VISUAL FIELD AND PATHWAY (Neuroanatomy).

Neuroanatomy

1. The optic nerve and Visual pathway The optic nerve 1

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3. Introduction The optic nerve carries visual impulses from the retina to the visual cortex. Consists of 3 orders of neurons. First order of neuron is formed by the central processes of the bipolar cells. Second neuron is formed by the central processes of the ganglion cells. These fibres form the optic nerve proper The third order of neuron is formed by the axons of the cells of the lateral geniculate nucleus 3

4. Course The central processes of the ganglion cells emerge through the optic disc of the retina to the back of the eye ball as the optic nerve. Here the optic nerve is surrounded by CSF and meninges . It enters the middle cranial fossa through the optic canal to reach the optic chiasma . 4

5. Course cont’d At the chiasma , the fibres from the nasal part of the retina cross (decussate) to the opposite side while the fibres from the temporal side continues on the same side The fibres exit the chiasma as the optic tract which terminates at three destinations as follows: 5

6. Course cont’d 1. The lateral geniculate body 2. The pretectal nuclei (for pupillary reflex) 3. The superior colliculus (body reflex to light) From the lateral geniculate body, the fibres of the third order of neurons (optic radiation) pass through the retrolentiform part of the internal capsule to the visual cortex on the medial surface of the occipital lobe. 6

7. VISUAL PATHWAY 7

8. Introduction Visual pathway is the path of travel of impulses carrying visual signals from the retina to the visual cortex Visual field is the whole area (or field of vision) one can see with one eye Visual field of both eyes greatly overlap Visual field is divided into 4 quadrants The two eyes record slight difference in the relative position of objects in the visual field; this is important in appreciating depth 8

9. Introduction cont’d Image from one side of the visual field form on the opposite side of the retina, because of the inverted image nature of image formed by the lens 9

10. Introduction cont’d Impulses generated from the photoreceptors pass to --> the optic nerve --> optic chiasma --> optic tracts --> lateral geniculate nucleus (relay) --> optic radiation --> ( retrolentiform part of internal capsule) --> visual cortex 10

11. The retina Rods and cones are photoreceptors Cones responds best to bright light, and are responsible to color discrimination. They concentrate more on the central part of the retina, especially at the macula lutea where the sharpest image is produced Rods are more in number, located more peripherally in the retina, and responds more to poor light 11

12. The retina cont’d Rods & cones are modified neurons Impulses generated here travel to the optic nerve, through the layers of the retina 12

13. Optic nerve, chiasm & tracts Optic tract carries all impulses from same half of the two retinas Optic tract carries all impulses from the opposite half of the visual field 13

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15. Lateral geniculate body It is a r elay centre for optic tract fibres Grey matter comprises 6 laminae Fibres from same eye terminates in laminae 2,3 & 5 Fibres from opposite eye ends in laminae 1,4 & 6 15

16. Lateral geniculate body cont’d Fibres from peripheral parts of retina enter the anterior part, while fibres from central part of the retina enter the posterior part Fibres from upper half of the retina enter the lateral part while fibres from the lower half enter the medial part 16

17. Lateral geniculate body cont’d 17

18. Geniculocalcarine tract & visual cortex Geniculocalcarine tracts = optic radiation Pass through the retrolentiform part of internal capsule The visual cortex receives fibres from half of the two retinas on same side just as lateral geniculate body 18

19. Clinical correlates 1. Visual loss Hemianopia = loss of vision from half of the visual field (either right of left) Same side involved in both fields = homonymous Different sides involved in both fields = heteronymous Quadrantic anopia = loss of vision from one quadrant of visual field 19

20. Lesion of the optic nerve = complete blindness on the same side 20

21. Lesion of the chiasma = bitemporal heteronymous hemianopia 21

22. Lesion of the optic tract, LGB or visual cortex = homonymous hemianopia on the opposite side 22

23. 2. Eye reflexes a. Pupilary light reflex When light is shone on the eyes, pupil of the same eye constrict (direct pupilary reflex), also pupil of the other eye constrict (consensual reflex) 23

24. Eye reflexes cont’d b. Accomodation reflex : when the eye looks at near objects, the pupil also constricts c. Corneal reflexes : when the cornea is touched with a piece of cotton wool, both eyes will close. Impulse from this travels through the opthalmic division of trigeminal nerve --> nucleus of trigeminal nerve --> motor nucleus of facial nerve --> facial nerve --> palpebral muscles 24