Anatomy and Physiology of the Eye

Anatomy of the Visual Pathway

Most fibers of the optic tract (about 90%) terminate in the lateral geniculate body, which is the thalamic nucleus on its respective side. In this area, axons of retinal ganglion cells synapse onto the next neurons.

Next, the optic radiation (radiatio opticae), also known as Gratiolet's optic radiation, is formed. These structures run deep within the tissues of the temporal and parietal lobes.

The endpoint of Gratiolet's optic radiation is the primary visual cortex. It is localized on the medial surface and the pole of the occipital lobes, specifically within the calcarine sulcus (sulcus calcarinus). Secondary cortical areas surround it, receiving impulses from the primary cortex. Their main function is to ensure the conscious and subjective perception of the received visual information.

Topical Diagnosis of Lesions

The cortical centers of the occipital lobes process information coming from the contralateral visual fields. This is a key rule of topical diagnosis.

• Damage to the optic tract, lateral geniculate body, occipital lobe cortex, or the entire optic radiation results in contralateral hemianopia. For instance, a lesion on the right side will lead to the loss of the left visual field, and vice versa.
• In isolated lesions of Gratiolet's optic radiation, the clinical picture varies depending on the location. If the parietal lobe is affected (where the upper half of the radiation from the lower retinal quadrants runs), the contralateral inferior visual field is impaired. If the lesion is in the temporal lobe (lower half of the radiation from the upper retinal quadrants), the contralateral superior visual field is disrupted.

Assessment of Visual Functions and Differential Diagnosis

The examination begins with taking the medical history: the physician determines the rate of symptom development and their connection to other conditions (somatic, neurological, or psychiatric). Visual acuity is tested for each eye separately.

In neurology, two main terms are used to describe visual impairments:

• Amblyopia — partial reduction of visual acuity.
• Amaurosis — complete blindness.

Vision loss requires differential diagnosis. The most frequent causes are ophthalmological in nature (myopia, hyperopia, astigmatism, lens pathologies). In such cases, corrective lenses restore vision. However, if lenses do not help and fundus examination reveals normal refractive media and retina, a neurological pathology is diagnosed. This indicates that the process is localized in the optic chiasm or optic nerve.

The Role of the Facial Nerve in Eye Physiology

The facial nerve directly affects the state of the eyeball. Its motor branch innervates the facial expression muscles, while its secretory branch innervates the lacrimal and salivary glands.

Facial neuropathy (which can result from infections, otitis media, tumors, facial trauma, or as a complication of anesthesia) leads to paralysis of the facial muscles. In terms of the eyes, this manifests with the following symptoms:

• Lagophthalmos ("hare's eye") — inability to completely close the palpebral fissure.
• Bell's phenomenon — when the patient attempts to close their eyes, the eyeball reflexively rolls upward and outward.
• Ophthalmological complications — due to incomplete eyelid closure and constant tearing, severe dry eye develops.

Class and exam questions

What else to study on this topic

• Etiology of facial nerve neuropathy (infections, trauma, iatrogenesis)
• Functional facial expression tests to evaluate the facial nerve
• Differential diagnosis of refractive errors (myopia, hyperopia, astigmatism)
• Anatomy of the parotid plexus (pes anserinus major)
• Symptom complex of facial muscle paralysis (facial asymmetry, smoothed folds)

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.