AP2 3.1: Vision and the Eye Flashcards
The five special senses
vision, smell, taste, hearing, and equilibrium (balance) are the senses that have specialized organs containing specialized receptor cells, which carry their impulses by way of specialized somatic and visceral afferents.
The other sense
The other sense, touch, is a somatic sense that does not have a specialized sense organ. Instead, touch uses general receptors composed of modified dendrites of sensory neurons. Touch includes pressure, vibration, pain, heat, and the combined information is carried in general somatic afferents and general visceral afferents.
organ responsible for sight
The organ responsible for sight is the eye, and it consists of three layers. The outer fibrous layer includes the sclera and cornea.
sclera
the Eye
The posterior portion is known as the sclera. also known as the “white of the eye.
cornea
the Eye
The anterior portion is known as the cornea. The cornea is the transparent part of the eye where light enters.
Aqueous humor
the Eye
Aqueous humor fills the spaces between the cornea and the lens.
The middle layer of the eye
the Eye
The middle layer of the eye includes choroid, the ciliary muscle, and the iris.
choroid
the Eye
The darkly-colored posterior choroid prevents light from dispersing throughout the eye. The choroid is highly vascular and supplies blood to the other layers of the eye.
ciliary body
the Eye
The anterior ciliary body changes the shape of the lens, allowing it to focus.
iris
the Eye
The iris is anterior to the ciliary body and contains the circular colored portion of the eye. The iris controls the amount of light let into the pupil, a hole in the center of the iris. The iris uses its muscle fibers to contract or dilate based on the amount of light in the environment.
Vision Pathway
Light passes into eye moving, progressively through the cornea, aqueous humor, lens, and vitreous humor to the posterior surface of the eye on the retina.
vitreous body
the Eye
The interior of the eye, posterior to the lens, is called the vitreous body. It is a chamber filled with vitreous fluid, which helps to hold the retina firmly to the choroid.
retina
the Eye
The inner sensory layer includes the retina. The retina contains containing two types of photoreceptors, cells that are sensitive to light. The more numerous cells are rods, which are stimulated in dim light. Rods are more sensitive to light but do not generate sharp or color images. The cones operate in bright light, helping to generate sharp color images.
lens
the Eye
The lens is located posterior to the iris and pupil.
Photoreceptors in the retina
Vision Pathway
Photoreceptors in the retina send a signal through the optic nerve on to the optic chiasm, located at the base of the hypothalamus.
optic tracts
Vision Pathway
The medial fibers of the optic nerve cross to the other side when they reach the optic chiasm where the optic tracts are formed. The optic tracts terminate in the thalamus in a region called the lateral geniculate nucleus (LGN). The information then continues from the thalamus through the optic radiations to the primary visual area of the occipital lobe. Finally, in the occipital lobe, the incoming sensory information is interpreted as vision.
binocular visual field
Overlapping information in the nasal visual fields allows for 3-D vision, called the binocular visual field.
nasal visual field
Vision Pathway
Each eye receives information from the left and right sides of the body. The nasal visual field of each eye is closest to the nose.
visual field
Each eye has a left visual field (purple in Figure 3.4) and a right visual field (orange in Figure 3.4).
peripheral vision field
Vision Pathway
The peripheral vision field is on the lateral side of each eye.
Each eye has a left and a right visual field
Sensory information gathered from the left side of the body (bilateral left visual fields) will eventually be interpreted in the right side of the brain and is reversed for the right visual fields. Information from the left visual field of each eye is gathered onto the ride side of each retina. The information from each retina is carried through the optic nerves to the optic tract. Only the medial fibers of the optic nerves cross at the optic chiasm. The left side of the brain receives information from the medial side of the left eye and the lateral side of the right eye (bilateral right visual fields). The situation is reversed for the right side of the brain, with the right side of the brain receiving information from the bilateral left visual fields.
Eye Muscles
Eye Muscles
Eye movements are controlled by six muscles, called the extrinsic eye muscles. The medial rectus, inferior rectus, superior rectus, and inferior oblique are all innervated by the third cranial nerve, the oculomotor nerve.
medial rectus
Eye Muscles
The medial rectus turns the eye medially.
inferior rectus
Eye Muscles
The inferior rectus moves the eye medially and depresses it.
superior rectus
Eye Muscles
The superior rectus moves the eye medially and elevates it.
inferior oblique
Eye Muscles
The inferior oblique is responsible for eye elevation, lateral movement, and external rotation.
lateral rectus
Eye Muscles
Finally, the sixth muscle, the lateral rectus, is controlled by the sixth cranial nerve, the abducens nerve, and turns the eye laterally.
superior oblique
Eye Muscles
The fifth muscle is the superior oblique, which is controlled by the fourth cranial nerve, the trochlear nerve. The superior oblique is responsible for eye depression, lateral movement, and internal rotation.
Visual System Pathology
Visual impairments occur from damage to any part of the vision pathway.
Cranial nerve damage
Visual System Pathology
Cranial nerve damage can also cause vision problems. Several different cranial nerves supply the muscles to move the eye. For example, the abducens nerve is responsible for the lateral movement (left and right) of the eyes. Damage to the abducens nerve would cause diplopia, or double vision. One test for abducens nerve damage is to have a person look left and right while keeping the head still. If the lateral rectus does not receive nervous signals from the abducens nerve, the eye is unable to rotate laterally when looking to the same side. The affected eye deviates medially when looking forward because of the imbalance in muscle tone between the medial and lateral recti.
Damage inside the brain
Visual System Pathology
Damage inside the brain, such as damage from a CVA or a brain tumor, can also cause vision impairments. For example, if the right optic nerve is damaged, the right eye will be unable to see.
Cataracts
Visual System Pathology
Cataracts are lenses that slowly become hardened and cloudy over time. Cataracts make vision look blurry because the light is unable to enter the lens clearly to be refracted onto the retina.
