Neuroanatomy: Brainstem and Cranial Nerves

Brainstem 1

• The brainstem is divided into three parts: midbrain, pons, and medulla oblongata.
• Major landmarks include the cerebral peduncles, interpeduncular fossa, pyramids, pons, olive, inferior pontine sulcus.
• The mammillary body is part of the hypothalamus.

Cranial Nerve I: Olfactory Nerve

• Function: Special sensory for olfaction (smell).
• Pathway: Begins in nasal epithelium, where olfactory sensory neurons detect odor molecules. Axons pass through the cribriform plate of the ethmoid bone to the olfactory bulb. Synapses occur within the olfactory bulb, with specialized neurons processing initial smell information. Axons from the olfactory bulb bundled to form the olfactory tract, which carries processed signals to higher brain regions, including the olfactory cortex, amygdala, and other associated areas.
• Target structures: Olfactory cortex, piriform cortex, and limbic system.

Cranial Nerve II: Optic Nerve

• Function: Special sensory for vision, transmitting visual signals from the retina to the brain.
• Pathway: Begins at retinal ganglion cells, forming optic nerve. Passes through optic canal, meeting at optic chiasm where nasal fibers cross. Continues as optic tract, projecting to the lateral geniculate nucleus (LGN) of the thalamus. Signals relayed via optic radiation to primary visual cortex in occipital lobe.
• Nuclei and Targets: Primary visual cortex (occipital lobe). Also projections to the superior colliculus (visual reflexes), pretectal area (pupillary light reflex), and suprachiasmatic nucleus (circadian rhythms).
• Extraocular muscles are not directly controlled, but play critical role in reflexes and perception.

Cranial Nerve III: Oculomotor Nerve

• Function: Controls most eye movements, eyelid elevation, pupil constriction, and lens accommodation.
• Origin: Interpeduncular fossa between cerebral peduncles in the midbrain.
• Nuclei: Superior rectus, inferior rectus, medial rectus, inferior oblique muscles, and the levator palpebrae superioris muscle. Also responsible for pupil constriction (Edinger-Westphal nucleus).

Cranial Nerve IV: Trochlear Nerve

• Function: Innervates the superior oblique muscle, responsible for depression and intorsion of the eye.
• Nucleus: Located in the midbrain, below the inferior colliculus.

Cranial Nerve VI: Abducent Nerve

• Function: Innervates the lateral rectus muscle, which abducts the eye.
• Nucleus: Located in the caudal pons, near the floor of the fourth ventricle.

Cranial Nerve V: Trigeminal Nerve

• Function: Mixed nerve with sensory and motor functions
• Sensory: Sensation from the face, scalp, oral and nasal cavities, anterior two-thirds of the tongue via the chorda tympani branch.
• Motor: Controls mastication muscles (temporalis, masseter, and pterygoids).
• Nuclei: Mesencephalic nucleus (proprioception), main sensory nucleus (touch and pressure), spinal nucleus (pain and temperature), motor nucleus. three divisions (ophthalmic, maxillary, mandibular)

Cranial Nerve VII: Facial Nerve

• Function: Mixed nerve with motor, sensory, and parasympathetic roles.
• Motor: Responsible for facial expressions.
• Sensory: Taste from the anterior two-thirds of the tongue (Chorda tympani).
• Parasympathetic: Innervates the submandibular and sublingual salivary glands, stimulating saliva production and the lacrimal gland, stimulating tear production.

Cranial Nerve VIII: Vestibulocochlear Nerve

• Function: Purely sensory nerve.
• Vestibular Nerve: Transmits information about balance and spatial orientation (from vestibular apparatus in inner ear).
• Cochlear Nerve: Transmits auditory information (from the cochlea converting sound waves into neural signals).

Cranial Nerve IX: Glossopharyngeal Nerve

• Function: Mixed nerve with sensory, motor, and parasympathetic roles.
• Sensory: Taste from the posterior third of the tongue, general sensation from pharynx and posterior tongue, blood pressure (carotid sinus).
• Motor: Controls stylopharyngeus muscle (swallowing).
• Parasympathetic: Innervates the parotid salivary gland. Also receives information from the carotid sinuses (blood pressure) and the carotid body (monitoring blood composition).

Cranial Nerve X: Vagus Nerve

• Function: Mixed nerve with motor, sensory, and parasympathetic roles.
• Motor: Controls muscles of pharynx and larynx (swallowing and speech).
• Sensory: Visceral sensory input from thoracic and abdominal viscera, taste from epiglottis.
• Parasympathetic: Innervates various involuntary muscles of the thoracic and abdominal viscera.

Cranial Nerve XI: Accessory Nerve

• Function: Primarily motor nerve with two roots.
• Cranial Root: Assists vagus nerve in motor control of certain muscles of the pharynx, larynx, and soft palate.
• Spinal Root: Innervates the trapezius and sternocleidomastoid muscles.

Cranial Nerve XII: Hypoglossal Nerve

• Function: Motor nerve controlling movements of the tongue (intrinsic and extrinsic muscles).

Brainstem 2

• Conduit for ascending/descending tracts, important reflexes (respiration, cardiovascular, consciousness).
• Contains nuclei for cranial nerves III through XII.

Brainstem Nuclei- Specific Structures, and Functions

• Raphe nuclei: Mood, sleep, and other central nervous system functions.
• Locus coeruleus: Attention, stress, alertness.
• Cerebellar peduncles: Travel to and from cerebellum to coordinate movement, motor control.
• Reticular formation: Arousal, wakefulness, consciousness, filters sensory input, includes Raphe and Locus Coeruleus nuclei.
• Pontine nuclei: Relays motor signals among cerebellum, brain cortex.
• Midbrain: Auditory and visual reflexes, contains substantia nigra. Visual reflex centers.
• Substantia Nigra: involved in motor function, motivation, reward (degeneration in Parkinson's)

Brainstem Compression & Coma

• Caused by trauma, tumors, stroke, increased pressure, or mass effect.
• Disrupts RAS leading to loss of arousal, consciousness causing coma.