Which cranial nerve innervates the lateral rectus muscle of the eye?

Appeared in: KPSC Staff Nurse - 2021

Explanation

The 6th cranial nerve, also known as the abducens nerve, is responsible for innervating the lateral rectus muscle.
The primary function of the lateral rectus muscle is to abduct the eye, which means moving it outward, away from the nose.
A widely used mnemonic to remember the innervation of the eye muscles is '(LR6SO4)AO3': Lateral Rectus by CN6, Superior Oblique by CN4, and All Others by CN3.

Option A: The 3rd cranial nerve (oculomotor) innervates the majority of the extraocular muscles, but specifically excludes the lateral rectus and the superior oblique.
Option B: The 4th cranial nerve (trochlear) has a singular function: it exclusively innervates the superior oblique muscle.
Option C: The 5th cranial nerve (trigeminal) is primarily a sensory nerve for the face and provides motor function to the muscles of mastication (chewing). It does not innervate any of the extraocular muscles.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Innervation of extraocular muscles as background academic context rather than a clinical decision trigger.
Assessing the six cardinal fields of gaze by having a patient follow a finger in an 'H' pattern is a fundamental nursing skill used to test the integrity of cranial nerves III, IV, and VI.
A deficit in the abducens nerve (CN VI) is called abducens palsy. It results in the inability to abduct the affected eye, causing it to deviate medially (inward squint or esotropia) and leading to horizontal double vision (diplopia) that worsens when looking toward the affected side.
Abducens palsy can be an early sign of increased intracranial pressure (ICP) because of the long course the nerve takes within the skull, making it vulnerable to compression.

How to Approach the Question

First, identify the core of the question: it's asking for the specific nerve supply to a single muscle, the lateral rectus.
Recognize this as a direct recall question based on anatomical knowledge of the cranial nerves and their functions.
Access memory aids for cranial nerve functions. The mnemonic '(LR6SO4)AO3' is highly effective for this topic.
Break down the mnemonic: 'LR6' stands for Lateral Rectus innervated by Cranial Nerve VI (Abducens).
'SO4' stands for Superior Oblique innervated by Cranial Nerve IV (Trochlear).
'AO3' stands for All Other extraocular muscles innervated by Cranial Nerve III (Oculomotor).

Concept Tested & Keywords

Q16-HuotmrK6ah3kdHTQFK

E6 Anatomy Applied Ashalatha 4e p. 426-428

E6 Physiology Ganong 27e Vol 2 Part 3 p. 5-7

E6 Medicine Harrison 22e Part 1 p. 276-278

Practise the full KPSC Staff Nurse - 2021

Attempt every question from this paper in a timed mock, then review the full solution for each one.