February 22, 2016

Blood Supply and Venous Drainage of Spinal Cord

by Sharadsaini on

The spinal cord is supplied with arterial blood by descending arteries that run the length of the spinal cord, and by radicular arteries that arise at the segmental level. The descending arteries comprise the paired posterior spinal arteries, and the unpaired anterior spinal artery. The posterior spinal arteries arise either from the posterior inferior cerebellar arteries, or from the vertebral arteries. They descend the cord on the dorsal (posterior) surface, medial to the dorsal roots. The posterior spinal arteries are of variable diameter as they descend, and at some points become so fine that they seem to be discontinuous. These arteries supply the dorsal horns and the posterior (dorsal) columns.

The anterior spinal artery is formed by a confluence of the anterior spinal arteries at medullary level. The vessel descends the cord in the midline, and supplies the midline rami to the lower medulla oblongata. It also gives off sulcal branches, which enter the spinal cord via the anterior median fissure. Some of the sulcal branches are given off from the anterior spinal artery alternately to right or left, or the sulcal artery may itself divide to form right and left branches. These sulcal branches supply the spinal cord central gray matter, the lateral and anterior (ventral) columns, the lateral and ventral horns, and the basal portion of the dorsal (posterior) horn.

The radicular arteries arise from segmental vessels, including the ascending and deep cervical arteries, and the intercostal, lumbar, and sacral arteries. The radicular arteries gain access to the cord via the intervertebral foramina, and then divide into anterior and posterior radicular arteries which run together with the ventral and dorsal nerve roots, respectively. They supply the main arterial input to the thoracic, lumbar, sacral, and coccygeal spinal segments. In the cervical region, blood is supplied equally by left and right radicular arteries, while in thoracic and lumbar regions of the cord the radicular arteries occur more commonly on the left side.

There may be from two to ten anterior radicular arteries. One anterior radicular artery is larger than the others; this is the artery of the lumbar enlargement, or the artery of Adamkiewicz, which may arise from an intercostal artery or a lumbar artery anywhere from segments T8-L3. This artery commonly runs on the left side of the cord together with lower thoracic or upper lumbar spinal roots. The posterior radicular arteries divide on the dorsolateral surface of the cord and join the paired posterior spinal arteries. Their distribution with respect to left or right of the cord is not as marked as that seen with the anterior radicular arteries.

The distribution of the spinal veins is similar in general to that of the spinal arteries. There are two major longitudinal venous trunks running along the cord in the midline, the anterior and posterior  spinal veins. These receive cord venous drainage via the sulcal veins. The posterolateral and posteromedial veins drain the dorsal horns and posterior funiculi. Ventral areas of the cord are drained by  anteromedian and anterolateral veins. The internal vertebral venous plexus drains into the external vertebral venous  plexus and from there into the ascending lumbar, azygos, and hemiazygos veins.

Blood Supply

Cerebral Hemispheres: Cellular Architecture

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Cellular ArchitectureThe gray matter of the cerebral cortex contains about 10 billion neurons. It varies in thickness from about 4.5 mm at the crest of a gyrus, to about 1.5 mmin the recess of a sulcus. The cortex contains five different types of cells: pyramidal, fusiform, horizontal cells of Cajal, stellate, and cells of Martinotti. The cortex has been divided into six layers according to the density and arrangement of the different types of cells.

The most superficial layer is the  molecular (plexiform) layer. It has a dense network of tangentially oriented fibers and cells, made of axons of cells of Martinotti, stellate cells, and apical dendrites of pyramidal cells and fusiform cells. Afferent fibers from the thalamus terminate here, as do many commissural fibers. This is a layer of intense synapsing. The external granular layer has several small stellate and pyramidal cells, and the external  pyramidal layer has larger pyramidal cell bodies than in more superficial layers. Their apical dendrites reach into the  molecular layer, and their axons descend into the white matter as projection, commissural or association fibers.

The internal granular layer is densely packed with stellate cells, and there is a horizontal band of fibers called the external band of Baillarger. The internal pyramidal (ganglionic) layer contains medium- sized and large pyramidal cells. In between these cells are cells of Martinotti and stellate cells. There is also another band of fibers called the inner band of Baillarger. The internal pyramidal layer in the precentral gyrus of the motor cortex contains very large pyramidal cells, called Betz cells. The axons of these cells contribute about 3-4% of the pyramidal or corticospinal tract. The innermost layer of the cerebral cortex is the multiform layer of polymorphic cells. Most of the cells in this layer are fusiform cells but pyramidal cells and cells of Martinotti are also present.

Pyramidal cells are so called because of the shape of the cell body. The apex is oriented towards the outer layers and from it a thick apical dendrite projects upwards, giving out several collaterals. Dendrites possess many dendritic spines for synapsing with other cells. An axon projects down from the base of the cell body and may terminate in deeper cortical layers, but more usually descends into the white matter as a projection, commissural, or association fiber.

Stellate cells have small polygonal cell bodies and radiate several dendrites and a short axon which may terminate in the same or a neighboring layer. Horizontal cells of Cajal are small cells horizontally oriented in the superficial layers. Fusiform cells (fusiform means spindle shaped or tapering at both ends) are oriented perpendicular to the layers, have dendritic projections from each pole, and occur principally in deeper cortical layers. Cells of Martinotti are small multipolar cells, with an axon projecting upwards to the surface, and short dendrites.

The bands of Baillarger are made up principally of collateral nerve fibers given out by incoming afferents, and of stellate cells and horizontal cells of Cajal. They include some pyramidal and fusiform collaterals as well. They are prominent in sensory cortical areas because of high densities of thalamocortical fiber terminations. The outer band of Baillarger is especially prominent in the visual cortex, where it is sometimes called the stria of Gennari.

Cellular ArchitectureCellular Architecture

Tracts of Cerebral Hemispheres

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Tracts of Cerebral HemispheresCerebral tracts are association, commissural, or projection in nature. In the brain, commissures run from one part to the corresponding part on the opposite side of the brain. Projection fibers carry information to and from the cortex. Association fibers connect different cortical areas.

The corpus callosum is the major commissure of the cerebrum. It is a massive band of myelinated nerve fibers and most of them interconnect symmetrical  regions of the cerebral cortex. The different regions of the corpus callosum are termed the splenium, at the posterior end, the body, which is the main part, and the genu, which is the Latin word meaning ‘knee’ and is the bend at the anterior part of the corpus callosum. From the corpus callosum, fibers radiate out to the cerebral cortex. The corpus callosum forms part of the roof of the lateral ventricle and also the floor of the longitudinal fissure. The corpus  callosum carries the interhemispheric  transfer of memory, sensory experience, and learned discrimination. Damage to the corpus callosum does not appear to affect performance, except that destruction of the splenium causes alexia, or the inability to understand written words. This may be due to disconnection of the verbal processing in the left hemisphere from visual processing in the right hemisphere.

The anterior commissure is a compact fiber bundle that crosses the midline in front of the columns of the fornix and connects the olfactory bulbs and regions of the temporal gyri. The hippocampal commissure is a transverse commissure linking the posterior columns of the fornix.

Projection fibers are afferents carrying information to the cerebral cortex, and efferents carrying information away from it. The most prominent are the corona radiata, which radiate out from the cortex and then come together in the brain stem. These fibers become highly condensed in the internal capsule, which runs medially between the caudate nucleus and the thalamus and laterally between the thalamus and the lentiform nucleus. The anterior limb of the internal capsule carries connections between the frontal lobe and the basal part of the pons and between the prefrontal cortex and the mediodorsal nucleus of the thalamus. The posterior limb of the internal capsule carries fibers between the ventral posterior nucleus of the thalamus and the primary somatosensory cortex and also carries corticospinal and corticobulbar fibers.

The association fibers connect different areas of the cerebral cortex. Some are relatively large, such as the superior longitudinal  fasciculus, which connects the occipital and frontal lobes. Part of the fasciculus, the arcuate fasciculus connects temporal and frontal lobes, and is important for language. The inferior longitudinal fasciculus connects   the temporal and occipital lobes, and is involved in visual recognition function.

Tracts of Cerebral Hemispheres

Cerebral Hemispheres: Internal Structures

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Cerebral HemispheresThe cerebral hemispheres contain the lateral ventricles, white matter, which consists of nerve fibers embedded in the neuroglia, and the basal nuclei (basal ganglia).

Each hemisphere possesses a lateral ventricle, which is lined with a layer of ependyma and filled with cerebrospinal fluid (CSF). The ventricle has a body located in the parietal lobe, and horns, the anterior, posterior and inferior horns,  which extend into the frontal, occipital and temporal lobes respectively. The body of the ventricle has a floor, roof, and a medial  wall. The body of the caudate nucleus forms the floor of the ventricle, and the lateral margin of the thalamus and the inferior surface of the corpus callosum form the roof.

The basal nuclei or ganglia are masses of gray matter lying inside each cerebral hemisphere. These masses are the amygdaloid  nucleus, claustrum, and the corpus striatum.

The corpus striatum lies lateral to the thalamus and is divided phylogenetically into the neostriatum, which consists of the caudate nucleus and the putamen, and the paleostriatum, which consists of the globus pallidus. The caudate nucleus and the putamen are separated almost completely by a band of fibers called the internal capsule. The caudate nucleus has a large head and a tail, rather like a tadpole, and the tail ends in the amygdaloid nucleus in the temporal lobe. The globus pallidus lies medial to the putamen, and consists of medial and lateral segments.

The putamen and globus pallidus are sometimes referred together as the lentiform nucleus, although in more modern textbooks the term lentiform is being disregarded as archaic terminology. The caudate nucleus lies laterally to the lateral ventricle and to the thalamus.

The corpus striatum has important connections with the substantia nigra, thalamus and the subthalamus. The major afferent inputs to the corpus striatum are  from the substantia nigra, the thalamus and the cerebral cortex. Nigrostriatal fibers are dopaminergic, and have both excitatory and inhibitory effects. Degeneration of this system results in Parkinson’s disease. The thalamostriatal projections arise in the intralaminar nuclei of the ipsilateral thalamus. The corticostriatal afferents are extensive; there are afferents from motor areas of the frontal lobe to the putamen. Fibers from cortical association areas project to the caudate nucleus. The most prominent white matter (see also next spread) consists of the  association and the commissural fibers connecting the corresponding regions of the hemispheres.

Cerebral Hemispheres

Cerebral Cortex: Surface Features

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Cerebral CortexThe cerebral cortex is phylogenetically the youngest part of the brain, and carries out a huge range of discriminative and cognitive processes relating to affective behavior, motor function, somatosensory perception, integration, and mnemonic function. It is structurally highly organized in layers of nerve cells and processes.

In external appearance, the cerebral cortex has a convoluted, corrugated appearance both in situ and after removal  from the cranial vault. The surface of the cortex is deeply folded, which greatly increases   its surface area. The visible crest of a fold is called a gyrus, and the invisible depression between folds is called a sulcus. One of the main landmark sulci of the brain is the central sulcus (or sulcus of Rolando). Some sulci are relatively deep, and are termed fissures. The main fissures include the very large one separating the two hemispheres, the interhemispheric fissure. Another is the fissure that runs approximately horizontally along the lateral surface of the brain, called, appropriately, the lateral fissure or fissure of Sylvius. Viewed laterally, the surface is composed of four lobes: the frontal lobe, the parietal    lobe, the temporal lobe, and the occipital lobe.

The frontal lobe is the largest of the cortex and extends from the central sulcus to the front or rostral end of the cortex. Several gyri can be distinguished on the frontal lobe surface. These are the precentral  gyrus, which is defined by the central and precentral sulci, and which holds within itself the motor cortex, rostral to the precentral sulcus are the superior, middle, and inferior frontal gyri.

The parietal lobe is the area immediately behind or caudal to the central sulcus and it runs caudally, approximately to the parieto-occipital sulcus. It is bounded ventrally by the lateral fissure. The somatosensory cortex  is contained within the postcentral gyrus in the parietal lobe, just caudal to the central sulcus. Caudal to the postcentral gyrus is the superior parietal lobule, and ventral to that is the inferior parietal lobule.

The temporal lobe lies ventral to the lateral fissure, and includes the inferior, middle, and superior temporal gyri. These run approximately parallel to the lateral fissure. The superior temporal gyrus holds the area of the primary auditory cortex.

The occipital cortex occupies the most caudal end of the brain and may be considered to lie caudal to a line drawn  through the parieto-occipital sulcus and the occipital notch. The visual cortex is located in this lobe, around the calcarine sulcus.

Although not shown here, there is an area of cerebral cortex called the insula, which is buried, deep in the lateral fissure. Its borders are defined by the frontal, parietal and temporal cortex. The rostral  end of the insula is a poorly understood part of the limbic system and the caudal end of the insula is involved in somatosensory processing.

There is also a limbic lobe, defined by Broca. The limbic lobe is made up of the cingulate, hippocampal and parahippocampal gyri. The cingulate gyrus is a primitive form of the cerebral cortex, having fewer layers of cells and is involved in the mediation of behavioral components  of endocrine, olfactory, skeletal, and visceral function, and in aspects of memory.

Cerebral Cortex

Thalamic Nuclei: Projections to Cerebral Cortex

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Thalamic NucleiThere are highly precise point-to-point reciprocal connections between thalamic nuclei and the cerebral cortex. All thalamic nuclei except the reticular nucleus send ipsilateral projections to the cerebral cortex, and all cortical areas receive inputs from the thalamus. Thalamic nuclei that communicate with cortical regions are termed specific nuclei. All the specific nuclei lie in the ventral tier of the lateral nuclear group. The thalamus projects efferents to the cortex in the thalamic peduncles.

The ventral posterior nucleus projects efferents via thalamocortical projections through the posterior limb of the internal  capsule and the corona radiata. which terminate in the primary somatosensory cerebral cortex in the postcentral gyrus. There is a lesser projection to the secondary somatosensory area at the inferior end of the postcentral gyrus. The ventral anterior nucleus projects widely to the frontal cortex, including the supplementary motor area. The ventral  lateral nucleus projects mainly to the motor and premotor areas of the cerebral cortex.

The anterior nuclear group is the most anterior part of the thalamus and is actually part of the limbic system. It receives inputs fromthe mamillary bodies of the hypothalamus via the mamillothalamic tract, and projects principally to the cingulate gyrus, which is seen on the medial surface of the cerebral hemisphere. This nuclear group appears to be associated with emotional status and recent memory.

The ventral lateral nucleus lies caudal to the anterior nucleus. This nucleus projects to the frontal lobe, including the areas of the primary and premotor cortex.

The bilateral lateral geniculate nuclei (also called the lateral geniculate bodies) form small but noticeable swellings or eminences near the posterior pole of the thalamus, just ventral to the pulvinar. These nuclei are the termination site of fibers of the optic tract from the retina, and are thus part of the visual system. Each nucleus projects efferents to the primary visual cortex in the occipital lobe via the retrolenticular portion of the internal capsule, and through the optic radiation.

The medial geniculate nucleus receives fibers carrying auditory information  from the inferior colliculus. The medial geniculate nucleus projects this information to the primary auditory cortex in the temporal lobe via the retrolenticular portion of the internal capsule and the auditory radiation.

The medial (mediodorsal) nuclear group receives inputs from the amygdala, hypothalamus, and from other thalamic nuclei. This nuclear group projects extensively  and reciprocally to the prefrontal cortex and mediates emotion and mood.

The intralaminar nuclei lie in the internal medullary lamina of the thalamus. These nuclei include the centromedian nucleus and parafascicular nucleus. These  nuclei receive afferents from the spinothalamic and trigeminothalamic tracts, and also from the brain stem reticular formation. They send efferents to the basal ganglia, namely the caudate nucleus and the putamen. They also project very extensively to the cerebral cortex. Lesions to these nuclei result in a reduction of the level of consciousness and the perception of pain.

Thalamic Nuclei

Thalamic Nuclei

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Thalamic NucleiThe thalamus is the largest mass of CNS nuclei and lies at the center of the brain. It consists of two bilateral egg-shaped lobes on opposite sides of the third ventricle. Their upper surfaces comprise the floor of each lateral ventricle, and their lateral surfaces are contiguous with the posterior limb of the internal capsule. The thalamus contains within it several nuclei with very diverse and often independent functions. The thalamic nuclei may be somatosensory, receiving inputs fromsensors of the somatosensory system and the special  senses. From these nuclei there are projections to the primary sensory cortex (see next spread). Motor nuclei receive inputs from the cerebellum and the basal ganglia.

Each thalamus has a Y-shaped internal medullary lamina consisting of nerve fibers which are some of the afferent and efferent connections of the thalamic nuclei. The lamina divides each lobe into three main nuclear masses: posteromedial (or mediodorsal), anterior and lateral. Lateral to these nuclear masses is athin, shield-like layer of  neurons called the  reticular nucleus. The reticular nucleus is the only thalamic nucleus that does not correspond with the cortex. Lying posteriorly (at the back) of the thalamus are the lateral and medial geniculate bodies. For convenience, the thalamic nuclei may be grouped as relay or specific, association and non-specific.

Specific nuclei are those which correspond reciprocally with the sensory and motor areas of the cerebral cortex. The ventral posterior nucleus is the termination site for fibers of the lemniscal system. A somatosensory homunculus has been mapped in the lateral and medial divisions  of this nucleus. The head is mapped medially, and the trunk laterally. In both divisions, nociceptive inputs occur towards the back of the homunculus, tactile inputs lie in the middle, and proprioception lies at the front. In other words, there is modality segregation. The ventral anterior nucleus receives inputs from the globus pallidus

The lateral geniculate nucleus receives afferents from the retina, and the medial geniculate nucleus receives afferents from the ear.

The association nuclei are (i) the anterior nucleus, which receives inputs from the mammillothalamic tract and may be involved in memory, (ii) the mediodorsal or posteromedial nucleus, which receives afferents from the limbic and olfactory systems and seems to mediate mood and judgment, and (iii) the pulvinar and lateral posterior nuclei, which are grouped as a single nucleus and receive afferents from the superior colliculus.

The non-specific nuclei include the intralaminar medullary nuclei and the reticular nucleus. The nuclei of the intralaminar medulla (see above) seem to be a rostral projection of the brain stem reticular formation involved in arousal. The reticular nucleus is separated from the other nuclei by the external medullary lamina; it receives collaterals from the thalamocortical fibers as they pass  through on their way to the cerebral cortex. The reticular nucleus in turn projects efferent GABAergic inhibitory fibers to the  corresponding thalamic nuclei from which it received the collaterals.

Thalamic Nuclei