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TSM24: BRAINSTEM AND CRANIAL NERVES 1 07/10/08 LEARNING OUTCOMES Describe the gross structure of the brainstem The brainstem is situated inferior to the cerebrum and consists of the medulla, pons and midbrain It contains the cranial nerve nuclei, fibres of all the ascending and descending pathways and makes up part of the circulatory pathway of the cerebrospinal fluid (CSF) MEDULLA OBLONGATA The medulla represents the transition between spinal cord and brainstem at the foramen magnum Rostrally it broadens out and joins the pons The ventral median sulcus extends rostrally along the midline of the ventral surface of the medulla o There are two prominent ridges immediately lateral to the sulcus called the pyramids o Lateral to the pyramids are two smaller oval-shaped eminences called the olives The dorsal median sulcus extends rostrally along the midline of the dorsal surface of the medulla o Lateral to the sulcus are the gracile and cuneate fasciculi respectively whose fibres terminate rostrally in the corresponding nuclei in the gracile and cuneate tubercles o The floor of the fourth ventricle on the dorsal surface narrows rostrally at the dorsal sulcus demarking the obex PONS The pons has a prominent ventral bulge divided in the midline by the basal sulcus o Lateral to the ventral (or basal) surface are the middle cerebral peduncles which join the pons to the cerebellum The dorsal surface is covered almost completely by the floor of the fourth ventricle MIDBRAIN The midbrain is immediately caudal to the optic tracts and the rostral-most part of the brainstem It is divided in the ventral midline by a deep depression called the interpeduncular fossa o The mamillary bodies are bilateral prominences at the rostral end of the fossa The majority of the ventral surface of the midbrain is made up of the bilateral cerebral peduncles The dorsal surface has two pairs of bilateral bulges at the rostral end, the superior and inferior colliculi Describe the origins of the cranial nerves from the brainstem CNI and CNII originate from the forebrain and so are not listed here CNIII Oculomotor Arises medially from the caudal ventral surface of the midbrain CNIV Trochlear Arises from the caudal dorsal surface of the midbrain immediately inferior to the inferior colliculi CNV Trigeminal CNVI CNVII CNVIII CNIX CNX CNXI Abducens Facial Vestibulocochlear Glossopharyngeal Vagus Accessory CNXII Hypoglossal Arises laterally from the ventral surface of the pons as motor and sensory roots respectively Arises medially from the caudal ventral border of the pons Arises laterally to the abducens nerve Arises laterally to the facial nerve Arises laterally from the rostral dorsal surface of the medulla Arises caudally to the glossopharyngeal nerve Arises caudally to the vagus nerve as cranial and spinal roots respectively Arises medially and in between the vagus and cranial accessory nerves