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UPSC MainsMEDICAL-SCIENCE-PAPER-I201212 Marks
Q2.

Tabulate the extrinsic and intrinsic muscles of eyeball along with their nerve supply and action(s).

How to Approach

This question requires a detailed anatomical understanding of the muscles controlling eyeball movement. The answer should be presented in a tabular format for clarity and conciseness. Focus on accurately listing both extrinsic and intrinsic muscles, their respective nerve supplies (crucial for clinical relevance), and their primary actions. A systematic approach – listing extrinsic muscles first, followed by intrinsic – is recommended. Mentioning the clinical significance of damage to these muscles can add value.

Model Answer

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Introduction

The human eye's ability to move and focus relies on a complex interplay of muscles. These muscles are broadly categorized into extrinsic and intrinsic groups. Extrinsic muscles originate outside the eyeball and are responsible for gross movements like looking up, down, left, and right. Intrinsic muscles, located within the eyeball, control the shape of the lens for accommodation and pupillary constriction/dilation. Understanding these muscles, their innervation, and actions is fundamental to ophthalmology and neurology, as their dysfunction can lead to various visual disturbances. This response will detail these muscles in a tabular format, outlining their nerve supply and actions.

Extrinsic Muscles of the Eyeball

These muscles originate from the bony orbit and insert into the sclera. They control the movement of the eyeball in various directions.

Muscle Nerve Supply Primary Action(s)
Superior Rectus Oculomotor Nerve (CN III) Elevation, Intorsion, Adduction
Inferior Rectus Oculomotor Nerve (CN III) Depression, Extorsion, Adduction
Medial Rectus Oculomotor Nerve (CN III) Adduction
Lateral Rectus Abducens Nerve (CN VI) Abduction
Superior Oblique Trochlear Nerve (CN IV) Intorsion, Depression, Abduction
Inferior Oblique Oculomotor Nerve (CN III) Extorsion, Elevation, Abduction

Intrinsic Muscles of the Eyeball

These muscles are located within the eyeball and control the shape of the lens and the size of the pupil.

Muscle Nerve Supply Action(s)
Sphincter Pupillae Parasympathetic fibers of Oculomotor Nerve (CN III) Pupil constriction (miosis)
Dilator Pupillae Sympathetic fibers Pupil dilation (mydriasis)
Ciliary Muscle Parasympathetic fibers of Oculomotor Nerve (CN III) Accommodation (lens shape change for near vision)

Clinical Correlation

Damage to the nerves supplying these muscles can result in diplopia (double vision) or strabismus (misalignment of the eyes). For example, damage to the abducens nerve (CN VI) leads to inability to abduct the eye, resulting in esotropia (inward deviation of the eye). Similarly, damage to the oculomotor nerve (CN III) can cause ptosis (drooping eyelid), dilated pupil, and impaired eye movements.

Conclusion

In conclusion, the precise and coordinated action of both extrinsic and intrinsic muscles is crucial for normal vision. The extrinsic muscles facilitate eye movements, while the intrinsic muscles control accommodation and pupillary response. Understanding their nerve supply is paramount, as nerve damage can lead to significant visual impairments. A thorough knowledge of these muscles is essential for diagnosing and managing various ophthalmological conditions, ensuring optimal visual function.

Answer Length

This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.

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Additional Resources

Key Definitions

Diplopia
Diplopia, commonly known as double vision, is a visual disturbance where a single object appears as two images. It can be monocular (affecting one eye) or binocular (affecting both eyes).
Accommodation
Accommodation refers to the eye's ability to adjust its focus to maintain a clear image of objects at varying distances. This is achieved by changing the shape of the lens, controlled by the ciliary muscle.

Key Statistics

Approximately 1-3% of the population experiences strabismus, with the highest prevalence in children under the age of 5.

Source: National Eye Institute (NEI), 2023 (Knowledge Cutoff)

Globally, an estimated 2.2 billion people have a vision impairment or blindness, of which at least 1 billion could have been prevented.

Source: World Health Organization (WHO), 2019 (Knowledge Cutoff)

Examples

Horner's Syndrome

Horner's syndrome, resulting from damage to the sympathetic pathway, presents with miosis (pupil constriction), ptosis (drooping eyelid), and anhidrosis (lack of sweating) on the affected side, demonstrating the role of sympathetic innervation in pupillary dilation.

Frequently Asked Questions

▶What is the difference between adduction and abduction?

Adduction is the movement of the eye towards the nose, while abduction is the movement of the eye away from the nose. They are opposing movements controlled by the medial and lateral rectus muscles, respectively.

Topics Covered

AnatomyPhysiologyOphthalmologyEyeballMusclesNervesEye Movement
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