Beatrice Lang
The nervous system of animals and the control of muscle movement are governed by electrical interactions. Electrical muscle stimulation (EMS) involves sending electrical impulses through the skin to target nerves or muscles. This stimulation may provide benefits such as helping repair tissue and strengthening muscles. The electrical impulses mimic what occurs when someone contracts and releases a muscle naturally. The ability of cells to communicate electrically requires that the cells expend energy to create an electrical gradient across their cell membranes. This charge gradient is carried by ions, which are differentially distributed across the membrane.
| Characteristics | Values |
|---|---|
| Electrical impulses in muscles | Electrical muscle stimulation |
| Involves sending electrical impulses through the skin to target nerves or muscles | |
| Mimics the natural contraction and release of a muscle | |
| Used to treat pain, injuries, and diseases | |
| Promotes blood flow and strengthens muscles | |
| Affects pain signals, reducing discomfort | |
| Types include TENS and EMS | |
| TENS is recommended for pain relief and to increase blood flow | |
| EMS helps muscles respond to natural signals to contract and can be used to strengthen or retrain a muscle | |
| How electrical impulses are carried | Through neurons or nerve fibers |
| Neurons conduct signals from the brain or spinal cord to the muscle | |
| Electrical signals travel along the neuron's axon, branching through the muscle and connecting to individual muscle fibers at a neuromuscular junction | |
| The ability of cells to communicate electrically is dependent on the creation of an electrical gradient across their cell membranes | |
| This charge gradient is carried by ions, which are differentially distributed across the membrane | |
| The sodium-potassium ATPase uses cellular energy to move K+ ions inside the cell and Na+ ions outside, creating a concentration gradient | |
| The opening of Na+ channels triggers the movement of positively charged ions into the cell, making it electrically positive | |
| This depolarization is associated with the conduction of electrical impulses |
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What You'll Learn
The nervous system and muscle movement
The nervous system plays a crucial role in muscle movement, with electrical interactions governing the control of muscles. This process involves the transmission of electrical impulses, or action potentials, along nerves and muscles, resulting in muscle contraction and relaxation.
Each skeletal muscle fibre is controlled by a motor neuron, which conducts signals from the brain or spinal cord to the muscle. These electrical signals travel along the neuron's axon, branching through the muscle and connecting to individual muscle fibres at neuromuscular junctions. The ability of cells to communicate electrically is made possible by the creation of an electrical gradient across their cell membranes, carried by ions with differential distribution across the membrane.
The sodium-potassium ATPase pump plays a vital role in this process by maintaining the concentration gradient of ions. It uses cellular energy to move potassium ions (K+) inside the cell and sodium ions (Na+) outside, creating a small electrical charge. The high concentration of K+ inside the cell and Na+ outside establishes a resting membrane potential, which is essential for generating action potentials.
When an action potential is generated, voltage-gated ion channels open, allowing Na+ to flow into the cell, resulting in depolarization. This change in polarity creates an electrical current that propagates along the nerve fibre, triggering the release of neurotransmitters from the synaptic terminal. These neurotransmitters diffuse across the synaptic cleft and bind to receptor molecules on the motor end plate of the muscle fibre, initiating muscle contraction.
Electrical muscle stimulation (EMS) is a technique that utilizes electrical impulses to stimulate muscle fibres and nerves. It can be used to treat various muscle issues, promote blood flow, strengthen muscles, and reduce pain. EMS helps muscles respond to natural signals to contract, aiding in muscle rehabilitation and strengthening after surgery or injury.
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Ion channels and depolarization
Ion channels are essential for the basic physiological function of excitable cells, including nerve, skeletal, cardiac, and smooth muscle cells. The movement of ions through these channels is critical for the depolarization and repolarization of the cell membrane, which is fundamental to the generation and transmission of electrical impulses.
Depolarization refers to the process of the cell membrane becoming more positively charged. In a resting state, the cell membrane is typically more negatively charged on the inside compared to the outside. During depolarization, this voltage changes, with the inside becoming more positive. This change in voltage is primarily due to the influx of positively charged sodium ions (Na+) through open sodium channels.
The opening of sodium channels is often triggered by the binding of acetylcholine (ACh) to receptors on the skeletal muscle cell. This binding opens the cation-selective pores, allowing Na+ to flow into the cell and initiate an action potential spike. The action potential then propagates along the cell membrane, causing the cell to contract.
In cardiac muscle cells, the action potential arises from specialized pacemaker cells in the sinoatrial node, which have automatic action potential generation capability. These cells produce approximately 60–100 action potentials per minute, resulting in a resting heart rate within the same range. The rapid depolarization of the cell membrane during phase 0 of the cardiac action potential is due to the influx of Na+ through open sodium channels.
Following depolarization, the cell undergoes a brief delay known as the absolute refractory period. During this time, there is a change in the permeability of the cell membrane, with potassium channels opening and allowing K+ to leave the cell. This movement of potassium ions out of the cell causes the membrane potential to return to a more negative state, known as repolarization.
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Action potential and the sinus node
The rhythmic electrical activity of the heart's natural pacemaker, the sinoatrial node (SAN), determines cardiac beating rate (BR). SAN cells are characterised by their lack of a true resting potential and instead, they generate regular, spontaneous action potentials.
The action potentials in SAN cells are sometimes referred to as "slow response" action potentials. This is because, unlike non-pacemaker action potentials in the heart, the depolarising current is carried into the cell by slow Ca++ currents instead of fast Na+ currents. There are no fast Na+ channels or currents operating in SAN cells, which results in slower action potentials in terms of how rapidly they depolarize.
SAN action potentials are divided into four phases. Phase 4 is the spontaneous depolarization (pacemaker potential) that triggers the action potential once the membrane potential reaches a threshold between -40 and -30 mV. Phase 0 is the depolarization phase of the action potential, followed by phase 3 repolarization. Once the cell is completely repolarized at about -60 mV, the cycle is spontaneously repeated.
The changes in membrane potential during the different phases are brought about by changes in the movement of Ca++ and K+ across the membrane through ion channels that open and close at different times. When a channel is opened, there is increased electrical conductance of specific ions through that ion channel. Closure of ion channels causes ion conductance to decrease. As ions flow through open channels, they generate electrical currents that change the membrane potential.
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Neurotransmitters and receptors
Neurotransmitters are chemical molecules that carry messages or signals from one nerve cell to another target cell. They are part of the body's communication system. Each type of neurotransmitter binds to a specific receptor on the target cell, like a key fitting into its partner lock. After binding, neurotransmitters trigger a change or action in the target cell, such as an electrical signal in another nerve cell, muscle contraction, or the release of hormones from a cell in a gland.
There are three possible actions that neurotransmitters transmit in their messages, depending on the specific neurotransmitter. Excitatory neurotransmitters, for example, "excite" the neuron, causing it to "fire off the message" and continue passing it along to the next cell. Examples of excitatory neurotransmitters include glutamate, epinephrine, and norepinephrine.
Acetylcholine (ACh) is a neurotransmitter released by motor neurons that plays a crucial role in muscle contractions. When ACh is released by the synaptic terminal, it diffuses across the synaptic cleft to the motor end plate, where it binds with ACh receptors. As ACh binds, ion channels open, allowing Na+ ions to enter the muscle cell. This reduces the voltage difference between the inside and outside of the cell, a process called depolarization.
The depolarization then spreads along the sarcolemma, creating an action potential as adjacent sodium channels open in response to the change in voltage. This action potential moves across the entire cell, generating a wave of depolarization. The enzyme acetylcholinesterase (AChE) breaks down ACh in the synaptic cleft to prevent unwanted extended muscle contraction.
After delivering their message, neurotransmitter molecules must be cleared from the synaptic cleft to avoid continuous stimulation. This can occur through diffusion, reuptake by the nerve cell, or degradation by enzymes within the synapse.
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Electrical muscle stimulation
EMS has been shown to improve muscle strength and enhance neuromuscular adaptations. It achieves this by activating large motor units before small motor units, which is the reverse of conventional exercise. The intensity of the electrical stimulation is important, with higher intensities leading to more efficient results. A generally accepted guideline suggests that an EMS frequency between 50 and 75 Hz is the most effective.
EMS has a range of applications, including pain management, physical therapy, and aiding in weight loss. It can help treat pain related to injuries, diseases, and chronic conditions such as osteoarthritis. By delivering electrical currents to the nerves, EMS may reduce pain signals and provide relief. It can also increase blood flow to the affected area, promoting healing and reducing discomfort.
Additionally, EMS has been found to be beneficial for improving muscle force-generating ability and enhancing athletic performance. A study on elite athletes concluded that EMS significantly enhanced their strength levels, offering a promising alternative to traditional strength training. Furthermore, EMS has been shown to reduce abdominal obesity, subcutaneous fat mass, and body fat percentage without modifying diet or exercise routines.
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Frequently asked questions
Electrical muscle stimulation is a process that involves sending electrical impulses to the muscles through the skin. This process may promote blood flow, strengthen the muscles, and affect pain signals, reducing discomfort.
Electrical impulses are carried through muscles via neurons. Each skeletal muscle fiber is controlled by a motor neuron, which conducts signals from the brain or spinal cord to the muscle. Electrical signals travel along the neuron's axon, which branches through the muscle and connects to individual muscle fibers at a neuromuscular junction.
Electrical impulses may help improve blood flow and stimulate the muscle fibers or nerves. This stimulation may provide benefits such as helping repair tissue and strengthening the muscles.
