ANATOMY OF THE BRAINSTEM BY L. S. K ( Ph.D. In View) DEPARTMENT OF HUMAN ANATOMY Brainstem Located between the cerebrum and the spinal cord Midbrain Provides a pathway for tracts running between higher and lower neural centers. Pons Consists of the midbrain, pons, and medulla oblongata. Each region is about an inch in Medulla length. oblongata Microscopically, it consists of deep gray matter surrounded by white matter fiber tracts. Produce automatic behaviors necessary for survival. Vertical Columns of Cranial Nerves Roots DEVELOPMENT DEVELOPMENT of the brain stem Midbrain: The midbrain develops from mesencephalon. Cells within the midbrain multiply continually and be compressed to form cerebral aqueduct. Pons: The pons develops from the anterior part of the metencephalon, but it also receives a cellular contribution from alar part of the myelencephalon. Medulla: develops from the caudal myelencephalic part of the rhombencephalic vesicle Midbrain Connects the pons and cerebellum with the forebrain. It is about 0.8 inch(2cm) in length The midbrain is traversed by a narrow channel called cerebral aqueduct filled with CSF. Passes through the tentorial notch. RELATIONS Laterally are Parahippocampal gyri, Optic tracts, Posterior cerebral artery, Basal vein, Trochlear nerve and Geniculate bodies Anteriorly to interpeduncular structures Posteriorly To the splenium of corpus callosum, great cerebral vein, pineal body, posterior ends of the right and left thalami. STRUCTURE OF MIDBRAIN EXTERNAL FEATURES Structure of Midbrain EXTERNAL FEATURES The midbrain comprises two lateral halves called- Cerebral peduncles; which is again divided into 1. anterior part- Crus cerebri 2. posterior part -Tegmentum by a pigmented band of gray matter, substantia nigra The central narrow cavity is called the cerebral aqueduct or aqueduct of Sylvius, which connects the 3rd and 4th ventricles. The tectum is the part of the midbrain posterior to the cerebral aqueduct; it has four small surface swellings referred to previously; these are two superior and two inferior colliculi. TECTUM & TEGMENTUM ANTERIOR SURFACE Anteriorly, it presents two large bundles of fibers, one on each side of the midline, called crus cerebri. The oculomotor nerve emerges from the medial aspect of the crus of the same side The crus cerebri bounds the interpeduncular fossa (posterior perforated substance ) on either side. POSTERIOR SURFACE Posteriorly, the midbrain presents four rounded swellings called colliculi- One Superior and inferior colliculi one on each side. Each colliculus is laterally related to a ridge called brachium. o Superior and inferior brachium arises from respective colliculi o Superior brachium connects the superior colliculus to lateral geniculate body and the optic tract o Inferior brachium connects the inferior colluculus to medial geniculate body o In the midline below the inferior colliculus, the trochlear nerve emerges which then winds round the side of the midbrain to reach its ventral aspect Lateral view INTERNAL STRUCTURE Transverse section at the level of Superior Colliculi Superior colliculi Substantia nigra Oculomotor nucleus Pretecta nucleus Red nucleus Mesencephalic nucleus of trigeminal n. Edginger-westphal nucleus Tracts Motor Corticospinal Corticonuclear Temporopontine Frontopontine Medial longitudinal fasciculas Decussation of rubrospinal tract Sensory Trigeminal Spinal Medial Transverse Section at the level of Inferior Colliculi Substantia nigra- lamina of grey matter made up of deeply pigmented cells, concerned with muscle tone Troclear nucleus Mesencephalic nucleus of trigeminal nerve- receives proprioceptive mpulses from muscles of mastication, facial and ocular muscles and the teeth Inferior colliculus Tracts Motor Corticospinal Corticonuclear Temporopontine, parietopontine & occipitopontine Frontopontine Medial longitudinal fasciculus Sensory Lateral Trigeminal Spinal Medial lemnisci Decussation of superior cerebellar peduncles Blood Supply Artery – posterior cerebral artery superior cerebellar artery basilar artery Veins – Drained by basal or great cerebral veins PONS It is situated anteriorly to the cerebellum measuring about 1 inch in length. Connects the medulla oblongata to the midbrain. Its also connects two cerebellar hemispheres and its name comes from this bridge like appearance. FUNCTIONS OF PONS Arousal Controls autonomic functions Relays sensory information between the cerebrum and cerebellum Sleep Maintaining posture 24 External Features Has 2 surfaces, VENTRAL and DORSAL Anterior surface of Pons Pons has a convex anterior surface marked by transversely running fibers which laterally form a bundle called middle cerebellar peduncle. Main Features: The trigeminal nerve (has 2 roots, motor and sensory) emerges from the anterolateral surface at its junction with middle cerebellar peduncle. Presents a basilar sulcus in the midline which lodges basilar artery In the groove between Pons and the medulla oblongata, there emerge, from medial to lateral, abducent, facial and vestibulocochlear nerves. Posterior Surface Triangular in shape and form the superior ½ of the floor of the fourth ventricle. Laterally bounded by the Superior cerebellar peduncles. Posterior surface contains median sulcus which is bounded by madial eminence on both sides. Inferior end of medial eminence is slightly expanded to form facial colliculus, which is produced by facial nerve root. The upper end of sulcus limitans presents a bluish-gray coloration called substantia ferruginosa. Area vestibule lies lateral to sulcus limitans. Structure of Pons Posterior part- Tegmentum Anterior part- Basis pontis traversed by running fibers of Trapezoid body. Transverse Section Through the Cranial Part Transverse Section Through the Cranial Part MEDIAL LEMNISCUS - It is situated in the most anterior part of the tegmentum THE LATERAL and SPINAL LEMNISCI lie at the lateral extremity of the medial lemniscus MOTOR NUCLEUS OF TRIGEMINAL N.- situated beneath the lateral part of the fourth ventricle within the reticular formation SENSORY NUCLEUS OF TRIGEMINAL N.- situated on the lateral side of the motor nucleus and is continuous inferiorly with the nucleus of the spinal tract. Pontine nuclei TRAPEZOID BODY- made up of fibers derived from the cochlear nuclei and the nuclei of the trapezoid body. They run transversely in the anterior part of the tegmentum Tracts Motor Corticospinal Corticonuclear Transverse pontine fibers Medial longitudinal fasciculas Sensory Lateral Spinal Medial lemnisci Transverse section through the Caudal part Transverse section through the Caudal part Medial lemniscus (accompanied by the spinal and lateral lemnisci) Facial nucleus-lies posterior to the lateral part of the medial lemniscus. The fibers of the facial nerve wind around the nucleus of the abducent nerve Abducent nucleus Medial longitudinal fasciculus- situated beneath the floor of the fourth ventricle on either side of the midline the main pathway that connects the vestibular and cochlear nuclei with the nuclei controlling the extraocular muscles (oculomotor, trochlear, and abducent nuclei). Medial vestibular nucleus- lateral to the abducent nucleus and is in close relationship to the inferior cerebellar peduncle Spinal nucleus of trigeminal nerve- and its tract lie on the anteromedial aspect of the inferior cerebellar peduncle Trapezoid body- made up of fibers derived from the cochlear nuclei and the nuclei of the trapezoid body. They run transversely in the anterior part of the tegmentum Pontine nuclei Tracts Motor Corticospinal Corticonuclear Transverse pontine fibers Medial longitudinal fasciculas Sensory Spinal tract of trigemial nerve Lateral Medial lemnisci Blood Supply MEDULLA OBLONGATA External structure of medulla Most inferior region of the brain stem. Becomes the spinal cord at the level of the foramen magnum. Medulla is broad above, joins with pons narrow below, continous with spinal cord Length is about 3cm, width is about 2cm at its upper end Surfaces shows series of fissures .Anterior median fissure .Posterior median fissure Medulla oblongata. A: Anterior view. B: Posterior view EXTERNAL FEATURES VENTRAL DORSAL Pyramid Posterior median sulcus Olive Postrolateral sulcus Anterior median fissure. Gracilis tubercle Anterolateral/pre-olivary Cuneate tubercle sulcus Fasciculus gracilis Post olivary sulcus Fasciculus cuneatus Rootlets of CN IX-XII External surface of medulla Ventral surface of medulla oblongata contains Pyramid •Elevation between anterior median and anterolateral sulcus •Formed due to decussation of corticospinal fibres. Olive •Oval swelling between anterolateral posterolateral sulcus,half an inch long •Produced by large mass of gray matter called inferior olivary nucleus The posterior part of medulla contains Fasciculus gracilis medially ending in rounded elevation ,called nucleus gracilis Fasciculus cuneatus laterally ending in rounded elevation,called nucleus cuneatus Posterior part of the medulla forms the floor of the fourth ventricle Tuberculum cinereum, longitudinal elevation in the lower part of medulla, lateral to fasciculus cuneatus. Internal Structure of Medulla Cross section at three levels Level of pyramidal decussation INTERNAL FEATURES Internally, the medulla is generally studied at the following levels; At the level of the pyramidal decussation At the level of the sensory decussation At the level of the olivary nucleus At level just below