Action Potential

John A. White , in Encyclopedia of the Human Brain, 2002

I. Basic Properties of the Action Potential

The basic properties of the action potential can be studied using a microelectrode constructed from a glass capillary tube with a fine tip and containing artificial intracellular solution. This microelectrode, inserted into the cell body or axon of a neuron ( Fig. 1a, inset), measures the value of membrane potential relative to the extracellular space. At rest, typical values of membrane potential range from −40 to −90   mV. Passing positive electrical current into the cell depolarizes it (i.e., makes membrane potential less negative). In response to small depolarizing stimuli, the neuron's response is small as well (Fig. 1a, bottom). In response to larger stimuli, above a threshold value, the response is fundamentally different; the membrane potential quickly rises to a value well above 0   mV and then falls over the course of 1–5   msec to its resting value (Fig. 1a, middle). Often, the falling phase of the action potential undershoots resting potential temporarily. The action potential is said to be all-or-nothing because it occurs only for sufficiently large depolarizing stimuli, and because its form is largely independent of the stimulus for suprathreshold stimuli. In some neurons, a single action potential can be induced by the offset of a hyperpolarizing stimulus (Fig. 1b). This phenomenon is called anodal break excitation or rebound spiking.

Figure 1. Basic properties of the action potential. (a) Traces show responses of a simulated space-clamped squid axon (T=6.3°C) to intracellularly injected current pulses of duration 0.5   msec (top trace). The simulated recording configuration is shown in the inset. Sufficiently large inputs evoke all-or-nothing action potentials (middle trace). The response is minimal to subthreshold stimuli (bottom trace). The inset shows the basic recording configuration. (b) A simulation demonstrating anode-break excitation in response to the offset of a hyperpolarizing current pulse (duration=10   msec). (c) Current threshold (the minimal amplitude of a current step necessary to evoke an action potential) plotted vs stimulus duration. (d) Simulation results demonstrating refractoriness. Two current pulses (duration=0.5   msec each) were delivered to the model, with interstimulus interval (ISI) varied systematically. The first pulse had magnitude twice the threshold for evoking an action potential. The y-axis shows the magnitude of the second pulse necessary to evoke a spike. For ISI<15   msec, threshold is above its normal value (dashed line). During the relative refractory period (RRP), threshold is elevated; during the absolute refractory period (ARP), it is not possible to evoke a second action potential.

The value of threshold depends on the duration of the stimulus (Fig. 1c); brief stimuli are required to be larger to evoke an action potential. Threshold also depends on more subtle features of the stimulus, such as its speed of onset. For a short time after an action potential has occurred, it is impossible to evoke a second one (Fig. 1d). This period is referred to as the absolute refractory period (ARP). After the ARP comes the relative refractory period (RRP), in which an action potential can be evoked, but only by a larger stimulus than was required to evoke the first action potential. Stimulation by an ongoing suprathreshold stimulus leads to repetitive firing at a rate that is constant once any transients have settled out(Fig. 2a). The rate of repetitive firing increases with increasing depolarization (Fig. 2bb), eventually approaching the limit imposed by the ARP.

Figure 2. Spike rate depends on the magnitude of applied current. (a) Simulated traces of space-clamped squid giant axon (T=6.3°C) to constant applied current. (b) Firing rate increases with increasing applied current. Note that the minimal firing rate is well above zero spikes/sec.

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.

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The voltage-gated channels of Na+ action potentials

Constance Hammond , in Cellular and Molecular Neurophysiology (Fourth Edition), 2015

4.1.1 The different types of action potentials

The 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 4.2 a). 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 4.2 b–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.

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NETWORKS | Cellular Properties and Synaptic Connectivity of CA3 Pyramidal Cells: Mechanisms for Epileptic Synchronization and Epileptogenesis

R.K.S. Wong , R.D. Traub , in Encyclopedia of Basic Epilepsy Research, 2009

Properties of CA3 pyramidal cells

Action 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).

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Brain Machine Interfaces: Implications for Science, Clinical Practice and Society

Rebecca A. Parker , ... Bradley Greger , in Progress in Brain Research, 2011

Electrophysiological recording data

Action potential recordings were sorted using a PCA-based t-distribution algorithm (Shoham, 2003). A threshold for action potentials was subsequently imposed at 70   μV. t-tests were performed to quantify changes in the number of electrodes which recorded well-isolated action potentials over time (using the first and last 30 datasets in Felines 1, 3, and 4 and the first and last 10 datasets in Feline 2). Student's t-test was also applied to acute pre- and poststimulation number of electrodes which recorded action potentials. The distribution of number of electrodes which recorded action potentials across all microstimulation sessions during pre-stimulation recordings was compared to the immediate post-stimulation distribution for Felines 3 and 4.

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Cytology of the nervous system

Jahangir Moini MD, MPH , ... Mohtashem Samsam MD, PhD , in Epidemiology of Brain and Spinal Tumors, 2021

Threshold and the all-or-none principle

Action potentials are stimulated when a graded potential depolarizes the axolemma to a specific level. A threshold is a membrane potential at which an action potential is initiated. An axon threshold is usually −60 to −55   mV. This relates to a depolarization of 10−15   mV. A stimulus altering the resting membrane potential from −70 to −62   mV produces a graded depolarization, not an action potential. Removal of a stimulus causes membrane potential to return to its resting level. Local currents are initiated by graded depolarization of the axon hillock. They cause depolarization of the initial axon segment.

For excitable membranes such as axons, a graded depolarization is similar to pressure on a gun's trigger. The action potential is similar to the firing of a gun. Every stimulus bringing the membrane to threshold creates identical action potentials. If a stimulus exceeds threshold, the action potential is independent of the intensity of the depolarizing stimulus. This all-or-none principle applies to all excitable membranes. The stimulus either triggers a typical action potential or none at all.

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Cellular Sinoatrial Node and Atrioventricular Node Activity in the Heart

H.J. Jansen , ... R.A. Rose , in Encyclopedia of Cardiovascular Research and Medicine, 2018

Atrioventricular Node Action Potentials

APs within the AVN are unique compared with other regions in the heart. Specifically, AVN myocytes generate spontaneous APs that are characterized by much lower V max 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 V max, 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).

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Propagation of the Action Potential

Joseph Feher , in Quantitative Human Physiology (Second Edition), 2017

The Velocity of Nerve Conduction Varies Directly with the Axon Diameter

The 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 Type Diameter (μm) Conduction Velocity (m   s−1) Physiological Function
Aα 12–22 70–120 Somatic motor
Aδ 1–5 12–30 Pain, sharp
C 0.5–1.2 0.2–2 Pain, 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.

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Membrane Potential and Action Potential

David A. McCormick , in From Molecules to Networks (Third Edition), 2014

Summary

An action potential is generated by the rapid influx of Na + ions followed by a slightly slower efflux of K+ ions. Although the generation of an action potential does not disrupt the concentration gradients of these ions across the membrane, the movement of charge is sufficient to generate a large and brief deviation in the membrane potential. Action potentials are typically initiated in the axon initial segment and the propagation of the action potential along the axon allows communication of the output of the cell to its distal synapses. Neurons possess many different types of ionic channels in their membranes, allowing complex patterns of action potentials to be generated and complex computations to occur within single neurons.

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Histophysiology

Jahangir Moini , Pirouz Piran , in Functional and Clinical Neuroanatomy, 2020

Continuous propagation

Action potentials move via continuous propagation in unmyelinated axons. The axolemma is organized in adjacent segments. Continuous propagation occurs as follows:

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The membrane potential briefly becomes positive at the peak of the action potential.

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A local current develops as sodium ions begin moving in the cytoplasm and ECF.

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The local current spreads out in all directions, depolarizing the nearby membrane areas. The axon hillock could not to respond with an action potential since it has no voltage-gated sodium ion channels.

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The process continues as if in a chain reaction.

Each time there is development of a local current develops, the action potential moves in one direction: forward. This is because the previous axon segment is still in the absolute refractory period. Therefore, action potentials move away from their generation site and do not reverse direction. Over time, the furthest parts of the plasma membrane are affected.

Messages are relayed from one location to another. Distance does not affect this process. The action potential that reaches the axon terminal is exactly the same as the one generated at the initial axon segment. Though the events at each location take about a millisecond, each event must be repeated at every step along the way. For another action potential to occur at the same location, another stimulus must be applied.

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Action Potential Initiation and Conduction in Axons

J.H. Caldwell , in Encyclopedia of Neuroscience, 2009

Action potential initiation and propagation processes in vertebrate axons are based on three proteins, two ion channels, voltage-dependent sodium channels and voltage-dependent potassium channels, and an ion pump, the sodium–potassium pump, which maintains sodium and potassium concentration gradients. Except for initiation involving sensory neurons, whereby action potentials begin near the sensory receptor, initiation takes place at the axon hillock and initial segment of the axon where sodium channels are concentrated. Conduction of the action potential is continuous in unmyelinated axons and is fast and saltatory in myelinated axons where sodium channels are concentrated at the nodes of Ranvier.

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