Brainstem & Cranial Nerves I-VI

Study Notes

The brainstem is a stalk-like structure connecting the spinal cord to the forebrain
Cranial nerves I-VI are discussed in this study material.
Learning outcomes include identifying brainstem parts, cranial nerve exit points, describing cranial nerve components, course, relations, distributions, and clinical applications.

Identify the different parts of the brainstem
Identify the cranial nerves by their exit points on the brainstem
Describe cranial nerves I, II, III, IV, V, VI and their important connections, pathways and distributions.
Explain the clinical applications of each cranial nerve

Functional Component: Special Sensory (smell)
Course and Distribution:
- Originates from the olfactory mucosa in the nasal cavity roof
- Passes through the cribriform plate of the ethmoid bone
- Enters the olfactory bulb in the anterior cranial fossa
- Travels via the olfactory tract to various olfactory areas in the brain.
Clinical Application: Anosmia (loss of smell) can be caused by old age or trauma to the cribriform plate (leading to CSF leakage).

Functional Component: Special Sensory (vision)
Course and Distribution:
- Optic tracts emanate from the lateral geniculate nucleus (within the thalamus), from each hemisphere.
- Forms the optic chiasm.
- Exits through the optic canal
- Supplies the retina
Various fibers, nasal and temporal visual fields have different pathways; nasal fibers cross at the chiasm to the opposite side, while temporal fibers remain ipsilateral.
Clinical Application: Visual field defects can result from lesions at various locations along the optic pathway, such as the optic chiasm or optic tract.

Functional Component: Somatic motor (eye movement) and visceral motor (pupil and accommodation).
Course and Distribution:
- Emerges from nuclei in the midbrain.
- Exits through the superior orbital fissure.
- Divides into superior and inferior divisions:
  - Superior division: supplies superior rectus and levator palpebrae superioris muscles
  - Inferior division: supplies inferior rectus, medial rectus, and inferior oblique muscles.
- Provides parasympathetic innervation to the ciliary ganglion, which then innervates the sphincter pupillae and ciliary muscles.
Clinical Application: Oculomotor nerve palsy can cause ptosis, as well as paralysis of the medial rectus muscle leading to abduction of the eyeball.

Functional Component: Somatic motor (eye movement)
Course and Distribution:
- Originates from the posterior surface of the midbrain.
- Passes anteriorly in the lateral wall of the cavernous sinus.
- Exits through the superior orbital fissure.
- Supplies the superior oblique muscle
Clinical Significance: Damage may affect the eye's downward and inward movement.

Functional Component: Sensory and motor functions
Course and Distribution:
Divides into three main branches: ophthalmic (V1), maxillary (V2) and mandibular (V3).
Major functions of each:
- Ophthalmic (V1): Sensations from the forehead, scalp, upper face and nasal cavity
- Maxillary (V2): Sensations from the cheek, upper teeth
- Mandibular (V3): Sensations from the lower teeth, jaw muscles, and anterior tongue. Mandibular nerve carries both sensory and motor functions.
Clinical significance: Trigeminal neuralgia may manifest as facial pain.

Functional Component: Somatic motor
Course and Distribution:
- Originates from the pons
- Courses through the cavernous sinus, lateral to the internal carotid artery.
- Exits through the superior orbital fissure.
- Supplies the lateral rectus muscle
Clinical Significance: Lesions in the nerve can lead to paralysis of the lateral rectus muscle, causing the affected eye to turn inward.