Electrical Stimulation

14 October 2022

Electrical Stimulation

Abstract

Electrical stimulation (ES) is a wonderful technology that has the potential to be increasingly used for human regeneration, augmentation, and treatment. This stimulation can be used on excitable tissues such as nerves and muscles for various applications. In this paper, we discuss the application of some electrical stimulations. Our vision is to make it more efficient to utilize all kinds of stimuli and exceptionally as wearable technologies.

Keywords: Electrical stimulation, wearable technologies, regeneration, augmentation

Introduction

Numerous techniques have imperative impacts on stimulation. Stimulation includes electrical, magnetic, optical, and mechanical [1]. Stimulation can be divided into invasive and non-invasive methods. Here we investigate non-invasive stimulation, which has various kinds, and we separately explain it in relevant papers. Considering electrical stimulation is our goal in this paper.

Electric currents are actually the flow of charged particles in conductive media. These currents, such as ionic liquids and nerves, exist naturally in the human body. It means that different body organs have electrical functions, including the heart, nervous system, and bones. Hence, as there are endogenous electric fields, we can use external electric fields for different purposes. Electricity will directly impact each cell and tissue that passes through, and this kind of response and effect depends on the type of tissue and the nature of the current applied. For example, the nerve conducts electricity very well. In contrast, the tendon and fat have a higher impedance or resistance to the flow of electricity. [2] [3] In the following, we will discuss different types of stimulation on different muscles and nerves, examine how it works, and present their different categories.

The first category of ES is EMS (Electrical Muscle Stimulation), NMES (Neuromuscular Electrical Stimulation), FES (Functional Electrical Stimulation), TENS (Transcutaneous Electrical Nerve Stimulation), ETS (EMG Triggered Stimulation), RETS (Reciprocal EMG Triggered Stimulation). They have been used not only for patients with neurological damage and disorders but also for rehabilitation and augmentation. In the following, we will examine some examples of them.

Transcutaneous Electrical Nerve Stimulation (TENS)

It is a non-invasive peripheral stimulation technique [4]. TENS is a stimulation device in which a current is applied across the skin to stimulate nerves Figure 1. TENS uses electrode pads attached to the skin’s surface to administer pulsed electrical currents via the skin using a battery-operated device and electrodes Figure 2.

The location of the TENS electrodes can be different, for example, directly on the area, on the peripheral nerves, on the nerve root of the spinal cord, or on points that cause nerve stimulation. The different method of the treatment needs various electrode place. For example, the electrode can be placed near the source of pain, and then signals are sent to nerve fibers to block or reduce the pain signals that pass through the central nerve system. TENS stimulates afferent sensory fibers to elicit the production of neurohormonal substances such as endorphins, enkephalins, and serotonin (i.e., Gate Control Theory) [5] [6].

Pain Pathway and Nociceptor Fiber

We can target particular types of nerves based on the parameters we select. We should concentrate on our A-alpha fibers if our objective is to change the tone of our muscles. If pain management is our goal, we will try to focus on our A-beta or A-gamma fibers. [7]

Peripheral pain receptors are known as nociceptors. These receptors act based on harmful and damaging stimuli to the central nervous system through nerve fibers. Two types of afferent nerve fibers are known to conduct painful stimuli: Fast fibers (5–30 m/s), known as myelinated A-delta (Aδ) fibers that conduct noxious mechanical stimuli transfer heat, harmful-cold. The next type is slow fibers (0.5-2 m/s); These more diffuse, unmyelinated C-type fibers transmit noxious mechanical, thermal, and chemical stimuli Figure 3. These two fiber types explain why you feel intense pain at first and the pain subsides over time. A-beta (Aβ) fibers are myelinated and respond to non-painful stimuli such as vibration and light touch. Although stimulation of this fiber plays a role in pain relief.

In the dorsal root ganglia (DRG) and then centrally to the dorsal horn of the spinal cord, nociceptive inputs travel via the afferent nerve fibers of the pseudounipolar neuron, where they synapse with second-order neurons, mainly in the Rexed laminae I (marginal zone)/II (substantia gelatinosa). In addition, before entering the dorsal horn of the spinal gray matter, a group of fibers travels caudally and rostrally to one to two spinal levels in the Lissauer tract. The second-order neuron’s axons then decussate over the spinal cord’s anterior white commissure before ascending via the spinothalamic tract to the thalamus’s ventral posterolateral nuclei. Third-order neuron fibers go through the corona radiata and internal capsule to connect with synapses inside the somatosensory cortex. The ability to localize pain in the body is made possible by signals to the somatosensory cortex, which is somatotopically organized into a homunculus. [8]

The majority of e-stim regimens work by depolarizing a particular target nerve.

– A-alpha fibers (motor)

– A-beta fibers (sensation)

– A-gamma/c- fibers (pain)[9]

One of the significant features of TENS is that it can activate selectively large diameter Aβ fibers without concurrently activating small diameter Aδ and C (pain-related) fibers or muscle efferent Figure 4.

Figure 3 Nociceptor fiber types and the cerebral pain pathway. The diameter and voltage output of A fibers are the biggest. Small, unmyelinated fibers are known as type C fibers. Type A neurons have myelination and are of moderate voltage and diameter [8].

Figure 4 TENS current and different fibers, It causes segmental analgesia by preferentially activating Aβ afferents [10].

How Does Transcutaneous Electrical Nerve Stimulation (TENS) Work?

It has two different mechanisms of action. According to one idea, the electric current activates nerve cells that prevent the passage of pain signals, altering pain perception. Thus, when we don’t have pain, it means that there is an even balance between the activity of the neurons that turn on pain and those that turn off pain. The alternative viewpoint contends that nerve stimulation increases the body’s natural painkiller and endorphin levels. The endorphins then suppress the perception of pain. Therefore, In the first place, TENS reduces pain by stimulating sensory nerves, then it stimulates and activates the body’s natural anti-pain mechanisms, such as the release of natural morphine (Gate Control Theory).

According to the Gate Control Theory, pain signals from the peripheral nervous system are controlled by gates on the nerve fibers between the brain and the peripheral nervous system Figure 5.

Figure 5 Mechanoreceptors can be activated by “rubbing the skin” to close the pain gate. This causes activity in Aβ afferents with a large diameter, which prevents the spread of harmful information. As a result of the “pain gate” is closed, the brain receives less toxic information, which lessens pain perception. The segmental organization of the relevant neural circuitry. Using electrical currents, traditional TENS seeks to stimulate Aβ fibers. The pain gate may also be closed by stimulating the brain’s pain-inhibitory pathways, which descend to the spinal cord via the brainstem (extrasegmental circuitry). These routes become active when small diameter peripheral fibers (Aδ) are physiologically aroused, such as during motivational tasks. The purpose of AL-TENS is to activate the descending pain-inhibitory pathway by stimulating small-diameter peripheral fibers.

Various Methods

Based on the different pulse amplitude (mA), frequency (pulses per second – PPS), width or duration (μs), and pattern of the currents, different TENS are defined. Various TENS devices use other pulse wave forms such as monophasic, symmetric biphasic, and asymmetric biphasic, Figure 6. Figure 7, is shown a picture of a standard TENS device’s output characteristics.

We have three kinds of TENS, high frequency, low frequency, and brief-intense. High frequency is for the sensory level, with the pulse frequency between 60-100 PPS, a short pulse duration of less than 100 µ sec and intensity between 12-30 mA. This technique is effective for myofascial pain, post-operative pain, Inflammatory conditions, and pain associated with musculoskeletal disorders. This technique actually activates gate pain modulation at the spinal cord level and also stimulates the large-diameter sensory nerve fibers. The low frequency is for the motor level, the pulse frequency is between 2-4 PPS, the long pulse duration is 150-250 µ sec and with the intensity more than 30 mA. It uses in chronic pain, pain caused by muscle spasms, pain due to damage to deep tissues, and Myofascial pain. This method activates the motor fibers and the small diameter of nociceptors (release of β-endorphin), stimulates the pituitary gland, and releases the β-endorphin. Brief-intense is for noxious level and motor level. The frequency pulse is greater than 100 PPS, the long pulse duration is 300-1000 µ sec, and the intensity more than 30 mA. The application of this technique relieves pain through activating mechanisms in the brain stem [11] [12].

High TENS (Activate A-delta fibers)
Low TENS (release of β-endorphins from the pituitary)
Brief-Intense TENS (noxious stimulation to active C fibers)6

Figure 6 various pulse waveforms used in TENS [6].

Figure 7 TENS device’s output characteristics.

In this paragraph we will mention some application of Transcutaneous electrical nerve stimulation (TENS) from different studies. In the paper [13] to control lower back pain, an experiment was conducted on two groups of people. For the first group, the electrodes were placed on the area of concern (Lower back/Gate-control). And for the second category in the kidney meridian point (Kidney meridian/Endogenous) Figure 8. The results showed that both groups experienced a reduction in pain. Nevertheless, in the second group, pain relief was experienced for longer.

Figure 8 left: lower back pad placement, right: kidney meridian pad placement [13].

It may be used to reduce acute postoperative pain [6]. Intense perioperative pain within the first few days after hip fracture surgery is common and is related to negative consequences such as restricted and delayed ambulation [10]. Another paper it illustrates, it is a good method for relieves pain in older person [14]. It is also use for myofascial [12], Musculoskeletal pain, Bone fractures [15] Dental procedures, Low back, Arthritis, Stump and phantom, Peripheral nerve injuries, Relief of chronic pain, Improving blood flow, Reduction of symptoms associated with Raynaud’s disease and diabetic neuropathy, Postoperative pain [6] [10] temporomandibular disorder [16]. TENS can be used for controlling chronic pain, reduction of posttraumatic and acute pain and managing the postsurgical pain [6].

Neuromuscular Electrical Stimulation (NMES)

In this method, electrodes are placed on specific muscle points to sending electrical impulses to the nerves, in order to contract the muscles. This method is often used to retrain the muscles to contract in the set. A small switch can be added to this device so that the device can be turned off and on quickly during various activities such as walking.

NMES has utilized when person have muscle weak, muscle atrophy or other impairments in structure or function, in this condition persons need the modification of neuromuscular activation to get better control.

Different stimulation parameters are used in the NMES therapy. The waveform of the stimulation pulse can generally take the shape of monophasic, biphasic, or burst (polyphasic) waves, as seen in Figure 9A. While the current intensity is measured in milliamperes, the pulse width is typically between 150 and 300 μs. You can separate the pulse waveforms into sine waves, rectangle waves, etc. Contrarily, biphasic waves can also be classified as symmetrical, asymmetrical, or unbalanced Figure 8B. [17]

In following paragraph, we will mention some application of NMES from various studies.

Muscle atrophy is one of the major problems of the elderly; this physiological process causes the loss of skeletal muscle mass and causes many problems, such as falls, fractures, and slow walking for these people.

In muscles, the factor that causes their contraction and coordination is the nerve input of the muscle, which changes with age. The final motor neuron system associated with muscle contraction is the motor unit (MU), which consists of an efferent motor neuron and all the muscle fibers it innervates. In old muscles, the number of MUs decreases, and as a result, it causes the denervation of some muscle fibers and the loss and reduction of muscle function.

As mentioned, skeletal muscle contraction can be induced using electrical stimulation, which happens due to the depolarization of motor neuron axons and their branches. Unlike motor nerve stimulation that activates all muscle fibers in a MU, direct muscle stimulation nonselectively activates fibers near the stimulating electrodes, which may not include whole Mus. Muscle or nerve stimulation is mainly dependent on proximity to stimulating electrodes, and axon depolarization also relies on membrane resistance. Simply put, NMES induces an inappropriate and non-physiological regulation of MU recruitment.

In this article, it is shown that NMES has positive effects on nerve and muscle regeneration in terms of morphological, molecular and functional aspects in elderly people. Through the tropomyosin-related kinase receptor B (trkB) and its downstream pathways, neuromuscular electrical stimulation (NMES) stimulates axonal outgrowth and the reinnervation process at the neuromuscular junction (NMJ) Figure 10 [18].

Figure 10 Muscle reinnervation and nerve regeneration [18].

This study [19], was conducted on 12 elderly people over a period of 8 weeks, it was shown that NMES improves the regeneration capacity of skeletal muscles. And as a result, the skeletal muscle strength and mobility of elderly people stimulated with NMES were significantly improved. NMES improved this effect by increasing proliferation of myogenic progenitor cells and fusion with mature myofibers. Increased cytoplasmic free Ca2+ concentration along with MYOD, MYOG, and microRNA-mediated regulation could be associated with decreased O2·- production, which, in turn, favors myogenic regeneration.

Other applications of NMES [20] [21]

The electrical stimulation can increase strength and range of motion, and offset the effects of disuse
To enhance performance in healthy muscle
contract paralyzed muscles
increase muscle strength
improve spasticity via disynaptic reciprocal inhibition
facilitate motor relearning
improves the regeneration capacity of skeletal muscles
improve muscle strength and jumping performance in volleyball athletes

Functional Electrical Stimulation (FES)

Functional electrical stimulation (FES) attaches electrodes to the muscle. This applied electrical charge stimulates the muscle to perform its normal movement. Damage to the muscle may be paralyzed or weaken due to brain or spinal cord damage. This method is mainly used as a treatment for foot drop in MS.

The FES device consists of a small control box, a battery, and an electrode. This tool is placed in different organs according to efficiency, but in most cases, it is attached below the knee like a wearable. In this case, the electrodes can stimulate the nerve that stimulates the muscle that raises the leg. For example, when walking, when the leg is supposed to be lifted, the FES device stimulates the nerve and lifts the leg. And when the foot hits the ground, the stimulation stops.

As a result, it uses artificial low-level electrical impulses applied to the nerve system to replace or block the damaged neurons and signals, restoring lost function. It is also beneficial for stroke, Parkinson’s disease, spinal cord injury, and pain; furthermore, it can be used to reduce chronic pain, such as phantom pain. In this system, the stimulator blocks the pain signals being sent to the brain implanted or external neuromodulation. [22]

One of the differences between TENS and FES is that TENS is used to stimulate sensory nerves and often to reduce pain. In contrast, FES is often used to stimulate motor nerves to achieve muscle contractions. [23]

In this study [24], it was shown that functional electrical stimulation (FES) had been widely used to induce muscle contraction in rehabilitation exercises after spinal cord injury (SCI). In the context of spinal cord injury, at least three scenarios have been proposed in which Hebbian learning may explain the positive effects of FES (Figure 11). In general, FES can be used to improve breathing, circulation, hand strength, mobility, and metabolism after SCI.

Figure 11 Hebbian theory’s justification for functional electrical stimulation. Damaged ascending and descending neuropathways keep deteriorating after SCI, resulting in paretic and paralyzed muscles. By directing adaptive plasticity, stimulating injured networks via spared fiber, and initiating neuronal plasticity via cortical neuron-controlled FES, FES can be utilized to strengthen the functional connections between the brain’s connections and affected muscles (sensory pathways are shown in blue and motor pathways are shown in red) [24].

CPGs are neural circuits that respond to chemical signals (i.e., neurotransmitters) and electrical signals (i.e., FES) to produce rhythmic motor outcomes. The L2 lumbar spinal cord circuits control hip, knee, and leg muscles. CPG commands can be transmitted to motor neurons. CPG can elicit lower limb movement without input from the brain, making it an excellent therapeutic target Figure 12.

Figure 12 It is suggested that FES also stimulates the locomotor center of CPG (sensory pathways are shown in blue, and motor path ways are shown in red) [24].

Electrical Stimulation and VR

Various studies show that we can use the benefits of VR with ES spontaneously. It means that utilize VR with customize tasks help us to enhance the efficient=cy of ES. For example, the results of this article [25] show that the simultaneous use of VR and FES can have better results in people who had a stroke. This experiment, which was conducted on 48 people for 3 months, also showed that the effect of FES along with VR on rehabilitation and improvement of motor function is very useful.

Another test was conducted on a 41-year woman who suffered spinal infarction and paraplegia after acute thoracoabdominal aortic dissection. This experiment was carried out for 60 days using VR and electrical stimulation as a wearable. The study’s results suggested enhancing the lower-limb movement and trunk function indices. Patients with spinal cord infarction were better able to walk and transfer thanks to enhanced limb and trunk function [26].

Figure 13 Left: self-training of sitting balance. The patient wears a head-mounted display and is done a task in virtual reality. Right: CT angiography of the thoracoabdominal aorta in three dimensions [26].

Conclusion

In this article, we took a glimpse at some of the electrical stimulations that were used on organs and muscles. We brought up that many of these tools can be used as wearable technologies, they can also be used for different purposes in VR. The following articles will deal with other types of electrical stimulation with the same insight.