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Chapter 3

PSYC55 - Cognitive Neuroscience Ch. 3 (pg.59-87)

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Pare, Dwayne

Chapter 3 (p 59-87)  Two prominent features found in Einstein’s brain o His Sylvian fissure (division that separates the temporal love form the frontal and parietal lobes) had an unusual anatomical organization. His brain showed strange confluence of the Sylvian fissure with the central sulcus on the brain’s lateral surface. Most brains have Sylvian fissure that projects posteriorly to end in an area surrounded by the supramarginal gyrus o His inferior parietal lobe was actually larger, and thicker in lateral to medial extent  Increased size of Einstein’s inferior parietal cortex might have been related to his intellectual capacity Neuroanatomy  Neuroanatomy is the study of the nervous system’s structure concerned with identifying the parts of the nervous system and describing how the parts are connected  Investigations occur at one of two levels o Gross neuroanatomy = focuses on general structures and connections visible to the naked eye o Fine neuroanatomy (microscopic anatomy) = describes organization of neurons and their connections and subcellular structure Methods in Neuroanatomy  In order to do a gross dissection, the first task is to decide how to view the brain  The brain is not only enclosed in the bony skull, it also is surrounded by dura mater (a dense layer of collagenous fibers)  After the dura mater, there are prominent structures that can be seen such as o Gyri o Primary sulci o Fissures o Gradual narrowing of the brainstem o Folding of the cerebral cortex  Gray matter forms continuous cortical sheath enshrouding a homogenous mass of white matter o Gray mater appears gray in preserved brains, even though it is actually reddish-pinkish in live brains. It contains cell bodies of neurons and glial cells o White matter is actually milky white in the living brain due to the fatty myelin surrounding the axons  It is also possible to follow the trajectory of axons grouped in large fiber tracts (bundles of axons) and they course through the brain and connect with different regions  Dissection reveals the arcuate fasiculus which connects the Wernicke’s area to the Broca’s area  Microscopic examination reveals that white matter is actually composed of millions of individual fibers, each surrounded by myelin  Histology is the study of tissue structure through microscopic techniques  Primary concern for neuroanatomists is to identify the patterns of connectivity in the nervous system in order to lay out the neural pathways that allow info to get from one place to another  There is tremendous convergence and divergence in the nervous system  Neighboring and distant connections between 2 cortical regions are called corticocortical connections  Inputs that originate in subcortical structures such as the thalamus would be regerred to as thalamocortical connections; the reverse would be called corticothalamic or coticofugal projections  Cell staining allows different levels of analysis o At a higher level of resolution, we can examine subcellular organelles o Fluorescent stains and UV light help visualize structures that take up the stains  Tract-tracing methods permit the connections between different neurons and brain regions to be identified  Degeneration is a method used to trace axonal pathways that are degenerating following damage or disease  At the simplest level, one can look for missing myelinated axons in damaged brains  At a more advanced level, the Marchi stain is used which selectively stains the myelin in degenerating axons  A more modern but now common method makes use of the horseradish peroxidase (HRP) enzyme o HRP is a retrograde tracer that is taken up by the axons when it is injected at axon terminals and transported back to their cell bodies o HRP provides a tool to visualize where the input to a particular neural region originates o Researchers who want to know which subcortical structures project to the primary visual cortex can use HRP as a retrograde transport tool, injecting it into the input layers of the visual cortex o It is then absorbed the axonal channels and diffuses up the axon to the cell body o Chemical treatment turns the HRP to a variety of colors  Anterograde tracers are absorbed at the dendrites or soma and then diffuse along the axons  Radioactively labeled materials are used as tracers and autoradiography is then used to visualize the pattern of staining o Tissue is sliced, labeled, and placed on a photographic material to let the radioactivity expose the photographic emulsion, producing a picture of the distribution of label in the tissue  Retrograde and anterograde tracers allow researchers to identify the input to a specific region and determine where the axons from a particular region terminate  Despite the commonalities across neurons, they are very heterogeneous  Brodmann used staining method to devise a cytoachitectonic map of the brain o He partitioned cortical areas according to differences in cell morphology, density, and layering and introduced a numbering scheme Gross and Functional Anatomy of the Nervous System  Nervous system has 2 major subdivisions o CNS (brains and spinal cord) – command-control portion o PNS (everything outside the CNS) – courier network that delivers sensory info to CNS and projects it onwards Cerebral Cortex  Cerebral cortex has 2 symmetrical hemispheres that consist of large sheets of layered neurons  It sits over the core structures (limbic system and basal ganglia) and surrounds structures of the diencephalon  Forebrain = cerebral cortex + basal ganglia + diencephalon  The folds of the cortex function to pack more cortical surface into the skull and provides a saving in space and reducing axonal distances and neuronal conduction time as well. The folding also brings nearby regions closer together  Cortex appears grayish due to the high amount of cell bodies  Cerebral hemispheres have 5 main divisions (lobes), considering the limbic system a lobe o Frontal o Parietal o Temporal o Occipital  Central sulcus divides frontal and parietal lobes and the Sylvian fissure separates the temporal from frontal and parietal  The parieto-occipital sulcus separates the occipital lobe from the temporal and parietal  Left and right cerebral hemispheres are separated by the interhemishpheric fissure (longitudinal fissure)  Connections between the 2 hemispheres are made through the corpus callosum which is the largest white matter commissure in the nervous system o Commissure = term for white matter tracts that cross from the left to right side or vice versa of the CNS  Cytoarchitectonics is another way of subdividing the brain which has to do with how cells in a region appear morphologically and are arranged (Brodmann) o Require histological analysis of tissue of different regions of the cerebral cortex o The goal is to define the extent of regions in which the cellular architecture look similar and might represent a homogenous area of the cortex  Brodmann identified ~52 regions of the cerebral cortex where as other identified ~ 200 areas o A combination of both is more effective way of dividing the cerebral cortex  Brodmann system often seems unsystematic o In some regions, the numbering system has a rough correspondence with the relations between areas that carry out similar functions (ex. vision areas 17,18,19)  Different nomenclatures often do not refer to the same area with a one-to-one mapping  It is also possible to divide the cerebral cortex according to the general patterns of layering o 90% of the cortex is composed of neocortex (contains 6 main cortical layers with high degree of speiclization) o Each layer is anatomically and functionally distinct  Neocortex includes areas such as o primary sensory o motor cortex o association cortex  Mesocortex (6 layers; between neocortex and allocortex) is the paralimbic region which includes o cingulate gyrus o parahippocampal gyrus o insular cortex o orbitofrontal cortex  allocortex has 1-4 layers of neurons and includes o hippocampal complex (archicortex) o primary olfactory cortex (paleocortex)  cognitive brain systems are often composed of networks whose component parts are located in different lobes of the cortex  most functions of the brain rely on both cortical and subcortical components  frontal lobe plays major role in planning and executing movements and has 2 main subdivions o motor cortex  begins in depths of central sulcus and extends in anterior direction  includes anterior portion of central sulcus and much of precentral gyrus  also includes premotor cortex and supplementary motor cortex  output layer of motor cortex has the large pyramidal neurons (Betz’s cells) which are the largest neurons in the cortex o prefrontal cortex  take part in complex aspects of planning and executing behavior  include dorsolateral prefrontal cortex, orbitofrontal cortex, and anterior cingulate and medial frontal regions  somatosensory cortex is posterior to the central sulcus and encompasses the postcentral gyrus  receives input from the somatosensory relays of the thalamus and represent info about touch, pain, temperature and limb proprioception o primary somatosensory cortex o secondary somatosensory cortex  somatosensory info coming into the thalamus and going to the primary somatosensory cortex transverses 2 main pathways: o anterolateral system for pain and temperature sense o dorsal column-medial lemniscal system for info about touch, proprioception and movement  the 2 systems take different pa
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