hemispheres

Cerebral Hemispheres: Cellular Architecture

by Sharadsaini on

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

by Sharadsaini on

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

by Sharadsaini on

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