A forebrain region {cerebrum}| {cerebral hemispheres} {cerebral cortex} above midbrain includes telencephalon and diencephalon. Cerebrum initiates behavior, causes consciousness, stores memories, and controls internal stimuli.
size
Human cerebral cortex is 2000 square centimeters in area and 300 cubic centimeters in volume.
neurons
Cerebral cortex has more than one billion neurons. Half are pyramidal cells. Surface folding increases area and density. Cerebrum has 100,000 to 300,000 neurons per cubic millimeter.
parts
Mammal cerebrum includes neocortex and hippocampus.
layers
Mammal cerebral-cortex layers average 2 millimeters thick, differing in thickness among the 52 Brodmann areas. Layers are on outside, with axon fibers on inside. Macrocolumns and their minicolumns go through all layers.
Limbic system typically has three-layered allocortex. Cingulate gyrus and insula have three to six cortex layers in juxtallocortex. Human cerebral hemispheres have six-layered neocortex. Top three layers are only in genus Homo and act as a unit. Cortex has only one inhibitory-cell layer.
layers: general
Layer 1 has horizontal cells. Layer 2 has small, round, granular cells. Layer 3 has pyramidal cells. Layer 4 has closely packed granular cells. Layer 5 has large and numerous pyramidal cells and has large spindle neurons, which begin after birth in anterior cingulate and frontal area FI and are for attention and self-reflection. Layer 6 has spindle-like small cells.
layers: detail
Top layer 1 contains pyramidal cell apical dendrites from other layers in macrocolumn and axons from other cortical areas, with few neuron cell bodies. Layers 2 and 3 have superficial pyramidal cells. Layers 1, 2, and 3 {superficial layers} receive from their column, other cortex, and thalamus matrix neurons. Layer 4 has many excitatory spiny stellate cells but few pyramidal neurons. Layer 4 has sublayers IVa, IVb, IVc, and IVc in visual cortex. Layers 5 and 6 {deep layers} have pyramidal cells, some with dendrites to layer 1, that send to cortex, thalamus, superior colliculus, and spinal cord.
layers: input and output
Layers 1 and 2 and layer-3 upper part receive from other cortical-area layer 4. Layers 1 and 2 and layer-3 upper part send to other cortical-area layer 5. Layer-3 lower part receives from outside cortex and sends to layers 1 and 2 and layer-3 upper part. Layers 2 and 3 mostly connect to layers 2 and 3, either laterally or through U-shaped fibers going down into white matter and then back up. Layers 2 and 3 also send to layer-3 lower part and to layer 4 for feedforward responses. Some layer-2-and-3 superficial neurons send output to layers 5 and 6. Layer 4 receives from layer 6 and from outside cortex. Layer 4 sends mainly to layers 1 and 2 and layer-3 upper part. Layer 5 receives from layers 1 and 2 and layer-3 upper part, from whole cortex. Layer 5 sends to layer 6, spinal cord, brainstem, basal ganglia, and hypothalamus. Layer 5 neurons do not project to other cortical areas, thalamus, or claustrum. Layer 6 receives from layer 5. Some layer-6 neurons receive from layer 4C. Layer 6 sends short vertical axons back to layer 4 and outputs to thalamus. Some layer-6 neurons send to thalamus, lateral geniculate nucleus, and claustrum.
layers: connections
Lateral axons are within all layers. Ascending and descending fibers connect all layers. Adjacent cerebral cortex areas always connect to each other. Distant cortical regions connect reciprocally.
input
Cerebrum receives from higher brainstem and limbic system. Brainstem or limbic system damage reduces cerebrum activity, and people enter dreamy state.
Ascending fibers to cerebral cortex have slow, long lasting NMDA receptors.
Excitatory input comes from ipsilateral cerebral cortex, and inhibitory input comes from contralateral cortex. Cerebral cortex mainly inhibits lower brain. It does not control older brain parts but interacts with them.
Cortical motor areas receive input from association areas, corticospinal tract, thalamus, post-central gyrus somaesthetic area, and frontal lobe motor areas.
input: topography
Cortical neurons separated by less than several hundred microns receive similar input and send similar output.
input: multisensory
Cortical neurons for multisensory information lie next to cortical areas for one sense. Superior-temporal, intraparietal, frontal, and prefrontal lobes are for multisensory convergence [Bruce et al., 1986].
input: synapses
70% of excitatory synapses on cerebral-cortex superficial pyramidal neurons are from less than 0.3 millimeters away. Few come from outside cerebral cortex. Average cortical-neuron effect on other neurons is 0.050 to 5 millivolts. Probability of one synapse causing a spike is 0.1 to 0.5.
Cortical neurons have dendritic trees with diameter 0.3 millimeters. Most neurons receive 100 synapses from 100 neurons and send to 100 other neurons. 8000 neurons eventually affect cortical neurons.
input: processing
Training, learning, and willing have widespread cortical activity and take one second.
Consciousness involves coordinated synchronized impulses in cerebral cortical neurons for over 100 milliseconds. Perhaps, impulses are high-frequency bursts, rate codes, oscillations at 40 Hz, or other synchronized impulses. Visual consciousness involves cortical layers 5 and 6. Cortical layer 4 receives input. Cortical layers 2 and 3 are for unconscious processing.
output
Motor and sense cortex sends axons to cerebellum, basal ganglia, and hippocampus, which send axons to thalamus and cortex, with no reciprocity.
Cerebral-cortex descending fibers can cause lower-brain-neuron long lasting subthreshold depolarization.
Cerebrum primary sense areas send to nearby secondary areas and nowhere else. Secondary sense areas send to other-hemisphere corresponding area, other same-hemisphere secondary areas, and cerebral association areas. Association areas interconnect.
output: synchronization
Cerebral-cortex superficial-pyramidal-cell axons travel horizontally in same cortical layer 0.4 to 0.9 millimeters and then make terminal clusters on other superficial pyramidal cells. The skipping pattern aids neuron-activity synchronization.
output: divergence
As signal travels farther into cerebrum, neuron receptive-field sizes increase and features to which neurons respond become more complex, because later areas receive input from several earlier areas.
output: feedback
Later areas send signals back to earlier areas.
damage
People with no cerebrum can sleep, awake, smile, and cry. They feel no danger or hunger and have no spontaneous behavior.
Damage to cortex causes poor memory retrieval and poor habit inhibition. Cortex loss does not affect general consciousness.
Biological Sciences>Zoology>Organ>Nerve>Brain>Cerebrum
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Date Modified: 2022.0224