7. The Visual System
Revised August 8, 2007
The objectives of this chapter are to:
- Describe the organization of the visual system.
- Describe the neuroanatomy of two important visual reflexes, the pupillary light reflex and the near response.
It is assumed that you have already studied the gross anatomy and histology of the eye.
I. The Visual Pathway
Eyeball and retina: The visual pathway begins in the eyeball (fig 7a). Light waves pass through the cornea and lens to impinge on the retina. The receptors for the visual system are specialized neurons called photoreceptors, rods and cones, which are located in the retina. The retina contains several cell types that are arranged in distinct layers. The outermost layer, i.e., the layer adjacent to the choroid, is the retinal pigment epithelium (#52211). What is the origin of these cells (fig 7b)? What is their functional significance? Embedded in the pigment epithelium (#9927) (c) are the processes (b) of the photoreceptor cells whose cell bodies are shown in (a). The entire retina, other than the pigment epithelium, is termed the neural retina. Retinal detachment occurs along the junction between the pigment epithelium and the photoreceptors in the neural retina (fig 7a).
The cell bodies of the photoreceptors make up the outer nuclear layer of the retina. The photoreceptors make contact with the bipolar cells, whose cell bodies constitute the inner nuclear layer of the retina (#52207, #7962). Bipolar cells, in turn, end on ganglion cells of the retina, whose cell bodies make up the ganglion cell layer (#52204, #5341). The layered arrangement of the retina is modified in the fovea, the retinal area that is essential for acute color vision (#367). The fovea is structurally specialized in the following ways: (1) the ganglion cell (#4395) and bipolar cell (#4396) layers are thinner here than elsewhere in the retina; (2) the cell bodies of the bipolar and ganglion cells are displaced laterally and (3) the fovea is avascular (#5338). What is the name of the photoreceptor in the central retina (#4397)? How do the two types of photoreceptors differ functionally?
Ganglion cell axons: Ganglion cell axons penetrate the back of the eyeball in an area called the optic disc (#6914 B, #5337) and form the optic nerve (#6914 A), cranial nerve II. Here the ganglion cell axons become myelinated. What cells form their myelin sheaths? If severed, do these axons regenerate? It is also at this point that the optic nerve becomes invested by the meninges. The thick outer coat of cranial nerve II, the dura mater (#7600), is continuous with the sclera (#52213). Near the center of the optic nerve is the central retinal artery (#52202). What is its origin? (#12396) Its distribution? How does the optic nerve enter the middle cranial fossa (#5435, #6934)?
Follow the intracranial course of the ganglion cell axons by identifying, in order, the optic nerve (#11711), optic chiasm (#11712), and optic tract (#11713). The optic nerves partially decussate in the optic chiasm. Which axons decussate? The optic nerve and tract both consist of ganglion cell axons. How are the two structures different? The relationship of the optic chiasm (Fig 7c) and tract to the hypothalamus (#6258) and cerebral peduncle (fig 7d, #6265) is best seen in sections through the brain. Note that the chiasm is just ventral to the rostral part of the third ventricle (#4318, #6253). What is the spatial relationship between the optic chiasm and the pituitary gland (#4980)? Pituitary tumors (#12192, #7967) can press on the chiasm and cause a specific visual field defect. What is the name of this deficit?
Lateral geniculate nucleus and axons: The optic tract goes around the cerebral peduncle to end in the lateral geniculate nucleus (fig 7d), the relay nucleus of the thalamus for the visual system. This nucleus is best seen from the dorsal surface of the brain stem (fig 7e). The relationship of the lateral geniculate nucleus (#6465) to the cerebral peduncle (#6463) is illustrated in sections through the brain. It is evident in these sections that the lateral geniculate nucleus is a layered structure (#8290, #6275). Axons from ipsilateral and contralateral ganglion cells end in different layers.
Axons from the lateral geniculate nucleus project to primary visual cortex, the cortex bordering the calcarine sulcus. This projection is called the optic (visual) radiation or geniculocalcarine tract. The axons leave the lateral geniculate nucleus and enter the most caudal part of the posterior limb of the internal capsule (fig 7f). The optic radiations pass through the temporal and parietal lobes (#7923) to the lateral side of the atrium (trigone) and occipital horn of the lateral ventricle (#5372, #6519, #5240). Especially note the relationship between the radiations and the lateral ventricle. What artery(ies) supply(ies) the optic radiations as they course deep in the white matter to the occipital lobe?
Cortex: The optic radiation, like thalamic axons in general, ends in the cortex as a band of myelinated fibers in layer IV. However, only in the primary visual cortex (V1) can this band be seen with the naked eye as the line of Gennari (#6282, #8987). In fact, the term striate ("striped") cortex is another name for primary visual cortex.
Primary visual cortex is also referred to as either calcarine cortex or Brodmann’s area 17. Although a small part of calcarine cortex is located on the lateral surface (#4217), most of it occurs on the medial surface of the occipital lobe (#4275). There is a point-to-point representation of the retina in the calcarine cortex. This is called the retinotopic organization of the cortex. What area of the retina is represented anteriorly (i.e., closer to the splenium) in the calcarine cortex? What retinal area is represented posteriorly (i.e., nearer the occipital pole) in the calcarine cortex? Are all areas of the retina equally represented? What cerebral artery supplies area 17 (#12519)? What is the origin of this artery (#5969)?
Adjacent to primary visual cortex (V1) are the visual association cortical areas, including V2 and V3 (= area 18, or 18 & 19, depending on the author) (#4350). Lesions of V1, V2 and V3 produce identical visual field defects.
Beyond V3, visual information is processed along two functionally different pathways. Injury isolated to the cortical areas in these two pathways does not disturb the visual fields; that is, visual sensation remains intact.
1. The occipitoparietal (“where”) pathway processes position and motion. A person with a unilateral (mainly right) posterior parietal lesion (angular and supramarginal gyri) suffers from hemispatial neglect, ignoring the contralateral external world and the contralateral side of her own body.
2. The occipitotemporal (“what”) pathway, to the undersurface of the temporal lobe (fusiform or occipitotemporal gyrus), identifies objects, symbols and colors. People with lesions of the left inferior occipitotemporal area cannot recognize objects by visual inspection alone. Such individuals can describe a key by palpation, but cannot identify and name it by sight. This condition is called associative visual agnosia for objects. Lesions in this vicinity can also cause inability to recognize words or to read despite intact spelling and writing (pure alexia, alexia without agraphia).
With a similar lesion on right, the patient cannot recognize familiar or famous faces (prosopagnosia) or may not recognize familiar places and streets. What artery supplies the inferior occipitotemporal area? What visual field deficit occurs if the lesion extends to the inferior bank of the calcarine cortex? Based on the above information, what findings might you expect with bilateral infarcts of this area?
Axial (Horizontal) Sections
A series of three axial (horizontal) sections are shown in fig 7g, fig 7h and fig 7i and help to visualize the visual pathway. The three sections are ordered from inferior to superior (toward the vertex, the top of the skull).
Beginning with the most inferior section at the level of the lateral geniculate nucleus (fig 7g), observe the relationship between the optic tract and cerebral peduncle (#6285). The tract can be followed into the lateral geniculate nucleus (#6286). The lateral geniculate nucleus gives rise to the optic radiation. The inferior axons of the optic radiation begin with an initial rostral loop (Meyer's loop) over the temporal horn to its lateral side (fig 7g) before proceeding to the inferior bank of the calcarine cortex. The superior axons of the optic radiation pass directly, without looping, to the superior bank of the calcarine cortex.
The section for fig 7h, enlarged in fig 7j-b, is closer to the vertex than fig 7g. The cerebral peduncles are no longer present, but these same axons are now part of the internal capsule, which separates the thalamus from the putamen and globus pallidus of the basal ganglia. In fig 7j-b the medial and lateral geniculate nuclei and the pulvinar of the thalamus are labelled.
Fig 7i is even closer to the vertex. The optic radiations are seen extending back from the internal capsule. Follow them as they course lateral to the occipital horn of the lateral ventricle and enter the occipital lobe.
In axial (horizontal) sections, be able to locate the thalamus (#6298), the putamen and globus pallidus of the basal ganglia (#6297), and the posterior limb of the internal capsule that separates them. What cortical regions does the splenium of the corpus callosum (#4443) connect?
II. Visual Reflexes
There are two visual reflexes that are clinically important and whose reflex arcs must be understood.
- Pupillary light reflex (This link plays an animation. Please enlarge the window.)
- Near response (near-point reaction, near reflex), which has three components
- Accommodation of the lens
- Convergence of the eyes
- Constriction of the pupil
1. Pupillary light reflex: This reflex depends on input to the pretectal region (fig 7j, fig 7k) from the retina. The pretectal region (pretectum) (#6307) is adjacent to the posterior commissure (#6305). Collaterals of ganglion cell axons leave the optic tract to go to the pretectum (and also to the superior colliculus) (#4359). As these collateral axons pass over the surface of the brain stem they are called the brachium of the superior colliculus. The relation of the pretectal region (#6307) to the corpus callosum, thalamus, and inferior temporal lobe can be seen in myelin (#6306) or gross coronal sections (fig 7l). What is the significance of the pretectal area in the light reflex? What is the direct light reflex? What is a consensual reflex?
2. Near response: This reflex (see movie) depends on input to the near-response area of the midbrain from visual cortex pathways. The visual association cortex projects to parietal cortex and the frontal eye fields (#74245). These latter cortical areas project to the near-response area, dorsal and dorsolateral to the oculomotor nucleus.
Common efferent arm – CN III: The pupillary light reflex and the near response have a common efferent path – the oculomotor nerve (cranial nerve III) (#4729). The efferent arm of these reflexes consists of pre- and postganglionic parasympathetic neurons. The Edinger-Westphal nucleus (#6310) contains the preganglionic cell bodies. Depending on the author, this nucleus is either part of the oculomotor nucleus (oculomotor nuclear complex) or an adjacent separate nucleus. The oculomotor nucleus is at the level of the superior colliculus near the midline ventral to the aqueduct and periaqueductal gray. The axons forming nerve III are seen in #6312. Note their relationship to the cerebral peduncle and the red nucleus (fig 7k). The postganglionic axons end in the sphincter pupillae of the iris and the ciliary muscle. Where are the postganglionic cell bodies located (#52066)? What is the action of the ciliary muscle? Which reflex (pupillary light reflex or near-response) also uses somatic motor neurons to extraocular muscles?
What is the role of the reticular formation, the intermediolateral cell column of the spinal cord, and the superior cervical ganglion in pupillary dilatation? Be sure you understand the anatomy of the pupillary light reflex and near response as well as pupillary dilatation. Can one reflex occur without the other?
- Accommodation and Near Response (the movie has sound) from Univ. of Toronto
- Extraocular Eye Movements, drag the control icon with your mouse to move eyes. Univ. of Toronto
- Orbit Autonomic Innervation from David Morton
- Parasympathetic reflex for pupillary constriction, CN III from HEAL
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