Membrane Potential and Action Potential PDF

Faculty of Dentistry 234): General Physiology Membrane potential Dr. Mai Adawi Lecturer of Physiology Faculty of Medicine 1  The membrane potential Electr...

Faculty of Dentistry 234): General Physiology Membrane potential Dr. Mai Adawi Lecturer of Physiology Faculty of Medicine 1  The membrane potential Electrical potentials exist across the membranes of virtually all cells of the body. In addition, some cells, such as nerve and muscle cells, are capable of generating rapidly changing electrochemical impulses at their membranes, and these impulses are used to transmit signals along the nerve or muscle membranes. Measuring the Membrane Potential: Measurement of the membrane potential of the nerve fiber using a microelectrode. Two electrodes. One inside the nerve fiber and the other one on the outside of the nerve membrane  What is meant by the resting membrane potential of a nerve fiber RMP = -90 mv ??? That is, the potential inside the fiber is 90 millivolts more negative than the potential in the extracellular fluid on the outside of the fiber.  The Resting membrane potential is defined as the electrical potential difference (voltage) across the cell membrane (between inside and outside of the cell) under resting condition. It is also called membrane potential, transmembrane potential, transmembrane potential difference or transmembrane potential gradient.  The Resting membrane potential The magnitude of the resting membrane potential depends mainly on two factors: (1) differences in specific ion concentrations in the intracellular and extracellular fluids, and (2) differences in membrane permeabilities to the different ions, which reflect the number of open channels for the different ions in the plasma membrane. The resting membrane potential of large nerve fibers when not transmitting nerve signals is about –90 millivolts. That is, the potential inside the fiber is 90 millivolts more negative than the potential in the extracellular fluid on the outside of the fiber. The resting membrane potential of small neurons is about – 70 mv The membrane in the resting state is said to be polarized because there is potential difference between both sides If the inside becomes more positive it is said the membrane is depolarized If the potential becomes more negative it is said to be hyperpolarized Faculty of Physiotherapy What is the physiological ionic basis of RMP?? Na / K pump unequal distribution of ions Selective permeability of cell membrane.  The Ionic basis of Resting membrane potential Development and maintenance of resting membrane potential in a muscle fiber or a neuron are carried out by movement of ions, which produce ionic imbalance across the cell membrane (unequal distribution of ions around the membrane). This results in the development of more positivity outside and more negativity inside the cell. Ionic imbalance is produced by two factors: 1. Sodium-potassium pump 2. Selective permeability of cell membrane.  The Ionic basis of Resting membrane potential 1. Sodium-potassium pump Sodium and potassium ions are actively transported in opposite directions across the cell membrane by means of an electrogenic pump called sodium- potassium pump. It moves three sodium ions out of the cell and two potassium ions inside the cell by using energy from ATP. Since more positive ions (cations) are pumped outside than inside, It leads to negativity inside and positivity outside the cell.  The Ionic basis of Resting membrane potential 2. Selective permeability of cell membrane Permeability of cell membrane depends on the transport channels. Which are selective for the movement of some specific ions. Inside the cells there are large negatively charged protein which cant cross the membrane adding to the negativity of the inside. There is channel protein in the nerve membrane through which potassium and sodium ions can leak, called a potassium sodium (K+-Na+) “leak” channel. this channels are far more permeable to potassium than to sodium, normally about 100 times as permeable as sodium.  The Ionic basis of Resting membrane potential 2. Selective permeability of cell membrane Inside the cells: K is present at high concentration than outside Na and cl are present at high concentration at the outside of the membrane In neurons, the resting membrane potential depends mainly on the movement of K because the membrane is more permeable to K than other ions  What is meant by action potential? It is a sudden change in the membrane potential. It is a propagated, non graded impulse, that obeys all or non law. They are large rapid changes in the membrane potential that spread rapidly along the nerve fiber membrane. A series of electrical changes that occur in the membrane potential when the muscle or nerve is stimulated. Properties of action potential propagated Action potential Obeys all Non- or non graded law  Action potential Each action potential begins with a sudden change from the normal resting negative membrane potential to a positive potential and then ends with an almost equally rapid change back to the negative potential.  Action potential curve 1. Latent Period: The period immediately after applying the stimulus. 2. Depolarization: starts after the latent period. Initially, it is very slow and the membrane is depolarized for about 15 mV from -70 to about -55 or -50 mv After the initial slow depolarization for 15 mV, the rate of depolarization increases suddenly. The point at which, the depolarization increases suddenly is called firing level.  Action potential curve 3. Overshoot: From firing level, the shoots up (overshoots) beyond the zero potential (isoelectric base) up to +35 mV. 4.Repolarization: When depolarization is completed (+35 mV), the repolarization starts. Initially, the repolarization occurs rapidly and then it becomes slow. 5. After hyperpolarization: After reaching the resting level, it becomes it is of small amplitude (1-2 mv) more negative beyond resting level. This is called after hyperpolarization or positive after potential. After this, the normal resting membrane potential is restored slowly. Ionic basis of Action Potential 16  Ionic basis of Action Potential At Depolarization Opening of voltage gated sodium (Na) channels → Na influx (ENTRY) → +ve ions inside→ overshoot +ve side This making the inside of the cell more positive than the outside At the peak,the Na channels start to close and voltage-gated K channels start to slowly open → efflux of K+ out of the cell, causing repolarization. They remain open for longer duration after restoring the membrane potential. It causes efflux of more K+ producing more negativity inside. It is the cause for hyperpolarization. FINALLY: The Na1–K1 pump gradually restores the concentration gradients disrupted by action potentials. Summary of events of Action Potential Ionic basis of Action Potential  Types of changes in membrane potential 1. Polarization: Any time membrane potential is other than 0 millivolts (mV), in either the positive or the negative direction. 2. Depolarization: The membrane becomes less polarized; the inside becomes less negative than at resting potential.This term also refers to the inside even becoming positive. 3. Repolarization. The membrane returns to resting potential after having been depolarized. 4. Hyperpolarization. The membrane becomes more polarized; the inside becomes more negative than at resting potential  The Graded Potential There are two basic forms of electrical signals: (1) Graded potentials, which serve as short-distance signals. When the stimulus with sub threshold is applied, only electrotonic potential (graded) develops and the action potential does not develop. Electrotonic potential is non-propagated. (2) Action potentials, which signal over long distances. It develops in a nerve fiber when it is stimulated by a stimulus with adequate strength. Adequate strength of stimulus, necessary for producing the action potential in a nerve fiber is known as threshold or minimal stimulus. Action potential is propagated.  The Graded Potential Definition: It is a non-propagated local response that develops in the nerve fiber when a subthreshold stimulus is applied. They are local changes in membrane potential that occur in varying grades or degrees of magnitude or strength.  The Properties of Graded Potential Properties: 1- Graded: It does not obey all-or-none law. If the intensity of the stimulus is increased gradually every time, there is increase in the amplitude till the firing level is reached. The stronger the triggering event, the larger the resultant graded potential.  The Properties of Graded Potential Properties: 2-The graded potentials die out over short distances. This current is lost across the plasma membrane and the spread of a graded potential (gradually decreases).  Examples of Graded Potential 1-Synaptic potential 2- Neuromuscular junction (end plate potential) 3- Receptor potential  Conductivity of The nerve Definition: Conductivity is the ability of nerve fibers to transmit the impulse from the area of stimulation to the other areas. Action potential is transmitted through the nerve fiber as nerve impulse. Normally in the body, the action potential is transmitted through the nerve fiber in only one direction. The impulse is automatically conducted throughout the neuron without further stimulation by one of two methods of propagation: contiguous conduction or saltatory conduction.  Contiguous conduction Contiguous conduction Involves the spread of the action potential along every patch of membrane down the length of the axon. This occurs in unmyelinated nerve fibers  Saltatory Conduction Saltatory conduction is the form of conduction of nerve impulse in which, the impulse jumps from one node to another. Conduction of impulse through a myelinated nerve fiber is about 50 times faster than through a nonmyelinated fiber. It is because the action potential jumps from one node to another node of Ranvier instead of travelling through the entire nerve fiber. Conduction Unmyelinated nerve fiber Myelinated nerve fiber Faculty of Dentistry Faculty of Dentistry References: 1.Essentials of Medical Physiology. K Sembulingam PhD and Prema Sembulingam PhD sixth edition 2.Ganong’s Review of Medical Physiology. Kim E. Barrett (editor), 26th edition, 2019. Lange Basic Science 3.Guyton and Hall Physiology Review,, freely downloaded https://www.medicinebau.com/uploads/7/9/0/4/79048958/ physiology_review.pdf Faculty of Dentistry THANK YOU