Medical brain notes 24

  The Visual Pathway          The peripheral receptors for light are situated in the  retina . Nerve fibres arising in the retina constitute the  optic nerve . The right and left optic nerves join to form the  optic chiasma  in which many of their fibres cross to the opposite side. The uncrossed fibres of the optic nerve, along with the fibres that have crossed over from the opposite side form the optic tract .         The optic tract terminates predominantly in the  lateral geniculate body . Fresh fibres arising in this body form the  geniculo-calcarine tract  or  optic radiation  which ends in the visual areas of the cerebral cortex (Fig. 18.1). The Retina        The retina has a complex structure. It contains photoreceptors that convert the stimulus of light into nervous impulses. These receptors are of two kinds,  rods  ...

  The Retina        The retina has a complex structure. It contains photoreceptors that convert the stimulus of light into nervous impulses. These receptors are of two kinds,  rods  and  cones  (Fig. 18.2). There are about seven million cones in each retina. The rods are far more numerous, and number more than 100 million.          The cones respond best to bright light. They are responsible for sharp vision and for discrimination of colour. They are most numerous in the central region of the retina which is responsible for sharp vision. This area is about 6mm in diameter. Within this region there is a yellowish area called the  macula lutea . The centre of the macula shows a small depression called the  fovea centralis .         Medial to the central area, there is a circular area called the  optic disc . This area is devoid of photoreceptors ...

  The Visual Field and Retinal Quadrants        When the head and eyes are maintained in a fixed position, and one eye is closed, the area seen by that eye constitutes the  visual field  for that eye. Now if the other eye is also opened the area seen is more or less the same as was seen with one eye. In other words the visual fields of the two eyes overlap to a very great extent. On either side, however, there is a small area seen only by the eye of that side. Although the two eyes view the same area, the relative position of objects within the area appears somewhat dissimilar to the two eyes as they view the object from slightly different angles. The difference though slight, is of considerable importance as it forms the basis for the perception of depth ( stereoscopic vision ).         For convenience of description, the visual field is divided into right and left halves. It may also be divided into upper ...

  Optic Nerve, Optic Chiasma & Optic Tract        The optic nerve is made up of axons of the ganglion cells of the retina. These axons are at first unmyelinated. The fibres from all parts of the retina converge on the optic disc. In this region the sclera has numerous small apertures and is, therefore, called the  lamina cribrosa  (crib = sieve). Bundles of optic nerve fibres pass through these apertures. Each fibre acquires a myelin sheath as soon as it pierces the sclera. The fibres of the nerve arising from the four quadrants of the retina maintain the same relative position within the nerve. The fibres arising from the macula are numerous and form the  papillomacular bundle . Close to the eyeball, the macular fibres occupy the lateral part of the nerve, but by the time they reach the chiasma they lie in the central part of the nerve.             The fibres of the optic nerve ...

  The Lateral Geniculate Body        Some aspects of the structure of the lateral geniculate body have been considered. It has been seen that the grey matter of this body is split into six laminae (Fig. 13.17). Fibres from the eye of the same side end in laminae 2, 3, and 5; while those from the opposite eye end in laminae 1, 4, and 6. The macular fibres end in the central and posterior part of the lateral geniculate body, and this area is relatively large (Fig. 18.3). Fibres from the peripheral parts of the retina end in the anterior part of the lateral geniculate body. The upper half of the retina is represented laterally, and the lower half of the retina is represented medially. Specific points on the retina project to specific points in the lateral geniculate body. In turn, specific points of this body project to specific points in the visual cortex. In this way a point to point relationship is maintained between the retinae and the visual cortex. ...

  Geniculocalcarine Tract and Visual Cortex         Fibres arising from cells of the lateral geniculate body constitute the geniculocalcarine tract or optic radiation. These fibres pass through the retrolentiform part of the internal capsule. The optic radiation ends in the visual areas of the cerebral cortex (areas 17, 18, 19).         Some fibres of the radiation loop forwards and downwards into the temporal lobe before turning backwards into the occipital lobe. The highest fibres may loop into the parietal lobe.         The cortex of each hemisphere receives impulses from the retinal halves of the same side (i.e., from the opposite half of the field of vision). The upper quadrants of the retina are represented above the calcarine sulcus, and the lower quadrants below it. The cortical area for the macula is larger than that for peripheral areas. It occupies the posterior part ...

  Pupillary reflexes        Light thrown on an eye causes the pupil of that eye to contract. This is called the  direct pupillary light reflex.  At the same time the pupil of the other eye also contracts. This is called the  consensual light reflex.  The pathway for the light reflex is shown in Fig. 18.4. Impulses from the retina (of theeye on which light is thrown) travel through the optic nerve, chiasma and optic tracts. Near the lateral geniculate body the fibres concerned pass into the midbrain and end in the pretectal nucleus. Axons arising from this nucleus reach the Edinger Westphal nuclei of both sides. Fibres arising in these nuclei supply the sphincter pupillae after relay in the ciliary ganglion. The consensual reflex may be explained by the following. 1.                   Fibres of each optic nerve enter both optic tracts as a result of ...