Brainstem: Cranial Nerves & Anatomy

• The brainstem, located at the base of the cerebral hemispheres, is a compact structure that carries nearly all information between the brain and the body
• Serves as the corridor for major sensory, motor, cerebellar, and cranial nerve pathways
• It houses important nuclei controlling cranial nerves, consciousness, cerebellar circuits, muscle tone, posture, and essential functions
• Small lesions can cause substantial deficits affecting motor, sensory, and neuroregulatory modalities
• Understanding brainstem anatomy is a powerful clinical tool for diagnosing and treating brainstem disorders
• In this chapter, the surface features of the brainstem will be reviewed, along with the functions and course of each cranial nerve
• The cranial nerves and central pathways mediating eye movements and pupillary control will be focused on
• The vascular supply and internal structures of the brainstem, including major ascending and descending tracts is discussed

Cranial Nerves

• Learning the cranial nerves requires memorization initially, but they become familiar due to clinical relevance
• Cranial nerves have both sensory and motor functions
• Two useful review strategies: listing cranial nerves in numerical sequence to discuss functions and sensory and motor cranial nerve nuclei are listed
• Integrating knowledge of the peripheral and central course of the cranial nerves is clinically relevant

Surface Features of the Brainstem

• The brainstem consists of midbrain, pons, and medulla, and is located in the posterior fossa of the cranial cavity
• Its rostral limit is the midbrain-diencephalic junction, meeting thalamus and hypothalamus at the level of the tentorium cerebelli
• Midbrain joins pons at the pontomesencephalic junction, and pons meets medulla at the pontomedullary junction
• The caudal limit is the cervicomedullary junction, at the level of the foramen magnum and pyramidal decussation
• The cerebellum attaches to the dorsal surface of the pons and upper medulla, and only implies midbrain, pons, and medulla
• On the dorsal surface of the midbrain are two pairs of bumps: superior and inferior colliculi which form the tectum ("roof") of the midbrain
• The ventral surface is composed of the cerebral peduncles and the interpeduncular fossa between them
• The pons is dorsally limited by the fourth ventricle, and more dorsolaterally, it is attached to the cerebellum by large white matter tracts (superior, middle, and inferior cerebellar peduncles)
• The pyramids descend on the ventral surface of the medulla from the pontomedullary junction to the pyramidal decussation
• The medulla is divided into rostral and caudal portions
• In the rostral medulla, the inferior olivary nuclei bulge laterally to the pyramids
• In the caudal medulla, the inferior olivary nuclei disappear, and the posterior columns and posterior column nuclei become visible dorsally
• The floor of the fourth ventricle extends from the pons to the rostral half of the medulla which several bumps are noticeable
• The facial colliculi, formed by the abducens nuclei and fibers of the facial nerve, are visible
• The hypoglossal trigone and vagal trigone are formed by the hypoglossal nucleus (CN XII) and the dorsal motor nucleus of CN X, respectively
• Rostrally, the fourth ventricle joins the cerebral aqueduct, which runs through the midbrain
• Caudally, the fourth ventricle drains into the subarachnoid space via the foramina of Luschka (laterally) and foramen of Magendie (midline)
• The fourth ventricle ends caudally at the obex, marking the entry to the spinal cord central canal, which is normally closed in adults

Cranial Nerves I–XII

• The cranial nerves emerge roughly in numerical sequence from I through XII proceeding from anterior to posterior
• The first tworanial nerves do not emerge from the brainstem but connect directly to the forebrain
• Cranial nerves III–XII exit the brainstem either ventrally or ventrolaterally
• CN IV, the one exception, exits from the dorsal midbrain
• CN III, VI, and XII (exiting ventrally near the midline) and CN IV (exiting dorsally) form a functional group that innervates somatic motor structures
• Oculomotor nerves (CN III) emerge ventrally from the interpeduncular fossa of the midbrain between the posterior cerebral artery and the superior cerebellar artery
• Trochlear nerve (CN IV) exits dorsally from the midbrain, with fibers crossing over
• Trigeminal nerve (CN V) exits from the ventrolateral pons
• Abducens nerve (CN VI) exits ventrally at the pontomedullary junction
• Facial nerve (CN VII), vestibulocochlear nerve (CN VIII), glossopharyngeal nerve (CN IX), and vagus nerve (CN X) exit ventrolaterally from the pontomedullary junction and rostral medulla
• CN VII, CN VIII, and CN IX exit the brainstem from a region called the cerebellopontine angle,
• Spinal accessory nerve (CN XI) arises laterally from multiple rootlets along the upper cervical spinal cord
• Hypoglossal nerve (CN XII) exits the medulla ventrally, between the pyramids and inferior olivary nuclei

Skull Foramina and Cranial Nerve Exit Points

• Olfactory nerves exit via the cribriform plate, and the optic nerve exits via the optic canal
• The superior orbital fissure transmits CN III, IV, VI, and V₁ into the orbit for eye movements
• The ophthalmic division of CN V conveys sensation for the eye and upper face
• The maxillary (CN V2) and mandibular (CN V3) divisions of the trigeminal nerve exit via the foramen rotundum and foramen ovale, respectively
• Both provide sensation to the remainder of the face
• CN VII and CN VIII both exit the intracranial cavity via the internal auditory meatus to enter the auditory canal to innervates the inner ear deep within the t
• CN VII exits the skull to reach the muscles of facial expression via the stylomastoid foramen
• The jugular foramen transmits CN IX, X, and XI Hypoglossal nerve (CN XII) exits the skull via the hypoglossal canal, which lies just in front of the foramen magnum

Sensory and Motor Organization of the Cranial Nerves

• The cranial nerves are analogous to some ways to the spinal nerves, having both sensory and motor functions
• Motor cranial nerve nuclei are located more ventrally, while sensory cranial nerve nuclei are located more dorsally
• During embryological development, the cranial nerve nuclei lie adjacent to the ventricular system
• Three motor columns and three sensory columns of cranial nerve nuclei develop, which subserve a different motor or sensory cranial nerve function through the length of the brainstem
• Each column can be described as general versus special, somatic versus visceral, and afferent versus efferent