Beatrice Lang
Electrical activity is an indispensable concept in biology and medicine. For example, the heart's pumping action is controlled by an electrical conduction system that coordinates the contraction of the heart chambers. Electrical activity in the heart can be measured using an electrocardiograph (ECG), which is an indispensable clinical diagnostic tool. Electrical activity also plays a crucial role in the brain, where it is associated with nerve impulses and various cognitive tasks. Techniques such as multi-electrode recordings and calcium imaging have been used to measure and understand brain electrical activity.
| Characteristics | Values |
|---|---|
| Heart's pumping action | Regulated by electrical impulses |
| Electrical conduction system | Coordinates the contraction of the heart chambers |
| Electrical stimulus | Generated in the sinus node or SA node |
| SA node | Sends out a regular electrical pulse 60 to 100 times per minute |
| Electrical pulse | Travels through the conduction pathways and causes the ventricles to contract and pump out blood |
| Brain electrical activity | Measured using electrodes, multi-electrode arrays, calcium imaging, voltage-sensing molecules, and other techniques |
| Electrical activity in neurons | Correlated with movement in mice |
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What You'll Learn
Electrical activity in the heart
The heart is a pump made of muscle tissue. Its pumping action is controlled by an electrical conduction system that coordinates the contraction of the heart chambers. The heart's electrical system is an example of an electrical activity.
An electrical stimulus is generated in a special part of the heart muscle called the sinus node or sinoatrial node (SA node). The sinus node is a small mass of special tissue in the right upper chamber of the heart (right atrium). In an adult, the sinus node sends out a regular electrical pulse 60 to 100 times per minute. This electrical pulse travels down through the conduction pathways and causes the heart's lower chambers (ventricles) to contract and pump out blood. The right and left atria are stimulated first and contract to push blood from the atria into the ventricles. The ventricles then contract to push blood out into the blood vessels of the body.
The original electrical impulse travels from the sinus node across the cells of the heart's right and left atria. The signal travels to the AV node (atrioventricular node), which is located between the atria and the ventricles. In the AV node, the impulses are slowed down for a very short period, allowing the atria to contract a fraction of a second before the ventricles. After passing through the AV node, the electrical current then continues down the conduction pathway, through a pathway called the bundle of His, and into the ventricles. The bundle of His divides into right and left pathways (bundle branches) to give electrical stimulation to the right and left ventricles.
The electrocardiograph (ECG) is an indispensable clinical diagnostic tool that provides a detailed picture of both normal and abnormal heart function. The standard ECG uses 3, 5, or 12 leads, with the greater number of leads providing more information. The continuous ambulatory electrocardiograph uses a portable device called a Holter monitor that continuously monitors heart electrical activity for a period of 24 hours during the patient's normal routine.
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Brain electrical activity
Traditionally, scientists have measured brain electrical activity by inserting electrodes into the brain, recording the activity of one neuron at a time. While this method is effective, it is also labour-intensive and time-consuming. To overcome these limitations, researchers have developed new techniques, such as using voltage-sensing molecules that fluoresce when brain cells are electrically active. This approach, created by MIT researchers, involves embedding a light-sensitive protein into neuron membranes. The protein emits a fluorescent signal that indicates the voltage of a particular cell, allowing scientists to study how neurons behave millisecond by millisecond as the brain performs different functions.
Another technique, calcium imaging, allows for dense sampling of neural electrical activity, but it measures calcium levels, which is an indirect and slower method. Additionally, multielectrode arrays can monitor electrical activity from multiple neurons simultaneously, but they cannot capture the activity of all neurons within a piece of brain tissue.
The development of new techniques, such as the use of voltage-sensing molecules and light-sensitive proteins, has enabled researchers to study brain electrical activity in greater detail. By imaging the electrical activity in the brains of living organisms, scientists can gain a better understanding of how individual neurons work together in larger circuits, ultimately improving our knowledge of brain function and its relationship to behaviour and cognition.
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Calcium ions and cardiac muscle electrical activity
The heart is a pump made of muscle tissue. Its pumping action is controlled by an electrical conduction system that coordinates the contraction of the heart chambers. The electrical conduction system of the heart is a complex process that involves the generation of an electrical stimulus in a special part of the heart muscle called the sinus node or sinoatrial node (SA node). This electrical stimulus then travels through the heart's conduction pathways, causing the heart's lower chambers (ventricles) to contract and pump out blood.
Calcium ions play a critical role in the physiology of cardiac muscle. Their influx through slow calcium channels accounts for the prolonged plateau phase and absolute refractory period, which are essential for proper cardiac muscle function. The absolute refractory period for cardiac contractile muscle lasts approximately 200 ms, and the relative refractory period lasts about 50 ms, totalling 250 ms. This extended period is vital as it ensures that the heart muscle contracts to pump blood effectively, and the contraction follows the electrical events. Without this extended refractory period, premature contractions would occur in the heart, which would not be compatible with life.
Calcium ions also play a crucial role in the troponin-tropomyosin complex. This complex combines with the regulatory protein troponin, removing the inhibition that prevents the heads of the myosin molecules from forming cross-bridges with the active sites on actin. This process provides the power stroke of contraction. The precise control of intracellular calcium cycling depends on the relationships between the various channels and pumps involved.
Additionally, calcium ions are essential for excitation-contraction coupling in the heart. This process links the electric excitation of the surface membrane (action potential) to contraction. Calcium ions bind to troponin, resulting in the sliding of thick and thin filaments, cell shortening, and the development of pressure within the ventricle, leading to the ejection of blood. The force of contraction depends on the amount of calcium bound to troponin, which is influenced by factors such as the magnitude and duration of the rise of intracellular calcium concentration.
In summary, calcium ions are essential for the proper functioning of cardiac muscle. They contribute to the prolonged plateau phase, the absolute refractory period, and the formation of the troponin-tropomyosin complex, facilitating contraction. Calcium ions also play a key role in excitation-contraction coupling, influencing the force of contraction and the ejection of blood from the heart.
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Electrodes and electrical activity
Electrodes are electrical conductors that are used to transmit electricity and detect electrical impulses from the body. They are used to make contact with a non-metallic part of a circuit, such as a semiconductor, an electrolyte, a vacuum, or a gas. Electrodes are essential components of batteries and electrochemical cells, where they can be called cathodes or anodes depending on the direction of the electric current.
In the field of electrophysiology, electrodes play a crucial role in studying the electrical properties of biological cells and tissues. They can be used for intracellular recording, where a fine electrode impales a cell, or in a non-invasive setup like electroencephalography (EEG) to record brain activity. EEG involves placing small metal disc-electrodes in a cap over the scalp, which can be uncomfortable and time-consuming due to the need to reduce skin resistance.
Electrodes are also used in medical applications for diagnosing neural and muscular problems. For example, they are used in electrocardiograms (ECG) to record heartbeats and detect abnormalities, as well as in electromyography (EMG) to monitor muscle activation. In addition, electrodes can be used for stimulation, such as activating muscles for improved exercise routines (EMS) or suppressing pain through transcutaneous electrical neurostimulation (TENS).
The size of the electrode determines its sensitivity and application. Smaller electrodes can be inserted into a single cell to directly observe intracellular electrical activity, while larger electrodes are sensitive to the net activity of many cells, termed local field potentials. The largest electrodes, such as uninsulated needles, are used in clinical and surgical neurophysiology to detect specific types of synchronous activity within millions of cells.
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Electrical activity in neurons
Neurons, or nerve cells, use electrical activity to exchange and process information. This is done through biochemical signalling pathways, which are not well-suited to rapidly transmit signals over larger distances. The electrical gradients across cellular membranes are exploited, and these gradients result from the uneven distribution of ions between the intra- and extracellular space. The ongoing activity of ion transporters and pumps, as well as the semi-permeable nature of the membrane to certain ions, creates a membrane potential that allows for the fast flow of ions across cellular membranes. This flow occurs through specialised membrane-spanning proteins called ion channels.
The membrane potential, or voltage difference, across the cell membrane is typically around -65mV, with the potential inside the cell being more negative than outside. This potential is used to transmit and integrate signals, sometimes over large distances.
In neurons, some ion channels are voltage-gated, creating an electrical feedback loop that can generate stereotyped, regenerative depolarizing responses. These voltage-gated channels are crucial for the function of neurons, allowing them to transmit signals rapidly and effectively.
The electrical activity of neurons can be modelled using equivalent electrical circuits, providing insights into the time-dependent aspects of membrane potential and how it varies over the spatial extent of a neuron. These models help us understand the underlying physical principles of neuronal electrical activity and its role in signal transmission and integration.
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Frequently asked questions
Electrical activity refers to the dynamic anatomy of electrical impulses in the nervous system.
Electrical activity in the brain can be measured by inserting an electrode into the brain. Multi-electrode arrays can monitor electrical activity from multiple neurons at once. A new imaging technique uses a voltage-sensing molecule that lights up when brain cells are electrically active, providing a clear picture of brain cell activity.
The heart's pumping action is controlled by an electrical conduction system. An electrical stimulus is generated in a part of the heart muscle called the sinus node or SA node. This electrical pulse travels through the conduction pathways and causes the heart's lower chambers (ventricles) to contract and pump out blood.
An ECG or electrocardiogram is a clinical diagnostic tool that provides a detailed picture of heart function. It uses an electrocardiograph to measure electrical activity in the heart.
Epileptic seizures are believed to be caused by abnormal electrical activity in the brain. Researchers are studying how to eradicate epileptic seizures by understanding the electrical activity that occurs during these seizures.
