Cell Junctions Overview

Cell junctions are connections between neighboring cells or between a cell and the extracellular matrix.
Three primary types of cell junctions:
- Occluding junctions
- Communicating junctions
- Anchoring junctions

Occluding junctions prevent intercellular exchange of substances and inhibit the movement of ions and molecules.
Example: Tight junctions, or zonula occludens, found in epithelial and endothelial cells.

Located at the apical margins of various cells, including those in the intestinal mucosa, renal tubules, capillary walls, and choroid plexus.
Functions:
- Strength and Stability: Holds neighboring cells firmly together, providing structural integrity.
- Selective Permeability (Gate Function): Acts as a barrier for small molecules and a complete barrier for larger molecules, regulating ion and water interchange.
- Fencing Function: Prevents lateral movement of membrane proteins and lipids, maintaining distinct cell surface regions.
- Maintenance of Cell Polarity: Keeps proteins localized to the apical region.
- Blood-Brain Barrier: Creates a selective barrier in brain capillaries, allowing only lipid-soluble substances to pass.

Communicating junctions enable the exchange of ions and small molecules between adjacent cells.
Examples: Gap junctions and chemical synapses.

Facilitate the passage of ions and smaller molecules, found in the heart and basal part of intestinal epithelial cells.
Functions:
- Allows the transfer of glucose, amino acids, and ions under a molecular weight threshold.
- Facilitates rapid chemical messenger exchange.
- Enables quick propagation of action potentials, a rapid voltage change across a membrane.

A junction between nerve fibers or between a nerve fiber and a muscle fiber where signals are transmitted via chemical transmitters.

Provide mechanical strength and attachment between cells or between cells and the extracellular matrix.
Critical for structural integrity, especially in tissues experiencing mechanical stress like heart muscle and skin epidermis.

Actin Filament Attachments:
- Adherens Junctions: Connect cells to each other.
- Focal Adhesions: Connect cells to the extracellular matrix.
Intermediate Filament Attachments:
- Desmosomes: Connect neighboring cells for strength.
- Hemidesmosomes: Anchor cells to the extracellular matrix.