetic) fibres, which pass ventrally to the ipsilateral Edinger-Westphal nucleus, some fibres crossing the posterior commissure to the contralateral Edinger-Westphal nucleus. An efferent limb:

There is an efferent two neuron pathway from the Edinger-Westphal nucleus to the pupillary sphincter. Fibres travel in the third nerve and synapse in the ciliary ganglia. Short ciliary nerves supply the ciliary muscles and sphincter pupillae.

Afferent limb lesions do not cause anisocoria. The swinging light test is needed to detect a relative afferent pupillary defect.

Efferent limb lesions cause anisocoria, with a larger pupil on the affected side. A pupil size greater than 4 mm is regarded as being dilated. Physiological anisocoria is usually associated with a difference in pupil size not greater than 1 mm.

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