Midbrain Anatomy (Chapter 5) PDF

Anatomy of the brain stem Aleko Bairamovi Brain Stem Located between the cerebrum (forebrain) and the spinal cord Consists of the midbrain, pons, and medulla oblongata. Functions of the brainstem : It serves as a conduit for the ascending tracts and descending tracts connecting the spinal cord to th...

Anatomy of the brain stem Aleko Bairamovi Brain Stem Located between the cerebrum (forebrain) and the spinal cord Consists of the midbrain, pons, and medulla oblongata. Functions of the brainstem : It serves as a conduit for the ascending tracts and descending tracts connecting the spinal cord to the different parts of the forebrain It contains important reflex centers associated with the control of respiration and the cardiovascular system and with the control of consciousness It contains the important nuclei of 3’d to 12th cranial nerves. Brainstem Thalamus Cerebellum Pons Medulla ob longata Spinal cord Midbrain The midbrain (also known as the mesencephalon) is the smallest of the three regions of the brainstem, measuring around 2cm in length. As it ascends, the midbrain travels through the opening in the tentorium cerebelli. (Tentorium cerebelli – separates the occipital lobes from the cerebellum. It contains a space anteromedially for passage of the midbrain – the tentorial notch.) Frontal lobe Central s u l c u s Parietal callosum Parietooccipital lcus Thalamus Hypothalamus Occipital lobe Cerebellum (d) M e d u l l a o b lo n g a t a Sginat cord General features of the mid brain The midbrain has 2 surfaces : Anterior surface posterior surface posterior surface of the midbrain On the posterior surface are four colliculi (corpora quadrigemina). These are rounded eminences that are divided into superior and inferior pairs by a vertical and a transverse groove. The superior colliculi are centers for visual reflexes, and the inferior colliculi are lower auditory centers. On the lateral aspect of the midbrain, the superior and inferior brachia ascend in an anterolateral direction. The superior brachium passes from the superior colliculus to the lateral geniculate body and the optic tract. The inferior brachium connects the inferior colliculus to the medial geniculate body. The Brain Stem The Midbrain Trochlear nerve In the midline below the Inferior colliculi, trochlear are inferior nerves emerge. These the diameter nerves that around smallaspect of wind the midbrain to enter thethe lateral lateral wall of the cavernous sinus Anterior surface of the midbrain On the anterior aspect of the midbrain, there is a deep depression in the midline, the interpeduncular fossa, which is bounded on either side by the crus cerebri. Many small blood vessels perforate the floor of the interpeduncular fossa, and this region is termed the posterior perforated substance. The oculomotor nerve emerges from a groove on the medial side of the crus cerebri and passes forward in the lateral wall of the cavernous sinus. — — — Internal Structure of the Midbrain Midbrain is divided into dorsal and ventrai parts at the level of cerebral aqueduct Dorsal portion (tectum) The tectum includes 4 colliculi (coropra quadrigemina) 2 inferior colliculi and 2 superior colliculi The ventral portion (cerebral peduncles) It consists of 2 thick nervous cords extending the forRbrain E to the pons Each cerebral peduncle divides into 3 parts midbrain tegmentum (dorsal part of cerebral peduncle) substantia nigra ( pigmented grey mater ) crus cerebri ( ventral part) The cerebral aqueduct, which connects the third and fourth ventricles and lined by ependyma and is surrounded by the central gray matter. Cross sections of the midbrain structures of the midbrain, 2 transverse sections are made in the midbrain Upper level (level of the superior colliculi) Lower level (level of inferior colliculi) Medial longitudinal fasciculus CORTICOSPINAL AND CORTICONUCLEAR TRACTS Transverse Section of the Midbrain at the level of the Superior Colliculi The following structures are seen at this level Nuclei Cranial nerve nuclei: oculomotor nucleus, Edinger-Westphal nucleus, mesencephalic nucleus of trigeminal nerve Superior colliculus, red nucleus Motor tracts Corticospinal and corticonuclear tracts Temporopontine, frontopontine, Medial longitudinal fasciculus, MLF Decussation of rubrospinal tract Sensory tracts Nuclei at the level of superior colliculi Cranial nerve nuclei: oculomotor nucleus, It contains the somatic efferent fibers that control the medial rectus, inferior rectus, superior rectus, inferior oblique, and the striated muscle in the levator palpebrae superioris. Edinger-Westphal nucleus, The Edinger–Westphal nucleus supplies preganglionic parasympathetic fibers to the eye, constricting the pupil, accommodating the lens, and convergence of the eyes. mesencephalic nucleus of trigeminal nerve - contains the primary sensory neurons that innervate the muscle spindle of the masticatory and other muscles in the oro-facial region, as well as a small number of periodontal mechanoreceptive afferents. Red nucleus Within the tegmentum, at the level of the oculomotor nervenucleus Walking Coordination of movement and gait Preservation of muscular tone Receives and relays input from the cerebral cortex and the cerebellum Origin of the rubrospinal tract and rubroolivary tracts The red nucleus The red nucleus is a rounded mass of gray matter situated between the cerebral aqueduct and the substantia nigra. Its reddish hue, seen in fresh specimens, is due to its vascularity and the presence of an iron-containing pigment in the cytoplasm of many of its neurons. Afferent fibers from (1) the cerebral cortex through the corticospinal fibers, (2) the cerebellum through the superior cerebellar peduncle. (3) the lentiform nucleus, subthalamic and hypothalamic nuclei, substantia nigra, and spinal cord. Efferent fibers leave the red nucleus and pass to (1) the spinal cord through the rubrospinal tract (as this The superior colliculus is a large nucleus of gray matter that lies in the upper level of the midbrain. It forms part of the visual reflexes. It is connected to the lateral geniculate body by the superior brachium. It receives afferent fibers from the optic nerve, the visual cortex, and the spinotectal tract. The efferent fibers form the tectospinal and tectobulbar tracts, which are probably responsible for the reflex movements of the eyes, head, and neck in response to visual stimuli. The oculomotor nucleus The oculomotor nucleus is situated in the central gray matter close to the median plane, just posterior to the medial longitudinal fasciculus. The fibers of the oculomotor nucleus pass anteriorly through the red nucleus to emerge on the medial side of the crus cerebri in the interpeduncular fossa as oculomotor nerve. The afferent pathway for the light reflex ends in the pretectal nucleus. This is a small group of neurons situated close to the lateral part of the superior colliculus. After relaying in the pretectal nucleus, the fibers pass to the parasympathetic nucleus of the oculomotor nerve (Edinger-Westphal nucleus). emerging fibers then pass to the oculomotor nerve. The Transverse Section of the Midbrain at the Level of the interior Colliculi The following structures are seen at this level Nuclei Cranial nerve nuclei: trochlear nucleus, mesencephalic nuclei of trigeminal nerve Inferior colliculus Motor tracts Corticospinal and corticonuclear tracts, Temporopontine, frontopontine, Medial longitudinal fasciculus Sensory tracts Lateral, trigeminal, spinal, and medial lemnisci. decussation of superior cerebellar peduncles A lateral orientation is a position away from the midline of the body The inferior colliculus, consisting of a large nucleus of gray matter, lies beneath the corresponding surface elevation and forms part of the auditory pathway. It receives many of the terminal fibers of the lateral lemniscus. The pathway then continues through the inferior brachium to the medial gentculate body. The trochlear nucleus is situated in the central gray matter close to the median plane just posterior to the medial longitudinal fasciculus. The emerging fibE'rs of the trochlear nucleus pass laterally and posteriorly around the central gray mattE'r and leave the midbrain just below the inferior colliculi. The fibers of the trochlear nerve now decussate completely in the superior medullary velum. The mesencephalic nuclei of the trigeminal nerve are lateral to the cerebral aqueduct. The decussation of the superior cerebellar peduncles occupies the central part of the tegmentum anterior to the cerebral aqueduct. The rRticular formation is smaller than that of the pons and is situated in the tegmE'ntum lateral and posterior to the red nucleus Medial lemniscus ascends posterior to the substantia nigra and carries proprioceptive and fine touch sensations from the opposite side of the body Lateral lemniscus: is located posterior to the trigeminal lemniscus and carries auditory sensations from the opposite and partly from the same side. Spinal lemniscus: situated lateral to the medial lemniscus and carries pain, temperature and crude touch sensations from the opposite side of the body Trigeminal lemniscus: which carries the general sensations from the opposite side of the head and face The substantia nigra is a large motor nucleus situated between the tegmentum, and the crus cerebri and is found throughout the midbrain. The nucleus is composed of medium-size multipolar neurons that possess inclusion granules of melanin pigment within their cytoplasm. The substantia nigra is concerned with muscle tone toandthe is cerebral cortex, spinal cord, connected hypothalamus, and basal nuclei. The crus cerebri contains important descending tracts and is separated from the tegmentum bv the substantia nigra. The corticospinal and corticonuclear fibers occupy the middle two-thirds of the crus. The frontopontine fibers occupy the medial part of the crus, and the temporopontine fibers occupy the lateral part of the crus. These descending tracts connect the cerebral cortex to the anterior gray column cells of the spinal cord, the cranial nerve nuclei, the pons, and the cerebellum aou Mesenceghalic nucleus Tegrnenlum Lalerai IemnisaJs Nucleus of i ochlear nerve TriqomiMl lamrii S Spnal lemn›scvs Gre s corticonuclear libels Decussalén ol superior Curved Special visceral efferent the first central auditory structure to receive input from the cochlea via the auditory nerve Loss of muscle control