Understanding Membrane Potential and Action Potentials in Excitable Cells
Membrane potential, resting potential, and action potentials play crucial roles in the functioning of excitable cells like neurons, muscle cells, and endocrine cells. Voltage-gated channels, depolarization, and repolarization are key processes involved in generating and propagating action potentials. The all-or-none principle dictates that action potentials are generated with specific stimuli, and the stages of action potentials involve sequential changes in ion channel permeability. Explore the fascinating world of cell excitability through membrane potential dynamics and action potential mechanisms.
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Presentation Transcript
Membrane potential Resting potential Action potential
Membrane potential Membrane potential ( transmembrane potential or membrane voltage) is the difference in electrical potential between the interior and the exterior of a biological cell. Typical values of membrane potential range from 40 mV to 100 mV.
Excitable cells Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and endocrine cells, as well as in some plant cells. Action potentials in neurons are also known as "nerve impulses . It sends the messages from our muscles to our brains and back, as well as all the thought processes in our brain. We could stimulate an excitable cell chemically, electrically, or mechanically.
Voltage gated channels Action potentials are generated by special types of voltage-gated ion channels embedded in a cell's plasma membrane. Two types of channels are present: 1. Voltage gated Na+channels 2. Voltage gated K+channels
Resting membrane potential to threshold level (-70 to -50 mv) 1. Opening of voltage gated Na+channels (electrical stimulus) 2. Opening of mechanically gated Na+channels (mechanical stimulus) 3. Opening of ligand gated Na+channels (chemical stimulus)
All or none principle Action potential will either be generated or not no gradations or intensities or possible Suprathreshold stimulus will elicit same action potential as elicited by threshold stimulus Subthreshold stimulus will not elicit action potential
Stages of action potential 1. Depolarization (-50 to +40 mv) Opening of voltage gated Na+channels About 5000 fold increase in Na+permeability Voltage rises and crosses zero (overshoot)
Stages of action potential 2. Repolarization (+40 to -70) Opening of voltage gated K+channels Closure of voltage gated Na+channels
Stages of action potential 3. Hyperpolarization Some voltage gated K+channels remain open even after RMP (-70 mv) is restored Potential decreased more than resting level Na+ -K+pump restores RMP from hyperpolarization
Re-establishment of ionic gradients During action potential Na+& K+ionic gradients reverse. In this condition cells contain: Large amount of Na+(due to massive Na+influx) Too less amount of K+(due to massive K+ efflux) Na+ -K+pump re-establishes ionic gradients (recharges the nerve fiber)
Refractory period 1. Absolutely refractory period Period during which a 2nd action potential can not be generated. This can be elicited: From start of depolarization to initial 1/3 of repolarization After closure, the inactivation gates do not reopen until RMP is restored It is mostly of 0.4 ms in large myelinated nerve fibers. 2. Relative refractory period Period during which 2nd action potential can be generated but with stronger than normally required stimulus. This can be elicited: From end of initial 1/3 of repolarization to start of after depolarization (middle 1/3rd) Some voltage gated Na+channels regain their resting configuration During this period K+efflux continues.
Refractory period limits frequency of action potentials. Longer the refractory period, less will be the frequency Absolutely refractory period of large myelinated nerve fiber is 0.4 ms, therefore, frequency of action potential is 2500/second Determine direction of action potential Action potentials can not be summated
Local anesthetics Procaine, Tetracaine etc block voltage gated Na+channels, thus No action potential occurs No nerve signal from periphery to brain No sensation of pain
Propagation (conduction) of action potential Propagates along nerve fiber as nerve signal or nerve impulse Means of communication between neurons or nerves and muscles. Causes muscle contraction
Conduction of nerve impulse Nerve impulse conduction is always unidirectional Chemical synapses are unidirectional Ensure one way transmission of nerve impulse
Types of nerve fibers (based upon myelination) Myelinated fibers Covered by myelin sheath Large diameter fibers (A fibers) carrying touch and pressure sensations to CNS Somatic motor fiber to skeletal muscle Unmyelinated fibers Not covered by myelin sheath Small diameter fibers (C fibers) carrying dull pain sensation to CNS Postganglionic autonomic fibers
Myelin sheath Fatty material Produced by Schwann cells Wraps around the axon in multiple layers Insulates the nerve fiber Ionic exchange can not take place through myelin sheath
Nodes of Ranvier Parts of myelinated nerve fiber devoid of myelin sheath Present after every1-3 mm of myelinated part of nerve fiber Are in contact with ECF Have abundance of voltage gated Na+channels Sites of action potential generation
Propagation The action potential generated at the axon hillock propagates as a wave along the axon. The currents flowing inwards at a point on the axon during an action potential spread out along the axon, and depolarize the adjacent sections of its membrane. If sufficiently strong, this depolarization provokes a similar action potential at the neighboring membrane patches.
Types of conduction Contiguous conduction Occurs in unmyelinated fibers Every part of nerve fiber undergoes depolarization Slow speed of impulse conduction More energy consumption Saltatory conduction In myelinated nerve fibers Depolarization occurs only at nodes of ranvier Myelinated parts do not depolarize Activation jumps from node to node
