Cerebral White Matter & Limbic System (PDF)

Objectives: 1. Identify the types of cerebral white matter & their connections. 2. Study the parts & relations of the corpus callosum & its clinical importance. 3. Identify the parts & relations of the projection fibers. 4. Identify the parts & connections of the limbic system, its functions & relat...

Objectives: 1. Identify the types of cerebral white matter & their connections. 2. Study the parts & relations of the corpus callosum & its clinical importance. 3. Identify the parts & relations of the projection fibers. 4. Identify the parts & connections of the limbic system, its functions & related disorders.  Types of cerebral white matter fibers Association (arcuate) fibers connect one part of the cerebral cortex to another part in the same hemisphere. Commissural fibers cross the midline and connect one part of a hemisphere to the corresponding part of the opposite hemisphere. Projection fibers connect the cerebral cortex to the subcortical nuclei in the cerebral hemispheres and with nuclei in the diencephalon, brainstem and spinal cord.  Association Fibers Short: U-shaped, pass from one part of a gyrus to another part of the same gyrus or adjacent gyrus of the same lobe. Long: run long distances between cortical areas of different lobes forming visible nerve bundles: 1. Superior longitudinal fasciculus largest bundle of nerve fibers. It connects the anterior part of the frontal lobe to the occipital and temporal lobes. 2. Cingulum: within the white matter of 3. Inferior longitudinal fasciculus: from the occipital to temporal lobes. the cingulate gyrus& connects frontal and parietal lobes with the 4. Uncinate fasciculus: from inferior frontal lobe gyri to temporal lobe. parahippocampal gyrus.  Commissural fibers: Corpus callosum (Parts in sagittal plane) 1. Rostrum: tapered anterior end connecting the orbital cortices. 2. Genu: sharp backward bend from the rostrum connecting frontal cortices. Body (trunk): 3. Body (trunk): largest, convex upwards connecting parietotemporal cortices. 4. Splenium: thick rounded posterior end connecting occipital cortices. The isthmus is the area between the body & splenium. Commissural fibers: Corpus callosum (Fibers of parts in horizontal plane) Tapetum from Latin tapete ‘carpet Commissural fibers: Corpus callosum (Fibers of body in coronal plane) Commissural fibers: Corpus callosum (Cover) A thin layer of grey matter lies in contact with the dorsal surface of the corpus callosum and is continuous laterally with the grey matter of the cingulate gyrus. This layer is called the supracallosal gyrus or indusium griseum (Latin=grey membrane). It contains two longitudinally directed strands of fibers termed respectively the medial and lateral longitudinal striae (of Lancisi).  Commissural fibers: Corpus callosum (median relations) Dorsally: Inferior sagittal sinus in lower free border of falx cerebri Caudally: Inferior sagittal sinus joins great cerebral vein → straight sinus→ transverse sinus Ventrally: septum pellucidum, interventricular foramen (of Monro), fornix (&its commissure), pineal body. Commissural fibers: Corpus callosum (Paramedian relations) Dorsally: pericallosal branch of ACA, cingulate gyrus Caudally: post. horn of lateral ventricle. Ventrally: ant. horn & body of lateral ventricle. Midline commissurotomy (Corpus callosotomy) & split-brain syndrome Prevents epileptic seizures from spreading from one half of the brain to the other. When the corpus callosum is severed during an experimental procedure, the experimenter can ask each side of the brain the same question and receive two different answers. William Perrine Van Wagenen (1897–1961) first performed it in in 1940  Commissural fibers: Anterior commissure Anterior bundle interconnect olfactory bulbs/tracts of the 2 hemisphere. Posterior bundle interconnect the parahippocampal gyri and other parts of the temporal lobe (for memory coordination).  Commissural fibers: Posterior & Habenular commissures (diencephalon) 1. Habenular commissure: in the superior (proximal) lamina of stalk of the pineal gland. It coordinates response to anxiety, pain, reward, stress, sleep …etc. 2. Posterior commissure: in the inferior (distal) lamina of the stalk of the pineal gland, may be involved in the pupillary light reflex  Projection fibers Ascending (afferent)→ Coticopetal Descending (efferent)→Corticofugal Cerebral cortex Corona radiata Internal capsule Cerebral peduncles (crura cerebria) Brainstem Pyramidal decussation Spinal cord Corona radiata: parts & switches!!!  Internal capsule: Parts & fibers Arrangement of fibers in the Internal Capsule Part Descending tracts Ascending tracts o Anterior thalamocortical Anterior limb Frontopontine fibers fibers Corticonuclear & o Anterior part of the Corticospinal fibers (head superior thalamocortical Genu and neck) fibers Cosrticospinal fibers (upper and lower limbs and o Superior thalamocortical trunk) Posterior limb fibers Corticopontine fibers Corticorubral fibers Some parietopontine and Retrolentiform occipitopontine fibers o Optic radiation Occipitocollicular fibers Some parietopontine and Sublentiform o Auditory radiation temporopontine fibers Pin-point lesions of the internal capsule can lead to widespread derangements of the body. Summary CEREBRAL WHITE MATTER FIBERS ASSOCIATION COMMISSURAL PROJECTION Short Long Corpus callosum Corona radiata External capsule Extreme capsule Cingulum Anterior commissure Internal capsule Uncinate fasciculus Posterior commissure Cerebral peduncle Superior longitudenal Habenular commissure bundle Inferior longitudenal Fornix commissure bundle  The Visual Pathways 1. The temporal nerves of the retina receive the same part of the visual field as the nasal fibers of the opposite retina. 2. As the optic nerve leaves the optic canal, it joins its fellow at the optic chiasma & the nasal fibers of each retina cross to the contralateral optic tract. 3. Now each optic tract carries the temporal fibers of the ipsilateral retina & nasal fibers of the contralateral retina. i.e. the right optic tract carries stimuli from the left ½ of the visual field & the left tract carries visual stimuli from the right ½ of the visual field. 4. The optic tract passes from the chiasma around the cerebral peduncle (crus cerebri) to reach the side of the thalamus (where it divides into 2 roots: → The lateral (larger) root carries visual fibers that enter the lateral geniculate body to synapse& then spread as the optic radiation through the retrolentiform part of the internal capsule. The Parietal part of the radiations carry fibers from the upper retinal quadrants (lower visual fields) & ends in the lower lip of the calcarine sulcus. The Temporal radiations (Meyer's loop) carry fibers from lower retinal quadrants (lower visual fields) & ends in the upper lip of the calcarine sulcus.  The Visual Pathways → The medial smaller root of the optic tract forms the superior brachium which passes to the midbrain and synapse in the superior colliculus (general light reflex) and pretectal nucleus (pupillary light reflex) to mediate the light reflexes.  The Auditory Pathways: Bilateral Cerebral Perception Organ of corti cells axons form the afferent (cochlear) Ear fibers of the vestibulocochlear nerve Cochlear fibers end in the cochlear nuclei and 2 sets of fibers arise. Medulla oblongata Dorsal fibers cross the pontine tegmentum to the contralateral lateral lemniscus, Ventral fibers synapse bilaterally in superior olivary nucleus complex to the bilateral lateral lemnisci. The lateral lemniscus ends at the inferior colliculus of Midbrain the midbrain The medial geniculate body (MGB) receives the Metathalamus inferior brachium from the inferior colliculus of the midbrain. Cerberal The auditory radiation arises from the MGB, passes through the sublentiform part of the internal capsule Hemisphere to reach the auditory cortex (temoral lobe).  The Limbic System: LIMBUS = Border / Edge…. term limbic system is loosely used to include a group of structures that lie in the border zone between the cerebral cortex and the hypothalamus surrounding the corpus callosum….concerned with regulating behaviors related to the objectives of primitive life (Sustenance, shelter and sex). The Limbic System: Cortical areas This cortex has been termed the "limbic lobe" because it makes a rim surrounding the corpus callosum, following the lateral ventricle. These Cortical regions include: Parahippocampal gyrus & uncus→ (Memory conversion/Learning). Insular cortex →(Social Reward) Cingulate gyrus. Orbital frontal cortex & Subcallosal gyrus. The Limbic System: Subcortical areas (Hippocampal formation & fornix)  The Limbic System: hippocampal formation Located in the temporal lobe: behind amygdala & medial to the inferior horn of the lateral ventricle (deep to the uncus) & consists of (in coronal section): Dentate g., hippocampal g. proper, subiculum (connector) & (para)hippocampal g.  The Limbic System: Fornix [from Latin= ‘arch or vaulted chamber’]. The fornix is a “C” shaped major tract of the hippocampal formation that begins from each hippocampus as the crus or posterior pillar of the fornix. The two crua (of right and left) come together and form the hippocampal commissure (by which the 2 hippocampi communicate with each other). The fibers decussate in the commissure and continue as a conjoined mass beneath the corpus callosum called the body of the fornix which reaches the anterior commissure. From the body, two conjoined columns of the fornix arch down anterior to the thalami and each divides into anterior and posterior fibers. The anterior fibers run to the septal nuclei near the lamina terminalis while the posterior fibers run to the mamillary bodies.  The Limbic System: Affectors & Responders The olfactory bulb/tract. (Smell) The hypothalamus. (Hormonal response) The amygdala (Rage/Aggression). The septal nuclei (Pleasure/Addiction). The anterior thalamic nucleus (Emotion) Mammillary bodies (facial recognition) Primitive needs for SATISFACTION Primitive needs Affecters  → Olfactory Temporal cortex pathways Limbic system related disorders Functions and disorders of the limbic system Component Main function Related disorders Signals the cortex of motivationally significant Anxiety, autism, depression, post- Amygdala stimuli (e.g. reward/fear) & social functions traumatic stress disorder, phobias (e.g. mating). Learning (Conversion of short-term memories Anterograde amnesia, Alzheimer’s Hippocampal formation to long-term memories) disease, navigational disability Neuropsychiatric disorders Septal nuclei Reward & pleasure center (Schizophrenia, Bipolar, OCD) Mammillary bodies Formation of recognition memory Prosopagnosia (face blindness) Insular cortex Social emotions Addictive behavior