Vision and Sensory Systems

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Questions and Answers

Explain why diplopia occurs in strabismus (squint).

Diplopia (double vision) occurs in strabismus because the misalignment of the eyes causes images to fall on non-corresponding points of the retinas. The brain cannot fuse these disparate images, resulting in the perception of two images.

Describe briefly the clinical testing for visual acuity.

Clinical testing for visual acuity typically involves using standardized eye charts, such as the Snellen chart. The patient reads letters of decreasing size from a set distance (usually 6 meters or 20 feet), one eye at a time, to determine the smallest line they can read accurately.

Describe briefly the disorders of the pupil due to defective visual reflexes.

Disorders include Horner's syndrome (miosis, ptosis, anhidrosis due to sympathetic pathway lesion), Adie's tonic pupil (dilated pupil with poor light reaction, slow accommodation), and Argyll Robertson pupil (bilateral small pupils that constrict with accommodation but not with light, often seen in neurosyphilis).

Enumerate the various disorders of vestibular system. Add a note on vestibular function tests.

Disorders include Benign Paroxysmal Positional Vertigo (BPPV), Meniere's disease, vestibular neuritis, labyrinthitis, and acoustic neuroma. Vestibular function tests include caloric testing (irrigating ear canals with warm/cold water), electronystagmography (ENG)/videonystagmography (VNG) to record eye movements, and rotational chair testing. Signup and view all the answers

Explain why we stop perceiving a particular smell after sometime.

This phenomenon is called olfactory adaptation or olfactory fatigue. Olfactory receptors become less responsive to a continuous stimulus over time. This rapid adaptation allows the olfactory system to remain sensitive to new or changing odors in the environment. Signup and view all the answers

Photoreceptor potential is hyperpolarizing and not depolarizing in nature. Explain.

In the dark, photoreceptors (rods and cones) are relatively depolarized due to an influx of Na+ ions through cGMP-gated channels (the 'dark current'). When light strikes, it activates a G-protein cascade that leads to the hydrolysis of cGMP, closing these channels. The reduction in Na+ influx causes the cell membrane to hyperpolarize. Signup and view all the answers

Deficiency of retinol interrupts photoreceptor pigment regeneration (Visual cycle). Explain.

Retinol (Vitamin A) is a precursor for retinal, the chromophore component of rhodopsin (in rods) and photopsins (in cones). During phototransduction, retinal changes shape (isomerization) and detaches from opsin. Regeneration requires reconversion of all-trans retinal back to 11-cis retinal, a process dependent on the availability of retinol supplied via the retinal pigment epithelium (RPE). Retinol deficiency impairs this regeneration, leading to reduced photoreceptor sensitivity, especially in low light (night blindness). Signup and view all the answers

Explain why a person is not able to see clearly for a few minutes when entering inside a cinema hall.

This is due to dark adaptation. In bright light outside, the photoreceptors (especially rods) are bleached and less sensitive. Entering a dark cinema requires regeneration of rhodopsin in the rods, which takes several minutes. Cone adaptation is faster but cones function poorly in low light. Visual sensitivity gradually increases as rhodopsin regenerates, allowing clear vision in the dark. Signup and view all the answers

Explain why pupillary reflexes are preserved in the visual pathway lesions affecting the lateral geniculate body.

The pupillary light reflex pathway bypasses the lateral geniculate nucleus (LGN). Afferent fibers from the retina travel via the optic nerve and optic tract, but fibers mediating the light reflex project to the pretectal nucleus in the midbrain, not the LGN (which relays visual information to the cortex). Therefore, lesions affecting the LGN or visual cortex impair vision but spare the pupillary light reflex. Signup and view all the answers

Explain why macular vision is typically unaffected in lesions affecting optic radiations and visual cortex.

The macula, responsible for central, high-acuity vision, has a large and often dual representation in the visual cortex, receiving blood supply from both the middle cerebral artery (MCA) and posterior cerebral artery (PCA). Lesions, particularly those caused by PCA occlusion affecting the main visual cortex, often spare the macular representation area due to this collateral blood supply from the MCA, preserving central vision. Signup and view all the answers

Explain why patients with gigantism can develop visual blurring.

Gigantism is often caused by a pituitary adenoma secreting excess growth hormone. As the pituitary gland is located near the optic chiasm, an enlarging adenoma can compress the optic chiasm. Compression typically affects the nasal retinal fibers (crossing fibers), leading initially to bitemporal hemianopia (loss of peripheral vision in both temporal fields). If compression progresses or affects other parts of the visual pathway, it can lead to generalized visual blurring and decreased acuity. Signup and view all the answers

Explain why visual acuity is highest at the fovea.

Visual acuity is highest at the fovea due to: 1) High density of cone photoreceptors. 2) Absence of rods. 3) Minimal convergence, with nearly a 1:1 ratio of cones to bipolar cells to ganglion cells, allowing for fine detail discrimination. 4) Thinning of overlying retinal layers, reducing light scattering before it reaches the photoreceptors. Signup and view all the answers

Presbyopia is not a true refractive error. Justify

True (A) Signup and view all the answers

Describe the physiological basis of the hearing mechanism.

Sound waves enter the ear canal, vibrate the tympanic membrane, transmit through the middle ear ossicles (malleus, incus, stapes) to the oval window. This vibration creates pressure waves in the cochlear fluid (perilymph and endolymph), causing the basilar membrane to vibrate. Hair cells on the organ of Corti, located on the basilar membrane, are stimulated by this vibration, bending their stereocilia against the tectorial membrane. This bending opens ion channels, depolarizing the hair cells and releasing neurotransmitters, which generate action potentials in the auditory nerve fibers, transmitting signals to the brain for interpretation as sound. Signup and view all the answers

Give the common causes of conductive deafness and their physiological basis.

Common causes include: 1) Cerumen impaction (earwax blockage): Prevents sound waves from reaching the eardrum. 2) Otitis media (middle ear infection): Fluid accumulation restricts ossicle movement. 3) Otosclerosis: Abnormal bone growth fixing the stapes to the oval window, impairing sound transmission. 4) Eardrum perforation: Reduces the surface area for vibration. The physiological basis is the impedance of sound wave transmission through the outer or middle ear. Signup and view all the answers

Give the physiological basis of Reading or close work becomes progressively difficult with the advancing age.

This difficulty is due to presbyopia. With age, the lens loses elasticity and becomes harder (lens sclerosis), and the ciliary muscle may weaken. This reduces the eye's ability to increase its refractive power (accommodate) to focus on near objects. Consequently, the near point of vision recedes, making reading and close work blurry without corrective lenses. Signup and view all the answers

Give the physiological basis of Argyll Robertson pupil.

The Argyll Robertson pupil is characterized by bilateral small pupils that constrict poorly or not at all to light (light-near dissociation), but constrict normally during accommodation (near response). The exact lesion site is debated but is thought to be in corresponds to bilateral damage to the pretectal nuclei in the midbrain, which interrupts the afferent pathway for the pupillary light reflex before it reaches the Edinger-Westphal nucleus, while sparing the pathways for accommodation originating from the visual cortex. Signup and view all the answers

Ultraviolet and infrared rays are not perceived by the human eye. Give reasons.

The human eye perceives light within the visible spectrum (approx. 400-700 nm). Ultraviolet (UV) rays (<400 nm) are largely absorbed by the cornea and lens, preventing them from reaching the retina; even if they did, photoreceptors lack sensitivity to these wavelengths. Infrared (IR) rays (>700 nm) do not have enough energy per photon to trigger the photochemical reaction (isomerization of retinal) in photoreceptors required for vision. Although high-intensity IR can be felt as heat, it's not detected visually. Signup and view all the answers

Give the physiological basis for when light is thrown into one eye, the pupil of the opposite eye constricts.

This is the consensual pupillary light reflex. Afferent signals from the illuminated retina travel via the optic nerve and tract to the pretectal nucleus in the midbrain on both sides (due to partial decussation and bilateral projections). Each pretectal nucleus projects to both Edinger-Westphal nuclei (parasympathetic preganglionic neurons). Efferent signals travel via the oculomotor nerve (CN III) to the ciliary ganglion, and then postganglionic fibers cause constriction of the sphincter pupillae muscle in both irises. Therefore, light shone in one eye causes constriction in both the direct (ipsilateral) and consensual (contralateral) pupil. Signup and view all the answers

Aircraft pilots wear red goggles in bright light. Give reasons.

Pilots wear red goggles before flying at night or in low-light conditions, often while preparing in a brightly lit room. Red light does not significantly bleach rhodopsin, the photopigment in rods responsible for night vision, because rods are relatively insensitive to long (red) wavelengths. Cones, however, can adapt to the red light, allowing the pilot to see and function. This preserves rod sensitivity, allowing for faster dark adaptation and better night vision once the goggles are removed and the pilot enters the dark cockpit or flies at night. Signup and view all the answers

Explain why presbyopia occurs in normal individuals as age advances, usually after the age of 40 years.

Presbyopia occurs due to age-related changes in the eye's accommodative system. Primarily, the lens progressively hardens and loses elasticity over time (lens sclerosis). Additionally, the strength of the ciliary muscle, which contracts to allow the lens to become more convex for near focus, may diminish. These factors reduce the eye's ability to increase its refractive power sufficiently to focus on near objects, leading to blurred near vision, typically becoming noticeable around age 40-45. Signup and view all the answers

Flashcards

Diplopia in Strabismus

Double vision occurring when the eyes are misaligned.

Clinical Testing for Visual Acuity

Assessing sharpness of vision, often using a Snellen chart.