Action potentials are rapid changes in membrane potential which spreads along the cell membrane. The cells which can generate action potentials are called as excitable cells, neurons, muscle cells, and certain endocrine cells or excitable cells. In general, at resting membrane potential, the cell membrane is negative. That means more negative ions near the inner side of the membrane as compared to the extracellular side. And this is referred as a polarized state. And during an action potential, the negativity of the membrane potential decreases and it becomes close to zero. The cell is said to have lost its polarity and it is called as a depolarized state. And the restoration of the membrane potential from a depolarized state to resting membrane potential is called as a repolarization. And a cell membrane goes more negative than the resting membrane potential. We call this as hyperpolarization. So, in general an action potential has a depolarization, repolarization, and certain cells also have an off-riper polarization. The movement of positive ions inside the cell is called as an inward current and that is responsible for depolarization. And the movement of positive ion outside is called as an outward current, this leads to repolarization. If you record this potential changes at a given point in a cell membrane, we would record as spike like this, a depolarization followed by a repolarization. But once the action potential is initiated at one point on the membrane, it spreads along the cell membrane. As a wave of depolarization followed by a repolarization spreads all along the cell membrane. An action potential is initiated by opening of sufficient number of voltage-gated ion channels. And this is induced by a stimulus which causes them to open. And once these ion channels open, they cause further depolarization which actually opens more and more number of voltage-gated channels. And this causes further depolarization. So, this leads to a positive feedback cycle and this is responsible for the depolarization during an action potential. And it is stimulus which can elicit an action potential is called as a threshold stimulus. So, stimulus which is any change which opens adequate number of ion channels and induces this positive feedback cycle and generates an action potential is called as a threshold stimulus. And a stimulus which cannot generate an action potential is called as a sub-trishol stimulus. In a sub-trishol stimulus there is no action potential when the threshold stimulus is given, there is an action potential. Even the stimulus strength is more than the threshold level. It will also result in action potential. But with the similar magnitude as what is produced in a threshold potential. This is because in action potential all the available voltage-gated channels are already open. So, there is no further increase in opening when a highest stimulus strength is given. So, this is referred as all or non-principal. Either the action potential is there or not there. There is no half action potential or a small action potential. The morphology of action potential is decided by the type and the number of ion channels present in the given cell. For example, a neuron has a very short action potential lasting only few milliseconds. Whereas a cardiac muscle action potential lost 200 to 300 milliseconds and that also has a pletive. These properties are decided by the presence of specific types of voltage-gated ion channels. Though different cells have different morphology of action potential, a given cell has the same morphology of action potential throughout its cell membrane. That is wherever we record along the membrane in a given cell, the action potential morphology is going to look similar. That is because in a given cell the pattern of distribution of voltage-gated ion channels are similar throughout its membrane. So, the magnitude of action potential does not change throughout the membrane. Even if a neuron is a meter long, the action potential is going to look similar from the beginning to the end. And this is called as non-discriminate self-propagation of action potential which is brought out by uniform distribution of voltage-gated ion channels throughout the cell membrane. In the next video, we are going to discuss about ionic basis of action potential, refractory period and accommodation. Stay tuned.