Action Potential
An action potential is a brief electrical impulse that travels along the membrane of a neuron or muscle cell. It is generated by the movement of ions across the cell membrane, resulting in a rapid change in membrane potential. This process is essential for communication within the nervous system and muscle contraction.
8 Key excerpts on "Action Potential"
eBook - ePub- Anne Feltz, Anne Feltz(Authors)
- 2020(Publication Date)
- Garland Science(Publisher)
...The Action Potential is an all or none transient depolarization, which is initiated when the membrane potential reaches a critical threshold, around −50 mV to −40 mV depending on the cell type. It is initiated at the axon hillock (also called the initial segment) where the axon comes out of the cell body. From one stimulation to the next, it is of constant amplitude and duration. An Action Potential is a depolarization of almost 100 mV above the resting potential, the depolarization going beyond 0 mV with a reversal of the polarity at the peak. There is an overshoot where the membrane potential becomes positive for a short while (see Figure 1.12c). The Action Potential is a short event, of around 1-ms duration. A second Action Potential can only be formed after a few-ms delay: the Action Potential depolarization is followed by a refractory period which determines the maximal firing frequency of a neuron. The molecular basis of the Action Potential and, in particular, of how its all-or-none property is generated will be examined in Chapter 5. Once initiated, an Action Potential propagates almost unchanged along the axon as along a cable. The local spike depolarization spreads around its initiation site toward the surrounding region, which in turn reaches the threshold and generates an Action Potential further down along the axon. Due to the refractory period consecutive to the Action Potential, its propagation is always oriented away from its initiation site toward the other end of the axon. This propagation can be as fast as 100 m/s, allowing the very fast transfer of information required for the motor command of the hand or leg, for example. 1.2.2.3 Firing properties of a neuron encode the transmitted information. If the Action Potential is an all-or-none event, how can a message of distinct amplitude and duration be conveyed? The characteristics of an incoming stimulus on a neural is encoded in a volley of Action Potentials...
eBook - ePubClinical Neurophysiology: Basis and Technical Aspects
Handbook of Clinical Neurology Series
- (Author)
- 2019(Publication Date)
- Elsevier(Publisher)
...Section I Basic physiological and recording concepts Chapter 1 Generation and propagation of the Action Potential Manoj Raghavan; Dominic Fee; Paul E. Barkhaus * Department of Neurology, Medical College of Wisconsin, Milwaukee, WI, United States * Correspondence to: Paul E. Barkhaus, M.D., Professor of Neurology, Department of Neurology, Medical College of Wisconsin, 9200 W. Wisconsin Ave., Milwaukee, WI 53212, United States. email address: [email protected] Abstract The Action Potential is a regenerative electrical phenomenon observed on excitable cell membranes that allows the propagation of signals without attenuation. It is the cornerstone of neurophysiology. This chapter is a review of the Action Potential and its relationship to the signals that are studied in clinical neurophysiology. The first section traces the history of key scientific discoveries over the last 250 years that have led to our present-day understanding of the electrical properties of nerve and muscle. The second section considers the molecular and biophysical mechanisms that are responsible for the electrical potentials that can be measured across all eukaryotic cell membranes, but specifically in neurons, nerves, and muscle. Mechanisms underlying propagation Action Potentials within the nervous system are also examined...
eBook - ePubBehavioral Neuroscience
An Introduction
- Carl W. Cotman, James L McGaugh(Authors)
- 2014(Publication Date)
- Academic Press(Publisher)
...The resting potential in neurons is fundamentally the equilibrium potential for the K + ion gradient. During an Action Potential, the resting potential is broken down. A suprathreshold depolarizing stimulus signals the voltage sensitive Na + channels to open. Sodium ions enter the axonal interior and change the potential to +50 mV, which is the equilibrium potential for Na +. Sodium ion channels rapidly close; however, additional K + channels open and restore the membrane potential to its resting level. In the course of signaling, this process repeats many times. The neuron interior gains Na + ions and loses a few K + ions. The ion gradient is restored and maintained by a Na + –K + pump which uses cellular ATP to drive the ions against their gradients and maintain the ionic gradients across the neuronal membrane. While Action Potentials are the language neurons use to pass signals along their length, they use synaptic potentials to talk to other neurons or cells. In Chapter 5 we shall look at the nature of synaptic potentials. Key Terms Action Potential: A transient all-or-nothing change in the membrane potential which propagates along the axon like a wave. Action Potentials, unlike electrotonic potentials, do not decay because they are self-regenerative. Burst duration codes: Information is contained in a short-lived burst of Action Potentials. Capacitance of the membrane: A property of the membrane which allows charge to be stored and separated. It introduces a distortion in the time course of passively conducted signals. Without capacitance all changes in potentials would be instantaneous. Coulomb: A unit of electrical charge. When 1 coulomb of charge flows for 1 second the quantity of current is called an ampere. Current: The rate of flow of charge...
eBook - ePub- Alan Longstaff, Michael R. Ronczkowski(Authors)
- 2011(Publication Date)
- Taylor & Francis(Publisher)
...SECTION B – NEURON EXCITATION B1 Membrane potentials Key Notes Excitable cells Excitable cells are able to produce Action Potentials, brief reversals of the electrical potential across their plasma membrane. Excitable cells include neurons and muscle cells. Resting potentials The resting potential is the voltage across the plasma membrane of an unstimulated excitable cell. All membrane potentials are expressed as inside relative to outside. Resting potentials are inside negative, and range from about –60 mV to –90 mV in neurons. The resting potential is caused largely by the tendency for potassium ions to leak out of the cell, down their concentration gradient, so unmasking a tiny excess of negative charge on the inside of the cell membrane. Other ions (e.g., sodium) make a small contribution to the resting potential. Action Potentials An Action Potential (nerve impulse) is a short–lived reversal of the membrane potential triggered by a stimulus that causes the potential to fall (depolarization). In neurons the spike lasts less than 1 ms and peaks at about +30 mV. The after–hyperpolarization that follows lasts a few milliseconds. Action Potential properties Action Potentials are triggered at the axon hillock and propagated along the axon. They obey the all–or–none rule; a stimulus must be sufficiently large to depolarize a neuron beyond a threshold voltage before it will fire and all Action Potentials in a given cell are the same size. The latent period is the short delay between the onset of the stimulus and the Action Potential. Neurons become inexcitable to further stimulation during the spike and harder to excite during the after–hyperpolarization. These constitute the absolute and relative refractory periods respectively...
eBook - ePub- Gordon L. Fain, Margery J. Fain(Authors)
- 2015(Publication Date)
- Harvard University Press(Publisher)
...PART TWO Active Propagation of Neural Signals 5 Action Potentials: The Hodgkin-Huxley Experiments T HE PASSIVE SPREAD or decremental conduction of electrical signals down the axons and dendrites of nerve cells described in Chapter 2 is one way messages can be communicated from one part of the nervous system to another. Passive spread is essential for the conveyance of synaptic potentials within the dendritic tree, and some neurons (like the starburst amacrine cell of Plate Fig. 2.2) appear to use only decremental conduction to transmit electrical signals. These cells are probably exceptions. Many neurons in the CNS are too large to be able to rely only on passive spread. Pyramidal cells can have long axons that convey signals from the brain to the spinal cord. Some other mechanism must exist for the reliable transmission of electrical signals over such extended distances. This mechanism is active propagation mediated by Action Potentials, which are also called spikes. An Action Potential is a large, regenerative depolarization produced by voltage-gated channels. In an axon, spikes are initiated by a depolarization of the membrane—for example, from excitatory synaptic input. This depolarization facilitates a change in conformation of proteins called voltage-gated Na + channels, leading to the opening of a pore selective for Na + (Fig. 5.1A). As Na + channels open, the relative permeability of the membrane to Na + increases and the membrane depolarizes, because the membrane potential (ignoring Cl −) is given by where α = P Na /P K. When the external Na + concentration is larger than the internal Na + concentration, any increase in P Na will produce a positive change in membrane potential. Fig. 5.1 Action Potentials. (A) Most voltage-gated Na + channels are closed at the resting potential of a typical neuron, but depolarization— for example, by excitatory synaptic input—causes some of the channels to undergo a change in conformation...
eBook - ePub- Paul Johns(Author)
- 2014(Publication Date)
- Churchill Livingstone(Publisher)
...These have a common motif with six membrane-spanning alpha helices. Voltage-gated ion channels have an aqueous pore with a selectivity filter that only permits certain types of ion to pass. A voltage sensor enables the channel to open or close in response to changes in the membrane potential. The structures of the voltage-gated sodium and potassium channels are similar, but the sodium channel also has an inactivation gate. When this is closed, the channel becomes unresponsive until the membrane is repolarized. The Action Potential The electrical changes that occur during an Action Potential are illustrated in Figure 6.9. The membrane first depolarizes rapidly from its normal resting value of –70 mV with a slight overshoot to a positive value of around +30 mV. The normal membrane polarity is thus briefly reversed. The membrane very quickly repolarizes to its normal (negative) value and there is a slight undershoot, before eventually returning to baseline. The timescale is about 1–2 milliseconds. Fig. 6.9 The Action Potential. Properties of the Action Potential An Action Potential may be initiated in the neuronal cell body (where excitatory and inhibitory influences from other nerve cells have been integrated) or in sensory nerve endings in response to a sufficiently strong graded potential (triggered by mechanical, thermal or other forms of stimulation). Nerve impulse generation To trigger an Action Potential, a stimulus must be large enough to depolarize the neuronal membrane to a particular threshold value (typically –55 mV). Once this point has been reached a full Action Potential will occur. It is not possible for an Action Potential to vary in magnitude like a graded potential: a full Action Potential either occurs or does not occur. This is referred to as the ‘ all or none ’ law. Since each Action Potential depolarizes the adjacent membrane to threshold, the nerve impulse propagates along the full length of the axon like a row of falling dominoes (Fig...
eBook - ePub- Orjan G. Martinsen, Sverre Grimnes(Authors)
- 2014(Publication Date)
- Academic Press(Publisher)
...Chapter 5 Excitable Tissue and Bioelectric Signals Abstract This chapter focuses on bioelectricity, meaning the active electrical properties of biological tissue. Topics include basic cell physiology, Action Potentials, and neuron and axon transmission. Details are given on different receptors and how nerve signals are transferred. Keywords Action Potential; Axon; Cell; Channel; Neuron; Receptor In living tissue, important communication control is implemented by hormones and nerves. Hormones are slow broadcasting information carriers; nerves are quick prewired point-to-point information carriers. Some cells are not excitable, such as adipose, connective tissue, and blood. They are passive, not under nerve control, and only weakly polarized. However, nerve, muscle, and gland cells are polarized and excitable; within a 1/1000 s, such cells may react on trigger signals. The excitation of a cell is accompanied by an Action Potential. The Action Potential is the basic bioelectric event and signal source in the body. In section 4.1.4, the passive cell membrane was described. The membrane is a bilayer lipid membrane (BLM), as shown in Figure 4.6 and Figure 5.1. The membrane of a living cell is a most complex and dynamic system. The cell membrane itself is a major barrier to ion flux, but embedded in the membrane there are channels, transporters, and ion pumps. Electrically, they represent shunt pathways in parallel with the BLM, as shown in Figures 5.1 and 5.3. The water channels are a special class of channels, selectively allowing water flux (but without electric charge: no proton or ion flux) in response to osmotic gradients and therefore regulating cell volume swelling or shrinking. Figure 5.1 Bilayer lipid membrane (BLM) with embedded proteins. A channel macroprotein may form a water channel, an ion channel, or an adenosine triphosphate (ATP)–driven ion pump. Figure 5.2 The polarized excitable cell. 5.1...
eBook - ePubA History of the Brain
From Stone Age surgery to modern neuroscience
- Andrew P. Wickens(Author)
- 2014(Publication Date)
- Psychology Press(Publisher)
...However, its atomic structure means it is likely to gain an electron making it more negative (Cl–). As physiologists began to understand this type of pharmacology, they realised that different concentrations of ions inside and outside the nerve cell could be the key to explaining its electrical characteristics. Unfortunately, this was not easy to examine. Although some cells in the body, such as muscle, were known to have different levels of potassium and sodium ions in their interior and extracellular compartments, resulting in voltage differences, nerve cells were far too tiny for their ionic chemistry to be measured in this direct way. Despite these difficulties, in 1902, Bernstein attempted to explain how the movement of ions could explain the formation of an Action Potential. To begin, he postulated that the negative voltage inside the nerve cell was due to a greater amount of negative ions compared to its outside. Consequently, the nerve was in a state of disequilibrium. However, under normal circumstances, the membrane of the nerve cell acted as a barrier to stop ion flow, thereby maintaining this difference. Viewed in this way, the nerve cell was like a battery, storing both positive and negative charges, before its terminals were connected. Bernstein’s great insight, however, was to realise the nerve cell’s membrane might become permeable to ions. That is, if there was a temporary breakdown in the membrane’s resistance, then positive ions would move into the cell, causing its negatively charged interior to become neutral (zero). 4 If this happened, the sudden change in voltage would appear as a wave of relative negativity on its surface. This theory also neatly explained why the nervous impulse was much slower than an electrical current flowing through a cable. If Bernstein was correct, then the Action Potential moved in small jumps as the membrane temporarily became more permeable to ions along its axon...
