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The cell cortex, also known as the actin cortex or actomyosin cortex, is a specialized layer of cytoplasmic protein on the inner face of the plasma membrane of the cell periphery. It functions as a modulator of plasma membrane behavior and cell surface properties. In most eukaryotic cells lacking a cell wall, the cortex is an actin-rich network consisting of F-actin filaments, myosin motors, and actin-binding proteins. The actomyosin cortex is attached to the cell membrane via membrane-anchoring proteins called ERM proteins and it plays a central role in cell shape control. The protein constituents of the cortex undergo rapid turnover, making the cortex both mechanically rigid and highly plastic, two properties essential to its function. In most cases, the cortex is in the range of 100 to 1000 nanometers thick.
In some animal cells, the protein spectrin may be present in the cortex. Spectrin helps to create a network by cross-linked actin filaments. The proportions of spectrin and actin vary with cell type. Spectrin proteins and actin microfilaments are attached to transmembrane proteins by attachment proteins between them and the transmembrane proteins. The cell cortex is attached to the inner (cytosolic) face of the plasma membrane in cells where the spectrin proteins and actin microfilaments form a mesh-like structure much like a fishnet except that it can be broken and reformed.
In plant cells, the cell cortex is reinforced by cortical microtubules underlying the plasma membrane. The direction of these cortical microtubules determines which way the cell elongates when it grows.