The Membrane Potential and Action Potential PDF

The Membrane Potential and Action Potential Dr. Mahmoud Zaher  The Membrane Potential Resting Membrane Potential (Polarization ). Resting membrane is polarized which means that the inner side is negative relative to the outside which is positive. The reason for polarization i...

The Membrane Potential and Action Potential Dr. Mahmoud Zaher  The Membrane Potential Resting Membrane Potential (Polarization ). Resting membrane is polarized which means that the inner side is negative relative to the outside which is positive. The reason for polarization is the unequal distribution of ions on both sides of the membrane  The Membrane Potential Resting Membrane Potential (Polarization ). In a resting neuron, Na + concentrations are high in the ECF. , whereas K+ conc is high in the ICF. There are also large, negatively charged proteins and ions, such as phosphates (PO4 3−), in the cytosol that cannot cross the plasma membranes.  The Membrane Potential Resting Membrane Potential (Polarization ). These differences polarize the plasma membrane, meaning there are a net excess of positive charges on the ECF-side and a net excess of negative charges on the cytosol- side (polarization). This is called resting membrane potential (RMP). In neurons, the RMP is maintained at an average around −70mV.  Action Potential A. Depolarization Action potential exhibits an all-or- none response. Threshold stimulus is the weakest stimulus that will activate a neuron to produce a nerve impulse. Nerve impulses do not vary in their degree of electrical change, meaning every nerve impulse is identical.  Action Potential A. Depolarization and generation of an action potential. If the stimulus is strong The membrane potential enough, it changes the changes to + 30mV. This permeability of the switch in polarity is called membrane, and sodium ions depolarization and the plasma diffuse rapidly into the cell, membrane is now referred to causing the membrane as depolarized. polarity to be completely This sudden depolarization is reversed, and an action the nerve impulse, or action potential is initiated. potential.  Action Potential B. Propagation of the action potential. Depolarization of the first membrane area causes permeability changes in the adjacent area of the membrane, and the events of depolarization are repeated. Thus, the action potential propagates rapidly along the entire length of the membrane.  Action Potential C. Repolarization Immediately after depolarization, K + channels open and Na + channels close, allowing K + to diffuse into the ECF restoring the negative charge on the inside and the positive charge on the outside of the membrane. As a result, the membrane voltage changes from + 30mV to − 70mV. Thus the membrane is repolarized and the RMP is reestablished.  Action Potential D. Hyperpolarization Hyperpolarization is a change in a cell's membrane potential that makes it more negative. It is the opposite of a depolarization. It inhibits action potentials by increasing the stimulus required to move the membrane potential to the action potential threshold.  Action Potential E. Restoring the initial ionic conditions. The ionic conditions of the resting state are restored later by the activity of the sodium- potassium pump. Three sodium ions are ejected for every two potassium ions carried back into the cell. Action Potential Action Potential  Action Potential Transmission of the Signal at Synapses A synapse is a junction of an axon with either another neuron or an effector cell (muscle or gland). When the action potential reaches an axon terminal of the presynaptic neuron, the electrical change opens calcium channels. Calcium ions cause the tiny vesicles containing neurotransmitter (acetylcholine) to fuse releasing it into the synaptic cleft.  Action Potential Transmission of the Signal at Synapses Then, the neurotransmitters bind to receptors, ligand gated ion channel, on the postsynaptic membrane, which triggers a response in the postsynaptic neuron. The nerve impulse is carried along the axon of the postsynaptic neuron to the next synapse where synaptic transmission takes place again. Action Potential The Membrane Potential and Action Potential Clinical Notes: Multiple sclerosis. Multiple sclerosis (MS) is an autoimmune disease in which the person’s own immune system attacks a protein component of the sheath. It gradually destroys the myelin sheaths around CNS fibers by converting them to hardened sheaths called scleroses. As this happens, nerve signals do not always reach the intended target. The affected person may have visual and speech disturbances, lose the ability to control his muscles.. The Membrane Potential and Action Potential Clinical Notes: Local anesthetics Def: drugs that block pain. Ex: procaine and lidocaine, Uses: -skin during suturing of skin. -mouth during dental work. Mech:These drugs act by blocking the opening of voltage-gated Na+ channels. Action potentials cannot propagate to the CNS. The Membrane Potential and Action Potential Clinical Notes: puffer fish neurotoxin. One particularly lethal neurotoxin is tetrodotoxin (TTX), present in the viscera of puffer fish. TTX effectively blocks action potentials by inserting itself into voltage-gated Na+ channels so they cannot open. TTX kills by paralyzing the diaphragm, causing suffocation. The Membrane Potential and Action Potential Clinical Notes: puffer fish neurotoxin. The Membrane Potential and Action Potential Clinical Notes: Localized cooling. Localized cooling of a nerve can also produce an anesthetic effect because axons propagate action potentials at lower speeds when cooled. The application of ice to injured tissue can reduce pain because propagation of the pain sensations along axons is partially blocked. Thank You Dr. Mahmoud Zaher