The creation and conduction of action potentials represents a fundamental means of communication in the nervous system. Action potentials represent rapid reversals in voltage across the plasma membrane of axons. These rapid reversals are mediated by voltage-gated ion channels found in the plasma membrane. The distribution of voltage-gated channels along the axon enables the conduction of the action potential from the nerve cell body to the axon terminal. At the synapse, the electrical signal is converted to a chemical signal that is then propagated to the postsynaptic neuron. Show
1 2 3 4 5 6 7 8 CONCLUSIONWhen the neuronal membrane becomes depolarized, either via the delivery of an electric current or from a signal passing from an adjacent patch of membrane, voltage-gated sodium channels open and positively charged sodium ions—which are in much higher concentration outside of the cell—rush into the cell, producing a rapid reversal of the charge across the membrane. This spike of depolarization represents the action potential. The depolarization spreads to adjacent regions of the membrane, bringing these regions to threshold and thus propagating the signal along the axon. There is thus no loss of signal as an action potential travels along an axon. When the axon potential reaches the nerve terminal, it triggers the release of neurotransmitter across the synaptic cleft, propagating the signal to the next neuron in the circuit. Once initiated, the action potential propagates down the axon at an approximately constant velocity. The leading edge of the action potential depolarizes adjacent unexcited portions of the axon, eventually bringing them to threshold. In the wake of the action potential, the membrane is refractory, preventing reexcitation of previously active portions of the cell. In unmyelinated axons, the action potential travels smoothly, with constant shape and at constant velocity. In myelinated axons, conduction is saltatory: The action potential “jumps” nearly instantaneously from one node of Ranvier to the next, greatly increasing the speed of propagation. View chapterPurchase book Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B0122272102000042 The voltage-gated channels of Na+ action potentialsConstance Hammond, in Cellular and Molecular Neurophysiology (Fourth Edition), 2015 4.1.1 The different types of action potentialsThe action potential is a sudden and transient depolarization of the membrane. The cells that initiate action potentials are called ‘excitable cells’. Action potentials can have different shapes; i.e. different amplitudes and durations. In neuronal somas and axons, action potentials have a large amplitude and a small duration: these are the Na+-dependent action potentials (Figures 4.1 and 4.2a). In other neuronal cell bodies, heart ventricular cells and axon terminals, the action potentials have a longer duration with a plateau following the initial peak: these are the Na+/Ca2+-dependent action potentials (Figure 4.2b–d). Finally, in some neuronal dendrites and some endocrine cells, action potentials have a small amplitude and a long duration: these are the Ca2+-dependent action potentials.  Figure 4.1. Action potential of the giant axon of the squid. Action potential intracellularly recorded in the giant axon of the squid at resting membrane potential in response to a depolarizing current pulse (the extracellular solution is seawater). The different phases of the action potential are indicated. Adapted from Hodgkin AL, Katz B (1949) The effect of sodium ions on the electrical activity of the giant axon of the squid. J. Physiol. 108, 37–77, with permission.Figure 4.2. Different types of action potentials recorded in excitable cells. (a) Sodium-dependent action potential intracellularly recorded in a node of Ranvier of a rat nerve fiber. Note the absence of the hyperpolarization phase flowing the action potential. (b–d) Sodium–calcium-dependent action potentials. (b) Intracellular recording of the complex spike in a cerebellar Purkinje cell in response to climbing fiber stimulation: an initial Na+-dependent action potential and a later larger slow potential on which are superimposed several small Ca2+-dependent action potentials. The total duration of this complex spike is 5–7 ms. (c) Action potential recorded from axon terminals of Xenopus hypothalamic neurons (these axon terminals are located in the neurohypophysis) in control conditions (top) and after adding blockers of Na+ and K+ channels (TTX and TEA, bottom) in order to unmask the Ca2+ component of the spike (this component has a larger duration due to the blockade of some of the K+ channels). (d) Intracellular recording of an action potential from an acutely dissociated dog heart cell (Purkinje fiber). Trace ‘a’ is recorded when the electrode is outside the cell and represents the trace 0 mV. Trace ‘b’ is recorded when the electrode is inside the cell. The peak amplitude of the action potential is 75 mV and the total duration 400 ms. All these action potentials are recorded in response to an intracellular depolarizing pulse or to the stimulation of afferents. Note the differences in their durations. Part (a) adapted from Brismar T (1980) Potential clamp analysis of membrane currents in rat myelinated nerve fibres. J. Physiol. 298, 171–184, with permission. Parts (b–d) adapted from Coraboeuf E, Weidmann S (1949) Potentiel de repos et potentials d’action du muscle cardiaque, mesurés à l’aide d’électrodes internes. C. R. Soc. Biol. 143, 1329–1331; Eccles JC, Llinas R, Sasaki K (1966) The excitatory synaptic action of climbing fibres on the Purkinje cells of the cerebellum. J. Physiol. 182, 268–296; and Obaid AL, Flores R, Salzberg BM (1989) Calcium channels that are required for secretion from intact nerve terminals of vertebrates are sensitive to ω-conotoxin and relatively insensitive to dihydropyridines. J. Gen. Physiol. 93, 715–730, with permission.Action potentials have common properties; for example they are all initiated in response to a membrane depolarization. They also have differences; for example in the type of ions involved, their amplitude, duration, etc. View chapterPurchase book Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B9780123970329000042 NETWORKS | Cellular Properties and Synaptic Connectivity of CA3 Pyramidal Cells: Mechanisms for Epileptic Synchronization and EpileptogenesisR.K.S. Wong, R.D. Traub, in Encyclopedia of Basic Epilepsy Research, 2009 Properties of CA3 pyramidal cellsAction potentials in CA3 pyramidal cells are followed by prominent depolarizing afterpotentials. Depolarizing afterpotentials often reach threshold to recruit additional action potentials, causing the firing of a cluster (burst) of 3–4 action potentials. Bursts of action potentials are a common form of spontaneous activity of CA3 pyramidal cells, recorded in vitro and in vivo. Burst firing can be viewed as a signal amplification process in that a single suprathreshold excitatory synaptic potential can trigger multiple action potentials from CA3 pyramidal cells. In addition to somatic action potential firing, CA3 pyramidal cell dendrites can also generate independent bursts. The combined excitability of the soma-dendritic complex of CA3 pyramidal cell serves to increase the effectiveness of the recurrent synapses (between CA3 pyramidal cells) to synchronize the CA3 neuronal population (see below). View chapterPurchase book Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B9780123739612002150 Calcium WavesLionel F. Jaffe, in Encyclopedia of Biological Chemistry, 2004 Ultrafast Calcium WavesAction potentials are electrical waves that are propagated by voltage-gated ion channels in the plasma membranes of nerve, muscle, and other cells. In the best-understood action potentials, sodium ions flow in through these ion channels to extend the electrical field along a cell; however, in another important class of action potentials, calcium ions flow in; these are called calcium action potentials or ultrafast calcium waves. The speeds of calcium action potentials have been measured along neurons within systems that range from jellyfish up to guinea pig brains, along muscles that range from moth hearts up to guinea pig hearts and even along an insectivorous plant. Unlike sodium action potentials, whose speeds vary over a thousandfold range, calcium action potential speeds vary over a range of approximately 10–40 cm/s (at 20°C) and thus over only a fourfold range. Moreover, unlike the speeds of sodium action potentials, the speeds of calcium ones are unrelated to cell diameter. Why do calcium action potentials, or ultrafast calcium waves, have such a limited range of speeds? Perhaps evolution has driven them to be the fastest waves of calcium influx that avoid subsurface poisoning of the cell. View chapterPurchase book Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B0124437109000661 Cellular Sinoatrial Node and Atrioventricular Node Activity in the HeartH.J. Jansen, ... R.A. Rose, in Encyclopedia of Cardiovascular Research and Medicine, 2018 Atrioventricular Node Action PotentialsAPs within the AVN are unique compared with other regions in the heart. Specifically, AVN myocytes generate spontaneous APs that are characterized by much lower Vmax values compared to the working myocardium. Furthermore, APs in AVN myocytes are shorter in duration compared with ventricular myocytes, but not atrial myocytes (Munk et al., 1996). While AVN myocytes are spontaneously active, their APs are characterized by slower spontaneous beating rates, reduced DD slopes, more negative MDPs, and reduced AP thresholds compared with SAN myocytes (Marger et al., 2011b). Regional differences in AP morphology have been characterized in atrio-nodal cells (rod-shaped cells) and compact node or nodo-his cells (ovoid cells) isolated from the rabbit heart. Compact AVN cells are characterized by the presence of a rounded AP OS (Munk et al., 1996). Furthermore, these compact AVN cells have the lowest Vmax, OS, APD, and MDP compared with the other regions of the AVN (Munk et al., 1996). In contrast, AP morphology of atrio-nodal cells demonstrates characteristics of both AVN and atrial myocytes (Munk et al., 1996; Yuill and Hancox, 2002). View chapterPurchase book Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B978012809657499759X Propagation of the Action PotentialJoseph Feher, in Quantitative Human Physiology (Second Edition), 2017 The Velocity of Nerve Conduction Varies Directly with the Axon DiameterThe action potentials shown in Figure 3.3.1 do not have identical waveforms due to the stimulation artifact that dies out with distance along the axon. After this initial stimulation artifact decays away, all subsequent action potentials are essentially identical. The identical waveform of the action potential as it travels over the axon is a variant of the “all-or-none” description of the action potential. As the action potential appears later at longer distances from the point of initiation, we can define a conduction velocity of action potential propagation equal to the distance between the recording electrodes divided by the delay in time between action potentials recorded at the two sites. The velocity of action potential conduction has been determined for myelinated and unmyelinated fibers of different sizes (see Table 3.3.1). Table 3.3.1. Velocity of Nerve Impulse Conduction as a Function of Axon Size Nerve Fiber TypeDiameter (μm)Conduction Velocity (m s−1)Physiological FunctionAα12–2270–120Somatic motorAδ1–512–30Pain, sharpC0.5–1.20.2–2Pain, ache Within each category of nerve fiber, myelinated or unmyelinated, the conduction velocity varies with the diameter of the nerve. For myelinated fibers, the conduction velocity varies approximately in proportion to the diameter. In unmyelinated fibers, the conduction velocity varies approximately with the square root of the diameter. View chapterPurchase book Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B9780128008836000252 The Electrochemical Basis of Nerve FunctionT.M. Dwyer, in Fundamental Neuroscience for Basic and Clinical Applications (Fifth Edition), 2018 Threshold VoltageAPs are initiated when generator potentials cross a narrowly defined range—the threshold voltage. A millivolt less and no AP will be triggered; a millivolt more and the AP abruptly takes off, all because three events are competing among themselves. The closed sodium channels are opening at a rate that becomes faster and faster as the membrane is depolarized; however, at the same time, the newly opened sodium channels do not last forever, as they undergo inactivation. Finally, the membrane conductances to potassium and chloride will tend to damp out the signal. These dynamic changes also mean that the rate of depolarization is also important, as will be described more completely in the section on accommodation. The outcome of these competing events is the basis of the Hodgkin-Huxley formulation of the mechanism of the AP. What is the first event of an action potential quizlet?The first step with regards to the generation of an action potential is the opening of voltage-gated potassium channels at the axon hillock.
What are the 4 steps of an action potential?Terms in this set (4). Step 1 - Resting Potential. Sodium and potassium channels are closed. ... . Step 2 - Depolarization. Sodium channels open in response to a stimulus. ... . Step 3 - Repolarization. Na+ channels close and K+ channels open. ... . Step 4 - Resting Conditions. Na+ and K+ channels are closed.. What happens during an action potential?An action potential occurs when a neuron sends information down an axon, away from the cell body. Neuroscientists use other words, such as a "spike" or an "impulse" for the action potential. The action potential is an explosion of electrical activity that is created by a depolarizing current.
What are the 6 steps of action potential quizlet?Terms in this set (6). Step One: Reaching Threshold. ... . Step Two: Depolarization. ... . Step Three: Sodium Channels Close and Potassium Channels Open. ... . Step Four: Active Sodium and Potassium Pumps Begin to Start Repolarization. ... . Step Five: Hyperpolarization. ... . Step Six: Resting Potential.. |
