Human Anatomy Part 2 - K. A. Dubenko A. K. Kolomiitsev Yu. B. Chaikovskyi 2008

Special Part
Nervous system, systema nervosum - Special Part
Brain - Embryogenesis of the brain - Medulla oblongata

The Medulla Oblongata (bulbus, myelencephalon) is a part of the Brainstem that is incorporated into the Hindbrain (rhombencephalon) (Fig. 135 A, B). The medulla oblongata develops from the posterior Brain vesicle. It houses vital centers that regulate external Respiration, Circulation*, vomiting and swallowing Reflexes, and METABOLISM. The medulla oblongata performs crucial sensory Functions, receiving afferent impulses that carry information from the receptors of the facial Skin, mucous membranes, respiratory tract, Internal Organs, and the auditory system. This afferent input forms the basis for The Development of corresponding reflex responses. Through these pathways, the medulla oblongata controls Tongue movements and the secretion of salivary, gastric, and pancreatic glands.

Class="center">External Anatomy

The medulla oblongata is situated between the Pons and the Spinal Cord (Fig. 135 A). In shape, it resembles a truncated cone with a length of 25–30 mm, a transverse dimension of 10–12 mm, and a weight of 6–7 g. The thickened upper part of the medulla oblongata is demarcated from the pons anteriorly by the bulbopontine sulcus, sulcus bulbopontinus; posteriorly, in the region of the Rhomboid fossa, the boundary corresponds to the level of the medullary striae of the Fourth ventricle. The lower boundary of the medulla oblongata is the inferior margin of the decussation of pyramids or the exit point of the anterior and posterior roots of the 1st pair of cervical nerves, which corresponds to the level of the foramen magnum. As a direct continuation of the spinal cord, the medulla oblongata retains features of its Structure. The medulla oblongata has anterior (ventral), posterior (dorsal), and lateral surfaces, which contain longitudinal sulci (Fig. 135 A, B). These sulci are continuations of the corresponding sulci of the spinal cord. The anterior surface of the medulla oblongata is convex, directed anteriorly and inferiorly, and rests against the clivus on the internal cranial base. Along the midline of the surface runs the deep anterior median fissure, fissura mediana anterior, bordered laterally by the pyramids of the medulla oblongata, pyramides medullae oblongatae, which continue into the anterior funiculi of the spinal cord. The pyramids are formed by the fibers of the Pyramidal Tracts. Most of these fibers cross to the opposite side in the depths of the anterior median fissure, forming the decussation of pyramids, decussatio pyramidum [decussatio motoria]. Lateral to the pyramid, on the lateral surface of the medulla oblongata, is an elevation called the olive, oliva. The pyramid and the olive are separated from each other by the anterolateral sulcus, sulcus anterolateralis, from the depths of which the rootlets of the Hypoglossal nerve emerge. Posterior to the olive, from the posterolateral sulcus, sulcus posterolateralis, the rootlets of the glossopharyngeal (IX), vagus (X), and accessory (XI) Cranial Nerves emerge sequentially. On the surface of the medulla oblongata is the lateral funiculus, which continues into the inferior cerebellar peduncle, pedunculus cerebellaris inferior.

* The vasomotor center was discovered in 1871 by Professor of Histology F. V. Ovsyannikov (Kazan University)

The dorsal surface of the medulla oblongata is covered by the Cerebellum. The relief of the lower part of the posterior surface resembles that of the spinal cord, while the upper part faces the cavity of the fourth ventricle (Fig. 135 B). The upper part of the posterior surface of the medulla oblongata forms the lower half of the floor of the rhomboid fossa. Along the midline of the posterior surface runs the posterior median sulcus, sulcus medianus posterior. Lateral to this sulcus are the funiculi, which run upward, diverge, and become part of the inferior cerebellar peduncles that laterally bound the lower half of the rhomboid fossa. The intermediate sulcus, sulcus intermedius, divides each posterior funiculus into the gracile fasciculus, fasciculus gracilis, located medially, and the cuneate fasciculus, fasciculus cuneatus, located laterally. Near the inferior angle of the rhomboid fossa, the gracile and cuneate fasciculi terminate in elevations—the gracile and cuneate tubercles.

Fig. 135 A. Ventral surface of the brainstem

Fig. 135 B. Dorsal surface of the brainstem

1 - Pulvinar;2 - Corpus pineale; 3 - Colliculus superior; 4 - Colliculus inferior; 5 - N. trochlearis (IV); 6 - Velum medulare superius; 7 - Pedunculus cerebellaris superior; 8 - Sulcus medianus; 9 - Pedunculus cerebellaris medius; 10 - Striae medullares ventriculi quarti; 11 - Trigonum n. hypoglossi (XII); 12 - Trigonum n. vagi (X); 13 - Tuberculum nuclei cuneati; 14 - Tuberculum nuclei gracilis

Internal Anatomy

The medulla oblongata consists of gray and White matter. The white matter occupies mainly the anterolateral region of the medulla oblongata. The Gray matter is represented by clusters of Nerve Cells—the nuclei of cranial nerves V, VIII, IX, X, XI, and XII, and the nuclei of the reticular formation (Fig. 136), which differ from one another in shape and size, as well as the gracile and cuneate nuclei, which are located in the corresponding tubercles and transmit information (Proprioception) to the thalamus and cerebellum. Lateral to the cuneate Nucleus lies Monakow's nucleus (accessory cuneate nucleus), surrounded by bundles of fibers that form the inferior cerebellar peduncle. The cranial nerve nuclei are located symmetrically on the floor of the rhomboid fossa beneath the ependyma (in the dorsal part of the medulla oblongata). The motor nuclei are located closer to the midline, the sensory nuclei are more lateral, and the autonomic nuclei lie in between. The hypoglossal nucleus, nucleus nervi hypoglossi, is located in the inferior angle of the rhomboid fossa, deep within the hypoglossal trigone. In the region of the vagal trigone lies the parasympathetic dorsal motor Nucleus of the Vagus nerve, nucleus dorsalis nervi vagi. Somewhat deeper and superior to the vagal nucleus, within the reticular formation, lies the large motor nucleus ambiguus, which is shared with the Glossopharyngeal nerve. The motor nucleus of the Accessory nerve joins the inferior end of The Nucleus ambiguus, and the parasympathetic inferior salivatory nucleus, nucleus salivatorius inferior, is located laterally between the nucleus ambiguus and the inferior olivary nucleus. Lateral to the latter are the sensory nucleus of the solitary tract, nucleus solitarius, and the spinal trigeminal nucleus, nucleus spinalis inferior nervi trigemini. In the lateral angle of the rhomboid fossa, within the vestibular area, area vestibularis, lie the nuclei of the Vestibulocochlear nerve (superior, inferior, medial, and lateral vestibular nuclei, and anterior and posterior cochlear nuclei). Posterior and lateral to the pyramids is the inferior olivary nucleus, nucleus olivaris inferior, which has the appearance of a folded plate of gray matter open medially at the hilum of the inferior olivary nucleus, hilus nuclei olivaris inferior, and forms the olive, oliva, on the lateral surface of the medulla oblongata. The olivary nucleus consists of interneurons, has a connection with the cerebellum via the olivocerebellar tract, tr. olivocerebellaris, and together with it is responsible for equilibrium. Posterior to this nucleus lies the dorsal accessory olivary nucleus, nucleus olivaris accessorius dorsalis, and more medially is the medial accessory olivary nucleus, nucleus olivaris accessorius medialis.

Fig. 136. Internal Anatomy of the medulla oblongata. Transverse section at the level of the rostral part of the olive (after Rauber-Kopsch)

In the central part of the medulla oblongata lies the coordinating apparatus of the brain—the reticular formation, formatio reticularis (Fig. 137 A). The reticular formation originates in the upper part of the spinal cord and extends through the medulla oblongata, pons, Midbrain, as well as the central part of the thalamus and the Hypothalamus (M. P. Sukhetska, 1958). The reticular formation is characterized by the presence of a vast number of nerve fibers running in various directions, which makes this structure resemble a net under the Microscope. This led O. Deiters* to name it the reticular formation, formatio reticularis. Within this network lie groups of multipolar Neurons ranging in size from very small to very large. The axons of small neurons make numerous contacts within the reticular formation itself. The axons of large neurons often bifurcate and branch, with some branches descending into the spinal cord and others ascending to reach the thalamus and other Regions of the Diencephalon and cerebrum.

The reticular formation receives sensory fibers from many sources: the cerebellum, vestibular nuclei, the motor area of the Cerebral Cortex, the hypothalamus, and the spinoreticular tract.

Within the brainstem are anatomical structures whose excitation exerts a generalized tonic influence on the anterior regions of the brain. This group of structures is known as the ascending reticular activating system. It plays an important role in maintaining wakefulness, as well as in the mechanisms underlying The formation of integrated conditioned reflex Responses of the Organism. Along with the ascending activating system, descending reticulospinal systems are also distinguished, which exert a controlling influence on the reflex activity of the spinal cord.

Thus, the reticular formation is a complex reflex center that controls Muscle tone and stereotypical movements. Lesions of the reticular formation lead to movement disorders, as well as disturbances of consciousness, Sleep, and autonomic functions (S. G. Eliason, 1978).

The WHITE MATTER OF the medulla oblongata consists of short and long bundles of intrinsic fibers and bundles of long transit fibers. The latter establish a reciprocal connection between the spinal cord and the higher regions of the brainstem and the cerebrum (Fig. 137 A, B). Short intrinsic fibers connect the nuclei of the medulla oblongata with each other, while long ones connect them with adjacent regions of the brain. The intrinsic fibers of the medulla oblongata include fibers of proprioceptive sensitivity. The first neuron of this pathway terminates near the Cells of the ipsilateral gracile and cuneate nuclei (Fig. 137 A). The processes of the cells of these nuclei curve arcuately (internal arcuate fibers, fibrae arcuatae internae) and cross to the opposite side, decussating anterior to the central canal with the fibers of the opposite side. The collection of these internal arcuate fibers is called the medial lemniscus, lemniscus medialis.* After forming the decussation of the medial lemnisci, decussatio lemniscorum medialium, they proceed to the lateral nuclei of the thalamus, forming the bulbothalamic tract, tractus bulbothalamicus, which is situated between the olives. The decussation of the medial lemnisci constitutes the sensory decussation, decussatio sensoria (Fig. 137 B). In addition to this decussation, the long descending fibers of the pyramidal tracts passing through the medulla oblongata in transit form a second decussation (ventral/motor)—decussatio pyramidum (decussatio motoria). Furthermore, corticonuclear fibers, tr. corticonuclearis, which carry impulses from the neurons of the neocortex to the corresponding cranial nerve nuclei, terminate in the medulla oblongata. These tracts mediate the regulatory Influence of the cerebral cortex on physiological reactions associated with the function of the cranial nerve nuclei.

* Otto Deiters (1834-1863) was a German histologist. He described the Reticular Formation and proposed the term "reticular formation"; the lateral vestibular nucleus was named after him.

Fig. 137 A

Fig. 137 B. Section of the medulla oblongata at the mid-olivary level (diagram)

In addition to its conduction function, the medulla oblongata regulates complex, vital, unconditioned reflexes of a protective and physiological nature, namely: sucking, chewing, swallowing, sneezing, coughing, vomiting, lacrimation, and salivation.

Metencephalon

The metencephalon is derived from the rhombencephalon (hindbrain vesicle) and is part of the brainstem. It is divided into two parts: the pons and the cerebellum.