CHAPTER 11. THE VISUAL SYSTEM

OBJECTIVE:
    BE ABLE TO IDENTIFY THE ANATOMICAL FEATURES OF THE EYE, THE VISUAL PATH, THE VISUAL REFLEXES, AND THE OCULAR MOTOR SYSTEM IN GROSS SPECIMENS AND MICROSCOPIC SECTIONS.

The Eye

The eye occupies about one-third of the orbital cavity whose walls protect it from injury. The eyeball weighs 6-8 gm and measures 24 mm anteroposteriorly and 23.5 mm vertically and horizontally. The anterior and posterior poles of the eye are the central points of the corneal and scleral curvatures, respectively. The equator is an imaginary line surrounding the eyeball in the frontal plane midway between the two poles. The meridian is an imaginary line on the surface of the eye extending from pole to pole, thus crossing the equator at a right angle.

General Features 
(Fig. 11-1; Sl. 88)

Note that the eyeball consists of three tunics: the external or fibrous, the middle or vascular, and the internal or nervous. The fibrous tunic is protective and consists of the cornea anteriorly and the sclera posteriorly. The middle tunic, or uvea, is responsible for nutrition, focusing, and regulating the intensity of light. It includes: 1) the thin but highly vascular and pigmented choroid located deep to the sclera, 2) the ciliary body, an enlarged part of the uvea deep to the sclera near its junction with the cornea, and 3) the iris, a thin projection from the anterior part of the ciliary body. The free edge of the iris forms the rim of the pupil. The inner tunic is the retina and consists of a nervous part containing the photoreceptors and non-nervous parts that line the posterior surfaces of the ciliary body and iris. The nervous part gives rise to the optic nerve which emerges from the posterior part of the eyeball slightly medial to the posterior pole.

The contents of the eyeball are transparent and include the lens, the aqueous humor, and the vitreous body. The lens is a biconvex disc about 1 cm in diameter. Its posterior surface is more convex than the anterior. It is enclosed in a capsule that is anchored to the ciliary body by the suspensory ligament of the lens.

The aqueous humor is found in two chambers located in the anterior part of the eye, partially separated from one another by the iris. The anterior chamber is in front of the iris, that is, between the iris and cornea, and the posterior chamber is behind the iris, that is, between the iris and the lens and the suspensory ligament of the lens. The two chambers communicate through the pupil.

The vitreous body is a gelatinous mass in the posterior four-fifths of the eyeball behind the lens and its suspensory ligament. It is fairly adherent to the posterior surfaces of the lens and ciliary body.

Histology of the Human Eye - Anterior Parts

Observe the relationship of the structures in the anterior part of the eye in Slide 89. Note the boundaries of the anterior chamber: posterior surface of the cornea, anterior surface of the iris and the center of the lens, and the sclera and ciliary body in the filtration angle. The posterior chamber is limited by the posterior surface of the iris, the ciliary body, the suspensory ligament of the lens (not discernible), and the lens as far centrally as the rim of the pupil.

The Cornea (Sl. 90)

The cornea is avascular and consists of five layers.

The five corneal layers are:

  1. anterior epithelium - a non-keratinizing stratified squamous type that regenerates readily, is replaced continually, and is rich in free nerve endings.

    These nerve endings are the receptors that trigger the corneal reflex: corneal free nerve endings ----> long ciliary nerve ----> nasociliary nerve ----> ophthalmic nerve ---> trigeminal ganglion ----> sensory trigeminal root ----> spinal trigeminal tract ----> spinal trigeminal nucleus ----> facial nucleus ----> facial nerve ----> contraction of orbicularis oculis muscle
  2. anterior limiting membrane (Bowman's) - a modified layer of the underlying stroma
  3. stroma or substantia propria - forming about 90% of the cornea and composed of transparent regular bundles of connective tissue arranged in many layers (the irregular spaces in this preparation are artifactual)
  4. posterior limiting membrane (Descemet's) - a thickened basement membrane covered on its inner surface by
  5. mesothelium - a single layer of flattened cells.
Ciliary Body and Filtration Angle (Fig. 11-2; Sl. 91)

The ciliary body contains the ciliary muscles, three groups of smooth muscles that are difficult to distinguish from one another. All three extend from the ciliary body to the scleral spur which is located near the inner surface of the sclerocorneal junction at the filtration angle. The ciliary muscles are innervated by postganglionic parasympathetic fibers that arise in the ciliary ganglion and reach the eye in the short ciliary nerves.

Most externally are meridional fibers that pass longitudinally just deep to the sclera. Most internally are circular fibers that sweep acutely from the scleral spur around the base of the iris. Between these two are radial fibers that radiate fan-like toward the ciliary processes. Interspersed among the smooth muscle bundles are numerous elastic fibers, melanocytes, blood vessels, and nerves. In accommodating, focusing on near objects, contraction of the ciliary muscles pulls the ciliary body forward and narrows the circle formed by it, thereby decreasing the tension on the suspensory ligament of the lens. The inherent elasticity of the lens then permits its central part to become more curved, especially anteriorly, thus keeping the image focused on the retina.

Observe the filtration angle at the junction of the iris and cornea. The scleral spur, which gives attachment to the ciliary muscles, is at the outer border of the filtration angle.

Slightly anterior to the scleral spur is the sinus venosus sclera (canal of Schlemm). This sinus runs completely around the eye and is separated from the filtration angle by a trabecular meshwork (spaces of Fontana) through which the aqueous humor in the anterior chamber is absorbed.

The Iris (Fig. 11-2; Sls. 91, 92)

The peripheral border or root of the iris attaches to the ciliary body (Sl. 91) while its central or free margin (Sl. 92) forms the rim of the pupil.

The iris is covered anteriorly by mesothelium that is continuous with that of the internal surface of the cornea. Deep to this is an internal limiting lamella, a narrow layer of condensed stroma that contains abundant pigment cells in brown eyes. The stroma of the iris contains collagenous fibers, pigment cells (very few in blue eyes), and the sphincter pupillae muscle. This important smooth muscle is present in the posterior part of the stroma near the pupil.

The sphincter pupillae muscle is innervated by postganglionic parasympathetic fibers that travel from the ciliary ganglion in the short ciliary nerves. Like the ciliary muscle, the sphincter pupillae is controlled by preganglionic parasympathetic neurons in the Edinger-Westphal part of the oculomotor complex. Contraction of the pupillary sphincter narrows the pupil (miosis). This occurs in the light reflex when the intensity of light striking the retina increases. It also occurs in accommodation.

The posterior surface of the iris is covered by the iridial part of the retina, a two-layered pigmented epithelium that forms a black circular fringe at the margin of the pupil. Between the pigmented retinal layers and the stroma, along the entire length of the iris, is the posterior limiting lamella consisting of the thin dilator pupillae muscle.

The dilator pupillae muscle increases the diameter of the pupil (mydriasis). It is innervated by postganglionic sympathetic fibers from the superior cervical ganglion by way of the carotid plexus, nasociliary nerve, and the long ciliary nerves.

Histology of the Human Eye - Posterior Parts

Observe the relationships of the structures near the posterior pole of the eye in Slide 93. The sclera is very thick and consists of a feltwork of collagenous fibers. Deep to it is the thin, pigmented choroid separating the sclera from the nervous part of the retina. The posterior part of the retina contains two special structures: the central area and the optic disc. In the central area is the macula lutea (yellow spot). Its yellow color is thought to be due either to the presence of pigments in this area or a great reduction in the capillary bed. It measures about 2 mm horizontally and 1 mm vertically and at its center is the fovea centralis, a depression in the internal surface of the retina. The optic disc or papilla is where the optic nerve emerges. It is about 4 mm medial to the macula. Since there are no photoreceptors here, this area of the retina is insensitive to light and forms the blind spot in the visual field. Upon emerging from the eye, the optic nerve is surrounded by a cuff of sclera which becomes continuous with the dura mater and the leptomeninges. Near the emergence of the optic nerve, but external to the dura, the sclera is invaded by the ciliary arteries and nerves.

The Retina (Sl. 94)

The nervous part of the retina forms the inner layer of the posterior four-fifths of the eye. Identify ten parallel layers, numbered from outside to inside:

  1. pigment epithelial layer - a single layer of cells filled with melanin.
    Externally, the pigment epithelium rests on a basal lamina that is part of Bruch's membrane, the most internal layer of the choroid
    .
  2. rods and cones - these are the photoreceptors.
    Each rod and cone cell consists of four parts: an outer segment, an inner segment, a cell body, and a synaptic terminal. Actually, the "rod and cone layer" is made up of the outer and inner segments of these cells.
  3. external limiting membrane.
    A dense layer of junctional complexes where the bases of the inner segments of the rods and cones are attached to Müller's cells, the modified glial cells that support the retina.
  4. external nuclear layer - the cell bodies and nuclei of the rods and cones.
  5. external plexiform layer - the cell processes and synapses that occur chiefly between the photoreceptor cells and the dendrites of the bipolar cells.
  6. internal nuclear layer - the cell bodies of the bipolar neurons which are considered the first order neurons in the visual pathway.
    Local circuit neurons, the horizontal and amacrine cells, are interspersed among the bipolar neurons. Most of the cell bodies of the Müller cells are located in this layer also.
  7. internal plexiform layer - the axons of the bipolar cells and the dendrites of the ganglion cells synapse here.
  8. ganglion cell layer - the cell bodies of second order neurons in the visual pathway.
    These multipolar neurons are arranged in a single layer in most of the retina, but become progressively more densely-packed from the periphery to the macula lutea, where they form as many as ten rows.
  9. optic nerve fiber layer - the axons of the ganglion cells. Until they reach the optic disc these axons are unmyelinated, an optical advantage since myelin is highly refractile.
  10. internal limiting membrane
    The basal lamina of the Müller cells that separate the retina from the vitreous body.
Central Area of the Retina (Sl. 95)

The central area of the retina is a region about 5-6 mm in diameter in which all the layers are modified to a greater or lesser degree. It contains the macula lutea and the fovea centralis, the central pit of the macula. The pit is due to the absence of the internal cellular and plexiform layers, leaving only the photoreceptor cells and their nuclei.

Identify the fovea and note the absence of the inner layers of the retina. Due to their displacement, the portion of the retina surrounding the fovea is the thickest, chiefly due to the accumulation of ganglion and bipolar cells which are stacked in many layers.

The spongy degeneration in the fovea and in the bipolar and ganglion cell layer adjacent to it is due to postmortem changes. The detachment occurring between the pigmented and the photoreceptor layers frequently occurs in preparing the histological sections. Its location, however, mimics the site of retinal detachment in vivo
.

Optic Disc (Sl. 96)

The optic disc, or blind spot, is where the optic nerve fibers and the central retinal vessels pass through the posterior wall of the eye. It is about 1.5 mm in diameter and can usually be examined with an ophthalmoscope. Note the absence of the outer layers of the retina, including teh photoreceptors, at the optic disc.

The unmyelinated axons of the retinal ganglion cells radiate horizontally from all regions of the retina and converge at the disc. Here they interrupt the outer layers of the retina and the choroid and form small bundles that pierce the sclera in small orifices that form the lamina cribrosa sclerae. One opening larger than the rest transmits the central retinal artery and vein.

Observe the retinal vessels at the surface of the optic disc.

The central retinal artery divides into superior and inferior branches each of which then further subdivides into nasal and temporal branches. These branches ramify in the deepest part of the optic nerve fiber layer and each supplies its corresponding quadrant of the retina. Capillaries from these vessels invade and supply the inner retinal layers. The outer layers, including the rods and cones, are supplied by the choriocapillaris within the choroid.

Visual Path

Locate the optic nerves and chiasm in a brain specimen (Pls. 6, 7, 8, 10, 12, 15) and in Slide 43. Note the proximity of the pituitary stalk to the median part of the optic chiasm and the internal carotid arteries to its lateral parts.

Follow the optic tract in the brain specimen (Pls. 6, 7, 8, 10, 11) and in Slides 41, 40, 39, 38, 37, 36, 35 as it leaves the chiasm, passes posterolaterally along the hypothalamus and cerebral crus, and enters the ventral surface of the lateral geniculate body (Sls. 31, 32). Here, axons from the retinal ganglion cells finally reach the tertiary visual path neurons and synapse.

The lateral geniculate nucleus has a triangular shape, somewhat similar to a Napoleonic hat (Pls. 10, 28, 29).

It consists of six layers. Impulses from the ipsilateral and contralateral retinae pass to three layers each: layers 1, 4, and 6 receive fibers from the contralateral retina and layers 2, 3, and 5 receive ipsilateral fibers. The two ventral layers (5 and 6) are the magnocellular layers related to the "M" or "Where?" pathway, while the four dorsal layers (1-4) are the parvocellular layers related to the "P" or "What?" path.

The tertiary lateral geniculate neurons give rise to the geniculocalcarine tract or optic radiation which initially enters the retrolenticular part of the posterior limb of the internal capsule (Pl. 28; MRI 10, Sls. 31, 32, 49).

As it enters the internal capsule the optic radiation forms a conspicuous triangular area referred to as the zone of Wernicke. The fibers carrying impulses from the contralateral lower quadrant of the visual field proceed posteriorly and superiorly in the lateral wall of the posterior horn of the lateral ventricle. Those carrying impulses from the contralateral upper quadrant of the visual field pass anteriorly and inferiorly and form the loop of Meyer which arches over the roof and into the lateral wall of the inferior or temporal horn of the lateral ventricle. They then proceed posteriorly in the lateral wall of the posterior horn. Hence, geniculocalcarine fibers carrying impulses from the lower half of the contralateral field of vision course through the white matter of the parietal and occipital lobes, while those representing the upper half course through the white matter of the temporal and occipital lobes.

Observe the optic radiation near the outer wall of the posterior horn of the lateral ventricle (Pls. 31, 32, 33; MRI 11, MRI 12, MRI 13) where it forms the external sagittal stratum which is conspicuous because its fibers are more myelinated than others in the vicinity.

The optic radiation terminates in the primary visual cortex located in the walls of the calcarine sulcus (Pls. 5, 32, MRI 14). The lower part of the visual field is represented superiorly, in the cuneus, while the upper part is represented inferiorly, in the lingual gyrus. Moreover, impulses from the macular region of the retina are localized in the posterior half of the visual cortex, the paramacular more anteriorly, and those from the peripheral parts of the retina most anteriorly. Observe these areas in a brain specimen.

Light Reflex

Many of the structures involved in the light reflex may be traced on a gross brain specimen and in brainstem slides. The afferent limb corresponds to the first part of the visual pathway and includes the photoreceptor cells, bipolar neurons, ganglion neurons, optic nerve, optic chiasm, and optic tract. Near its termination, many of the optic tract fibers veer medially and bypass the lateral geniculate body, forming the brachium of the superior colliculus. This bundle can be followed in a brain specimen (Pl. 10) and in Slides 31 and 32 as it courses medially between the medial geniculate body and the ventral part of the pulvinar. This brachium terminates in the pretectal area (Sl. 33). Within the pretectal area are groups of pretectal nuclei, some of which comprise the light reflex center. Pretecto-oculomotor axons from the light reflex center either pass anteriorly toward the periaqueductal gray matter or they cross in the posterior commissure (Pls. 12, 28; Sl. 33) and then proceed to the contralateral periaqueductal gray. Thus, the consensual response is based on the crossing of some impulses in the optic chiasm and others in the posterior commissure.

The efferent limb of the light reflex originates in the Edinger-Westphal nucleus (Sls. 32, 33), the visceromotor part of the oculomotor complex. The Edinger-Westphal nucleus is comprised of preganglionic parasympathetic neurons whose axons travel via the ipsilateral oculomotor nerve to the ciliary ganglion. The postganglionic pupilloconstrictor fibers reach the eye in the short ciliary nerves and terminate on the sphincter of the iris (Sl. 92).

Accommodation Reflexes

The stimulus for accommodation originates in the occipital cortex. Corticotectal fibers arising here proceed to the brainstem by following, in a reverse direction, the optic radiation. Thus, the internal sagittal stratum, lying between the lateral wall of the posterior horn of the lateral ventricle and the external sagittal stratum, contains these occipitotectal fibers which then enter the retrolenticular part of the internal capsule, bypass the lateral geniculate nucleus, and reach the brachium of the superior colliculus by coursing medially ventral to the pulvinar (Sls. 31, 32).

An accommodation center is located at the superior colliculus-pretectum junction. Impulses from here pass to visceromotor and somatomotor neurons of the oculomotor complex (Sls. 31, 32, 33). The visceromotor neurons are located in the Edinger-Westphal nucleus which sends preganglionic parasympathetic fibers via the oculomotor nerve to the ciliary ganglion. The short ciliary nerves then carry the postganglionic parasympathetic fibers to the eye where some terminate on the ciliary muscles (Sl. 91) and others terminate on the constrictor muscle of the iris (Sl. 92). The somatomotor neurons involved in the accommodation responses are oculomotor alpha motoneurons whose axons pass via the oculomotor nerves to the medial recti muscles of the eyes.

Pupillary Dilation

Passive pupillary dilation occurs when the intensity of light is decreased, while active pupillary dilation occurs as a result of the emotional expressions associated with fear, rage, etc. and is a sympathetic nervous system response. The pathway is shown in Fig. 11-3 and the only CNS structure that may be precisely identified is the sympathetic cell column at C.8-T.1, the ciliospinal center. Our myelin stained sections do not include these levels.

Oculomotor System

Brainstem Gaze Centers

Two areas in the brainstem are associated with conjugate eye movements, a pontine center related to horizontal movements and a midbrain center related to vertical movements. Identify the pontine gaze center as the paramedian pontine reticular formation adjacent to the abducens nucleus (Sls. 21). The neurons in the pontine gaze center send their axons directly to the ipsilateral abducens nucleus (Sls. 31, 32, 33). Alpha motoneurons here produce abduction in that eye; interneurons in the abducens nucleus project axons through the contralateral MLF (Sls. 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33) to the oculomotor nucleus (Sls. 31, 32, 33) to produce adduction of the other eye.

Stimulation of the pontine gaze center produces gaze to the ipsilateral side, while lesions in this area result in paralysis of gaze toward the ipsilateral side.

The vertical gaze center is in the midbrain periaqueductal gray at the most rostral part of the oculomotor nucleus. It includes one or perhaps more of the accessory oculomotor nuclei (Sl. 33). and the right and left nuclei are interconnected via the posterior commissure.

Clinical evidence suggests that upward movements are represented more dorsally because with pressure on the tectum, often caused by a tumor of the pineal gland which is at the surface of the pretectum, upward gaze becomes paralyzed before downward.

Cortical Gaze Centers

Identify the frontal eye field (FEF) that is associated with saccades and is located chiefly in the posterior part of the middle frontal gyrus (Pl. 4). Its impulses travel in corticofugal fibers through the internal capsule (Sls. 47, 48, 49) to 1) to the gaze centers in the midbrain and pons, and the superior colliculus (Sl. 31), from whence corticofugal fibers project bilaterally to the gaze centers in the midbrain and pons, and 2) the superior colliculus. (Sls. 31, 32)

Stimulation of the FEF results in sustained gaze to the opposite side. An acute lesion of this center or of its corticofugal fibers in the internal capsule as in capsular stroke, results in gaze toward the side of the lesion, which is transient because the FEF in each hemisphere projects bilaterally to the brainstem gaze centers.

After a moving object is brought into focus, it is followed and kept in focus by smooth pursuit movements. Recent evidence suggests that the smooth pursuit movement pathway chiefly involves the posterior part of the lateral surface of the temporal lobe, although the superior parietal lobule and frontal eye field may also be involved. Projections from these cortical areas pass to dorsolateral pontine nuclei, which send fibers to the flocculus of the cerebellum and then to vestibular nuclei that project to the ocular motor nuclei (III, IV, and VI).

The primary visual and visual association areas in the occipital lobe control the near response associated with the accommodation reflexes. Occipitotectal fibers from these areas project to the vergence center.  

Because of the great importance of vision in man, eye movements are influenced by numerous centers in the cerebral cortex. Moreover, the connections of these centers occur bilaterally and over a wide variety of routes. Hence, although focal lesions in the cerebral hemispheres may impair eye movements temporarily, they do not do so permanently.

Superior Colliculus

Each superior colliculus (Sls. 31, 32) is a laminated structure consisting of alternating fibrous and cellular layers. The superficial layers are related chiefly to visual input from the retina and visual association areas. The intermediate layers receive input from the frontal eye fields, while the deep layers receive mainly auditory and somatosensory input from appropriate brainstem and spinal cord nuclei. Descending tectofugal fibers arise from the deeper layers and pass chiefly to brainstem centers related to conjugate eye movements. Tectospinal fibers descend just anterior to the medial longitudinal fasciculus to cervical spinal cord levels related to turning the head.