The action potential
is an electric signal. It corresponds to a fast change in the membran
potential, as a consequence of a fast depolarization followed by a repolarisation.
Because of an asymmetric distribution of ions across the plasma membrabe of all cells, put into place by ion pumps, and because of constant "leakage" of ions through membrane transporters, cells develop a membrane potential (see figure 2). Normally we define the membrane potential as a negative value (inside relative to the outside of the membrane). In the majority of cells the resting potential is around -70mV +/-10mV. What this means is that the flow K+ ions to the outside matches the flow of Na+ ions to the inside. The system is in equilibrium.Thos equilibrium can be disturbed by changing the permeability of the transporter proteins, in the case of neurons, by changing the permeability of the ion-channels fot K+, Na+ or Cl-. This is what neurotransmitters do, they modify ion-channel permeability, and by doing so they can generate, or prevent, action potentials. When the membrane potential reduces, say goiong from -70 to -50 mV, it depolarizes, when the potential increases, going from -70 to say -80mV, it hyperpolarizes.
Because of an asymmetric distribution of ions across the plasma membrabe of all cells, put into place by ion pumps, and because of constant "leakage" of ions through membrane transporters, cells develop a membrane potential (see figure 2). Normally we define the membrane potential as a negative value (inside relative to the outside of the membrane). In the majority of cells the resting potential is around -70mV +/-10mV. What this means is that the flow K+ ions to the outside matches the flow of Na+ ions to the inside. The system is in equilibrium.Thos equilibrium can be disturbed by changing the permeability of the transporter proteins, in the case of neurons, by changing the permeability of the ion-channels fot K+, Na+ or Cl-. This is what neurotransmitters do, they modify ion-channel permeability, and by doing so they can generate, or prevent, action potentials. When the membrane potential reduces, say goiong from -70 to -50 mV, it depolarizes, when the potential increases, going from -70 to say -80mV, it hyperpolarizes.
An essential aspect of excitable cells is that they carry membrane-potential sensitive Na+-channels (also known as voltage sensitive Na+-channels). This distinguishes them from non-excitable cells. When the membrane potential depolarizes to arround -40 mV, the volatge -sensituve Na+-channels open for a very brief period, in the order of 1 to100 milliseconds. The Na+ curtent briefly dominates and the potential changes from -40mV to +30mV. The abrupt change is called an action potential. It propagates very rapidly across the membrane of a neuron until it reaches the end of the axon and then dies out because the voltage-sensitive Na+ automatically close. A subsequant dominance of the K+-current brings the system back to a resting level (around -70 mV), a phenomenon referred to as membrane repolarisation. After a short refractory period, again in the order of milliseconds the process can be repeated. Numerous actions potentials in a row are called spikes. An active neuron is defined as one that produces lots of spikes a weak signal.
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