 Dear students, in this module, we shall discuss membrane potential and membrane excitation. You know that all electrical phenomena in the cell depend on transmembrane potential. This transmembrane potential is known as the membrane potential and is represented as Vm. The potential difference is actually electrochemical in nature because it is established by charged ions. This membrane potential is the basis of production of all signals that are used in communication and muscle contraction. The membrane potential arises because of two features found in all eukaryotic cells. One is the unequal distribution and maintenance of different concentrations of various ions inside and outside the membrane. And second is the presence of ion selective channels that are permeable to various ionic species. The potential difference is measured as an intracevular potential relative to the extracevular potential. It is expressed in millivolts, its values vary with the type and physiological state of the cell. Overall, its values range between minus 30 to 100 millivolt in non-excited cells. The potential difference or voltage that exists across the membrane is localized to the regions which are adjacent to the surface of the membrane on two sides. It is not present throughout the cell but it remains confined to the regions which are close to or adjacent to the outer and inner membrane surfaces. The potential difference acts as a source of potential energy with which the ions move across the membrane. This voltage gives rise to an electric field that is measured in terms of volts and distance so that E is equal to V over D. We can see here that the electric field is inversely proportional to the distance. If we consider that distance is the thickness of the membrane then it is only 5 nanometers as a result we can see that this short looking electric field is actually very large. We can measure and detect the membrane potential directly by voltage clamp method. In this method, two electrodes are used. One electrode is placed in the cytosol and the other in the extracellular medium. These electrodes detect voltage or potential difference and show the value of the membrane potential on a screen in modern digital equipments. Now we shall discuss the excitable membranes. Dear students, you can understand that a voltage or potential difference exists across the plasma membranes of all animal cells but only the excitable membranes that is the membranes of neurons, muscles and sensory cells can respond to changes in potential difference by producing action potentials. How Excitation Occurs When a stimulus is received on the surface membrane of an excitable cell it decreases the potential difference across the membrane. This decrease in potential difference is known as depolarization. As a result of depolarization, voltage-gated sodium channels open and as a result sodium ions influx in the cell. When sodium ions move in the cell, action potential triggers and starts. The opening of voltage-gated channels in response to stimulus generated depolarization and the resulting inflow of sodium ions into the cell is known as membrane excitation.