Structure of a gap junction, a. Electron micrograph showing the plasma membranes of two adjoining cells forming a gap junction. The unit membranes (arrows) approach one another, narrowing the intercellular space to produce a 2-nm-wide gap. x76,000. b. Drawing of a gap junction, showing the membranes of adjoining cells and the structural components of the membrane that form channels or passageways between the two cells. Each passageway is formed by a circular array of six subunits, dumbbell-shaped transmembrane proteins that span the plasma membrane of each cell. These complexes, called connexons, have a central opening of about 2-nm in diameter. The channels formed by the registration of the adjacent complementary pairs of connexons permit the flow of small molecules through the channel but not into the intercellular space. Conversely, substances in the intercellular space can permeate the area of a gap junction by flowing around the connexon complexes, but they cannot enter the channels, c. The diameter of the channel in an individual connexon is regulated by reversible changes in the confirmation of the individual connexins.
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The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.