January 10, 2016

Diencephalon

by Sharadsaini on

DiencephalonThe diencephalon extends from the region of the mamillary bodies and the posterior commissure at its caudal end to the interventricular foramen at its most rostral end. It forms the lateral wall of the third ventricle and is made up  principally of the hypothalamus, epithalamus,  thalamus, and subthalamus (also termed ventral thalamus). The thalamus lies above the hypothalamic sulcus, and the hypothalamus below it. The thalamus makes up the dorsal wall and the hypothalamus the ventral wall of the ventricle. Little can be seen of the diencephalon, since most of it is surrounded by the cerebral hemispheres, and it is best seen in sagittal section. The only part that is visible on the brain surface is in the ventral view, when the infundibulum, bilateral mamillary bodies and the tuber cinereum can be seen, as well as a surface rostral boundary, the optic chiasm. The mamillary body holds the mamillary nuclei of the hypothalamus.

                 In sagittal section, the hypothalamus is seen from the mamillary body at its caudal end to the interventricular foramen rostrally.  Functionally, the hypothalamus is critical for normal life, since it controls body temperature, fluid and water balance, and neuroendocrine function, and has an important role in the control of the autonomic nervous system and  emotional and sexual behavior. At the base of the hypothalamus is the infundibulum or pituitary stalk, which connects the hypothalamus to the pituitary gland through blood portal and nervous links. Several small but important nuclei  have been identified in the hypothalamus.

                The thalamus is the largest member of the diencephalon, and if it were dissected free might resemble a hen’s egg in shape. It is separated from the hypothalamus by a groove, the hypothalamic sulcus. There are two thalami, joined by amassa intermedia or interthalamic adhesion. The thalamus is a huge relay station, and has massive reciprocal connections with the cerebral cortex. The thalamus extends forward to the interventricular foramen, and is bounded laterally by the posterior limb of the internal capsule, and the head of the caudate nucleus. Internally, the thalamus consists of several nuclei, which project to the ipsilateral cerebral cortex, and the cortex in turn sends reciprocal fibers back to the areas from which it received them. Functionally, this relationship serves to control the organism’s response to inputs from the special and the general senses, and to ensure a proper motor response to them.

                Immediately below the thalamus lies the subthalamus, which is situated dorsolaterally to the hypothalamus. The epithalamus consists of the habenular nucleus and the pineal gland. The pineal gland synthesizes the hormone melatonin, which may modulate sleep waking rhythms, and in recent years melatonin  has been advocated to alleviate the condition known as jet lag.Diencephalon

Cerebrum

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cerebrum The cerebrum or forebrain is the largest part of the human brain and is housed in the concavity produced by the vault of the skull. It consists of the diencephalon and telencephalon.

                          The   diencephalon consists of the third ventricle and the structures that define its rostral, caudal, superior, and inferior  boundaries. It is situated in the midline of the brain, and most of its components are bilateral and symmetrically arranged, with free communication between the two sides of a given diencephalic structure.

                         The telencephalon consists of the cerebral hemispheres. These are two bilaterally and symmetrically arranged structures separated by a sagittal midline fissure, and are connected across their midline by the commissural fibers of the corpus callosum.

                     The structures of the diencephalon are dealt with in more detail in later sections; the components of the diencephalon can be summarized as consisting of the  third ventricle, and the major structures surrounding it, namely the thalamus, subthalamus, epithalamus, and the hypothalamus. Within each of these structures are nuclei, pathways and subsidiary structures which are considered in more detail later. The  thalamus is a complex, highly organized and compartmentalized relay station for ascending tracts, situated centrally in the cerebrum, and plays an important part in the integration of somatic and visceral function. The  hypothalamus forms the floor and part of the lateral walls of the third ventricle, and plays a critical role in endocrine, metabolic, autonomic, and emotional function. The subthalamus, which lies immediately below the thalamus, is concerned with the modulation of involuntary movement, and is considered to be one of the extrapyramidal motor nuclei. The  epithalamus consists of the pineal gland and thehabenular nuclei, which play a part in the integration of somatic and olfactory information.

                        Each  cerebral hemispherce of the telencephalon has a highly convoluted and folded surface covering of gray matter, the cerebral cortex, and inner core of white matter consisting of fiber tracts. Deep with the hemispheres are masses of gray matter, the basal nuclei (also called basal ganglia) and the lateral ventricles. The infoldings of the surface greatly increase the surface area of the cortex; these folds are termed  gyri (singular, gyrus), separated from each other by fissures called sulci.

                    The basal nuclei occur bilaterally and symmetrically in the hemispheres, and consist of the amygdaloid nucleus, situated in the temporal lobe, the claustrum and corpus striatum, which lies lateral to the thalamus. The corpus striatum is split by the internal capsule, a band of nerve fibers, into the caudate nucleus and lentiform nucleus. These nuclei are further  subdivided by nerve fiber sheets into other nuclei.cerebrum

Midbrain

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The midbrain can be divided into three main parts: the tectum (quadrigeminal plate); the tegmentum, which is a continuation of the pons tegmentum; and the very large crus cerebri, which contains  the corticofugal fibers. The midbrain contains two cranial nerve nuclei, the oculomotor and trochlear nuclei. The most   prominent nuclear mass in the midbrain is the substantia nigra, a huge area darkly  pigmented with melanin, a metabolic byproduct of dopamine breakdown. The substantia nigra, which sends dopaminergic  projections to the basal ganglia, is very important clinically since its degeneration produces a loss of dopamine terminations in the basal ganglia, resulting in the extrapyramidal  disorder Parkinson’s disease. The structure of the midbrain is most usually  demonstrated using transverse sections at the level of the inferior and superior colliculi.

                                 Transection at the level of the inferior  colliculus reveals that the pontine tectum or covering, i.e., the superior medullary velum, is now replaced by the inferior and  superior colliculi, swellings caused by the  masses of nuclei serving as relay stations for transmission of auditory and other signals to the brain. At this level the cerebral   aqueduct replaces the fourth ventricle and decussation of the fibers of the superior cerebellar peduncles is visible.

                             Several tegmental nuclear groups surround the cerebral aqueduct in the periaqueductal gray matter. These include the locus ceruleus, a pigmented cell mass which sends many norepinephrine-containing projections to the cerebellum and cerebral cortex. The locus ceruleus appears to be involved in modulation of cortical sensory and association areas, and in sleep activation. (Parts of several nuclei, including the nucleus ceruleus, are also seen in rostral sections of pontine areas; it is wrong to compartmentalize brain stemnuclei as strictly pontine or midbrain etc.) Also in this region is the mesencephalic nucleus of the trigeminal nerve, a collection of unipolar sensory neurons, and the dorsal nucleus of the raphe. The trochlear nucleus lies ventrally in the periaqueductal gray matter and sends efferents to the superior oblique muscle of the eye.

                               Several tracts can be seen in transverse section. The most prominent is the decussation of the cerebellar peduncles. The lateral lemniscus is seen where it enters the inferior colliculus and the medial lemniscus en route to the thalamus. Just medial is the ventral trigeminothalamic tract. Clustered medially are the dorsal trigeminothalamic tract, central tegmental tract, the medial longitudinal fasciculus, and the tectospinal tract. The ventrally placed crus cerebri contains the  massive descending corticospinal and corticobulbar tracts, and temperopontine fibers.

                    Transection at the level of the superior colliculi shows the prominent bilateral  red nucleus, so called because it appears pinkish red in freshly cut sections. The red nucleus runs continuous with the crossed superior cerebellar peduncle, and it is the origin of descending motor tracts, which decussate in the ventral tegmentum to become the rubrospinal tract.

                      The superior colliculi communicate through the posterior commissure and integrate auditory, cortical, spinal, and retinal afferents in the control of eye movements and reflex reflexes. The superior brachium carries the retinal inputs. The oculomotor nucleus lies ventrally in the periaqueductal gray matter, and its efferent projections cross the red nucleus, emerge in the interpeduncular fossa and  run to optic and extra-optic muscle.

Midbrain

 

The Cerebellum II: Cellular and Lobular Arrangement

by Sharadsaini on

lobesThe lobules and fissures of the cerebellum are more easily understood if it is imagined that the surface of the cerebellum has been flattened as shown opposite. Using this representation, many of the areas of the cerebellum can be quickly and easily drawn schematically, and their relationship to other cerebellar structures understood.

                                           The medial vermal cerebellum has been subdivided into lobes running down the middle. These are, from dorsal to ventral, the lingula, culmen, declive, folium, tuber, pyramid, uvula, and nodule. The various lobular subdivisions and the main lobes and fissures are also distinguished using this view.

                                            Most of the cerebellar cortex is buried in the folia, and only about 15% is visible. In section, the cerebellar cortex is seen to be uniformly structured throughout, with three clearly defined layers that contain five different types of neurons. The cerebellar cortical layers are, from the surface inwards, the molecular, Purkinje cell (sometimes called piriform), and the granular layers. The medullary layer lies beneath the granular layer.

                                          The molecular layer is relatively sparsely populated with two types of nerve cells: basket cells and outer stellate cells. The axons and dendrites of the outer stellate cells do not leave the molecular layer, and neither do the dendrites of basket cells. These processes run roughly horizontally in the layer, transverse to the long axis of the depth of the folia or infolding. The basket cell bodies are close to those of the Purkinje cells in the next layer, and project fibers that form basket shapes around the cell bodies of the Purkinje cells. Below this layer is the relatively narrow Purkinje cell layer. The Purkinje cells are large Golgi type I neurons; their cell bodies lie in rows along the folia, and their axons project to the intracerebellar nuclei. Some of these Purkinje axons in the archicerebellum project to the brainstem vestibular nuclei. Purkinje dendrites proliferate densely, transverse to the plane of the folia. Immediately below is the relatively wide granular layer, whose cells are very tightly packed and send axons up into the molecular layer, where they branch in Tshapes and run as parallel fibers along the horizontal axis of the folia. Each Purkinje dendritic tree may form synapses with up to half a million parallel fibers that have projected up from the granular layer. Also in the granular layer is a relatively small population of inhibitory Golgi neurons, which project their dendritic trees up into the molecular layer. One Golgi cell may synapse with a row of ten to twelve Purkinje cells, and it appears that Golgi cells do not overlap with respect to the innervation of the Purkinje cells.                       

                                        TCerebellum here are two main types of afferent input to the cerebellum, and both are excitatory. Each Purkinje cell is supplied by one climbing fiber from the contralateral inferior olive . The phylogenetically more ancient archicerebellum and paleocerebellum are served by the correspondingly older accessory olivary nuclear cells. The neocerebellum is supplied with fibers by the newer inferior olive. The second afferent input is through the mossy fibers from many different sources, including the pontine nuclei. These fibers diverge extensively, and one mossy fiber may serve several folia. The mossy fiber axons form multiple rosettes, which synapse with several granular cell dendrites. Inhibitory Golgi axons synapse in these rosettes. It follows, therefore, that since mossy fiber rosettes synapse with granular fibers, which in turn synapse with Purkinje cells, that one mossy fiber can indirectly affect electrical activity in very many Purkinje cells.

Cerebellum

by Sharadsaini on

The hindbrain or rhombencephalon consists of the medulla (myelencephalon), pons (metencephalon), and the cerebellum as its largest structure. The cerebellum consists of two hemispheres joined medially by a relatively narrow vermis, sits in the posterior cranial fossa of the skull beneath the tentorium cerebelli, and is separated from the medulla and pons by the fourth ventricle. The cerebellar cortex has many curved transverse fissures in the form of narrow infoldings called folia. Structurally, the cerebellum is covered by a cortex of gray matter with a medulla of white matter, which holds four intrinsic pairs of nuclei (see below). Observation of the superior surface shows two deep transverse fissures, the primary and the posterior superior fissures. Viewed from the ventral surface, the cerebellum is  divided approximately into superior and  inferior halves by the horizontal fissure. Three pairs of cerebellar peduncles connect the cerebellum to the three lower brain segments. The inferior, middle, and superior cerebellar peduncles connect it to the medulla, pons, and midbrain, respectively.

The superior vermis lies between the hemispheres as a longitudinal ridge; it is more clearly differentiated visually from the hemispheres on the ventral surface, where it is divided by fissures into the nodule, uvula, and pyramid. A stalk extends from the nodule on each side to the flocculus, which forms the flocculonodular lobe. The tonsil is a lobule that lies over the inferior vermis. The inferior medullary velum is exposed if the tonsil is removed.

From an embryological and functional viewpoint, the cerebellum can be divided into three main parts. (i) The archicerebellum, or flocculonodular node, is made of the pairs of flocculi and their peduncular connections. The flocculonodular node is the most ancient part of the cerebellum, present in fish as well as humans, and is connected with the vestibular nuclei and system. It is connected particularly with the dentate nucleus, one of the intrinsic medullary cerebellar nuclei. (ii) The paleocerebellum, or anterior lobe of the cerebellum, lies dorsal to the primary fissure. The lobe also includes the pyramid and uvula of the inferior vermis. The anterior lobe receives inputs via the spinocerebellar tract, originating in stretch receptors, and is the lobe most involved in the control of involuntary muscle tone. This lobe is connected principally to the globose and emboliform nuclei, which project to the red nucleus, and thence to the central tegmental, rubroreticular, rubsospinal, and rubrobulbar efferent pathways. The paleocerebellum evolved in terrestrial vertebrates, which need to use limbs to support the body against the pull of gravity; therefore its connections are mainly spinal, and its functions are concerned with such stereotyped movements such as posture, locomotion, and muscle tone. (iii) The neocerebellum, which, as its name implies, is the phylogenetically newest part of the cerebellum, communicates with the thalamus and motor cortex. This lobe is made up of virtually all the posterior lobe, except for the pyramid and uvula of the vermis. The neocerebellum modulates non-stereotyped, learned behavior such as the learning of manual skills.Cerebellum

Fourth Ventricle

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Fourth VentricleThe fourth ventricle is an expansion of the central canal of the medulla oblongata. It is a roughly tent-shaped cavity filled with cerebrospinal fluid (CSF), situated beneath the cerebellum, above the pons and above the rostral half of the medulla. Laterally, the ventricle is bounded by the superior and inferior cerebellar peduncles.

                                The roof or dorsal surface of the fourth ventricle consists of a sheet of white nonnervous tissue called the inferior medullary velum. A layer of pia mater covers the inner lining or ependyma. Situated at the caudal part of the roof is an opening, the median eminence or foramen of Magendie, which connects the subarachnoid space and the interior of the ventricle. The ventricle communicates with the subarachnoid space also through two lateral openings called the foramen of Luschka . Situated caudally above the ventricular roof is a double layer of pia mater, called the tela choroidea, which lies between the cerebellum and the ventricular roof. The tela choroidea is highly vascularized, and its blood vessels project through the roof of the caudal part of the ventricle to form the choroid plexus. The choroid plexus together with others situated in the lateral and third ventricles produce the CSF.

                                           The floor or rhomboid fossa of the fourth ventricle is formed by the rostral half of the medulla and the dorsal surface of the pons, and is divided longitudinally into symmetrical halves by the median sulcus. The floor is raised because of the nucleus and the fiber tracts that run beneath it in the pons and medulla. Thus, there is a slight swelling in the floor, the facial colliculus, caused by the fibers leaving the motor nucleus of the facial nerve as they arch over the abducens nucleus.  Although not shown here, there are other nuclei situated immediately belowthe floor of the fourth ventricle. These include the vagal and hypoglossal nuclei, together with their fiber connections.

                            The ventricle is filled with fluid, and if it is overfilled, as can occur through abnormal production of CSF, hydrocephalus may result. This is a clinically significant increase in the volume of CSF, due to the obstruction of the foramina of the roof of the fourth ventricle, or to displacement of the medulla by a tumor, or adhesions of tissues through meningitis, or through the presence of a congenital septum. The increase in fluid increases pressure on the  nuclei and fiber tracts immediately below the floor of the fourth ventricle, and can  result in autonomic and motor disturbances such as cardiac, respiratory, and vasomotor problems.

                                                Tumors may arise in the ependymal lining of the fourth ventricle, or in the pons, or the vermis of the cerebellum and may spread to the fourth ventricle. Alternatively tumors of ependymal origin may invade the cerebellum, causing locomotor disturbances.brain ventricles

Pons

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Ponspons

The pons (metencephalon) lies beneath (anterior to) the cerebellum and is around 2.6 cm in length. The pons has been arbitrarily divided into the dorsal or posterior tegmentum, and a basal or anterior part, sometimes referred to as the pons proper.

Transection of the caudal pons at the level of the facial colliculi shows the fourth ventricle prominently, as well as  the middle cerebellar peduncles. (The term colliculus refers to the visible swellings caused by the mass of the nucleus.) The superior and inferior cerebellar peduncles and the nuclei and spinal tracts of several cranial nerves are also visible. The medial lemniscus runs at the base of the tegmentum, and above it the area occupied by the reticular formation is now much larger than that of the medulla. The trapezoid body consists of fibers from the cochlear nuclei and the nuclei of the trapezoid nucleus in the pons; these convey, for example, auditory information arriving in the pons. Ascending and descending fiber tracts, such as the corticospinal tractcourse through the pons.

The basal (anterior or ventral) portion of the pons consists of transverse and longitudinal bundles of fibers. The fibers constitute, mainly, a massive relay system from the cerebral cortex to the contralateral cerebellar cortex.

Dorsolateral to the reticular formation, lying in the floor of the fourth ventricle are the vestibular nuclei, which receive afferent inputs concerning equilibrium and balance and which are then well placed to be relayed to the cerebellum. The cerebellum in turn sends afferents from Purkinje cells to the vestibular nucleus; these are inhibitory, and release the neurotransmitter !-aminobutyric acid (GABA). The vestibular nuclei project efferent fibers to the middle ear.

The motor nucleus of the facial nerve innervates facial muscles, and its functionis clearly manifested when the facial nerve is damaged. This results in partial paralysis of the facial muscles (Bell’s palsy), and possibly autonomic disturbances. Transverse section through the pons higher up (rostrally) reveals similar structural features, except that the motor and sensory nuclei of the trigeminal nerve are now clearly visible. The principal sensory nucleus of the trigeminal nerve lies lateral to the motor nucleus, and its sensory incoming fibers lie laterally to the efferent fibers of the trigeminal nerve, which leave the trigeminal motor nucleus. The superior cerebellar peduncle is now more prominent, as is the lateral lemniscus, which runs dorsolateral to the medial lemniscus.

Damage to the pons results, typically, in muscle paralysis or weakness of structures innervated by cranial nerves. For example, a childhood tumor of the pons called astrocytoma of the pons, is the most prevalent  brainstem tumor, and causes a number of symptoms that reflect the paralysis of the ipsilateral cranial nerve; thus there may be weakness (hemiparesis) of facial muscles due to damage to the facial nucleus. The pons may be damaged by hemorrhage of the cerebellar arteries or of the basilar artery, and, depending on whether the damage is unilateral or bilateral, will result in facial paralysis and contralateral paralysis of lower limbs, through damage to corticospinal fibers which traverse the ventral pons.mid pons

Transverse Section of Medulla Oblongata

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Higher transection of the medulla oblongata at the level of the middle of the olivary nuclei clearly shows the fourth ventricle, the roof of which is formed by the choroid plexus in the inferior medullary velum at the base of the cerebellum. The floor of the ventricles is pushed up by the hypoglossal and dorsal vagal nuclei. The reticular formation, a network of nerve cells in the brain stem, is nowclearly visible, as are the major fiber tracts.

                                      The pyramids, medial lemnisci, and tectospinal tract lie medially in section. The tectospinal tract carries descending fibers from the tectum, which is the roof of the midbrain, consisting of superior and inferior colliculi. Also prominent is the inferior vestibular nucleus, which lies just medial to the inferior cerebellar peduncle.

                                             The most prominent feature of the transverse section at this level is the convoluted inferior olivary nucleus, which has a massive input to the cerebellum through the olivocerebellar tract which constitutes most of the inferior cerebellar peduncle. If it could be dissected entirely, the inferior olive would resemble a collapsed purse or bag. Axons of olivary cells leave the nucleus and decussate to the other side of the medulla and sweep up into the peduncle. The fibers radiate to virtually all parts of the cerebellum and many have an excitable effect on cerebellar Purkinje cells. The inferior olivary complex has been divided into the principal, medial accessory and dorsal accessory olivary nuclei, based mainly on their cerebellar connections. For example, the fibers arising from the medial portion of the principal nucleus and those from the accessory nuclei terminate mainly in the vermis of the cerebellum.

                                            The olive receives descending corticoolivary fibers from the occipital, parietal, and temporal cortex, which terminate bilaterallymainly in the principal olivary nucleus. The principal olive also receives rubro-olivary fibers from the red nucleus, and fibers in the central tegmental tract from the periaqueductal gray matter in the midbrain, some of which also terminate in the medial accessory nuclei. The dorsal and medial accessory olives receive ascending fibers in the spino-olivary tract, which runs up the cord in the anterior (ventral) funiculus of the white matter.

                                           There are other nuclei at this level. The nucleus ambiguus is a longitudinal column of nerve cells within the reticular formation, extending through the medulla from the medial lemniscus to the midrostral portion of the inferior olive. The cells are multipolar motoneurons, and the efferents from this nucleus arch upward to join efferents from the dorsal vagal nucleus and from the nucleus of the tractus solitarius. Efferents from the rostral part of the nucleus ambiguus become visceral efferents of the lossopharyngeal nerve,which innervate the stylopharyngeus muscle. The more caudal portion of the nucleus gives rise to fibers of the spinal accessory nerve.The nucleus of the tractus solitarius gives rise to fibers, which, among other destinations, target the hypothalamic nuclei which release the peptide vasopressin. The reticular formation contains several important raphe nuclei which extend in the pons, and which project 5-HT neuronal processes to the midbrain, diencephalon and cerebral cortex. These central gray projections appear to mediate rhythmic processes such as arousal.

Medulla Oblongata

Medulla Oblongata

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Medulla Oblongata

The spinal cord becomes the medulla oblongata, which also contains white and gray matter, but the arrangement hanges, due to the embryonic expansion of the central canal to form the hindbrain vesicle, which will become the fourth ventricle. Development of the ventricle pushes dorsally situated structures more dorsolaterally. The transition is clearly seen in transverse section. The spinal cord becomes the medulla, which initially resembles the upper cervical segments. The substantia gelatinosa is now much larger in size and has become the spinal nucleus of the trigeminal nerve. In transverse section, descending fibers of the spinal trigeminal tract can be seen immediately dorsolateral to the nucleus. There is an increase in the amount of gray matter surrounding the central canal.

                                       At low medullary level, the most prominent sign of transition to medulla is the appearance of the decussation at the pyramids. This is where the descending corticospinal motor tracts cross over. These fibers cross ventral (anterior) to the central gray matter and project dorsolaterally across the base of the ventral horn of the medulla. The pyramidal decussation almost eliminates the spinal anterior median fissure. (In the human, approximately 90% of the descending corticospinal fibers decussate and descend the cord in the lateral corticospinal tract, while about 10% do not cross, and descend in the uncrossed lateral and ventral corticospinal tracts.) The decussation explains the contralateral control of body movements by the motor cortex. At this level can also be seen the tracts of the gracile and cuneate fasciculi, which are the CNS projections of the cells of the spinal ganglia, and the lower ends of the gracile and cuneate nuclei where they terminate. At this level are also the cut fibers of the ascending ventral (anteriorand lateral spinocerebellar tracts, which carry information from the sense organs in tendons and muscle spindles, the inferior olivary nucleus, and the spinal root of the accessory nerve.

                                       Transaction at a higher level of the medulla (B) reveals another prominent decussation, that of the medial emniscus. This is where fiber tracts from the ascending gracile and cuneate nuclei cross the midline of the medulla on their way up to higher centers. The nuclei are complex and arranged to correspond topographically with the body areas from which the ascending 0fibers come. Ascending fibers from the nuclei curve round the central gray matter and decussate to form the medial lemniscus. At this level, the spinal nucleus of the trigeminal nerve, which innervates the head region, is prominent, and immediately dorsolateral to it are the fibers of the descending trigeminal nerve. At both levels, the ascending spinocerebellar and spinothalamic tracts are both visible, and in B the medial accessory olivary nucleus lies medial to these tracts.

                                              In summary, the transition from spinal cord to medulla is marked by (i) the expansion of the central canal; (ii) decussation at the pyramids; (iii) formation of the medial lemniscus through the decussation of ascending fibers arising from the cuneate and gracile nuclei; (iv) dorsolateral displacement of the dorsal horn of gray matter; (v) appearance of cranial nerve nuclei and various relay nuclei projecting to the cerebellum.

Medulla Oblongata

Dorsal View

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Dorsal

The dorsal surface of the brain stem, and particularly that of the medulla and pons, is obscured by the cerebellum. When this is removed, the bilateral swellings caused by the ascending cuneate and gracile fasciculi can be seen, as well as the corresponding tubercles, which are the swellings caused by their nuclei. Dorsal to the olives are the inferior cerebellar peduncles, which climb to the lateral aspect of the fourth ventricle and then swing into the cerebellum between the middle and superior cerebellar peduncles. The inferior cerebellar peduncle receives fibers in the stria medullaris, a tract from the hypothalamic arcuate nucleus. The stria medullaris fibers pass dorsally through the midline of the medulla and cross the floor of the fourth ventricle.

                                             The floor of the fourth ventricle (also called the rhomboid fossa) is in part the dorsal surface of the pons; the dorsal surface of the pons (also called the tegmentum of the pons) forms the rostral half of the floor of the ventricle, and is divided longitudinally by a medial sulcus into two symmetrical halves. The ventricle is broadin the middle and narrows caudally to the obex, the most caudal end of the fourth ventricle, and rostrally towards the aqueduct of the midbrain. Caudally, the ventricle narrows into two triangles or trigones. Beneath the medial area of the ventricle are several motor nuclei; the rostral ends of both the vagal and hypoglossal nuclei lie beneath these trigones. There is a swelling at the lower end of the medial eminence, the facial colliculus, which is formed by fibers from the motor nucleus of the facial nerve. The roof of the fourth ventricle is tent-shaped and extends upwards towards the cerebellum. The roof is formed rostrally by the superior cerebellar peduncles and by a sheathcalled the superior medullary velum. The rest of the roof consists of another sheath, the inferior medullary velum, which is often found adhering to the underside of the cerebellum. The sheath may be incomplete, creating a gap called the median aperture of the fourth ventricle or the foramen of Magendie, which constitutes the main communication between the ventricular system and the subarachnoid space. The lateral walls of the fourth ventriclesare provided mainly by the inferior cerebellar peduncles. There are recesses in the lateral walls, which extend around the medulla, and these open ventrally as the foramina of Luschka, through which cerebrospinal fluid can enter the subarachnoid space.

                                            The dorsal surface of the midbrain is defined by four rounded swellings: the superior and inferior colliculi (the corpora quadrigemina). The colliculi make up the roof or tectum, and define the length of the dorsal surface, round 1.5 cm. The inferior colliculus is mainly a relay nucleus in the transmission of auditory impulses en route to the thalamus and cerebral cortex. The superior colliculus mediates control of voluntary eye movements and the head in response to visual and other forms of stimuli. The lateral surface of the midbrain is formed principally by the cerebral peduncle. Parts of the epithalamus (see, the habenular nuclei and the stria medullaris are seen ostral to the midbrain. The third ventricle of the diencephalon and the pineal body are also shown.

dorsal