The cerebral cortex is the most complex structure of the human brain. It has a wide range of functions, including the planning and initiation of motor activity, the perception and awareness of sensory information, learning, memory, conceptual thinking, awareness of emotions, and much more. The performance of all these functions is due to the unique multilayer arrangement of neurons. The cytoarchitectonics of the cerebral cortex is their cellular organization.
Structure
The cerebral cortex is made up of hundreds of billions of neurons, all of which are variations on just three morphological forms: pyramidal (pyramidal) cells, spindle cells, and stellate (granular cells). Other types of cells seen in the cortex are modifications of one of thesethree types. There are also horizontal Cajal-Retzius cells and Martinotti cells.
Pyramidal cells in the cytoarchitecture of the hemispheric cortex make up to 75% of the cellular component and are the main output neurons. They vary in size from small to giant. They usually have one apical dendrite that runs to the surface of the cortex and several basal dendrites. The number of the latter varies widely, but there are usually more than three to four primary dendrites that branch into successive generations (secondary, tertiary, etc.) They usually have one long axon that leaves the cortex and enters the subcortical white matter.
Spindle cells are usually located in the deepest cortical layer in the cytoarchitecture of the cerebral cortex. Their dendrites protrude towards the cortical surface, while the axon may be commissural, associational, or projective.
Star-shaped (granular) cells are usually small, and since their processes are projected in all planes, they resemble a star. They are located throughout the cortex, except for the most superficial layer. Their processes are very short and are projected locally into the cortex and can modulate the activity of other cortical neurons. Based on the presence of dendritic spines (small cytoplasmic protrusions), some of them are called spiny cells. Their dendrites have spikes and are mostly located in layer IV where they release glutamate, which is an excitatory neurotransmitter, so theyare functionally excitatory interneurons. Another cell type secretes gamma-aminobutyric acid (GABA), which is the most potent inhibitory neurotransmitter in the CNS, so they function as inhibitory interneurons.
Horizontal Cajal-Retzius cells are visible only in the most superficial part of the cortex. They are very rare, and only in small numbers can be found in the adult brain. They have one axon and one dendrite, both of which synapse locally in the most superficial layer.
Martinotti cells are multipolar neurons that are most densely located in the deepest layer of the cortex. Their numerous axons and dendrites move towards the surface.
Layers
By analyzing the cerebral cortex using Nissl staining techniques, neuroscientists have found that neurons have a laminar alignment. This means that neurons are organized in layers parallel to the surface of the brain, which differ in size and shape of neural bodies.
Cytoarchitectonics of the cerebral cortex includes six layers:
- Molecular (plexiform).
- Outer grainy.
- Outer pyramidal.
- Inner grainy.
- Internal pyramidal (ganglionic).
- Polymorphic (fusiform).
Molecular layer
It is the most superficial in the cytoarchitecture of the cortex, located directly under the pia mater encephali. This layer is very poor in the cellular component, which is represented by only a few horizontalCajal-Retzius cells. Most of it is actually represented by the processes of neurons lying in the deeper layers and their synapses.
Most dendrites originate from pyramidal and fusiform cells, while axons are actually terminal fibers of the afferent thalamocortical tract, which originates from the nonspecific, intralaminar and median nuclei of the thalamus.
Outer granular layer
It consists mainly of stellate cells. Their presence gives this layer a "grainy" appearance, hence its name in the cytoarchitectonics of the cerebral cortex. Other cell structures are shaped like small pyramidal cells.
Its cells send their dendrites to various layers of the cortex, especially the molecular layer, while their axons travel deeper into the cerebral cortex, synapsing locally. In addition to this intracortical synapse, the axons of this layer can be long enough to form association fibers that pass through the white matter and eventually terminate in various CNS structures.
Outer pyramidal layer
It consists mainly of pyramidal cells. The surface cells of this layer of cytoarchitectonics of the cerebral cortex are smaller compared to those that are located deeper. Their apical dendrites extend superficially and reach the molecular layer, while the basal processes attach to the subcortical white matter and then againproject into the cortex so that they serve as both associative and commissural corticocortical fibers.
Inner granular layer
In the cytoarchitectonics of the cerebral cortex, it is the main input cortical station (this means that most of the stimuli from the periphery come here). It consists mainly of stellate cells and, to a lesser extent, of pyramidal cells. Stellate cell axons remain local in the cortex and synapses, while pyramidal cell axons synapse deeper within the cortex or leave the cortex and connect with white matter fibers.
Stellate cells, as the dominant component, contribute to the formation of specific sensory cortical areas. These areas receive fibers mainly from the thalamus in the following order:
- Stellate cells of the primary sensory cortex receive fibers from the ventral posterolateral (VPL) and ventral posteromedial (VPM) nuclei of the thalamus.
- The primary visual cortex receives fibers from the lateral geniculate nucleus.
- Stellate cells from the primary auditory cortex receive projections from the medial geniculate nucleus.
When these sensory fibers "penetrate" the cortex, they turn horizontally so they can spread out and diffusely synapse with the cells of the inner granular layer. Because these fibers are myelinated and therefore white, they are highly visible in the gray matter environment.
Inner pyramidal layer
It consists mainly of medium and largepyramidal cells. This is the source of the output or corticofugal fibers. For this reason, it is most prominent in the motor cortex, from which it sends out fibers that mediate motor activity. The primary motor cortex contains a specific form of these cells called Betz cells.
Because we are talking about the cortical level of motor activity, these fibers form tracts that synapse with various subcortical motor centers:
- Corticothectal tract that reaches the midbrain tectum.
- The corticorubral tract that runs to the red nucleus.
- Corticoreticular tract, which synapses with the reticular formation of the brainstem.
- Corticopontal tract (from the cerebral cortex to the pontine nuclei).
- Corticonuclear tract.
- The corticospinal tract that leads to the spinal cord.
This layer also contains a horizontally oriented band of white matter formed by axons of the inner pyramidal layer that locally synapse within the layer, as well as with cells from layers II and III.
Polymorphic (fusiform)
This is the deepest layer of the cortex and directly overlies the subcortical white matter. It contains mostly spindle cells and less pyramidal and interneurons.
The axons of the spindle and pyramidal cells of this layer distribute corticocortical commissural and corticothalamic projection fibers that terminate in the thalamus.
Column organization
The cerebral cortex can also be functionally divided into vertical structures called columns. They are actually functional units of the cortex. Each of them is oriented perpendicular to the surface of the cortex and includes all six cell layers. This structure should also be considered within the framework of the cytoarchitectonics of the human cerebral cortex.
Neurons are closely connected within the same column, although they share common connections with neighboring and distant similar formations, as well as with subcortical structures, especially with the thalamus.
These columns are capable of remembering relationships and performing more complex operations than a single neuron.
Review of the cytoarchitectonics of the cerebral cortex
Each column has its supragranular and infragranular parts.
The first is formed on the most superficial layers I-III, and in general, this part is projected onto other columns, being interconnected with them. In particular, level III is associated with adjacent columns, while level II is associated with distant cortical ones. The infragranular part includes layers V and VI. It receives input from the supragranular regions of adjacent columns and sends output to the thalamus.
Layer IV is not functionally included in either of these two parts. It acts as a kind of anatomical boundary between the supragranular and infragranular layers, while from a functional point of view it has many functions. This layer receives input from the thalamus andsends signals to the rest of the corresponding column.
Thalamus, on the other hand, receives information from almost the entire cortex and many subcortical regions. With the help of these connections, it creates a feedback loop with the cortex, analyzing the information received from layer IV and sending it the appropriate signals. Thus, the integration of signals occurs both in the thalamus and in the cortical centers.
Each column can be partially or fully active. Partial activation implies that the supragranular layers are excited while the subgranular layers are inactive. When both parts are excited, this means that the column is fully active. The activation level reflects a certain level of function.
