Understanding Anodal and Cathodal Stimulation

Whether you practice neurophysiology in surgery, in the lab, or in the clinic, you probably use electrical stimulation to activate the nervous system on a daily basis. As you probably know, cathodal stimulation works best in some applications, while anodal stimulation works best in other applications.

Armed with this knowledge, you know precisely where to place electrodes on the body, and where to plug those electrodes in – black in cathode (-) and red in anode (+). But, what’s the difference? What exactly is anodal or cathodal stimulation, and why does one work better than the other in some applications?

Today I hope to answer some of those questions for you because I believe that understanding stimulus polarity is important, and it will make you a better neurophysiologist.

Before we talk about how stimulators work, it is important to have a basic understanding of how a battery works.


How a Battery Works

The correct term for what we frequently refer to as a “battery”, is a “cell”, but I’m going to use the word battery to keep it simple. So, a battery is a charge-separating device.  It stores electric energy by separating cations and anions into two separate compartments, or terminals (Figure 1).

  • Cations are positively-charged ions (+).
  • Anions are negatively-charged ions (-).
Figure 1:

Figure 1: In a battery, the flow of electrons goes from anode to cathode.

If you refer to the illustration in Figure 1, you will see that one terminal of the battery contains an excess of cations (+), and this is the positive terminal (+). Because it contains cations (+), the positive (+) terminal of the battery is called the cathode (+). The other terminal of the battery contains an excess of anions (-), and this is the negative terminal (-). Because it contains anions (-), the negative (-) terminal of the battery is called the anode (-).

When the battery is connected to a load, in this case a lightbulb, the device is powered by the flow of current. Conventional Current assumes that current flows out of the positive terminal, through the circuit and into the negative terminal. This was the convention chosen during the discovery of electricity, but they were wrong! Rather, Electrical Current is what actually happens, as electrons (-) flow out of the negative terminal (anode), through the circuit and into the positive terminal (cathode). 

The take-home message is that, in a battery, current flows from anode to cathode. To learn more about batteries, go here.

How an Electrical Stimulator Works

In an electrical stimulator, the flow of anions (-) and cations (+) is controlled by the mechanics of the circuitry within the stimulator.  The stimulator is unique in that the cathode is the negative pole (-) because it discharges anions (-), and the anode is the positive pole (+) because it discharges cations (+). At the end of the day, that’s the fundamental difference between a battery and a stimulator.

Depending on how we configure the polarity, the stimulator will discharge either cations or anions into the body part being stimulated.

In cathodal stimulation, anions (-) are discharged into the body as current flows from the cathode (-), through the tissue, and back to the anode (+).

In anodal stimulation, cations (+) are discharged into the body as current flows from the anode (+), through the tissue, and back to the cathode (-).

Now, let’s imagine that we place an electrical stimulator on the surface of the skin with a nerve bundle running underneath (Figure 2). Within the nerve bundle is a single nerve fibre (axon) upon which we will focus.

Nerve_Rest

Figure 2: A stimulator has been placed on the skin with the anode and cathode positioned over a nerve bundle. Within the nerve “bundle” we see a single nerve axon. Because of the relative concentrations of anions (-) and cations (+) in the intracellular and extracellular spaces, when the nerve is at rest, the inside of the cell is electrically negative compared to the outside of the cell.

At rest, the inside of a cell is more negative than the outside of a cell. This occurs because there is a slightly greater number of negative charges than positive charges inside of the cell (intracellular space), and a slightly greater number of positive charges than negative charge outside of the cell (extracellular space). Because of the electrical difference, the cell is said to be polarized – just like a magnet, one side is more positive and the other side is more negative. If the electrical gradient were suddenly reversed, the cell would be depolarized, and we might see an action potential.

Cathodal Stimulation of Peripheral Nerves

When we use the term cathodal stimulation, what we mean is that negatively-charged anions (-) flow from the cathode, into the tissue, and back to the anode (Figure 3). As the electrical current flows from cathode to anode, negative charges (anions) tend to accumulate on the outer surface of the nerve membrane as they will be repelled by the negatively-charged cathode. This makes the outside of the membrane more negative. Consequently the inside of the membrane becomes more positive due to accumulation of positive ions on the inside.This will result in depolarization, which, if sufficient in magnitude, will result in an action potential (nerve impulse or muscle activation).

 

Cathodal_Stimulation

Figure 3: Cathodal Stimulation.

Figure 3 illustrates activation of the axon under the cathode. As a result of stimulation, an action potential is sent in both directions along the length of the nerve, starting at the cathode. Something interesting happens underneath the anode, though! All of the negative charge from the extracellular space is attracted to the anode, leaving the outside of the cell excessively electrically positive relative to the inside of the cell. The cell is thus hyperpolarized under the anode, meaning that it is very, very difficult to activate.

If you apply the information above to the median nerve SSEP (Figure 4), then you can see why the anode is always distal, and the cathode is always proximal.

For median nerve stimulation: cathode between the tendons of the palmaris longus and the flexor carpi radialis muscles, 2 cm proximal to the wrist crease. The anode should be placed 2-3 cm distal to the cathode or on the dorsal surface of the wrist.

Figure 4: For median nerve stimulation, the cathode is placed between the tendons of the palmaris longus and the flexor carpi radialis muscles, 2 cm proximal to the wrist crease. The anode is placed 2-3 cm distal to the cathode, or on the dorsal surface of the wrist.

What happens when you accidentally reverse your stimulating electrodes when performing an SSEP test? The difficulty that you experience in attempting to acquire an SSEP is explained by the phenomenon of anodal blocking (Figure 3). Thus, when bipolar electrodes have tips in the same orientation as a fiber, a fiber will be depolarized under the cathode, and hyperpolarized under the anode. If the hyperpolarization is large enough, an action potential initiated under the cathode may not be able to propagate through the region of hyperpolarization. If this is the case, the action potential will propagate in only one direction.

Anodal Stimulation of Peripheral Nerves

When we use the term anodal stimulation, what we mean is that cations (+) flow from the anode, into the tissue, and back to the cathode (Figure 5). When applied to the surface of a nerve, anodal current will increase the concentration of cations (+) in the extracellular space under the anode. This will result in hyperpolarizationwhich, as I just mentioned, puts the cell in a heightened state of rest. So, what we see in Figure 5 is that the nerve axon becomes deactivated (hyperpolarized) under the anode.

Figure 5: Anodal Stimulation

Figure 5: Anodal Stimulation

The Importance of Cell Orientation

In all of the examples described thus far, the orientation of the cell under the stimulator has been horizontal with respect to the orientation of the anode and cathode (Figures 2-5). This is usually the case when stimulating nerves in the arms and legs.

What happens when the orientation of the cell is vertical with respect to the orientation of the anode and cathode? The answer is that things usually work exactly opposite to what we just discussed regarding horizontally-oriented cells.

This becomes particularly important in the brain where pyramidal cells of the cerebral cortex are vertically-oriented with respect to the surface where we stimulate.

Figure 6: Anodal stimulation activates a pyramidal cell, which has a vertical orientation with respect to the surface of the brain. Figure from Ranck (1975).

Figure 6: Anodal stimulation activates a pyramidal cell, which has a vertical orientation with respect to the surface of the brain. Figure from Ranck (1975).

Anodal Stimulation of Cerebral Cortex

Electrical stimulation of cerebral cortex is used for lots of reasons, but today I’m going to focus on motor evoked potentials (MEPs). If you use electricity (as opposed to a magnet) to evoke MEPs in your clinical practice, hopefully you know the following principle:

Whether you are stimulating the scalp over motor cortex, or directly stimulating the cortical surface, MEPs are always easiest to elicit and characterize when you use anodal, monopolar, pulse-train stimulation. Things change a little with subcortical stimulation, but that’s a topic for a different day.

Starting with Fritsch and Hitzig (1870), many researchers have shown that monopolar stimulation of the motor cortex is more effective with an anode, as opposed to a cathode. Also, monopolar anodal stimulation seems to activate pyramidal cells directly.

One proposed mechanism is that anodal current enters (and hyperpolarizes) dendrites at the surface of the brain, then leaves and depolarizes the axon or cell body. One way to think about this illustrated in Figure 7.

Figure 7. Mechanism of pyramidal cell activation following anodal stimulation of cerebral cortex.

Figure 7. Mechanism of pyramidal cell activation following anodal stimulation of cerebral cortex. Image adapted from work by Nair et al (2008), and Stephani & Luders (2011).

Anodal stimulation is just the injection of positively-charged ions under the electrode. Because opposites attract, negatively charged ions migrate to the the very surface of cortex under the anode. You can think of this a current sink and the consequence is hyperpolarization of the apical dendrites of the pyramidal cell. In order to compensate for this current sink, a current source is generated distally such that positively-charged ions congregate around the other end of the pyramidal cell. This results in depolarization (activation) of the cell body, the axon hillock and the initial segment of the axon, which forms the corticospinal tract.

Of course it isn’t that simple! Computational simulations paint a more complex picture. As Figure 8 illustrates, the neural response to stimulation is likely a complex pattern of depolarization and hyperpolarization throughout the neural geometry of the cell, which is dependent upon stimulation parameters and the neural positions relative to the electrode. Clearly, when the long axis of the cell is oriented vertically relative to the orientation of an anodal stimulation electrode, the computation simulation supports hyperpolarization of the apical dendrites and depolarization around the axon hillock.

Figure 8:

Figure 8: Computer simulation. Extracellular stimulation of a 3D reconstructed layer V pyramidal neuron A: Anodal stimulation. B: Cathodal stimulation. From Nair et al (2008).

It all comes down to the orientation of the cell!

Think about this… when you place your monopolar stimulating electrode over the motor cortex and deliver anodal stimulation, your lowest threshold CMAPs are from the vertically-oriented cells just below your electrode. If you do transcranial MEPs, your electrode is probably C3 or C4, right? And these electrode are just over the hand representation of the motor homunculus. You really have to jack up your intensity to get MEPs from the legs, right? This is because those “leg” cells are deep in the interhemispheric fissure and the cells are oriented horizontal to your anodal stimulating electrode. BUT, if you switch your polarity and deliver cathodal stimulation from the same electrode, MEPs from the legs are suddenly easy to elicit and hands become more challenging.  It all comes down to the orientation of the cell!

Thanks for reading. Hope you found this useful. As always, please feel free to leave feedback.

References

  • Fritsch GT, Hitzig E. 1870. Über die elektrische Erregbarkeit des Grosshirns. Arch Anat Physiol Med Wiss 300–32. Translation in Von Bonin G. 1960. Some papers on the cerebral cortex. Springfield (IL): Charles C Thomas.
  • Merrill DR, Bikson M, Jefferys JGR. Electrical stimulation of excitable tissue: Design of efficacious and safe protocols. J Neurosci Methods. 2005 Feb 15; 141(2):171-198.
  • Nair DR, Burgess R, McIntyre CC, Lüders H. Chronic subdural electrodes in the management of epilepsy. Clin Neurophysiol. 2008 Jan;119(1):11-28. Epub 2007 Nov 26. Review.
  • Ranck JB Jr. Which elements are excited in electrical stimulation of mammalian central nervous system: a review. Brain Res. 1975 Nov 21;98(3):417-40. Review.
  • Stephani C, Luders HO. Electrical Stimulation of Invasive Electrodes in Extratemporal Lobe Epilepsy. In: Koubeissi MZ, Maciunas RJ, eds. Extratemporal Lobe Epilepsy Surgery. Montrouge, France: John Libby Eurotext; 2011. 261-313. Print.

39 thoughts on “Understanding Anodal and Cathodal Stimulation

  • Reply Marat October 23, 2015 at 16:11

    Great material. I enjoyed reading it.
    THANK YOU

  • Reply Josh Lawson October 25, 2015 at 08:12

    I was just looking for a clear and concise explanation – perfect, thanks!

  • Reply Ahmed Imran October 25, 2015 at 09:29

    Really worth it…… Good job

  • Reply Ehsan October 25, 2015 at 16:04

    Great information. It was of big help for me.

  • Reply Sergio October 25, 2015 at 17:49

    Very clear and useful as always Richard, thanks.

  • Reply Karen Persely October 26, 2015 at 11:45

    Excellent! A simplified approach to understanding a complex, but fundamental concept, that constitutes essential knowledge. Thank you!

  • Reply Kristina Port October 26, 2015 at 18:48

    Richard, informative as usual. Thanks for sharing?

  • Reply Diane Bouchard October 27, 2015 at 06:05

    hi,

    if i understand well and you are monitoring a thoracic spinal fusion and want to maximise legs MEP’s you should stimulate cathodal?

    • Reply Richard Vogel October 27, 2015 at 15:54

      Hi Diane,
      It’s an interesting question that I’ve been thinking about recently. I wouldn’t quite recommend using this method yet, as I’m still testing it, but there may be some validity to it.

  • Reply Susan Morris, PhD October 27, 2015 at 07:50

    A very clear and thorough description; thanks for sharing it!

  • Reply Bettina Muench October 30, 2015 at 13:58

    Thank you for this article! This is a great explanation. What I had heard before when asking why we use anodal stimulation in some cases was only “because it works better” but not why.

    • Reply Richard Vogel November 1, 2015 at 08:21

      Glad you liked the post! I’m sorry to hear that you got the generic, “because it works better” response. Usually people say, “because that’s the way we always do it.” Neither response is acceptable.

  • Reply Liz Huber November 20, 2015 at 12:35

    Thank you thank you thank you. Well written and I appreciate your thoughtfulness for the IONM field in general.

  • Reply Happy Holidays from Neurologic Labs!NeurologicLabs December 2, 2015 at 11:44

    […] column mapping, spinal cord stimulators, speech & language mapping, and the difference between anodal & cathodal stimulation. All of these posts were extremely […]

  • Reply Lola February 18, 2016 at 17:04

    thank youuu ^^ i had hard time and was confused to understand these stimulations, your post was really helpful and easy to understand :))

  • Reply Mary Harvey April 24, 2016 at 08:57

    Thanks for inspiring an “aha moment” today, EXCELLENT.

    • Reply Richard Vogel April 25, 2016 at 08:51

      Thanks for the comment, and thanks for reading!!

  • Reply Suradip Das September 23, 2016 at 13:35

    Thanks for the detailed explanation of the concepts. I request your suggestions on a few points –
    1. I am working on recording CMAP from deep peroneal nerve. From your article I understand that during cathodic stimulation current flows from cathode to anode, however action potential flows from the cathode away from the anode. Since ours is a motor study should we place the cathode distal and the anode proximal (reverse of what you suggested for SSEP in median nerve) ?
    2. I would be using surface patch/disc electrodes to stimulate. How do I know which is the active stimulating electrode…both look the same! How do I ensure that its cathodic stimulation only ?

    3. We have been reversing the polarities of the stimulating electrodes to differentiate between stimulus artifact and actual signal. Our premise is that an artifact will flip with reversed polarity of stimulation electrodes whereas the actual signal will remain same. Do you think it is accurate ?
    Eagerly look forward to your thoughts.

    • Reply Richard Vogel October 10, 2016 at 11:06

      Hi, Here are the answers to your questions by number:
      1. Yes.
      2. It just depends on your stimulator setting, and into which port you plug your electrodes.
      3. Yes, many people do this, but it doesn’t always work. You may have to try other methods to reduce your stimulation artifact.

      Examples:
      Move your stimulating electrodes closer together.
      Move your recording electrodes closer together.
      Place a reference/ground between where you stimulate and where you record.
      Adjust filter settings. Usually, increasing the high pass filter works.

      Good luck.

  • Reply Jo October 27, 2016 at 18:19

    Thank you for the post! Given the fact that I am a Uni student and this was primarily aimed at neurologists, it was still exceptionally clear, easy to follow and understand. You’ve done a good job explaining what my friends and I found very confusing and difficult. Thanks again. Having discovered this, I will definitely have a look round your website.
    P.s. such a clean cut web design too!

    • Reply Richard Vogel November 3, 2016 at 00:06

      Thanks for your comments and feedback!! Hope you keep reading!

  • Reply rebecca clark-bash November 15, 2016 at 11:14

    Hi Rich, would you be open to me including this paper in the board prep material for students? I would do a copy and paste if it is acceptable. I understand if this isnt something you want given away in this manner. Crossing my fingers!

    • Reply Richard Vogel November 15, 2016 at 14:03

      Hi Rebecca, this website is free for anyone to read. The only restriction that I hold on reproduction is maintaining credit for what I’ve written, and for the images that I’ve created. You can feel free to use this material as long as my name is credited for original authorship. Also, keep in mind that Figures 4, 6 and 8 are reproduced for the works cited in text, and may have additional copyright restrictions if disseminated.

  • Reply Zaid Qaddoumi November 19, 2016 at 09:21

    One of the most interesting and helpful articles about the Cathodal and Anodal stimulation! Very clear and Simple 🙂

  • Reply Shir Shalom December 10, 2016 at 09:13

    OMG thank you so much for this gift of missing knowledge!

  • Reply Dr Gehan Abouelseoud March 2, 2017 at 02:31

    I would like to cite the information in the article in my paper. Do you have a formal publication containing the same information that I can cite.

    • Reply Richard Vogel March 2, 2017 at 21:33

      Some of the information cited in this article can be found in the “References” section at the bottom. You can also cite this webpage directly using MLA style as follows: Vogel, RW. “Understanding Anodal and Cathodal Stimulation.” Neurologiclabs.com. Published: 2/23/2015. Web. Accessed: 3/2/2017.

  • Reply JS March 15, 2017 at 02:52

    Just THANK YOU. As a dentistry student in Denmark this really made my day (week, month, semester!)

  • Reply Gabriele Fusco March 22, 2017 at 11:50

    Thank you for the post, very useful.
    If I correctly understood, for the brain modulation, anodal stimulation delivers constant positive-discharged current from the anode to the cathode depolarizing the resting membrane potential and increasing excitability in the target region located under the active electrode. Conversely, cathodal stimulation delivers constant negative-discharged current from the cathode to the anode causing hyperpolarization of the membrane potential, inhibiting the spontaneous neural firing rate….Is it correct?

    • Reply Richard Vogel May 18, 2017 at 10:36

      I think’s that’s a good summary. I don’t know about the “spontaneous” firing rate. That would have to be tested with long-term stimulation. With a discrete stimulus, one can certainly make the cells more likely or less likely to fire.

  • Reply Cameron Pfeffer May 5, 2017 at 01:45

    Nicely written first and foremost.

    A have been trying to understand cardiac cell cathodal and anodal stimulation better and perhaps it follows the same principles as the the nerve that you’ve eloquently described.

    A have a couple of questions that I hope you can answer:

    1. After the -‘ve anions have accumulated in the extracellular space during a cathodal stimulus, is the +’ve build up intracellularly due to +’ve ions flowing across the cell membrane (ie. out to in) before potentially reaching membrane potential? (Voltage gated ion channel allow +’ve to flow in the mechanism?)

    2. You explained separately cathodal and anodal stimulation and the respective flow of ions. In a cathodal (-‘ve flow) cardiac stimulus, we usually desire cathodal depolarisation, however we sometimes also get anodal depolarisation simultaneously. Could you describe what is happening intra/extra cellular around the hyperpolarised cells that cause this phenomenon (to re-iterate, I’m not talking about an anodal stimulus, whereby +’ve ions are used – this is still cathodal stimulus). Under the anode, you’ve explained that extracellularly there is an excessive of +ve ions but as these are outside the cell, it can’t cause depolarisation from beneath anode. My take on it is that adjacent cardiac cells must somehow be attracted to the hyperpolarised cells and become more +’ve charged inside to produce ‘anodal depolarisation’ but I can’t piece together where the +’ve ions would have come from for these adjacent cells.

    3. Is there a directional bias/preference for the ions during a stimulus towards the anode. Ie. do the ions uniformly depolarise beneath an electrode or are the ions biased to the side of the cathode electrode in the direction of the anode?

    Thanks so much

    • Reply Richard Vogel May 18, 2017 at 10:32

      Hi Cameron, excellent questions. I’ll do my best to answer. Regarding question #1: The mechanism of ion accumulation doesn’t have to be the result of ion channel activation (but it can be the result and/or the cause). Let’s just think about this in a single cell in a closed system. First, just consider the fact that the cell membrane is impermeable to ion flow. If, on the outside of the cell there is suddenly a very large accumulation of -‘ve, then there will be an equally large accumulation of +’ve on the other side of the membrane (in accordance with Newton’s Third Law). This can be large enough to depolarize/activate the cell and, in the case of a neuron, result in the generation of an action potential. I image the same may be true for muscle cells, but I’d have to do some research. Regarding question #2: I’d probably also have to do some research on this. Your explanation seems plausible, but I just don’t know the answer from my experience. Regarding question #3: Interesting question. To my knowledge, the would not be a bias because this seems to contradict Newton’s Third Law. I guess bias could potentially be introduced by the mechanisms by which a particular type of stimulator works, but I just don’t know. I think I’ll contact on my colleagues who is both a neurophysiologist and engineer and solicit his input. Stay tuned. Thanks for the great questions and sorry for the delay in responding! Rich

  • Reply François Kroll May 20, 2017 at 11:03

    Super useful, thanks!!

  • Reply Nicolas June 23, 2017 at 14:07

    Hi, in intracortical microstimulation (ICMS) protocols, the current is actually cathodal. This is because the stimulation is most often delivered at the depth of cortical layer V. This technique is widely used in fundamental research, especially in motor mapping.

    Great article by the way!

    • Reply Richard Vogel June 23, 2017 at 17:57

      Thanks for commenting. Can you send me some sample references? I’m always looking for something new to read. As a field, in the clinical realm, we’ve always found anodal to work for for cortical mapping and cathodal for subcortical white matter mapping. Very interested in the idea of intracortical stimulation!!

      • Reply Nicolas June 27, 2017 at 18:26

        Hi Richard,

        The original article explaining the method :
        Excitation of pyramidal tract cells by intracortical microstimulation: effective extent of stimulating current. Stoney et al. (1968 !). Journal of Neurophysiology 31-5: 659-669 https://www.ncbi.nlm.nih.gov/pubmed/5711137

        A typical example of the seminal microstimulation studies in the field of motor control. There are plenty like this one: Multiple Representations of Body Movements in Mesial Area 6 and the Adjacent Cingulate Cortex: An Intracortical Microstimulation Study in the Macaque Monkey. Luppino et al (1991). The Journal of Comparative Neurology 311:463-482 https://www.ncbi.nlm.nih.gov/pubmed/1757598

        A more recent article with a slightly different paradigm (paired-pulse protocol), but still seeking motor responses from cathodal stimulation:
        Interactions between rostral and caudal cortical motor areas in the rat. Deffeyes et al. (2015). Journal of Neurophysiology 113-10: 3893-3904 https://www.ncbi.nlm.nih.gov/pubmed/25855697

        These are all studies aimed at mapping motor-related areas, but ICMS is also used in vision research. Nowadays, since the maps of the major motor areas and their smallest subdivisions have already been detailed at lengths in several species, the technique is mostly used to quickly and precisely identify the areas in order to perform lesions, neuronal tracing studies, pharmacological or optogenetic protocols, or to implant arrays for chronic recordings of neuronal activity. The biggest advantage of ICMS over surface stimulation is its greater resolution. Obviously, the insertion of an electrode inside the cortex brings its lot of risks. The electrodes normally used, however, are much smaller than those used in deep brain stimulation, and their higher impedance luckily reduce the amount of current required to excite adjacent cells. I do not know how useful ICMS would be in a clinical setting, with the exception of brain-machine interfaces which I think require more localized feedback stimulation in the sensory cortex.
        Have a nice day!

        Nicolas

  • Reply Yoon Kihwa August 26, 2017 at 16:44

    Loved the ideal chemistry between the richness of factual knowlge and the spirit of reader friendliness in your writing. Thank you.

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