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Generation of the Action Potential
Published in Nassir H. Sabah, Neuromuscular Fundamentals, 2020
If the duration of the current pulse is TD, a certain minimum amplitude of the current pulse, imp, is required to generate an AP. It is found in practice that if the amplitude of the pulse is reduced, a longer duration of the pulse is needed to bring the membrane to threshold and generate an AP. A plot of imp vs. TD is the strength-duration relationship and has the general shape shown in Figure 3.13. The horizontal asymptote is the rheobase and represents the smallest amplitude of a current pulse of very long (theoretically infinite) duration that will generate an AP. The chronaxie is the value of TD for a current pulse whose amplitude is twice the rheobase.
Miscellaneous
Published in Bobby Krishnachetty, Abdul Syed, Harriet Scott, Applied Anatomy for the FRCA, 2020
Bobby Krishnachetty, Abdul Syed, Harriet Scott
Chronaxie is the duration a current must be applied to the nerve to initiate an impulse when the current level is twice the rheobase. The term is often used to describe excitability of different tissues and nerves and below is a table of the chronaxie time for different nerves (Table 7.2).
High frequency subthalamic nucleus stimulation
Published in Hans O Lüders, Deep Brain Stimulation and Epilepsy, 2020
Erwin B Montgomery, John T Gale, Kenneth B Baker
These findings stand in contrast to those of Dostrovsky et al.7 who, using pairs of micro-electrodes to simultaneously stimulate and record in the globus pallidus internal segment, reported a reduction in neuronal activity. Reconciliation may lie in the fact that different components of the neuron require independent consideration. DBS effects on the neuronal cell bodies may be dissociated from effects on the axon hillocks (initial segments) or first internodes between myelin segments. Computer modeling of thalamic neurons based on membrane properties, neuronal geometries, and conductance channels demonstrated that stimulation can result in hyper-polarization of the cell bodies and dendrites (the primary source of extracellular action potentials recorded with microelectrodes) yet still generate an action potential in the first intermyelin segment node.8 Ashby et al. provided evidence that excitation of axons by subthalamic DBS mediates DBS effects on electromyographic (EMG) activity,9 while Baker et al. demonstrated the axonal mediation of subthalamic DBS-evoked scalp poten-tials .10 Chronaxie experiments on therapeutic efficacy also are consistent with axonal mechanisms.11 It is possible, indeed probable, that neuronal activity recorded within the stimulated target may be reduced while efferent axonal activity may be increased.
Neuromuscular disorders in women and men with spinal cord injury are associated with changes in muscle and tendon architecture
Published in The Journal of Spinal Cord Medicine, 2023
Larissa Santana, Emerson Fachin-Martins, David Lobato Borges, Jonathan Galvão Tenório Cavalcante, Nicolas Babault, Frederico Ribeiro Neto, João Luiz Quagliotti Durigan, Rita de Cássia Marqueti
A universal pulse generator (Dualpex 071, Quark Medical LTDA) was used to assess the five target muscles through a reference electrode (anode) with an area of 100 cm2 placed on the ankle. An active pen electrode (cathode) with an area of 1 cm2 was used to find the motor-points.38 The rheobase was defined as the minimal current intensity necessary to reach the neuromuscular excitability threshold, applied with a rectangular pulse with an infinite duration, varying from 0 up to 69 mA, with increments of 1 mA until the point at which a slight but visible muscle contraction appeared. During the assessments, the stimulation protocol consisted of a rectangular pulse width of 1s and a rest interval of 2s.21 The chronaxie was represented by the shortest pulse duration required to reach the neuromuscular excitability threshold by a current with twice the intensity of the rheobase; values higher than 1000 µs indicated NED.21,24 The pulse width increased from 20 µs to 1s, using 100 µs increments until 1000 µs, followed by 1000 µs increments from this point.21 An exponential pulse monopolar current with a pulse width of 1000 µs was applied to define the accommodation values, increasing the electrical current from 0 to 69 mA with increments of one mA until the visible muscle contraction. The accommodation index (AI) was calculated as the ratio between the accommodation and rheobase (AI = accommodation/rheobase).21,25
Intra and inter-raters reliability and agreement of stimulus electrodiagnostic tests with two different electrodes in sedated critically-ill patients
Published in Physiotherapy Theory and Practice, 2020
Amaro Eduardo Tavares de Araujo, Paulo Eugênio Silva, Karina Livinode de Carvalho, Emerson Fachin-Martins, Nicolas Babault, João Luiz Quagliotti Durigan
Some NMES studies in the ICU reported the difficulty of evoking adequate contractions during the course of a critical illness (Rodriguez et al., 2011; Segers et al., 2014). The way in which neuromuscular excitability is determined (particularly chronaxie values) can be one of the strategies for solving this problem (Silva et al., 2017). Chronaxie values acquired through the SET are fundamental to the parameter/dose to be used in NMES treatment (Russo et al., 2007; Silva et al., 2017). Since NMES is performed with electrodes having areas ≥ 25 cm2, it is plausible to evaluate the chronaxie using this size of electrode. The current study demonstrated that the SET performed with electrodes of different areas did not influence the chronaxie results. The use of square cathode electrode positioned in the same area for NMES can reduce the time spent on these procedures. Therefore, the cathode square electrodes can be used and could improve the feasibility of NMES based on chronaxie (Silva et al., 2017).
Deep brain stimulation: new programming algorithms and teleprogramming
Published in Expert Review of Neurotherapeutics, 2023
Renato Puppi Munhoz, Ghadh Albuainain
Amplitude and PW are the waveform components related to current–distance relationship [27]. More specifically, the activation threshold of neural elements with unequal membrane excitability profiles varies in the sense that amplitude and PW have a nonlinear inverse correlation as the current required to activate neural elements decreases when PW increases, according to a curve known as chronaxie relationship [27,28]. Chronaxie is a measure of neural elements excitability defined as the pulse duration equivalent to double the minimal current amount necessary to excite a neural element, termed rheobase current [28]. In other words, chronaxie is defined as the pulse duration equivalent to the double rheobase current on the strength–duration curve. From a practical standpoint, it is well documented that larger, more densely myelinated axons (i.e. internal capsule) have lower chronaxies than neuron cell bodies and small-diameter axons (i.e. the surgical targets) near the electrode [29]. As such, although longer PW may have a role for the exploration of the effect of stimulation of relatively larger nuclei such as the GPi, shorter PW can widen therapeutic windows by more selectively stimulation of elements of interest rather than more distant, larger fibers such as the corticospinal and corticobulbar fibers passing through the internal capsule [27–30]. These experimental findings mirror clinical studies: Steigerwald et al. [30], for example, compared the effect of subthalamic nucleus (STN) DBS using 30 µs stimulation PW versus standard 60 µs, concluding that both parameters are equally effective on PD motor signs; however, shorter PW is more energy efficient, has a wider therapeutic window, and is less likely to present stimulation-related side effects. Previously, the most widely used IPG models limited the applicability of these advantages as the lowest range of PW permitted by the systems was 60 µs. Current new-generation IPGs, however, allow the use of PW as low as 10 or 20 µs depending on the manufacturer. Although this may seemingly look like a simple and basic innovation, in practice it is a valuable feature when a narrower and more selective electric field needs to be used in order to avoid adverse events while prioritizing therapeutic effects [8].