MCAT Social and Behavioral Sciences : Physiology of the Eye

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Example Question #1 : Environment And Sensation

Which of following is a true statement regarding light and the human eye?

Possible Answers:

Light will pass through the lens before passing through the cornea

The retina is a small disc located at the focal point at the back of the eye

Light will pass through the aqueous humor before passing through the vitreous humor

The pigments of the iris contain small numbers of photoreceptors

Correct answer:

Light will pass through the aqueous humor before passing through the vitreous humor

Explanation:

The first structure that light will encounter as it enters the eye is the cornea, a transparent protein disc designed to refract light toward the eye interior. The aqueous humor is the liquid medium between the cornea and the lens of the eye. Light will pass through the aqueous humor and the pupil before hitting the lens. The lens then focuses the light onto the back of the eye. The vitreous humor is the liquid medium between the lens and the retina at the back of the eye.

The iris is used to control the shape of the pupil, and does not contain photoreceptors. All photoreceptors are located on the retina, which lines the back wall of the eye. A small region known as the fovea centralis houses a large number of cones (photoreceptors) at the back of the eye, but is part of the greater retinal structure.

The only true statement given is that light will pass through the aqueous humor before passing through the vitreous humor.

Example Question #1 : Physiology Of The Eye

Diana takes the subway to and from school every day. Her family’s apartment, situated in a low-income neighborhood of New York City, is a thirty-minute walk from the subway station. During her trips to the subway, Diana loves to watch people work, play, and socialize. She feels very safe in her community because she trusts the people around her. 

Diana’s mother is having trouble seeing clearly. The lens of her eye appears white and cloudy. Which of the following most likely describes this situation?

Possible Answers:

Cataract

Kinestesis

Conjunctivitis

Astigmatism

Correct answer:

Cataract

Explanation:

The symptoms describe a “cataract.” Cataracts form when a protein coat builds up and cause a white barrier to form that limits vision. It is normally surgically removed. “Astigmatism” is a misshapen cornea that can also cause vision problems. “Conjunctivitis” is a disorder that is made evident by the inflammation of the sclera, or white part of the eye. Last, “glaucoma” describes damage caused to the optic nerve, usually by a buildup of excess fluid in the eye. 

Example Question #2 : Physiology Of The Eye

In myopia, which of the following abilities is impaired?

Possible Answers:

Ability to discern color

Ability to shift focus from distant objects to close objects

Ability to focus on distant objects

Ability to focus on close objects

Correct answer:

Ability to focus on distant objects

Explanation:

Myopia, often called nearsightedness, is an inability to being distant objects into clear focus. This may be due either to excess curvature of the cornea or to length of the eyeball itself causing the focal point of light rays to fall in front of the retina, rather than against it.

Example Question #3 : Physiology Of The Eye

Which of the following terms describes the inability to focus vision on close-up objects?

Possible Answers:

Strabismus

Amblyopia

Nystagmus

Presbyopia

Correct answer:

Presbyopia

Explanation:

“Presbyopia” is the term for the inability to focus the vision on close-up objects. It is most commonly caused by loss of elasticity of the lens of the eye as individuals age. 

On the other hand, the other choices are incorrect. “Amblyopia” is a general term used to denote cases in which one eye is communicating less visual information to the brain, resulting in a loss of vision in that eye In other words, the brain learns to ignore visual stimulation from the non-dominant eye. A common cause of amblyopia is “strabismus”—commonly known as “lazy eye.” It is defined as a lack of coordination between extraocular muscles that prevents the eyes from orienting in the same direction. In this case, there will almost always be a dominant eye, and if the condition is not corrected in childhood, the non-dominant eye generally suffers permanent vision loss. “Nystagmus” is a condition of involuntary eye movement (i.e. the eye seems to shudder, generally in a horizontal plane) that may or may not be associated with visual impairment. It can have numerous etiologies, including nervous system disorders, alcohol or drug reaction, congenital defect, or inner-ear disorder.

Example Question #4 : Physiology Of The Eye

Which of the following structures of the eye is not responsible for the refraction and focusing of light?

 

Possible Answers:

Retina

Cornea

Anterior chamber

Lens

Correct answer:

Retina

Explanation:

Light is refracted and focused by the cornea, the anterior chamber, and the lens. The retina is a thin layer of nervous system tissue, which receives focused light patterns and transmits those patterns to the brain via photoreceptor cells. 

Example Question #5 : Physiology Of The Eye

Which of the following is not a primary function of the extrinsic eye muscles?

Possible Answers:

Saccadic eye movements 

Tracking moving objects

Convergence 

Dilation of the pupils

Correct answer:

Dilation of the pupils

Explanation:

The extrinsic muscles of the eye are primarily responsible for the mechanical movement of the eyeball. The main movements of the eye are convergence during accommodation, saccadic movements, tracking, and maintenance of a horizontal position. Dilation of the pupils is dependent on intrinsic muscles of the eye.

Example Question #6 : Physiology Of The Eye

Which muscle is responsible for changing the shape of the lens during accommodation? 

Possible Answers:

Dilator pupillae

Medial rectus muscle

Ciliary muscle

Sphincter pupillae

Correct answer:

Ciliary muscle

Explanation:

The structure responsible for changing the shape of the lens during accommodation is the ciliary muscle. This is a ring of smooth muscle in the middle layer of the eye that is attached to the lens via the suspensory ligament. Contraction and relaxation of the ciliary muscle will cause the suspensory ligament to tighten (in the case of relaxation of the ciliary muscle) or relax (in the case of contraction of the ciliary muscle). This affects the thickness of the lens, allowing for accommodation.

The sphincter pupillae and dilator pupillae are both responsible for changing the shape of the iris, rather than the lens, and the medial rectus muscle is an extrinsic eye muscle that moves the whole eye medially.

Example Question #7 : Physiology Of The Eye

Which division of the nervous system controls the dilator pupillae and is known as the iris dilator muscle?

Possible Answers:

The somatic division of the peripheral nervous system

The sympathetic division of the autonomic nervous system

The parasympathetic division of the autonomic nervous system

The sensory division of the peripheral nervous system

Correct answer:

The sympathetic division of the autonomic nervous system

Explanation:

Control of the muscles responsible for dilation of the pupil is mediated by the sympathetic division of the autonomic nervous system. During states of fear, sexual arousal, or heightened attention, the pupil will dilate due to the stimulation of the dilator pupillae, or iris dilator muscle.

Example Question #8 : Physiology Of The Eye

Deficits in which cranial nerve could cause ptosis (i.e. the drooping of the upper eyelid)?

Possible Answers:

CN IV (the trochlear nerve)

CN VII (the facial nerve)

CN III (the oculomotor nerve)

CN VI (the abducens nerve)

Correct answer:

CN III (the oculomotor nerve)

Explanation:

The levator palpebrae superioris is the muscle responsible for elevating the upper eyelid. It is innervated by CN III, the oculomotor nerve. CN IV (the trochlear nerve) and CN VI (the abducens nerve) are both responsible for innervation of various other muscles of eye movement, while the facial nerve (CN VII) is primarily responsible for control of the muscles of expression, for taste, and for motor innervation to the muscles of the inner ear.

Example Question #9 : Physiology Of The Eye

Which of the following is the term for the process by which the eye's lens changes shape to bring close or distant objects into focus on the retina?

Possible Answers:

Convergence

Mydriasis

Contrast

Accommodation

Correct answer:

Accommodation

Explanation:

The lens of the eye must be able to change its shape in order to re-focus the vision on near or far objects as the eye moves. This process is called “accommodation.” On the other hand, “convergence” is the ability of the eyes to move toward one another in order to focus on objects that are very close up. “Mydriasis” is the term for dilation of the pupil, and "contrast" is a visible quality of difference in color or brightness of objects, and not a function of the eye itself.

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