Retina  optic tract  optic chiasm

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1. Each retinaoptic tractoptic chiasm (nasal fibers from ganglion cell axons decussate across the midline and join undecussated fibers from temporal half of other retina in the optic tract)  optic tract ends in lateral geniculate nucleus of thalamus  optic radiation (retrolenticular and sublenticular parts of internal capsule)  primary visual cortex above and below calcarine sulcus (striate cortex)
Retinotopic map – map of image falling on retina; large number of fibers representing fovea; in visual cortex – retina represented right side up (superior fields below calcarine sulcus, inferior fields above), fovea represented posteriorly at occipital pole (large), periphery anteriorly
2. Deficits in visual fields

- Fibers to upper back of calcarine sulcus go through retrolenticular part of internal capsule and to occipital lobe

- Fibers to lower bank go through sublenticular part and loop out into temporal lobe (Meyer’s loop) before turning posteriorly toward occipital lobe

- Damage in optic radiation or occipital lobe spares part of the large foveal representation (foveal or macular sparing)

3. – Receptive fields of lateral geniculate neurons are similar to those of ganglion cells: input from one eye, center-surround antagonism, some receptive fields are for color and others are for black/white contrast, lot of representation of small foveal receptive fields, LGN neurons project to striate cortex
- cortical cells for contrasts have a variety of receptive fields; cortical cells do not respond to diffuse illuminate because there are excitatory and inhibitory regions in receptive fields that cancel each other out; receptive fields are not circular which do not respond well to small spots of light

- Cortical cells respond to stripes or edges with a particular orientation; simple cells have excitatory and inhibitory regions in the shape of oriented bars; complex cells respond to oriented lines of a particular length

-other neurons are more concerned with color than with black/white contrast; they have circular receptive fields with antagonistic center-surround color properties
4. – visual cortex divided into hypercolumns; within hypercolumns all aspects of the information coming from a particular part of the contralateral visual field are sorted (to association areas); each hypercolumn includes two sets of orientation columns through which all stimulus orientations are mapped out for both eyes; among orientation columns are color-sensitive neurons

- Foveal hypercolumns correspond to small areas of visual field and peripheral hypercolumns to larger areas

- Form and color processed in ventral areas of occipital and temporal lobes

- processing of location and motion takes place in dorsal parts of occipital and parietal lobes

5. – Illumination of either retina causes both pupils to constrict

- Sphincter is strong; parasympathetic innervation from oculomotor nerve and ciliary ganglion; activated during pupillary light reflex

- Dilator is weak; sympathetic innervation via intermediolateral cell column of spinal cord  superior cervical ganglion; preganglionic sympathetics for dilator activated by long descending pathways from ipsilateral half of hypothalamus
Pupillary light reflex

- Response of illuminated eye is direct reflex; equal response of unilluminated eye is consensual reflex

- Afferent impulses travel over ganglion cells in optic nerve; half of them (nasal) cross in optic chiasm

- Fibers bypass lateral geniculate nucleus and travel through superior brachium to pretectal area (rostral to superior colliculus at midbrain-diencephalon junction; adjacent to posterior commissure

- Pretectal area projects bilaterally to Edinger-Westphal nucleus of oculomotor nucleus (preganglionic parasympathetics neurons reside there)  oculomotor nerve  ciliary ganglion  sphincter

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