 In this video we are going to discuss about occluding junctions and communicating junctions. Now communicating junctions are cap junctions. Cap junctions are cell junctions that enable transport of substances between the cells. So here there are two cells and via cap junctions substances can move from one cell to another. At cap junctions cells are situated very close to each other. In between the cells there is a space of approximately 3 nanometers but the membranes do not make physical contact directly. Now if we magnify the area we find that there are certain channels present on the membrane of the cells and channels on the membrane of both cells combined together. So these two cells are there. So channels of one cell are opposed with the channels of other cell forming kind of a pipeline through which substances can pass. So it forms a pipeline through which substances can pass between the cells. Now if we see a single channel it is made up of transmembrane proteins known as conixins. This is the membrane of the cell and this entire unit is forming a channel. Each unit is made up of subunit known as conixins and six conixins, six of them combine together to form the entire unit known as conixon. So the conixins are arranged such that they form a central pore. So conixon of one cell plasma membrane is opposed with the conixon of other cell plasma membrane through which molecules less than 1000 Dalton's can pass. Now gap junctions are present in cardiac muscle cells. So these are the cardiac muscle cells okay and they communicate with each other by gap junctions so that ions can pass from one cell to another via gap junctions. Because of this if single muscle fiber is stimulated the action potential can spread to other muscle fibers also that is why cardiac muscle cell behaves as a functional syncytium. That means if one cell is stimulated the entire cardiac muscle will be stimulated because of ions passing through the gap junction to other muscle fibers. Now some metabolites like ATP can also pass through gap junctions. This is important in the nutrition of cells of lens syncytium. So one example we saw cardiac muscle cell another example we are telling a vascular lens where metabolites are not provided by blood supply but they can pass through gap junction to other cells of the lens. Another junctions which we are going to talk about are tight junctions. We know that specialized transport processes exist for transport of substances across the membrane of the cells but wire substances not able to cross in between the cells from here in the space between the cells that is by paracelular path. This happens due to presence of tight junctions at the apical membrane in between two cells. This prevents paracelular transport of substances and allows only transcellular root. These tight junctions are present in epithelial cells of intestine, nepron where absorption as required occurs through transcellular path. Otherwise anything and everything can enter via paracelular path. Second example is urinary bladder where contents of the bladder that is urine should not leak out of it. Third they are present between endothelial cells of capillary in brain forming blood-brain barrier Pb. So this is CSF and endothelial cells of membrane there are tight junctions. So it's kind of a security system for neurons where the contents of the blood cannot leak out into CSF but because of this it also blocks access of drugs to CNS. So it becomes a problem when we want to treat any diseases of nervous system plus also in case of infection in brain these tight junctions are destroyed and substances can leak into CSF. That is why by studying CSF composition we can get information about certain diseases. So gap junctions allow transport of substances in between cells and tight junctions prevent transport of substances via paracelular root from the space in between the cells.