Physiology.pdf

Cell Junctions DEFINITION AND CLASSIFICATION Cell junction is the connection between the neighbouring cells or the contact between the cell and extracellular matrix. Cell junctions are classi ed into three types :-...

Cell Junctions DEFINITION AND CLASSIFICATION Cell junction is the connection between the neighbouring cells or the contact between the cell and extracellular matrix. Cell junctions are classi ed into three types :- 1. Occluding junctions 2. Communicating junctions 3. Anchoring junctions OCCLUDING JUNCTIONS Cell junctions which prevent intercellular exchange of substances are called occluding junctions. These junctions prevent the movement of ions and molecules from one cell to another cell. Example - Tight junctions ❖ Tight junctions : Tight junction is the intercellular occluding junction that prevents the passage of large molecules. It is also called zonula occludens. This type of junction is present in the apical margins of epithelial and endothelial cells in intestinal mucosa, wall of renal tubule, capillary wall and choroid plexus. ❖ Functions of Tight Junction 1. Strength and Stability : The tight junction holds the neighbouring cells of the tissues rmly and thus provides strength and stability to the tissues. 2. Selective permeability (Gate function) : The tight junction forms a selective barrier for small molecules and a total barrier for large molecules. Barrier function of tight junction regulates the interchange of ions, water and varieties of macromolecules between the cells. 3. Fencing function : Tight junction prevents the lateral movement of proteins (integral membrane proteins) and lipids in cell membrane and thus acts as a fence. 4. Maintenance of cell polarity : Fencing function of the tight junction maintains the cell polarity by keeping the proteins in the apical region of the cell membrane. 5. Blood-brain barrier : Tight junction in the brain capillaries forms the blood-brain barrier, which prevents the entrance of many substances from capillary blood into brain tissues. Only lipid-soluble substances like drugs and steroid hormones can pass through the blood-brain barrier. +91 7691895979 1 www.bamsstudies.in  fi fi  COMMUNICATING JUNCTIONS Cell junctions which permit the intercellular exchange of substances are called communicating junctions, i.e. these junctions permit the movement of ions and molecules from one cell to another cell. Example - Gap junction and Chemical synapse. ❖ Gap Junction : Gap junction is the intercellular junction that allows passage of ions and smaller molecules between the cells. It is present in heart, basal part of epithelial cells of intestinal mucosa. ❖ Functions of Gap Junction The channel permits the passage of glucose, amino acids, ions and other substances, which have a molecular weight less than 1,000. It helps in the exchange of chemical messengers between the cells. It helps in rapid propagation of action potential from one cell to another cell. Action Potential - A rapid sequence of changes in the voltage across a membrane. ❖ Chemical Synapse : Chemical synapse is the junction between a nerve ber and a muscle ber or between two nerve bers, through which the signals are transmitted by the release of chemical transmitter. ANCHORING JUNCTIONS Anchoring junctions are the junctions, which provide strength to the cells by acting like mechanical attachments. These junctions provide rm structural attachment between two cells or between a cell and the extracellular matrix. Anchoring junctions are responsible for the structural integrity of the tissues and are present in the tissues like heart muscle and epidermis of skin, which are subjected to severe mechanical stress. Depending upon this, anchoring junctions are classi ed into four types: 1. Actin lament attachment i. Adherens junction (cell to cell) ii. Focal adhesion (cell to matrix) 2. Intermediate lament attachment i. Desmosome (cell to cell) ii. Hemidesmosome (cell to matrix) +91 7691895979 2 www.bamsstudies.in fi fi fi fi fi fi fi +91 7691895979 3 www.bamsstudies.in