Neuro Anatomy Lecture 8 PDF

Neuroanatomy 2711212 Cerebrum (Part 3) Lecture 8 WHITE MATTER OF THE CEREBRUM The white matter of the cerebrum is a compact mass of a large number of myelinated nerve fibers. Types of fibers in white matter They are classified into the following three types: commis...

Neuroanatomy 2711212 Cerebrum (Part 3) Lecture 8 WHITE MATTER OF THE CEREBRUM The white matter of the cerebrum is a compact mass of a large number of myelinated nerve fibers. Types of fibers in white matter They are classified into the following three types: commissural, association and projection fibers 1. Commissural Fibers They interconnect the identical cortical areas of the two cerebral hemispheres (interhemispheric fibres). 2. Projection Fibers They connect the cerebral cortex to the subcortical centers (such as the corpus striatum, thalamus, brain stem) and spinal cord. 3- Association Fibers They interconnect the different regions of the cerebral cortex in the same hemisphere (intrahemispheric fibers). Commissural fibers: They interconnect the identical cortical areas of the two cerebral hemispheres (interhemispheric fibres). The important commissures of the brain are as follows: 1. Corpus callosum. 2. Anterior commissure. 3. Posterior commissure. 4. Hippocampal commissure. 5. Habenular commissure. 1. Corpus callosum: It is the largest commissure of the brain. External features of corpus callosum: Corpus callosum forms a massive arched interhemispheric bridge in the floor of the median longitudinal cerebral fissure connecting the medial surfaces of the two cerebral hemispheres. In sagittal section of cerebrum it is seen as C-shaped mass of white fibers on the medial surface. The concave inferior aspect of corpus callosum is attached with the convex superior aspect of the fornix by septum pellucidum. Parts of the corpus callosum: It is divided from before backwards the following four parts into 1. Genu: It is thick curved anterior extremity of corpus callosum. The fibers of genu sweep (curve) forwards on either side into the anterior parts of the frontal lobes, forming a forklike structure, the forceps minor. - Its fibers connect the anterior parts of the 2 frontal lobes together. 2. Rostrum: - Its fibers represent a downward thin prolongation from genu. - Its fibers connect the orbital surfaces of the 2 frontal lobes together. 3. Body (trunk): - It is the main (middle) part of the corpus callosum between genu and splenium. -Its fibers connect the posterior parts of the 2 frontal lobes together and parietal lobes. 4. Splenium: it is the massive posterior extremity of the corpus callosum. The fibers of the splenium connect the parietal (posterior parts), temporal, and occipital lobes of the two hemispheres. The fibers connecting the occipital lobes sweep backwards on either side above the calcarine sulcus forming a large fork-like structure, the forceps major. 2. Anterior commissure : It is a small round bundle of white fibers which crosses the midline in the upper part of the lamina terminalis, immediately in front of the anterior column of the fornix and interventricular foramen. It interconnects the olfactory regions of the two hemispheres 3. Posterior commissure: It is a bundle of white fibers which crosses the midline through the inferior lamina of the stalk of pineal gland. 4. Habenular commissure: It is a bundle of white fibers which crosses the midline through the superior lamina of the stalk of pineal gland. 5. Hippocampal commissure (commissure of fornix): It interconnects the crura of fornix and hippocampi of the two sides 2. Projection Fibers They connect the cerebral cortex to the subcortical centers (such as the corpus striatum, thalamus, brain stem) and spinal cord. The projection fibers include the corona radiata, internal capsule, and long tracts e.g pyramidal tracts Internal Capsule The internal capsule is a compact bundle of projection fibers between thalamus and caudate nucleus medially and lentiform nucleus laterally. It consists of ascending and descending nerve fibers. The ascending (sensory) fibers pass up from thalamus (thalamic radiation) to the cerebral cortex and descending (motor) fibers pass down from the cerebral cortex to the cerebral peduncle of the midbrain. The fibers of internal capsule are mainly responsible for the sensory and motor innervation of the opposite half of the body. Shape and parts of internal capsule In a horizontal section of the cerebral hemisphere, the internal capsule appears as a V-shaped compact bundle of white fibers with its concavity directed laterally internal capsule is divided into the following five parts: 1.Anterior limb: between head of caudate nucleus and lentiform nucleus. 2. Posterior limb: between thalamus and lentiform nucleus. 3.Genu: the bend between the anterior and posterior limbs with concavity of the bend facing laterally. 4. Retrolentiform part: behind lentiform nucleus. 5. Sublentiform part: below lentiform nucleus. Constituent fibers of internal capsule : Part Motor fibers Sensory fibers Anterior limb Corticopontine fibers Anterior thalamic radiation Genu Corticonuclear fibers Posterior limb Corticopontine fibers Superior thalamic radiation Corticospinal (sensory radiation) (pyramidal) fibers Corticorubral (extrapyramidal) fibers Retrolentiform Corticopontine fibers Posterior thalamic part radiation (optic radiation) Sublentiform Corticopontine fibers Inferior thalamic radiation part (auditory radiation) Arterial supply of internal capsule 1 The anterior part of the anterior limb: supplied by anterior cerebral artery. 2 The posterior part of anterior limb, genu and the posterior limb: supplied by middle cerebral artery. 3The posterior part of the posterior limb, sublentiform and the retrolentiform parts: supplied by anterior choroidal artery. This is a branch from the internal carotid artery. Lesions of internal capsule: Damage to the internal capsule caused by hemorrhage or infarction, leads to loss of sensations and spastic paralysis of the opposite half of the body (contralateral hemiplegia). 3- Association Fibers They interconnect the different regions of the cerebral cortex in the same hemisphere (intrahemispheric fibers). a. Short associative fibers: connect adjacent gyri with each other. b.Long associative fibers: these fibers connect distant gyri with each other. Examples: 1. Superior longitudinal bundle: passes along the superiomedial border. It connects the frontal pole with the occipital pole. Some fibers curve onnect the frontal and the temporal poles. 2.Inferior longitudinal bundle: passes along the inferolateral border. It connects the occipital pole with the temporal pole. 3. Uncinate bundle: curves around the lateral sulcus to connect the frontal lobe with temporal lobe. 4.Cingulum: It passes in the cingulate gyrus, and passes in the para- hippocampus to reach the uncus. It is a part of the limbic system 5. Fornix The fornix is a large bundle of association fibers, which connects the hippocampus with the mammillary body. It constitutes the sole efferent system of the hippocampus On the medial surface of the cerebral hemisphere, it is seen as an arched prominent bundle of white fibers below the corpus callosum, along the lower border of septum pellucidum. There is one fornix in each cerebral hemisphere but two are so closely related/fused beneath the middle of the body of corpus callosum that they are usually described as a single structure. Parts of the fornix: are fimbria, crura, body, and anterior columns. The crus of fornix: as it arches below the splenium and behind the thalamus. The two crura, one of each hemisphere unite in the midline beneath the trunk of corpus callosum to form the body of fornix. Anteriorly, the body of fornix divides into two columns, the columns of fornix.. They end in the mammillary body.