Материал: Kaplan USMLE-1 (2013) - Anatomy

Chapter 5 • The Brain Stem

Cranial Nerve Nuclei

Abducens nucleus

The abducens nucleus is found near the midline in the floor of the fourth ven­ tricle just lateral to the MLF.

Facial motornucleus

The facial motor nucleus is located ventrolateral to the abducens nucleus. Fibers from the facial nucleus curve around the posterior side of the abducens nucleus (the curve forms the internal genu of the facial nerve), then pass ventrolaterally to exit the brain stem at the pontomedullary junction.

Superiorolivary nucleus

The superior olivary nucleus lies immediately ventral to the nucleus of CN VII and receives auditory impulses from both ears by way ofthe cochlear nuclei. The cochlear nuclei are found at the pontomedullary junction just lateral to the infe­ rior cerebellar peduncle.

Vestibular nuclei

The vestibular nuclei are located near the posterior surface of the pons lateral to the abducens nucleus, and extend into the medulla.

Cochlearnuclei

The dorsal and ventral cochlear nuclei are found at the pontomedullary junction.

All of the fibers of the cochlear part of CN VIII terminate here.

Trigeminal nuclei

Motor Nucleus-Pons

The motor nucleus of CN V is located in the pons just medial to the main sensory nucleus ofthe trigeminal and adjacent to the point ofexit or entry ofthe trigeminal nerve fibers. These motor fibers supply the muscles ofmastication (masseter, tem­ poralis, and medial and lateral pterygoid (Figure IV-5-3).

Main Sensory Nucleus-Pons

The main sensory nucleus is located just lateral to the motor nucleus.

The main sensory nucleus receives tactile and pressure sensations from the face, scalp, oral cavity, nasal cavity, and dura.

Spinal Trigeminal Nucleus-Spinal cord to pons

The spinal trigeminal nucleus is a caudal continuation ofthe main sensory nu­ cleus, extending from the mid pons through the medulla to the cervical cord. Central processes from cells in the trigeminal ganglion conveying pain and tem­ perature sensations from the face descend in the spinal tract ofV and synapse on cells in the spinal nucleus.

MEDICAL 393

Chapter 5 • The Brain Stem

Cranial NervesV, VI, VII, and VIII

Four cranial nerves emerge from the pons. Cranial nerves VI, VII, and VIII emerge from the pontomedullary junction. The facial nerve is located medial to the vestibu­ locochlear nerve. The abducens nerve (CN VI) emerges near the midline lateral to the corticospinal tract. The trigeminal nerve (C V) emerges from the middle of the pons.

MIDBRAIN

The midbrain (mesencephalon) is located between the pons and diencephalon. The cerebral aqueduct, a narrow channel that connects the third and fourth ven­ tricles, passes through the midbrain. The inferior colliculi and superior colliculi are found on the dorsal aspect of the midbrain above the cerebral aqueduct. The inferior colliculus processes auditory information received bilaterally from the co­ chlear nuclei by axon fibers of the lateral lemniscus. The superior colliculi help direct movements ofboth eyes in gaze. The pretectal region is locatedjust beneath the superior colliculi and in front of the oculomotor complex. This area contains interneurons involved in the pupillary light reflex. The massive cerebral peduncles extend ventrally from the midbrain. 1be cerebral peduncles contain corticospinal and corticobulbar fibers. The interpeduncular fossa is the space between the cere­ bral peduncles.

The substantia nigra is the largest nucleus of the midbrain. It appears black to dark brown in the freshly cut brain because nigral cells contain melanin pig­ ments. Neurons in the substantia nigra utilize Dopamine and GABA as neu­ rotransmitters.

The medial lemniscus and spinothalamic tract and descending hypothalamic fi­ bers course together ventrolateral to the periaqueductal gray.

The MLF continues to be located near the midline, just beneath the cerebral aq­ ueduct.

The mesencephalic nuclei ofthe trigeminal nerve are located on either side ofthe central gray.

Cranial Nerve Nuclei

The trochlear nucleus is located just beneath the periaqueductal gray near the midline between the superior and inferior colliculi. The oculomotor nucleus and the nucleus of Edinger-Westphal are found just beneath the periaqueductal gray near the midline at the level of the superior colliculi.

Two cranial nerves emerge from the midbrain: the oculomotor (CN III) and the trochlear (C IV) nerves.

The oculomotor nerve arises from the oculomotor nucleus and exits ventrally from the midbrain in the interpeduncular fossa. CN III also contains preganglion­ ic parasympathetic axons that arise from the nucleus of Edinger-Westphal, which lies adjacent to the oculomotor nucleus.

Axons of the trochlear nerve decussate in the superior medullary velum and exit the brain stem near the posterior midline just inferior to the inferior colliculi.

MEDICAL 395