• cortex: outer grey matter of the cerebral hemispheres; 2000-2500 cm²; 14-16 billion neurons (but there are far more neurons in cerebellar cortex); arranged in layers rather than nuclei
• archicortex: the hippocampal formation; oldest phylogenetically; contains 3 layers
• paleocortex: next newest; located adjacent to hippocampus; contains 4-5 layers
• neocortex: accounts for 95% of human brain, contains six layers
• Cortical Layers – in the neocortex
• Layer I: almost no cell bodies; mostly made of apical dendrites of neurons in lower layers
• Layer II: contains small pyramidal neurons projecting to other cortical regions via association or commissural fibers
• Layer III: large pyramidal neurons projecting to other cortical areas; well-developed in association cortices; layer III is the primary source and target of cortico-cortical projections
• Layer IV: most dense layer of neurons, mostly granule cells which are the primary receiving cells of the cortex; well-developed in primary sensory cortices (often termed granular cortices)
• Layer V: contains the largest pyramidal neurons, main cortical output; well-developed in motor cortices (agranular cortex)
• Layer VI: contains multiform projection neurons, including pyramidal cells; output to thalamus, claustrum, other areas
• Cortical Cytology
• pyramidal neurons: primary output neurons of the cortex, found in layers II, III, V, and VI
• dendrites: two sets (1) apical dendrite: large, arises from apex of cell, (2) many smaller dendrites arise from the base
• axon: arises from the base of the cell body; emits large plexus of collaterals with four basic patterns
• (1) association neurons (III): project from cortex to ipsilateral cortex; superior and inferior longitudinal fasciculus, arcuate fascicle, cingulum, uncinate fasciculus
• (2) commissural neurons (II): project from cortex to contralateral cortex; corpus callosum, anterior/posterior commissure
• (3) projection neurons (V): project from cortex to subcortical structures; corticobulbar, corticospinal, corticopontine
• (4) granule cells (IV): most prominent in layer IV; stellate in shape; primary receiving cells of the cortex
• Cortical Columns: columns of cells vertically interconnected that extend from layer I to layer IV
• evidence for columnar arrangement: in a particular column of the sensory cortex, all cells: constitute a single perceptive field, respond to the same stimulus, discharge similarly
• functional output of the column is to pyramidal neurons (whose dendrites extend in the column and can receive up to 60K synapses within the column)

• primary motor cortex – precentral gyrus (area 4): contains very large pyramidal cells that project to the corticospinal and corticobulbar tracts; all of area 6 projects to the primary motor cortex; controls single muscles
• supplementary motor cortex – medial aspect of hemisphere; premotor area – lateral (area 6): involved in initiation and planning of more complex motor activities; control multiple muscles
• Broca’s speech area – anterior to area 6, lateral, dominant hemisphere (area 44): controls the motor aspects of speech
• frontal eye fields – anterior to area 6, medial (area 8): voluntary control of saccadic eye movements

• primary somatosensory cortex – postcentral gyrus (areas 3, 2, 1): input from VPL and VPM in dorsal thalamus; output to association cortex, contralateral primary sensory cortex; also dorsal thalamus, dorsal column nuclei (for regulation of incoming sensory information)
• primary visual cortex – occipital pole (area 17): input from lateral geniculate body, projects directly to visual association areas 18, 19 (extrastriate visual cortex) which lie just rostral
• primary auditory cortex – upper part of temporal lobe (area 41): input from medial geniculate body, projects to auditory association area 42 and 22 – which lie just inferior
• Wernicke’s area (posterior area 22): in dominant hemisphere – recognition of spoken language; in non-dominant – interpretation of emotional content of speech
• Piriform cortex – 43 – gustatory (taste)

Association Cortex – aka: interpretive cortex, higher order cortex; comprises 75% of cortex

• has columnar organization with well-developed layer III (main source and target of cortico-cortical projections)
• large proportion of association cortex in humans is what makes us so damn smart
• pre-frontal association cortex: deficits lead to problem-solving and emotional deficits (i.e. inability to concentrate, use judgement, foresight; unpredictable, bizarre behavior)
• parietal association cortex: perception of body image, attention to stimuli; deficit Þ neglect of contralateral body and surroundings
• temporal association cortex: recognition, naming familiar faces and objects
• limbic association cortex: input from higher order cortex and all other association areas; transfers input to amygdala and hippocampus where it is linked to emotions and responses to stimuli