Final answer:
Testing a patient's eye movements with the follow-the-pen test evaluates the combined function of the oculomotor, trochlear, and abducens cranial nerves, as well as referring to the optic nerve for visual perception.
Step-by-step explanation:
Testing Cranial Nerve Function
When asking a patient to follow the tip of a pen with their eyes, the test evaluates the function of several cranial nerves. Specifically, the eye movements are controlled by the oculomotor (CN3), trochlear, and abducens nerves, which together orchestrate the movement of the extraocular muscles. These movements involve testing for conjugate gaze and assessing functions such as convergence and accommodation. Dysfunction might manifest as a lack of coordination between the eyes, leading to symptoms like diplopia or double vision. An inability of one eye to follow the movement of the other is referred to as internuclear ophthalmoplegia. Similarly, a failure in accommodation or convergence is associated with deficits in the oculomotor nerve.
These cranial nerves are interconnected by the medial longitudinal fasciculus (MLF) for the coordination of gaze. A complex reflex involving these nerves, known as the vestibulo-ocular reflex, stabilizes gaze during head movements by coordinating equilibrium sensations with the eye movement systems.
The assessment also includes examining the optic nerve (CN2), which is crucial for visual acuity and field. The optic nerve can be tested through different visual field tests and a physical inspection with an ophthalmoscope to examine the optic disk.