The insula is a lobe found within the lateral fissure, while the limbic lobe, which includes the hippocampal formation, is situated on the medial side of the hemisphere. The hippocampal formation is tucked away within the parahippocampal gyrus of the temporal lobe and cannot be observed from the brain's surface. Interconnections between the cerebral hemispheres are facilitated by the cerebral commissures, with the corpus callosum being the primary connection for the bulk of the cortex. The anterior commissure connects certain parts of the temporal lobes. The cerebral cortex is composed of three types: neocortex, archicortex, and paleocortex, with the neocortex accounting for approximately 90% of the cerebral cortex in humans. The neocortex consists of six layers, while the archicortex has three layers, and the paleocortex has four to five layers. Pyramidal cells are the most abundant cell type in the neocortex, making up around 75% of neocortical neurons. Other cell types include stellate cells and various nonpyramidal neurons. Pyramidal cells have a large triangular cell body with a long apical dendrite directed towards the cortical surface, as well as basal dendrites. Their axons project into the subcortical white matter. Stellate cells, or granule cells, are interneurons with local connections. They possess small soma and numerous branched dendrites, with some having an apical dendrite. Stellate cells can be excitatory or inhibitory interneurons, with their axons remaining within the same cortical region. Excitatory interneurons are abundant in layer IV and have axons that ascend into the upper cortical layers. The neurotransmitter of pyramidal cells is an excitatory amino acid (glutamate or aspartate), while inhibitory interneurons utilize gamma-aminobutyric acid (GABA) as their neurotransmitter. Each layer of the neocortex has distinct cellular characteristics.
10.1 ). Buried within the lateral fissure is another lobe, the insula (see
Fig. 4.6A). A group of structures that make up the limbic lobe is on the medial aspect of the hemisphere, and its largest part, the hippocampal formation, is folded into the parahippocampal gyrus of the temporal lobe and cannot be seen from the surface of the brain.
Activity in the two hemispheres of the cerebral cortex is coordinated by interconnections through the cerebral commissures. The bulk of the cortex is connected through the massive corpus callosum (see
Figs. 4.9 10.1), and parts of the temporal lobes connect through the anterior commissure.
There are three types of cerebral cortex: neocortex, archicortex, and paleocortex. The neocortex has six cortical layers (Fig. 10.2 ). In contrast, the archicortex has only three layers, and the paleocortex has four to five layers. In humans, approximately 90% of the cerebral cortex is neocortex.
The Neocortex
Neuronal Cell Types in the Neocortex
A number of different neuronal cell types in the neocortex have been described (see
Fig. 10.2 ). Pyramidal cells are the most abundant cell type and account for approximately 75% of neocortical neurons. Stellate cells and various other types of nonpyramidal neurons make up the balance. Pyramidal cells have a large triangular cell body, a long •Fig. 10.1 Lateral(A) andmedial(B) illustrationsofthelefthemisphereofthehumancerebrumwiththemajorfeatureslabeledandthelobesindicatedbycolor.R,G,B,andSindicate,respectively,therostrum,genu,body,andspleniumofthecorpuscallosum.(FromHainesDE[ed].Fundamental Neuroscience for Basic and Clinical Applications. 3rded.Philadelphia:ChurchillLivingstone;2006.)
I. Molecular layer
II. External granular layer
III.External pyramidal layer
IV.
V.
VI.Polymorphic layer Outer band of Baillarger
Inner band of Baillarger
Fig. 10.2 An Area of Neocortex Stained by Three Different Methods. TheNisslstain(center) showsthecellbodiesofallneuronsandrevealshowdifferenttypesaredistributedamongthesixlayers.TheGolgistain(left) showsonlyasampleoftheneuronalpopulationbutrevealsdetailsoftheirdendrites.TheWeigertstainformyelin(right) demonstratesverticallyorientedbundlesofaxonsenteringandleavingthecortexandhorizontallycoursingfibersthatinterconnectneuronswithinalayer.(FromBrodmann
K.
Zellenbaues. Leipzig:JABarth;1909.) apical dendrite directed toward the cortical surface, and several basal dendrites. The cell’s axon emerges from the body opposite the apical dendrite, and those from the larger pyramidal cells project into the subcortical white matter. The axon may give off collateral branches as it descends through the cortex. The neurotransmitter of pyramidal cells is an excitatory amino acid (glutamate or aspartate). Stellate cells, often called granule cells, are interneurons with local connections. They have a small soma and numerous branched dendrites, although many have an apical dendrite and thus look like small pyramidal cells. Some are excitatory interneurons; these cells are abundant in layer IV of the cortex (described in the next section). Their axons remain in the same cortical region, and many ascend into the upper cortical layers. Some stellate cells are inhibitory inter-neurons whose neurotransmitter is gamma-aminobutyric acid (GABA).
Cytoarchitecture of Cortical Layers
Each of the six layers of the neocortex has a characteristic cellular content (see