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1. A thin, watery fluid found anterior to the lens, between lens and cornea.
2. The aqueous humor contains nutrients for the cornea and lens, both of which lack a blood supply. |
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1. A thick, jelly-like semifluid found posterior to the lens, between lens and retina.
2. The vitreous humor is important in maintaining the shape of the orbit. |
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| the white part of the eye. |
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| the clear portion of the outer layer found anteriorly. The cornea is a non-adjustable "lens" of the eye, whereas the lens proper, located in the middle layer, is adjustable. |
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| contains blood vessels and nerves |
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| the pigmented portion of the eye (blue, green, brown) consisting of smooth muscles for constricting or dilating the pupil. Pupillary constriction requires the action of the parasympathetic component of CN III. Pupillary dilation requires activity of sympathetic neurons from the spinal cord T1 and T2 segments. (Recall that lesions at T1 and T2 result in Horner's syndrome). |
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| suspended behind the iris diaphragm by fine "guy ropes" called zonula fibers that are anchored in the ciliary body. |
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| contains involuntary muscles that vary the tension exerted on the lens by zonula fibers. Adjustment of the shape of the lens to make it more rounded is called accommodation. This process enables one to focus on near objects, and is a function that is lost with age (usually around middle age). |
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the light-sensing part of the eye. The retina is actually an extension of the diencephalon.
There are numerous cells located in the retina (a variety of interneurons: amacrine cells, bipolar cells, horizontal cells) that help to process the neural signals of the visual system. |
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| are photoreceptors for black, white & gray vision; are important for night vision. |
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are photoreceptors for color vision and for high visual acuity
E. The photosensitive part of the photoreceptors (rods and cones) is at the most distal end of the light path that enters the eye through the cornea. Thus light has to travel through many cell layers of the retina before it reaches the light-sensitive part of the photoreceptor. Passage of light through all these cell layers does not reduce visual sensitivity or acuity because these superficial cell layers neither absorb nor distort light. |
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| The retinal ganglion cells |
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| the most anterior of the cells in the retina. Axons of these bipolar cells form the optic nerve (CN II). |
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| no photoreceptors here, that part of the retina |
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| The macula lutea (or simply macula) |
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| a region of the retina about 1 cm in diameter. In the center of the macula is the fovea, an area that is particularly rich in cones (no rods here). |
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| Region of highest visual acuity (all interneurons that are found in other regions of the retina are missing from this area) |
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| sharp, straight-ahead vision that is needed for driving and reading small print. |
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| Most fibers in each optic tract terminate (synapse) in the... |
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| lateral geniculate nucleus of the thalamus (which, like the medial geniculate nucleus for the auditory system, is a relay nucleus). |
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| neurons leaving the lateral geniculate pass anteriorly and laterally into the temporal lobe before they curve to pass posteriorly in the optic radiation |
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1. A combined visual and reflex pathway extends from the retina to the superior colliculus in the midbrain tectum.
2. This pathway is important in visual attention and detection of movement.
3. By activating neurons of the tectospinal pathway, the head may be oriented to a novel moving visual stimulus. |
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| Retinohypothalamic Pathway |
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Definition
1. Photic input via the retina reaches the hypothalamus in a region above the optic chiasm called the suprachiasmatic nucleus.
2. There is evidence that the suprachiasmatic nucleus of the hypothalamus is a master clock that sets the timing of many (but not all) circadian rhythms that are entrained to a 24 hour light cycle.
3. Examples of these rhythms include fluctuations in body temperature, hormone secretion, eating, drinking, altertness, and sleep-wake cycles.
4. Involved in sympathetic pupillary dilation |
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| The response of the stimulated eye is called |
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Definition
| direct light response (or reflex). |
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| The response of the unstimulated eye is |
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| consensual light response (or reflex |
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| Pupillary Constriction (Parasympathetic) |
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| Bright light directed into one eye causes the pupils of both eyes to constrict, following contraction of pupillary constrictor muscles. |
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| Pupillary Dilation (Sympathetic) involved in Horner’s syndrome |
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Definition
1. A descending sympathetic axonal pathway from the hypothalamus synapses on preganglionic neurons of the intermediolateral cell column (lateral horn) in the upper thoracic spinal cord (T1-T2).
2. These spinal cord neurons send axons to the superior cervical ganglia (part of the sympathetic chain ganglia).
3. The superior cervical ganglia neurons then send axons the pupillary dilator muscle of the iris.
4. Again no cortical involvement in this reflex. |
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| Accommodation-Convergence Reflex |
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Definition
1. This reflex involves adjustments of the lens by action of the ciliary body to bring a closely-placed object into focus
2. There is cortical involvement in that the individual selects the object to be brought into focus, and makes a conscious effort to focus.
3. After a visual stimulus (looking at a near object) reaches the visual cortex, impulses then descend to the superior colliculus to activate preganglionic parasympathetic neurons of the Edinger-Westphal nucleus. The pre-ganglioinic neurons synapse in the ciliary ganglion (as for the light reflex) and cause the smooth muscles, this time of the ciliary body, to round the lens for near vision (the lens is flattened for distance vision).
4. Immediately after the eyes shift from a distant object to a near one, and the lens thickens (rounds) to bring the object into focus by accommodation, two further events occur:
A. The eyes converge as the medial rectus muscles contract bilaterally (CN III function).
B. The pupils constrict to increase definition of the image (also a CN III function). |
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| The retina can be divided by a vertical line into |
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Definition
| temporal and nasal hemiretinas. |
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| The retina can also be divided by a horizontal line into |
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| superior and inferior hemiretinas. |
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| An age-related eye disease affecting the small part of the retina responsible for sharp, straight-ahead vision that is needed for driving and reading small print. This disorder usually occurs in people 65 years of age or older. |
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| separation of layers of the retina. These are normally only firmly adherent at the optic disk and at a region called the ora serrata. Retinal layers can separate following head trauma or may be due a familial problem with no trauma involved. The retina floats forward in the vitreous humor. To the patient, the detached retina looks like a curtain, blocking vision. Retina can be reattached with laser surgery. |
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| a blind spot sometimes due to small lesion in retina. The optic disk is a natural blind spot. Scotomas also accompany some forms of migraine headache. Sometimes the blind spot moves or "dances" and is accompanied by the appearance of bright images. This is called a scintillating photoscotoma. This may last for several minutes and is usually followed by the migraine headache. |
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| secondary to oxygen therapy can cause damage to the retina and blindness. Premature babies on oxygen therapy are susceptible to blindness if too much oxygen is given. |
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| Hypertension can cause compression of the retina |
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| Increased intraocular pressure due to excessive aqueous humor (due to insufficient drainage secondary to blockage in the drainage canal). Excess aqueous humor pushes the lens backward into the vitreous humor, which presses against the retina. Compression of retina and optic nerve can lead to blindness if not treated. |
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| Opacity of the lens of the eye. Can be a normal consequence of aging. |
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| Diabetes mellitus causes blood vessels of the retina (and other parts of the body) to become leaky. Protein escapes into the normally clear aqueous humor, making it cloudy and causing light to defract, blurring vision. In advanced stages, blindness results. |
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Definition
| loss of accommodation (near vision) due to aging. The lens becomes less distensible and will not round enough to focus on near objects. This is not a neural problem. |
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| Uncal herniation through the tentorial notch as a result of increased supratentorial intracranial pressure, causes |
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Definition
| causes compression of the midbrain resulting in fixed, dilated pupils, ptosis, and "down and out" eye (due to compression of CN III, oculomotor nerve). |
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| Interruption of the non-decussating optic tract fibers results in |
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Definition
| ipsilateral nasal hemianopsia (nasal visual field is lost on one side). |
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| A midline lesion of the optic chiasm results in |
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| bitemporal heteronymous hemianopsia (or hemianopia). That is, blindness in the nasal half of the retina or the temporal half of the visual field of each eye. The result is tunnel vision. |
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| interruption of the optic tract, lateral geniculate body, optic radiations, or visual cortex on one side (e.g., on the right side) results in |
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| contralateral homonymous hemianopsia. That is, blindness in the field of vision contralateral to the lesion (e.g., left side). |
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| A lesion of the cuneus (primary visual cortex above the calcarine sulcus) results in |
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Definition
| contralateral lower quadrantic anopsia because pathways from the upper temporal quadrant of the ipsilateral retina and upper nasal quadrant of the contralateral retina are interrupted. |
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| Cortical blindness may occur due to |
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Definition
| stroke, trauma, or near-drowning when blood or oxygen supply is interrupted to geniculostriate fibers (i.e., optic radiations) or striate cortex (primary visual cortex, area 17). This is due to a lesion of the posterior cerebral artery. The light reflexes may be intact, but the visual information never reaches the primary visual cortex. |
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| Uncal herniation through the tentorial notch as a result of increased supratentorial intracranial pressure, causes |
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Definition
| causes compression of the midbrain resulting in fixed, dilated pupils, ptosis, and "down and out" eye (due to compression of CN III, oculomotor nerve). |
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| Temporal lobe lesions may cause visual deficits (contralateral homonymous superior quadantanopia) because |
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Definition
| axons of Meyer's loop course through the temporal lobe on their way to the primary visual cortex. |
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