The optics of the eye focus light on specialized retinal photoreceptor cells, which transduce light into a neuronal signal. The visual field has a highly ordered representation on the retina and throughout the visual pathway from the retina to the cerebral cortex. Central processing occurs in the primary visual cortex and surrounding visual association cortex and allows interpretation of the complex qualities of vision (e.g., form, color, depth, motion, and distance).
The major structures of the eye are described as follows (Figure 2-23A):
The sclera is the white of the eye. It is composed of tough connective tissue, which is continuous with the dura mater around the optic nerve.
The retina forms the inner cellular layer around most of the eye. The fovea is a small depression in the center of the retina, which is surrounded by a region called the macula. Light is focused on this part of the retina, which has the highest level of visual acuity (i.e., the ability to distinguish between two-point light sources).
The optic nerve exits the eye posteriorly. The point of exit of the optic nerve causes a discontinuation of the retina and produces a small “blind spot” in the visual field.
The cornea is the curved transparent area where light enters the front of the eye.
The iris is the colored diaphragm running across the anterior part of the eyeball. The pupil is the aperture at the center of the iris, the diameter of which is controlled by muscles of the iris. The pupil dilates (mydriasis) in the dark to allow more light to enter the eye and constricts (miosis) in bright light to prevent the excessive entry of light.
The lens lies behind the cornea and consists of a transparent viscous gel encased in a capsule. The lens and cornea together constitute the optic apparatus of the eye. The ciliary muscle and suspensory ligaments determine the curvature of the lens.
The cavity of the eye contains fluids; the aqueous humor is in front of the lens, and the vitreous humor is behind the lens.
A. Anatomy of the eye. B. Secretion and reabsorption of aqueous humor. The ciliary epithelium secretes aqueous humor, which enters the anterior compartment of eye via the pupil and drains into the venous system via the canal of Schlemm.
Table 2-4 highlights examples of conditions that involve different structures of the eye.
Table 2-4Conditions Involving the Major Structures of the Eye ||Download (.pdf) Table 2-4Conditions Involving the Major Structures of the Eye
|Major Eye Structure ||Disease/Condition |
|Sclera ||Osteogenesis imperfecta: The sclera appear blue because the genetic defect in the type I procollagen allows the underlying (blue) choroid layers to be seen. |
|Retina ||Hypertensive retinopathy: Severity and duration of hypertension are the key determinants of retinopathy. One classification system is based on funduscopic examination findings: grade 1, arterial narrowing, “copper/silver wiring” ; grade 2, arteriovenous nicking; grade 3, flame-shaped hemorrhages, cotton-wool spots (infarcted zones), hard exudates (extravasated lipid); grade 4, papilledema. |
|Macula ||Tay-Sachs disease: A neurodegenerative disease caused by hexosaminidase A deficiency. Patients have a characteristic macular “cherry-red” spot. |
|Optic nerve ||Multiple sclerosis: Optic neuritis is a common initial presenting symptom of multiple sclerosis characterized by decreased visual acuity, ocular pain (especially with movement), and color desaturation. |
|Cornea ||Herpes: Dendritic ulcers of the cornea are characteristic of herpetic eye infections. |
|Iris ||Neurofibromatosis type I: A genetic condition that results in characteristic neurofibromas (benign Schwann cell tumors), café au lait spots (freckling of non-sun-exposed skin), and Lisch nodules (iris hamartomas). |
|Lens ||Marfan's syndrome: A genetic defect in fibrillin production that results in the classic clinical triad: ocular lens dislocation, aortic dilation, and long thin extremities. |
Fluid Compartments of the Eye
The intraocular fluids of the eye keep the eyeball distended. Vitreous humor is a stable gelatinous mass, but aqueous humor is continually secreted. Intraocular pressure is maintained within a normal range of approximately 15 mm Hg (±3 mm Hg) by a balance between the secretion and absorption of aqueous humor. The aqueous humor is secreted by the ciliary body and is absorbed via the canal of Schlemm.
Epithelial cells projecting from the ciliary body actively secrete aqueous humor into the space between the iris and the lens (the posterior compartment) (see Figure 2-23B).
Aqueous humor flows through the pupil into the space between the iris and the cornea (the anterior compartment).
The canal of Schlemm leaves the anterior chamber, at the angle between the iris and cornea, and drains into the extraocular veins.
Glaucoma is associated with a pathologic increase in intraocular pressure, which occurs as a result of an imbalance between aqueous humor production and drainage. Glaucoma may result in damage to the optic nerve and progressive loss of vision. Patients with acute closed-angle glaucoma, which results from rapid increases in intraocular pressure, present with a painful, red eye and are at risk of rapid permanent vision loss. Acute closed-angle glaucoma is considered a medical emergency! Open-angle glaucoma, on the other hand, is much more common and presents as an insidious onset of painless progressive loss of vision; typically, the peripheral vision is affected first, and the central vision is affected later.
Light rays bend as they enter the eye and become focused at the retina. The bending of light at a curved surface is called refraction. The degree of refraction depends on how curved the surface is and on the refractory index of the material. Materials with a high refractory index cause the light to slow more as it enters the eye, resulting in more refraction.
The cornea and lens are both convex surfaces that cause light to be focused toward a focal point; the distance beyond the lens where light focuses is the focal distance (Figure 2-24A). When an image is viewed through a convex lens, the projected image is turned upside down, with the two sides reversed (see Figure 2-24B). This occurs at the eye, but the brain is able to perceive images in their correct orientation.
Optical properties of the eye. A. The convex cornea and lens cause refraction of light and convergence of light rays to bring objects into focus on the retina. The focal distance is the distance from the front of the eye to the focal point. B. Image inversion by the optical apparatus of the eye.
The refractive power of a lens is measured using diopter (“D”) units. A spherical lens that focuses light 1 m beyond the lens has a power of +1 D; a lens with 10 times this refractory power (+10 D) focuses light 10 cm beyond the lens. A concave lens disperses light and has refractory powers described in negative dioptric units.
Mechanism of Accommodation
The total refractory power of the eye is approximately +60 D, with two thirds of refraction occurring at the cornea and most of the remaining refraction occurring at the lens. The lens has a variable curvature that provides refractive plasticity by contributing between +13 D (in its most flattened state) to +26 D (in its most curved state). The ability to voluntarily change the refractive power of the lens allows objects at different distances to be focused on the retina. For example, if a distant object moves toward the eye, the lens becomes more curved to keep the object in focus. Changing the shape of the lens to focus an object is called accommodation.
The mechanism of accommodation depends on the elastic properties of the lens and the tension placed on it by the suspensory ligaments. If a lens is isolated from the eye, it becomes almost spherical. However, inside the eye, the lens is placed under tension by the suspensory ligaments, and in the resting condition, the lens remains fairly flat. The ciliary muscle is located at the attachment of the suspensory ligaments to the eyeball; contraction of the ciliary muscles reduces tension in the ligaments and allows the lens to assume a more rounded shape. Contraction of the ciliary muscles is controlled by the parasympathetic nerves (see Autonomic Innervation of the Eye).
People with normal vision (emmetropia) focus distant objects clearly, with the ciliary muscle completely relaxed (Figure 2-25A). To focus objects at close range, the ciliary muscle is contracted to provide the appropriate amount of accommodation. There are four basic refraction errors:
Myopia (nearsightedness) and hyperopia (farsightedness). A. The normal eye focuses light from a distant point on the retina with the ciliary muscles relaxed. B. Myopia results when the eyeballs are longer than normal, causing light to be focused on a point in front of the retina. Myopia can be corrected using eyeglasses with a concave lens. C. Hyperopia results when the eyeballs are shorter than normal, causing light to be focused behind the retina. Hyperopia can be corrected using eyeglasses with a convex lens.
Myopia (nearsightedness) occurs when light from distant objects is focused in front of the retina with the ciliary muscle relaxed. There is no way for the maximally flattened lens to refract the light any less, resulting in a limiting “far point” for clear vision. However, accommodation still occurs to allow clear focus on closer objects. Myopia is usually caused by an eyeball that is too long; the refraction error can be corrected using eyeglasses with a concave lens (see Figure 2-25B).
Hyperopia (farsightedness) occurs when the resting position of the lens does not adequately refract the light from distant objects, causing the focal point to lie behind the retina. The hyperopic patient is able to voluntarily use the accommodation mechanism to increase the refractive power of the lens and bring distant objects into focus. The remaining range of ciliary muscle contraction available to accommodate near objects may be reduced, causing the “near point” to recede. Hyperopia is most commonly the result of a short eyeball; the refraction error is corrected using eyeglasses with a convex lens (see Figure 2-25C).
Presbyopia is the reduced ability to accommodate near or distant objects due to a dramatic decrease in the elasticity of the lens, which occurs with increasing age. In most people older than the age of 70, the curvature of the lens becomes fixed, resulting in the inability to focus on near objects (e.g., reading); distance vision is also limited because the lens flattens less than it does in younger people. Bifocal lenses, which have an upper half of the lens focused for distance vision and a lower part focused for near vision, are often prescribed.
Astigmatism is caused by incorrect curvature of the eye in one plane. Two different focal distances are produced, depending on the plane on which light enters the eye. Eyeglasses with a cylindrical lens are needed to correct the refraction error of astigmatism.
The retina is the site of phototransduction and is composed of the following cellular layers (Figure 2-26A):
The retinal pigment epithelium is the outermost layer of cells. It assists the photoreceptor cells by recycling the visual pigment molecules that are degraded during phototransduction. The pigment epithelium also absorbs stray light, preventing reflection of light back into the eye, which would otherwise disturb phototransduction.
There are two types of photoreceptor resting on the pigment epithelium: the rods and the cones.
Retinal bipolar cells form synapses with photoreceptor cells at one pole and with ganglion cells at the other pole.
Ganglion cells are the innermost layer of cells, and they are the output cells of the retina. The axons of ganglion cells become the optic nerve.
Horizontal cells have a radial orientation and form synapses in the outer layer of the retina with the photoreceptors and the bipolar cells.
Amacrine cells have a similar orientation to horizontal cells but are located in the inner layer of the retina, where they synapse with bipolar cells and ganglion cells.
The retina. A. Incoming light passes through several neuronal layers to reach the photoreceptor cells. The retinal output consists of action potentials in ganglion cells; the axons of ganglion cells are conveyed to the central nervous system via the optic nerve. B. Rods and cones. Photoreceptor cells have two major regions, an outer segment resting on the retinal pigment epithelium, and an inner segment that synapses with bipolar and horizontal cells. The outer segment contains stacks of membranous disks containing the visual pigment molecule rhodopsin.
Properties of Rods and Cones
Rods and cones both have an outer segment and an inner segment (see Figure 2-26B). The outer segment faces the retinal pigment epithelium and is composed of characteristic membrane folds, referred to as disks. The disk membrane contains a high concentration of the visual pigment molecule rhodopsin. The inner segment synapses with bipolar and horizontal cells. There are key differences in the properties of rods and cones that underlie different functional properties of the visual system:
Rods are monochromatic (single color) receptors, which are highly sensitive to light and allow objects to be seen in low intensity light. Dark adaptation (increased light sensitivity in response to prolonged darkness) occurs to a much greater extent in rods than in cones.
Cones are less sensitive to light and function best under high light intensity conditions, such as bright sunlight. There are three types of cone photoreceptors, with overlapping sensitivity to light of different wavelength (see Mechanism of Color Vision):
Blue (or S) cones absorb light of short wavelength.
Green (or M) cones absorb light of medium wavelengths most optimally.
Red (or L) cones absorb light at the long wavelength end of the visible spectrum.
In most cases, colored light excites each type of cone but to a different degree; it is therefore the pattern of stimulation from the three types of cones that encodes color.
The fovea only contains cone cells. Most cone cells in the fovea synapse with a single bipolar cell, which in turn synapses with a single ganglion cell. This creates very small receptive fields for the foveal bipolar and ganglion cells, thereby accounting for the high visual acuity of the fovea. Visual acuity decreases dramatically toward the periphery of the eye; a high proportion of rod cells are located in the periphery, and many rods converge on each ganglion cell. Therefore, central vision has a high resolution but is poor in dim light, whereas peripheral vision has low resolution but allows vision in low light.
Mechanism of Phototransduction
Phototransduction is the cascade of chemical and electrical events through which light energy is converted into a receptor potential (Figure 2-27). Rods and cones are unusual sensory receptors in that the receptor potential is a hyperpolarization. The phototransduction mechanism involves the following steps:
Mechanism of phototransduction. The membrane potential of photoreceptors is depolarized in the dark due to sustained opening of cyclic guanosine monophosphate (cGMP)-dependent cation channels. The absorption of light by the visual pigment rhodopsin stimulates the G protein transducin to increase cGMP phosphodiesterase activity. cGMP is broken down to guanosine monophosphate (GMP), which causes cation channels to close and results in a hyperpolarizing receptor potential.
The photoreceptor membrane potential is depolarized in the dark because cyclic guanosine monophosphate (cGMP)-activated nonselective cation channels are open. Continual Na+ influx is measured as the “dark current” of photoreceptors. Depolarization of the photoreceptor causes tonic release of the neurotransmitter glutamate.
Light is absorbed by the light receptor molecule rhodopsin. The rhodopsin molecule is formed from the transmembrane protein opsin and an attached retinal molecule (retinal is a vitamin A derivative).
Photons of light are absorbed by retinal, causing isomerization from 11-cis retinal to all-trans retinal. A resulting conformation change in opsin activates the rhodopsin pigment; activated rhodopsin is called metarhodopsin II.
Opsin is associated with the guanosine triphosphate (GTP)-binding protein transducin, which triggers a second-messenger response inside the photoreceptor.
When rhodopsin absorbs light, transducin is activated, resulting in the stimulation of a cGMP phosphodiesterase. cGMP is the diffusible second messenger that is responsible for maintaining the dark current. Activation of the phosphodiesterase increases the rate of cGMP breakdown and causes cGMP-activated cation channels to close.
Inhibition of the dark current causes the photoreceptor membrane to hyperpolarize, which reduces the tonic release of glutamate on bipolar cells.
Termination of phototransduction involves the inactivation of metarhodopsin II by the ever-present enzyme rhodopsin kinase. All-trans retinal and opsin separate soon after light is absorbed, and are recycled over a period of several minutes to regenerate rhodopsin.
Signal amplification is a key feature of the phototransduction mechanism, whereby a few photons of light produce a large change in intracellular cGMP concentration and closure of many nonselective cation channels.
The output from the retina occurs in the form of action potentials from retinal ganglion cells. Each ganglion cell may receive input from a single bipolar cell (e.g., at the fovea) or from the convergence of many bipolar cells (e.g., at the periphery of the eye). There is significant refinement of the visual signal by the retina before central pathways are involved. For example, our highly developed ability to detect contrasts and the edges of objects is based on the presence of different types of bipolar cell. Some bipolar cells respond to glutamate with a depolarization and others with a hyperpolarization.
Depolarization of the bipolar cell occurs when it expresses the ionotropic glutamate receptor. These bipolar cells are excited in the dark when photoreceptors produce the tonic dark current, and are therefore referred to as OFF bipolar cells.
Hyperpolarization of the bipolar cell occurs when it expresses metabotropic glutamate receptors, which convert glutamate into an inhibitory signal. These bipolar cells are referred to as ON bipolar cells because they are excited in the light (Figure 2-28A). Stimulation by light reduces glutamate release and weakens the inhibitory stimulus in ON bipolar cells.
Center-surround organization of bipolar cell receptive fields. A. Stimulation of an ON bipolar cell when light is shone in the center of its receptive field through a direct pathway between the photoreceptor and bipolar cell. B. Inhibition of an ON bipolar cell when light is shone in the area of the retina surrounding the receptive field center. Hyperpolarization of the photoreceptor results in hyperpolarization of the bipolar cell through an indirect pathway that includes horizontal cells.
A point light source moving across the retina will cause a variable pattern of action potential frequencies from ganglion cells, depending on whether the receptive field is excited or inhibited by light. This pattern of signaling helps the visual system to perceive contrasts such as edges and borders. The ability to perceive contrasts is greatly increased by the property of antagonistic center-surrounds in the receptive fields of bipolar cells. Whatever the electrical response of a bipolar cell is to light in the center of the receptive field, the opposite response occurs if light is applied to the immediate surrounding area (see Figure 2-28B). When a point light source moves across the retina, an ON bipolar cell is inhibited as the light moves across one edge of its receptive field surround. When the light moves to the center of the receptive field, the bipolar cell is strongly stimulated. However, when the light moves to the other side of the receptive field surround, the output signal is again decreased.
Dark and Light Adaptation
Retinal sensitivity must be altered in response to changing levels of light or images appear washed out and uniform in bright light or, conversely, images are invisible in dim light conditions. In sustained bright light, a large proportion of rhodopsin will dissociate to retinal and opsin. Therefore, the retina becomes adapted to light by having less visual pigment available for phototransduction. Conversely, the retina becomes more sensitive to light with increasing time spent in darkness. Rhodopsin stores are built up as all the available retinal and opsin are combined. Rods are responsible for the dark adaptation response, which takes 30–40 minutes of continuous darkness to reach a peak.
Vitamin A has many important functions in the body, including maintenance of healthy epithelia and vision. Vitamin A deficiency results in night blindness (rod cell dysfunction), xerophthalmia (dry eyes that are prone to ulceration and infection), and follicular hyperkeratosis (rough elevations of skin around hair follicles resembling goose bumps).
Mechanism of Color Vision
Color is determined by the wavelength of light. The phototransduction of light of differing wavelengths is achieved by the three different types of cone receptors. Traditionally, these receptors are referred to as blue, green, and red cones, but they are also called S (short), M (medium), and L (long) to reflect the differences in the wavelengths of light producing the most light absorbance. Differences in the light absorption spectrum for each type of cone arise from differences in the primary structure of the opsin protein, which is present in the visual pigment. The nervous system decodes color according to the relative stimulation of the three types of cones. In other words, a green object is not perceived as green because only M cones are activated; all cones may be activated to a greater or lesser extent, and the pattern of overall activation decodes the color.
“Color blindness” is a common condition in which there is a range of possible defects in color vision. Most commonly, a single type of cone receptor is missing, which results in the inability to distinguish some colors from others. X-linked recessive mutations are a common cause of defective color vision, resulting in a higher proportion of males than females with this condition.
Monochromacy (true color blindness) is the lack of two out of three of the cone receptor types and is rare.
Dichromacy is the lack of one type of cone receptor. The selective loss of the blue cone (tritanopia) is rare. Most commonly, either the red cone (protanopia) or the green cone (deuteranopia) is missing. There is extensive overlap in the spectral sensitivity of red and green cones for wavelengths of light between 500 and 600 nm. The loss of either red or green cones results in difficulty distinguishing between green, yellow, orange, or red colors. Patients have particular difficulty distinguishing between red and green and are therefore said to have red-green color blindness.
Anomalous trichromy is caused by defective (not missing) L cones with absorbance spectra between the normal L and M ranges.
The main visual pathway conveys signals from the retina to the primary visual cortex as follows (Figure 2-29):
Axons from retinal ganglion cells enter the optic nerve in each eye.
The optic nerves meet at the optic chiasm, where some axons cross the midline.
An optic tract leads from each side of the optic chiasm to the lateral geniculate body of the thalamus, where retinal ganglion cells synapse.
Second-order sensory neurons follow a course to the primary visual cortex via the optic radiation.
The main visual pathway. The binocular visual field is shown, divided into equal left and right halves. Light from the left visual hemifield stimulates the right half of each retina; light from the right visual hemifield stimulates the left half of each retina. Nerve impulses originating from stimuli in the left hemifield are transmitted to the right visual cortex, whereas those from the right hemifield are transmitted to the left visual cortex.
The visual fields of each eye overlap extensively to produce binocular vision. Images from each half of the visual field are processed by the contralateral side of the visual cortex (e.g., the left visual cortex is concerned with information from the right half of the visual field). As a result, axons from the nasal half of each retina must cross the midline at the optic chiasm, whereas axons from the temporal half of each retina remain on the ipsilateral side. For example, light from the right half of the visual field projects onto the nasal half of the right retina and will cross at the optic chiasm before continuing on to the left visual cortex, whereas the light that projects on the temporal half of the left retina will not cross at the optic chiasm and will continue on to the left visual cortex (Figure 2-29).
The visual pathway between the retina and the visual cortex can be interrupted at several locations, resulting in characteristic visual field defects. Table 2-5 summarizes the defects caused by different lesions in the visual pathway.
Table 2-5Visual Field Defects Caused by Lesions in the Visual Pathway ||Download (.pdf) Table 2-5Visual Field Defects Caused by Lesions in the Visual Pathway
|Site of Lesion ||Description of Visual Field Defect ||Defect ||Possible Cause |
|Optic nerve ||Blindness in the affected eye ||Monocular blindness ||Optic neuritis, retinal artery occlusion |
|Optic chiasm ||Loss of fibers crossing the midline from the nasal half of each retina causes loss of temporal visual field on both sides ||Bitemporal heteronymous hemianopia ||Pituitary tumor (e.g., craniopharyngioma) pressing on the optic chiasm from below |
|Optic tract ||Loss of fibers for the visual field on the opposite side to the lesion ||Homonymous hemianopia ||Brain tumor |
|Optic radiation ||Loss of fibers for the visual field on the opposite side to the lesion ||Homonymous hemianopia ||Brain tumor; occlusion of a branch of the posterior cerebral artery |
|Visual cortex (one side) ||Loss of visual processing for the visual field on the opposite side to the lesion ||Homonymous hemianopia with macular sparing ||Posterior cerebral artery thrombosis; however, the central (macula) vision is maintained due to collateral circulation between the posterior and middle cerebral arteries |
Visual fibers are not restricted to the main visual pathway. Information about light levels and the visual scene project to the following brain areas:
The suprachiasmatic nuclei, for control of circadian rhythms.
The superior colliculi of the midbrain, for the control of eye movements.
The pretectal nuclei of the midbrain, for the reflex control of eye movement associated with changing the focus of vision and for the pupillary light reflex.
Movements of the eyes must be extremely fast and accurate to serve the demands of the visual system. For example, the control of eye movement must be able to fix the gaze on objects at different distances, to maintain focus on objects as the head moves, and to follow moving objects in the visual field. The movement of both eyes must also be exactly integrated to maintain binocular vision. Movements of the eye are mediated by three pairs of muscles, which receive motor innervation from cranial nerves III (oculomotor), IV (trochlear), and VI (abducens) (Figure 2-30):
Extraocular muscles. The individual action of each muscle is shown for the right eye. The nerve supply of each muscle is shown in parentheses.
The medial and lateral rectus muscles, which move the eyes side to side.
The superior and inferior rectus muscles, which move the eyes upward and downward.
The superior and inferior oblique muscles, which prevent rotation of the eyeball and move the eyes upward and downward.
Fixation movements of the eye lock the gaze on a specific object, allowing it to be focused on the central part of the retina. Once a particular object is selected, the gaze is locked by the involuntary fixation pathway, which begins in the visual association areas surrounding the primary visual cortex. Involuntary fixation on the object is maintained by reflexes that are coordinated by the superior colliculus. The eyes continuously make small movements, which cause the image on the fovea to move slightly. If the image moves close to the edge of the fovea, there is a rapid reflex flicking movement of the eyes to restore the position of the image to the center of the fovea. The gaze is unlocked by the voluntary fixation pathway. The voluntary selection of a new object originates from an area in the frontal lobe, close to the premotor cortex. Cortical fibers descend to the brainstem nuclei that control cranial nerves III, IV, and VI.
Saccadic movements of the eyes are rapid movements in the position of the eyes (saccades). Saccadic movement is necessary when objects in the visual field are moving or when the head is moving. A succession of fixation points is selected to survey the “highlights” of the moving visual scene. Reading is an example of saccadic movement, where the brain becomes trained to use saccades to survey a static visual field for highlights. If an object is moving in a regular cycle, saccadic movements quickly become smooth “pursuit” movements as the central visual processing mechanisms adapt and produce programmed eye movements.
Nystagmus is defined as rhythmic oscillations of the eyes characterized by a slow drifting component and a fast saccadic component in the opposite direction. Although nystagmus can be physiologic, it can also indicate underlying serious pathologic conditions, including demyelinating diseases (e.g., multiple sclerosis), cerebellar or brainstem lesions, drug intoxication (anticonvulsants or alcohol), or vestibular dysfunction (e.g., Ménière's disease). A complete neurologic examination should be performed on all patients presenting with nystagmus.
To achieve binocular vision, the visual image of interest must be projected onto the fovea of both eyes simultaneously. This is achieved even during rapid saccadic eye movements, and the brain fuses the images from each eye into the perception of a single image. One major advantage of binocular vision is improved depth perception (stereopsis). Because the eyes are placed about 5 cm apart, the images projected onto the two retinae are not precisely the same, particularly for objects that are close to the eyes. This slight difference is computed by the brain to provide depth perception in the visual scene.
Strabismus refers to misalignment of the eyes, which results in two images being projected to the brain, causing diplopia (double vision) in adults. In children, however, uncorrected strabismus can result in amblyopia (reduced vision) of the misaligned eye. Amblyopia occurs because the developing brain has the ability to suppress the images from the deviated eye, thereby preventing diplopia at the expense of reduced vision in the affected eye.
Autonomic Innervation of the Eye
When the gaze changes to fix on a new object at a different distance from the eye, the lens must rapidly accommodate to focus the object on the retina. The brain cortical areas that control fixation movements of the eye also control accommodation of the lens. Cortical signals descend to the oculomotor nucleus in the midbrain (see Figure 2-7A). The oculomotor nucleus has two efferent parts:
A motor nucleus, giving rise to motor fibers to the extraocular muscles.
A visceral part (the Edinger-Westphal nucleus), giving origin to parasympathetic preganglionic fibers, which pass in cranial nerve III to the ciliary ganglion. Postganglionic neurons pass to the eye and innervate the ciliary muscles for accommodation of the lens.
The same pathway from the Edinger-Westphal nucleus is used by parasympathetic neurons that innervate the iris to bring about constriction of the pupil. The pupillary light reflex assists the eye in adapting to variable light levels; reflex constriction of the pupils occurs when light is shone into the eyes. Afferent signals from some retinal ganglion cells pass (via the optic nerve) to the pretectal nucleus of the midbrain; interneurons connect the pretectal area to the Edinger-Westphal nucleus on both sides. Efferent parasympathetic neurons innervate muscles of the iris, resulting in constriction of the pupil. The pupillary light reflex is consensual (i.e., light shone in one eye constricts both pupils). Brainstem damage can result in an absent pupillary light reflex and/or the presence of different sized pupils.
A detailed understanding of the pupillary light reflex pathway is essential to diagnosing the location of an associated lesion. Consider afferent versus efferent lesions in the pathway:
Afferent (optic nerve, CN II) lesion. When light is shone in the affected eye, the direct and consensual reflex is absent; when light is shone in the unaffected eye, the direct and consensual reflex is intact.
Explanation. The afferent nerve must be intact to trigger an efferent (pupillary constriction) response. Because neurons project from the pretectal area to the Edinger-Westphal nucleus on both sides, any afferent stimulus that reaches the pretectal nucleus will trigger an efferent response in both eyes (assuming an intact efferent pathway).
Efferent (CN III or Edinger-Westphal nucleus) lesion. Light shone in the eye of the affected side will trigger a consensual reflex but not a direct reflex; light shone in the eye of the unaffected side will trigger a direct reflex but not a consensual reflex.
Note: a lesion of CN III will result in weakness of the extraocular muscles, innervated by CN III, in addition to defects in the pupillary light reflex.
Sympathetic innervation to the eye is relayed from the first thoracic segment of the spinal cord and reaches the eye via the superior cervical ganglion. Postganglionic fibers travel along the outer surface of blood vessels to the eye. Sympathetic neurons innervate the iris to cause dilation of the pupil.
Horner's syndrome is caused by the interruption of the sympathetic nerves to the face and head and therefore has the following consequences:
Persistent constriction of the pupil on the affected side, due to loss of the sympathetic dilator response.
Persistent vasodilatation of blood vessels on the affected side, due to loss of sympathetic vasoconstriction.
Loss of sweating in the affected area, due to sweat glands being stimulated by sympathetic innervation.
Droop of the upper eyelid, due to loss of contraction of smooth muscle fibers in the eyelid, which receive sympathetic innervation.