Graded and Action Potentials PDF

Topic 5 Graded Potentials and Action Potentials 1 Graded Potentials Graded potentials are changes in membrane potential that are confined to a small region of the plasma membrane. They are called graded po...

Topic 5 Graded Potentials and Action Potentials 1 Graded Potentials Graded potentials are changes in membrane potential that are confined to a small region of the plasma membrane. They are called graded potentials because the magnitude of the potential change can vary (is “graded”). They are decremental: Potential change decreases as the distance from the site of the original event increases. Summation: Addition of graded potentials from several stimuli, that occur in rapid succession, before each graded potential has died out. Summation of several small potentials can aid in integration, and in reaching the threshold potential, so an action potential will occur. 2 Figure 6.15 Depolarization and Graded Potential Caused By a Chemical Stimulus 3 Figure 6.16 Graded Potentials Can Be Recorded Under Experimental Conditions in Which the Stimulus Strength Can Vary Access the text alternative for slide images. 4  Graded Potentials Small deviations from resting potential of -70mV hyperpolarization = membrane has become more negative depolarization = membrane has become more positive The signals are graded, meaning they vary in amplitude (size), depending on the strength of the stimulus and localized. Graded potentials occur most often in the dendrites and cell body of a neuron. Figure 6.17 Leakage of Charge (Predominately K+) Across the Plasma Membrane Reduces the Local Current at Sites Farther Along the Membrane From the Site of Initial Depolarization Access the text alternative for slide images. 6 Generation of an Action Potential An action potential (AP) or impulse is a sequence of rapidly occurring events that decrease and eventually reverse the membrane potential (depolarization) and then restore it to the resting state (repolarization). During an action potential, voltage-gated Na+ and K+ channels open in sequence. According to the all-or-none principle, if a stimulus reaches threshold, the action potential is always the same. A stronger stimulus will not cause a larger impulse. Figure 6.21 Changes in Membrane Potential With Increasing Strength of Excitatory Stimuli Access the text alternative for slide images. 8 Depolarization Phase of Action Potential Chemical or mechanical stimulus caused a graded potential to reach at least (-55mV or threshold) Voltage-gated Na+ channels open & Na+ rushes into cell in resting membrane, inactivation gate of sodium channel is open & activation gate is closed (Na+ can not get in) when threshold (-55mV) is reached, both open & Na+ enters inactivation gate closes again in few ten-thousandths of second only a total of 20,000 Na+ actually enter the cell, but they change the membrane potential considerably(up to +30mV) Repolarization Phase of Action Potential When threshold potential of -55mV is reached, voltage-gated K+ channels open K+ channel opening is much slower than Na+ channel opening which caused depolarization When K+ channels finally do open, the Na+ channels have already closed (Na+ inflow stops) Repolarization Phase of Action Potential K+ outflow returns membrane potential to -70mV If enough K+ leaves the cell, it will reach a -90mV membrane potential and enter the after- hyperpolarizing phase K+ channels close and the membrane potential returns to the resting potential of -70mV by action of the Na+/K+ pump An Action Potential The Action Potential: Summarized Resting membrane potential is -70mV Depolarization is the change from -70mV to +30 mV Repolarization is the reversal from +30 mV back to -70 mV) Refractory Period of Action Potential Period of time during which neuron can not generate another action potential Absolute refractory period even very strong stimulus will not begin another AP inactivated Na+ channels must return to the resting state before they can be reopened Relative refractory period a suprathreshold stimulus will be able to start an AP K+ channels are still open, but Na+ channels have closed Refractory Periods 2 The refractory periods limit the number of action potentials an excitable membrane can produce in a given period of time. Most neurons respond at frequencies of up to 100 action potentials per second, and some may produce higher frequencies for brief periods. Refractory periods contribute to the separation of action potentials so that individual electrical signals pass down the axon. The refractory periods also determine the direction of action potential propagation. 15 Figure 6.22 Absolute and Relative Refractory Periods of the Action Potential Determined by a Paired-Pulse Protocol Access the text alternative for slide images. 16 Propagation of Action Potential An action potential spreads (propagates) over the surface of the axon membrane as Na+ flows into the cell during depolarization, the voltage of adjacent areas is effected and their voltage-gated Na+ channels open self-propagating along the membrane The traveling action potential is called a nerve impulse Continuous versus Saltatory Conduction Continuous conduction (unmyelinated fibers) step-by-step depolarization of each portion of the length of the axolemma Saltatory conduction depolarization only at nodes of Ranvier where there is a high density of voltage- gated ion channels current carried by ions flows through extracellular fluid from node to node Figure 6.23 One-Way Propagation of an Action Potential (a) Action potential initiated in region 1 (b) Propagation of action potential to region 2 (c) Propagation of action potential to region 3 Access the text alternative for slide images. 19 Figure 6.24 Myelinization and Saltatory Conduction of Action Potentials Access the text alternative for slide images. 20  Saltatory Conduction Nerve impulse conduction in which the impulse jumps from node to node Speed of Impulse Propagation The propagation speed of a nerve impulse is not related to stimulus strength. larger, myelinated fibers conduct impulses faster due to size & saltatory conduction Types of Nerve Fibers Fiber types A fibers largest (5-20 microns & 130 m/sec) myelinated somatic sensory & motor to skeletal muscle B fibers medium (2-3 microns & 15 m/sec) myelinated visceral sensory & autonomic preganglionic C fibers smallest (.5-1.5 microns & 2 m/sec) unmyelinated sensory & autonomic motor Encoding of Stimulus Intensity How do we differentiate a light touch from a firmer touch? frequency of impulses firm pressure generates impulses at a higher frequency number of sensory neurons activated firm pressure stimulates more neurons than does a light touch Clinical Effects of Action Potential Inhibition The generation of action potentials is prevented by local anesthetics such as procaine (Novocaine) and lidocaine (Xylocaine) because these drugs block voltage-gated Na+ channels, preventing them from opening in response to depolarization. Without action potentials, graded signals generated in sensory neurons—in response to injury, for example—cannot reach the brain and give rise to the sensation of pain. 25 Synapses A synapse is an anatomically specialized junction between two neurons. Synapses can be chemical or electrical. At electrical synapses, the plasma membranes of the presynaptic and postsynaptic cells are joined by gap junctions. Thus, the electrical activity of the presynaptic neuron affects the electrical activity of the postsynaptic neuron. At chemical synapses, presynaptic neurons release neurotransmitters from their axon terminals, and the neurotransmitters bind to receptors on post-synaptic neurons. 26 Figure 6.26a Electrical Synapse (a) Electrical synapse 27 Figure 6.26b Diagram of a Chemical Synapse (b) Chemical synapse 28 Figure 6.27 Mechanisms of Signaling at a Chemical Synapse (a) Sequence of events at a chemical synapse (b) Mechanism of neurotransmitter release 29 Chemical Synapses Action potential reaches end bulb and voltage-gated Ca2+ channels open Ca2+ flows inward triggering release of neurotransmitter Neurotransmitter crosses synaptic cleft & binding to ligand-gated receptors the more neurotransmitter released the greater the change in potential of the postsynaptic cell Synaptic delay is 0.5 msec One-way information transfer Excitatory & Inhibitory Potentials The effect of a neurotransmitter can be either excitatory or inhibitory a depolarizing postsynaptic potential is called an EPSP it results from the opening of ligand-gated Na+ channels the postsynaptic cell is more likely to reach threshold an inhibitory postsynaptic potential is called an IPSP it results from the opening of ligand-gated Cl- or K+ channels it causes the postsynaptic cell to become more negative or hyperpolarized the postsynaptic cell is less likely to reach threshold Activation of the Postsynaptic Cell Excitatory chemical synapses generate an excitatory postsynaptic potential (EPSP). An EPSP is a depolarizing graded potential that decreases in magnitude as it spreads away from the synapse by local current. Its only function is to bring the membrane potential of the postsynaptic neuron closer to the threshold potential. Inhibitory chemical synapses generate an inhibitory postsynaptic potential (IPSP). An IPSP is a hyperpolarizing graded potential that lessens the likelihood that the postsynaptic cell will depolarize to the threshold potential and generate an action potential. 32 Figure 6.28 Excitatory Postsynaptic Potential (EPSP) 33 Figure 6.29 Inhibitory Postsynaptic Potential (IPSP) 34 Figure 6.31 Interaction of EPSPs and IPSPs at the Postsynaptic Neuron Access the text alternative for slide images. 35  Removal of Neurotransmitter Diffusion move down concentration gradient Enzymatic degradation Acetylcholinesterase in muscles for example Uptake by neurons (reuptake) or glia cells neurotransmitter transporters