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Chapter 16 Electrophysiology
Published in B H Brown, R H Smallwood, D C Barber, P V Lawford, D R Hose, Medical Physics and Biomedical Engineering, 2017
Nerves can conduct impulses in either the normal (orthodromic) or the opposite (antidromic) direction. The impulses are typically 1 ms in duration and the frequency of impulses in a single fibre may vary from zero up to about 200 pps. Neurones are not all the same size, but a typical cell body has a diameter of 100 μm and the axon may be up to 1 m long with a diameter of 15 μm. A large nerve trunk will contain many nerve fibres which are axons. The ulnar nerve runs down the arm and is very superficial at the elbow where it may be knocked and cause a characteristic feeling of pins and needles; this nerve trunk looks rather like a thick piece of string and contains about 20 000 fibres in an overall diameter of a few millimetres. The optic nerve within the head contains even more fibres and the brain which it supplies is estimated to contain approximately 109 neurones.
Deep brain stimulation programming strategies: segmented leads, independent current sources, and future technology
Published in Expert Review of Medical Devices, 2021
Bhavana Patel, Shannon Chiu, Joshua K. Wong, Addie Patterson, Wissam Deeb, Matthew Burns, Pamela Zeilman, Aparna Wagle-Shukla, Leonardo Almeida, Michael S. Okun, Adolfo Ramirez-Zamora
DBS involves applying an electrical field directly into a brain region and modulating connected circuitry. DBS may lead to the opening and closing of voltage-gated sodium channels and subsequent propagation of orthodromic or antidromic action potentials, ultimately affecting the release of neurotransmitters [19,20]. The clinical benefits of DBS are well known, however the time course ranges from seconds to minutes (i.e., tremor suppression) to weeks to months (i.e., improvement in dystonia, tics, obsessive-compulsive disorder). This variability in time course suggests multiple biological changes and potential mechanisms of action. It may also infer that we may not have achieved the most efficient form of stimulation for specific disorders or symptoms [20–22]. Since its introduction, the exact mechanism of action of DBS has been elusive, although several hypotheses have been proposed. We will summarize some of the most common hypotheses.
A novel fast-acting sub-perception spinal cord stimulation therapy enables rapid onset of analgesia in patients with chronic pain
Published in Expert Review of Medical Devices, 2021
Clark S. Metzger, M. Blake Hammond, Jose F. Paz-Solis, William J. Newton, Simon J. Thomson, Yu Pei, Roshini Jain, Michael Moffitt, Luca Annecchino, Que Doan
Paresthesia-based spinal cord stimulation (SCS) has been used for decades to treat chronic pain. Historically, antidromic activation of the nerve fibers in the dorsal columns has been thought by some to be capable of ‘closing the gate’ resulting in analgesia, while orthodromic activation elicits paresthesias [1]. The relationship between the induction of paresthesia and analgesia was therefore assumed to be linked given that the location of pain relief has been demonstrated to correlate with paresthesia overlap [2]. However, nearly 40 years after the inception of SCS, Yearwood, and Foster described SCS cases wherein analgesia was achieved at amplitudes below the paresthesia threshold, and current SCS approaches now exist that do not produce paresthesia, variously described as ‘paresthesia-free’, ‘paresthesia-independent’, or ‘sub-perception’ SCS [3–6]. Intriguingly, sub-perception SCS yields a slower onset of analgesia after turning on stimulation (typically several hours to days) in contrast to paresthesia-based SCS in which analgesia is usually observed within minutes [7–10]. This has contributed to various hypotheses regarding potential mechanisms of action that mediate pain relief produced by sub-perception-based approaches [10–12].
Burst and high frequency stimulation: underlying mechanism of action
Published in Expert Review of Medical Devices, 2018
Shaheen Ahmed, Thomas Yearwood, Dirk De Ridder, Sven Vanneste
SCS is being used to treat neuropathic pain, failed back surgery syndrome (FBSS), complex regional pain syndrome (CRPS), angina pectoris, and ischemic limb pain [10–12]. SCS is advantageous in part because it is minimally invasive, making it a safer and more cost-effective technique than surgical methods. Furthermore, SCS can achieve targeted pain relief and even reduce opioid use, all with little to no side effects [13]. Traditionally, SCS therapy is delivered via tonic stimulation, usually with a frequency between 40 and 50 Hz, an amplitude between 2 and 4 mA, and a pulse width that falls between 300 and 500 µs. The mechanism of action of SCS can be understood through both spinal and supraspinal mechanisms [14,15]. Electrical stimulation produces both orthodromic and antidromic action potentials. The action potential travels antidromically into the dorsal horn, where Aβ fibers synapse with the wide-dynamic-range neurons and release inhibitory neurotransmitters such as γ-amino butyric acid (GABA) and adenosine. The orthodromic potentials travel to the dorsal column, inducing inhibition via serotonergic and noradrenergic pathways [16,17].