Scientific Understanding of Consciousness
Consciousness as an Emergent Property of Thalamocortical Activity

Neocortex

The neocortex is part of the cerebral cortex (along with the archicortex and paleocortex, which are cortical parts of the limbic system). In humans, it is involved in higher functions such as sensory perception, generation of motor commands, spatial reasoning, conscious thought and language. (Wikipedia)

Typical cerebral cortex has a surface area of 2200 cm² and a thickness between 1.5 and 4.5 mm in humans. (Science, “From the Connectome to the Synaptome,” 26 Nov 2010, vol. 330 no. 6008, p.1200)

Two types of circuits in the brain.   (1) mediating circuits produce behaviors, (2) modulatory circuits act on the mediating circuits, regulating the strength of their synaptic connections. (Kandel; Search of Memory, 224)

Nervous systems are organized on multiple scales,    from synaptic connections between single cells,    to the organization of cell populations within individual anatomical regions,    and finally to large-scale architecture brain regions and their interconnecting pathways. (Sporns; Networks of the Brain, 35)

Minicolumn contains about 110 cells. This figure is almost invariant between different neocortical areas and different species of mammals, except for the striate cortex of primates, where it is 260. (Edelman and Mountcastle, Mindful Brain; Mouthcastle; Organizing Cerebral Function, 37)

Three major cytoarchitectural divisions of the neocortex. (1) granular cortex of the sensory areas, contains small, densely packed neurons in the middle layers; (2) agranular cortex of the motor and premotor cortical areas, (3) varying populations of granule cells, mostly 'association cortex'. Within each of these areas there are many subdivisions, both functional and anatomical. (Shepherd, Synaptic Organization of the Brain; Douglas; Neocortex, 459)

Link to — Cortical Layers diagram

 

Evolutionary

Human brain is three times as large as might be expected for a primate of equivalent weight. Human brain is not simply a scaled-up version of closest primate relative, the Bonobo chimpanzee. Greatest expansion is in the cortical structures, particularly the cerebellum and neocortex, (Douglas; Neocortex, 459)

Within the neocortex, the expansion is uneven. In comparison with nonhuman primates of equivalent body weight, the human association and premotor areas have expanded relative to the sensory areas, (Douglas; Neocortex, 459)

In all mammals the neocortex consists of a sheet of cells, about 2 mm thick. Conventionally, it is divided into 6 layers, but in many regions more than 6 laminae are in evidence. Each cubic millimeter contains approximately 50,000 neurons, (Douglas; Neocortex, 459)

Sensory and Motor Functions, Speech Production and Comprehension

In addition to the basic sensory and motor functions, the human cortex is intensely involved in high-level functions, such as speech production and comprehension, (Douglas; Neocortex, 461)

Sensory Areas, Motor and Premotor Areas, Association Cortex

Three major cytoarchitectural divisions of the neocortex. (1) granular cortex of the sensory areas, contains small, densely packed neurons in the middle layers; (2) agranular cortex of the motor and premotor cortical areas, (3) varying populations of granule cells, mostly 'association cortex'. Within each of these areas there are many subdivisions, both functional and anatomical., (Douglas; Neocortex, 459)

Spiny-Dendrite Cells and Smooth-Dendrite Cells

All three cytoarchitectural divisions of the neocortex contain the same two basic types of neurons: those whose dendrites bear spines (the stellate and pyaramidal cells) and those whose dendrites are smooth (smooth cells), (Douglas; Neocortex, 461)

Pyramidal cells form about 70% of the neurons. Smooth cells form about 20% of the neurons., (Douglas; Neocortex, 461)

Spiny cells are excitatory, whereas smooth neurons are inhibitory., (Douglas; Neocortex, 462

Spiny neurons are called this because their dendrites bear small processes called spines, which are usually club shaped, with a head of about 1µm diameter and a shaft, or neck, of about 0.1 µm diameter., (Douglas; Neocortex, 464)

Pyramidal Cells

Major subtype of spiny neurons are the pyramidal cells, which constitute about two-thirds of the neurons in the neocortex., (Douglas; Neocortex, 464)

Pyramidal neurons are found in all cortical layers except layer 1, (Douglas; Neocortex, 464)

The most prominent feature of pyramidal neurons is an apical dendrite that may extend through all layers of the cortex above the soma., (Douglas; Neocortex, 464)

Pyramidal cells are the major output neurons of the neocortex, (Douglas; Neocortex, 464)

Pyramidal cells participate both in connections between the different cortical areas and to subcortical structures such as the thalamus and superior colliculus., (Douglas; Neocortex, 464)

Pyramidal cells are a major provider of excitatory input to the area in which they are found: each pyramidal neuron has a rich collateral network that forms part of the local cortical circuitry, (Douglas; Neocortex, 464)

Proximal shafts of the dendrites of the spiny cell types are nearly devoid of spines, (Douglas; Neocortex, 464)

Spine density varies considerably between different types of neurons. At one extreme is the sparsely spiny neuron, which may bear fewer than 100 spines over the entire dendritic tree. These neurons form a small subclass of the inhibitory neuron population. At another extreme are neurons such as the Betz cell, a large pyramidal cell that is found in the motor cortex (area 4) and bears about 10,000 spines, (Douglas; Neocortex, 464)

The most prominent pyramidal cells in the neocortex are the Betz cells of area 4, the motor cortex. The Betz cells are very large pyramidal cells located in layer 5. Their axons form part of the pyramidal tract that descends to the spinal cord, (Douglas; Neocortex, 464)

Primary visual cortex has a distinct set of exceptionally large pyramidal neurons, called the solitary cells of Meynert. These pyramidal cells, which are found in the deep layers (5 or 6, depending on species), project to other cortical areas and down to the midbrain structures such as the superior colliculus and the pons, (Douglas; Neocortex, 465)

Within layer 5 in the visual cortex, two types of pyramidal cells have been distinguished: (1) a thick apical dendrite that ascends to layer 1 where it forms a terminal tuft. These neurons have a bursting discharge response. (2) a thin apical dendrite that terminates without branching in layer 2, and has a regular discharge, (Douglas; Neocortex, 465)

Shape of the dendritic tree may affect spike output pattern

Theoretical work has suggested that the shape of the dendritic tree is a major factor in controlling the pattern of spike output from neurons, (Douglas; Neocortex, 465)

A second group of spiny neurons, the spiny stellate neurons, are found exclusively in layer 4 of the granular cortex. They do not have the apical dendrite characteristic of pyramidal cells. Instead, dendrites of approximately equal length radiate from the soma, giving a star-like appearance, (Douglas; Neocortex, 465)

Occasionally spiny stellate neurons do project to other areas, but most have axonal projections confined to the area in which they occur, (Douglas; Neocortex, 465)

Smooth neurons tend to have elongated dendritic trees, both in the radial and the tangential dimension., (Douglas; Neocortex, 465)

The most prominent smooth neuron is the cortical basket cell, (Douglas; Neocortex, 465)

As with basket cells in the cerebellum and hippocampus, the axons of basket cells form nests or baskets around the somata of their targets, usually pyramidal cells, (Douglas; Neocortex, 465)

Thalamus projects to all cortical areas and provides input to most layers of the cortex. The densest projections are to the middle layers, where they form about 5-10% of the synapses in those layers, (Douglas; Neocortex, 466)

The main feature of thalamic input to the cortex is that it is highly ordered. The sensory inputs are represented centrally in a way that their topographic arrangement in the periphery is preserved, (Douglas; Neocortex, 466)

Thalamocortical mapping is achieved by preserving the nearest-neighbor relationships of the arrangements of the sensory or motor elements in the periphery. Such topographic projections are a ubiquitous feature of the cortex, (Douglas; Neocortex, 466)

Precision of topographic mapping varies between areas. Primary sensory and motor areas usually preserve the highest detail of topography, degrading progressively through secondary, tertiary and higher order areas of cortex, (Douglas; Neocortex, 466)

Subcortical Projections to the Neocortex

Although the thalamus is a major source of input to the neocortex, it is not the only one. More than 20 different subcortical structures projecting to the neocortex have been identified, (Douglas; Neocortex, 467)

Subcortical structures projecting to the neocortex include the claustrum, locus coeruleus, basal forebrain, the dorsal and median raphe, and the pontine reticular system, (Douglas; Neocortex, 467)

Neuromodulatory Subcortical Projections to the Neocortex

Three main types of monoamine-containing cortical afferents have been described: (1) the dopamine-positive fibers arising from the rostral mesencephalon, (2) the noradrenaline-containing axons originating from the locus coeruleus, (3) the serotonin (5-HT) fibers that originate from the mesencephalic raphe nuclei, (Douglas; Neocortex, 468)

Locus coeruleus, a small nucleus in the dorsal pons, projects to most of the neocortex. Neurons synthesize norepinephrine. Activity in locus coeruleus is involved with the arousal response induced by sensory stimuli, (Douglas; Neocortex, 468)

Raphe nuclei and pontine reticular formation are a complex of nuclei that contain the highest density of neurons that synthesize serotonin. These neurons project to all cortical areas with varying degrees of laminar specificity, (Douglas; Neocortex, 468)

The third monoamine projection to cortex is the dopaminergic pathway.The dopaminergic projection to the frontal cortex originates from the ventral tegmental area, the rostral mesencephalic groups, and the nucleus linearis. All layers except layer 4 receive dopaminergic input. Dopaminergic projections are strongest to the rostral cortical areas, especially the prefrontal cortex, (Douglas; Neocortex, 468)

Cortical Connections

Axons of cortical neurons do not extend more than a few millimeters laterally in an area, (Douglas; Neocortex, 475)

Neurons with similar functional properties are organized in 'columns' that extend from the cortical surface to the white matter, (Douglas; Neocortex, 475)

Output neurons from the cortex are generally pyramidal cells, (Douglas; Neocortex, 477)

Cortico-cortical connections arise mainly from the superficial cortical layers, and the subcortical projections arise from the deep layers, (Douglas; Neocortex, 477)

Within the deep layers, there is an output to regions that have a motor-related function, e.g, the superior colliculus, basal ganglia, brainstem nuclei, and spinal cord, (Douglas; Neocortex, 477)

Cortico-thalamic projection generally arises from the layer 6 pyramidal cells, (Douglas; Neocortex, 477)

Basic circuit for visual cortex (diagram), (Douglas; Neocortex, 477)

Neurotransmitters, Receptors, LTP

Long term potentiation -- brief tetanic stimulation of a set of input fibers potentiates synapses in hippocampal excitatory synapses for many hours, (Douglas; Neocortex, 484)

Processes of LTP and LTD have been studied in a number of different cortical areas, (Douglas; Neocortex, 485)

In the neocortex the main excitatory neurotransmitter is the amino acid glutamate, (Douglas; Neocortex, 485)

Activation of the NMDA receptor evokes a long-duration conductance change (tens of milliseconds), during which cations flow through the channel, (Douglas; Neocortex, 486)

A number of chemical substances have inhibitory effects on cortical neurons, but the most dominant inhibitor appears to be GABA. (Douglas; Neocortex, 488)

Establishment of the identity of cortical neurotransmitters has been one of the most tortuous activities of the last 40 years, (Douglas; Neocortex, 492)

 

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