The cerebral cortex is organized into distinct horizontal layers of neurons, known as cortical layers. This layered arrangement is fundamental to how the brain processes information. These layers work in concert to support higher brain functions, including sensory perception, thought, memory, and motor control. Their organization is established during early brain development through neural migration and differentiation.
Different Types of Cortex
While “cortical layers” often refers to the neocortex, the cerebral cortex comprises several types of cortex. The neocortex, also known as the isocortex, is the most evolutionarily recent and largest part of the human cerebral cortex, making up approximately 90% of its total surface area. It is distinguished by its six characteristic layers and is responsible for complex cognitive processes such as language, reasoning, and decision-making.
In contrast, older cortical regions, collectively termed the allocortex, have fewer, less distinct layers, typically three or four. The allocortex includes the archicortex, such as the hippocampus, which generally has three layers and plays a role in memory formation and spatial navigation. The paleocortex, found in areas like the olfactory cortex, also typically features three layers and is involved in processing smells.
The Six Layers of the Neocortex
The neocortex has six distinct layers, numbered I to VI from outermost to deepest, each with a unique cellular composition and connections. These layers include various neuron types, such as pyramidal cells, stellate cells, and interneurons, which contribute to their specialized functions.
Layer I (Molecular Layer)
Layer I, the outermost layer, is also known as the molecular or plexiform layer. It is sparsely populated with neuron cell bodies, containing less than 0.5% of all neurons. This layer primarily consists of a dense network of dendrites from pyramidal neurons in deeper layers, along with horizontally oriented axons and glial cells. It receives input from the thalamus and other cortical regions, playing a role in integrating cross-modal information, associative learning, and attention.
Layer II (External Granular Layer)
Layer II, the external granular layer, is densely packed with small neurons, often called granule cells. Along with Layer III, it receives significant input from other neocortical areas, particularly from the opposite hemisphere. Layer II is involved in inter-cortical connections, facilitating communication between different cortical regions.
Layer III (External Pyramidal Layer)
Layer III, the external pyramidal layer, contains medium-sized pyramidal neurons. Similar to Layer II, it receives input from other cortical regions and is a primary source of output to other cortical areas. Neurons in this layer contribute to cortico-cortical connections, integrating information across different cortical parts.
Layer IV (Internal Granular Layer)
Layer IV, the internal granular layer, is a primary recipient of sensory input from the thalamus, which relays sensory and motor signals to the cerebral cortex. This layer is densely packed with small stellate and pyramidal cells, and it is particularly prominent and thick in primary sensory cortices, such as the visual, auditory, and somatosensory areas. Layer IV processes and relays sensory information to other cortical layers, notably Layers II and III, for further processing.
Layer V (Internal Pyramidal Layer)
Layer V, the internal pyramidal layer, contains large pyramidal neurons. This layer serves as a major output pathway from the cortex to subcortical structures, including the basal ganglia, brainstem, and spinal cord. In the primary motor cortex, Layer V contains giant pyramidal cells called Betz cells, whose axons form the corticospinal tract, involved in voluntary motor control.
Layer VI (Multiform/Polymorphic Layer)
Layer VI, the deepest layer, is known as the multiform or polymorphic layer due to its heterogeneous neuron population. It contains various cell types, including pyramidal and multiform neurons. Layer VI sends efferent fibers back to the thalamus, establishing precise reciprocal connections that modulate thalamic signals and contribute to gain control.
Integrated Function of Cortical Layers
The distinct cortical layers do not function in isolation but operate together within highly organized vertical units called cortical columns. These columns span all six layers of the neocortex and are considered basic functional units that process specific types of information, such as sensory input from a particular whisker or visual stimulus. This columnar organization is a fundamental principle underlying the brain’s ability to perform complex cognitive functions.
Information typically flows systematically through these layers and columns. Sensory input from the thalamus primarily enters Layer IV, where initial processing occurs. This information is then relayed to Layers II and III for further processing and integration with information from other cortical regions. The processed information then descends to Layers V and VI, which serve as the main output layers, sending signals to subcortical structures and back to the thalamus. This intricate interplay within and between layers, along with the brain’s capacity for plasticity, allows for adaptive and flexible information processing, supporting complex behaviors, sensory perception, and motor control.