2 Membrane potentialMembrane 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.
3 Excitable cellsAction 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.
4 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+channels2. Voltage gated K+channels
5 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)
7 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
8 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)
9 Stages of action potential 2. Repolarization (+40 to -70)►Opening of voltage gated K+channels►Closure of voltage gated Na+channels
10 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
14 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)
15 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 restoredIt is mostly of 0.4 ms in large myelinated nerve fibers.2. Relative refractory periodPeriod 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 configurationDuring this period K+efflux continues.
17 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
18 Local anestheticsProcaine, Tetracaine etc block voltage gated Na+channels, thus►No action potential occurs►No nerve signal from periphery to brain►No sensation of pain
19 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
20 Conduction of nerve impulse ►Nerve impulse conduction is always unidirectional►Chemical synapses are unidirectional►Ensure one way transmission of nerve impulse
21 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 muscleUnmyelinated fibers►Not covered by myelin sheath►Small diameter fibers (C fibers) carrying dull pain sensation to CNS►Postganglionic autonomic fibers
23 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
24 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
25 PropagationThe 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.
26 Types of conduction Contiguous conduction ►Occurs in unmyelinated fibers►Every part of nerve fiber undergoes depolarization►Slow speed of impulse conduction►More energy consumptionSaltatory conduction►In myelinated nerve fibers►Depolarization occurs only at nodes of ranvier►Myelinated parts do not depolarize►Activation ‘jumps’ from node to node