cortical layers (laminae)

Cortical layers are the six horizontal sheets of cells, stacked one on top of another, that make up the neocortex — the wrinkled outer rind of the brain where much of our thinking, seeing, and movement is organized. If you sliced through this rind and looked at it under a microscope, you would not see a uniform paste of cells. Instead you would see bands, like the stripes in a layered cake, where the neurons (the brain's signaling cells) change in size, shape, and how densely they are packed. These bands are the layers, and anatomists number them from the surface inward as I (the outermost) through VI (the deepest, nearest the brain's white matter). The Latin word for such a layer is lamina, which is why the layered structure is called laminar.

Each layer has a job, set largely by what it connects to. Layer IV is the main mailbox: it receives incoming signals from the thalamus, a relay station that funnels information from the senses. Layers II and III mostly talk sideways to other patches of cortex, knitting distant regions together. Layers V and VI are the outboxes: their large neurons send commands down and out — to the spinal cord, muscles, and other deep brain structures, or back to the thalamus. Layer I, just under the surface, holds few cell bodies and is mostly a web of connecting fibers. Because every region of neocortex shares this same six-layer plan, yet tunes the thickness of each layer to its task (motor areas have a thin layer IV, sensory areas a thick one), the layers are a kind of universal blueprint that helps scientists map the cortex and compare one brain area — or one species — to another.