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Lecture 8

Lecture 8 Notes.docx

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Rutsuko Ito

Lecture 8 Notes Synaptic organization of neocortex Neocortex 1. Constitutes the top 2mm thick layer of cerebral hemisphere 2. Newest addition to cerebral cortex a. Older additions i. Archicortex -- Hippocampus ii. Paleocortex -- Olfactory cortex 3. Presence of neocortex distinguishes mammals from vertebrates 4. Involved in a. Intelligence i. More folds 1. Gyri 2. Sulci 3.  Increased surface area for information representation 4.  Decrease distance that axons would have to travel for communication ii. Better defined temporal lobe iii. Larger frontal lobe b. Executive control Human neocortex Topographical organization 1. Penfield a. Mapping of motor and somatosensory homunculi b. Non proportional mapping of body part c. Contralateral muscle contractions from stimulation of motor cortex Brodmann's areas 2. Division of the cortex on the basis of cytoarchitectural organization of neurons observed during nissl staining 3. Neocortex shows functional localization a. Different parts associated with different functions i. Divided into 1. Primary cortical areas a. Receive direct inputs from the motor and sensory organs of the brain 2. Associational areas a. Often located adjacent to primary cortical areas b. Important for integrating and interpreting information received from the primary cortical areas Prefrontal cortex 1. Highest level of cortical hierarchy a. Crucial for i. Decision making ii. Memory iii. Planning iv. Language v. Social skills 2. Division a. Dorsolateral prefrontal cortex i. DLPFC implicated in 1. Spatial working memory (right hemisphere) 2. Language (left hemisphere) b. Ventromedial prefrontal cortex i. Includes the orbitofrontal cortex ii. VMPFC implicated in 1. Object working memory 2. Decision making 3. Emotional processes 4. Morality 5. Social skills 3. Development of the PFC a. Postnatal development i. Functional maturation of the PFC does not occur in humans until 15-19 years of age (after puberty) 1. Alterations in synaptic connectivity in the PFC during these years implicated with development of schizophrenia b. In old age i. PFC and temporal lobes are the first to undergo involution (gradual decrease in size, volume, and density of cells) 1. Associated mainly with the shrinkage and disappearance of dendrites 4. Phineas Gage a. Bilateral damage to the ventromedial region of the frontal lobes b. Change in personality and mood DLPFC and working memory 1. Performance of delayed response tasks a. Retaining knowledge of information provided by a sensory cue over a brief delay i. To produce an appropriate behavioral response after the delay period 2. DLPFC neurons show persistent spatially tuned firing during the delay period at 90 degrees (the preferred direction for this neuron) but not at other spatial locations 3. Conclusion a. The cell provides a representation of the 90 degree location over the delay period when information is no longer available in the environment Domain specificity theory of working memory 1. DLFPC implicated in spatial working memory 2. VMFPC implicated in object working memory Study 1. Monkeys trained to perform oculomotor delayed response tasks in which spatial or pattern memoranda had to be remembered on independent trials 2. Spatial and pattern trials required identical responses but differed in the type of memory that guided those responses a. Spatial i. Stimuli presented to left or right of fixation ii. Gaze at fixation point iii. Fixation point disappears iv. Move to presentation of stimuli b. Pattern i. Stimulus pattern presented in center ii. Pattern required left or rightward eye movement 3. Found that ventral PFC neurons were most responsive over the delay period of pattern trials 4. Found that DLPFC neurons were most responsive over the delay period in the spatial trials Principles of neocortical organization Horizontal laminar organization 1. Division of neocortex into 6 horizontal layers a. Notable differences in the thickness of each layer b. Notable differences between different cortical regions i. Visual cortex 1. Prominent layer 4 ii. Motor cortex 1. Thick layer 6 Neocortical neurons 1. Principle neurons of the neocortex comprise 80% of the cell population a. Glutamatergic (excitatory) b. Spiny c. Cell types i. Pyramidal cells 1. Occupy all cortical layers except layer 1 2. Have apical and basal dendrites ii. Stellate / granule cells 1. Present only in layer 4 2. Major recipients of thalamic input 3. Project to layer 2 to target pyramidal neurons 2. Local GABAergic (inhibitory) make up 20% of the population a. Cell types i. Bitufted cells 1. Characteristic vertical axonal arborations that look like branching arcades ii. Double bouquet cells 1. Long axons that project to layer 5 iii. Small basket cells iv. Large basket cells v. Chandelier cells vi. Undesignated cell (long stringy cell) 1. Transmits neuromodulators a. Acetylcholine b. Neuropeptides vii. Neurogliaform cells Afferent / efferent projections 1. 90% of afferent inputs to neocortical neurons are of cortical origin a. 10% are of thalamic origin i. <1% are neuromodulatory 2. Thalamic (glutamatergic) neurons target spiny stellate cells in layer 4 to form a feed forward excitatory circuit a. They also make synaptic contacts with GABAergic neurons for feed forward inhibition 3. 90-99% of all axons remain within the cortex a. Giving rise to cortico-cortical connections remaining in layers 2/3 b. Pyramidal cells in layer 5 and 6 act as the output projections to subcortical structures i. Such as 1. Thalamus 2. Basal ganglia 3. Brainstem 4. Spinal cord 5. Superior colliculus Vertical columnar organization 1. Columns thought to represent a functional unit a. All cells within that particular unit being maximally attuned to by the same property of a stimulus 2. Evidence of columnar organization a. Barrel cortex of rats i. Each vibrissae connects to a barrel-like cell column in the cortex ii. Small regions of the visual field are analyzed by an array of complex cellular units called hypercolumns iii. A single hypercolumn represents the neural machinery necessary to analyze a discrete region of the visual field iv. Each contains 1. A complete set of orientation columns representing 360 degrees 2. A set of left and right ocular dominance columns a. Each ocular dominance column receives input from either the contralateral or ipsilateral eye via projections form cells in individual layers of the lateral geniculate nucleus that serve one or the other eye 3. Several blobs a. Regions of the cortex in which the cells are specific for color 3. Functional column organization a. Found in primary cortices b. Found in associational cortices i. E.g. IT stream of visual information processing implicated in facial recognition 1. Neurons that respond to similar properties of objects 4. Functional organization of PFC representing visuospatial working memory a. Inducing local injection of muscimol (GABA-A agonist) at 100 sites in the DLPFC of monkeys b. Observing the behavioral consequences in an oculomotor delayed response task c. At 82 sites, muscimol induced deficits in memory guided saccades to a few specific (usually contralateral) target locations that varied with the location of the injection site i. Deficits depended on 1. Delay length a. Longer delays associated with larger deficits in memory guided saccades b. Injection sites and affected spatial locations of the target showed a gross topographical relati
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