Electrophysiology: what it is and how it is investigated

Electrophysiology is responsible for analyzing and studying the electrical processes that take place in different organs, tissues and structures of our body, such as the heart, muscles or the brain. Its application in clinical practice helps us to observe and diagnose different pathologies and diseases.

In this article we explain what is electrophysiology and what the main techniques for recording electrical activity consist of.

• Related article: "Parts of the human brain (and functions)"

What is electrophysiology?

Electrophysiology is the science that studies the electrical properties of cells and the biological tissue of an organism. Although the best known study is related to the heart system, measurements (such as the change in voltage or the electrical current) in other types of body structures, such as muscles or the brain, through the use of electrodes that measure the activity electrical.

In the mid-19th century, the Italian physicist Carlo Matteuci was one of the first scientists to study electrical currents in pigeons. In 1893, the Swiss physiologist Wilhelm His, famous for being the founder of histology and inventor of the microtome (an instrument that allows biological tissue to be sectioned to be analyzed under a microscope), provided new findings in the field of electrophysiology cardiac. And already in 1932, Holzmann and Scherf, discovered and invented the electrocardiogram.

Currently, neuroscience is nourished by research and advances in new electrophysiological techniques that allow micro (from a simple ion channel) and macro (up to the complete brain) analysis of brain structures.

Advances in knowledge of the functioning of behavior and the human nervous system are based on studies in which electrical signals from individual neurons and large-scale neuronal groups are recorded. In neuropsychology, for example, it seeks to explore the correlations between certain areas of the brain and cognitive functions superiors or certain behaviors, hence the recording techniques of electrical activity used in electrophysiology are so important.

The electrical properties of cells

In electrophysiology, when we talk about the study of electrical properties, we refer to the ion flow analysis (an atom or a group of them with an electrical charge, which can be positive or cation, and negative or anion) and to the state of rest and activity of excitable cells (neurons, cardiac cells, etc.).

The excitability of a cell is a property that allows it to respond actively to the application of a stimulus, that is, any energetic variation in the environment. These stimuli can be of multiple types: mechanical, thermal, sound, light, etc. For example, in neurons, this excitability gives them the ability to change its electrical potential to transmit that nerve impulsethrough the axon to other neurons.

The membrane that covers the cell regulates the passage of ions from the outside to the inside, since they contain different concentrations of them. All cells have a potential difference between the inside and the outside of the cell, called membrane potential, which is due to the existence of ionic concentration gradients on both sides of the membrane, as well as differences in the relative permeability of the cell membrane to different ions present.

Furthermore, excitable cells perform their functions by producing electrical signals in terms of membrane potential changes, a key concept in electrophysiology. These electrical signals can be: brief and of great amplitude (such as action potentials), responsible for transmitting information quickly and over great distances; slower and lower voltage, with an integrating function; and low-voltage (such as synaptic potentials), which are caused by synaptic action.

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Types of electrophysiological readings

The recording of electrical activity can occur in different biological tissues and cells, as well as with different electrophysiology techniques.

The most common electrophysiological recordings They include: electrocardiogram, electroencephalography and electromyography. Next, we explain in more detail what each of them consists of.

1. Electrocardiogram

The electrocardiogram (ECG) is an electrophysiology technique that is responsible for recording the electrical activity of the heart. heart, through the study of voltage changes during a certain time (which does not usually exceed 30 seconds). A graph is usually recorded on the monitor, similar to a television screen, of the electrocardiograph.

The electrical activity of the heart that is collected in the ECG can be observed in the form of a tracing that presents different waves that correspond to the path of electrical impulses through the different structures of the device cardiac.

This test is a must for the study of heart problems such as arrhythmias, heart disease or acute episodes in coronary diseasesuch as myocardial infarction.

An ECG is performed as follows:

• The patient lies down and electrodes are placed on the arms, legs, and chest. Sometimes it is necessary to clean or shave the area.
• The electrocardiograph leads are connected to the subject's skin via electrodes attached to the ankles, wrists, and chest. This is how electrical activity is collected from different positions.
• The person should remain relaxed, quiet, with arms and legs immobile and with a normal breathing rhythm.

2. Electroencephalogram

An electroencephalogram (EEG) is an electrophysiology technique that detects and records electrical activity in the brain, through small electrodes fixed on the scalp of the person. This test is non-invasive and is commonly used in neuroscience to observe and study the functioning of the central nervous system and, more specifically, the cerebral cortex.

With this technique, neurological alterations that suggest diseases such as epilepsy, encephalopathies, narcolepsy, dementia or neurodegenerative diseases can be diagnosed. In addition, the EEG also makes it possible to identify normal and pathological rhythms of brain activity, as well like the waves that we usually have both in a waking state and in sleep: alpha, beta, delta, theta and gamma.

This test also frequently used in studies of sleep phases (polysomnography), to detect possible anomalies in the recordings of eye movement cycles rapid (REM) and normal (NREM) sleep cycles, as well as to detect other possible sleep disorders dream.

The EEG lasts approximately 30 minutes and can be performed in a hospital or in a neurophysiology unit. To perform it, the patient sits in a chair and the electrodes are attached (between 15 and 25 sensors) to the scalp, using a hair gel so that electrical activity is recorded correctly. And while the person is relaxed, the test is carried out.

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3. electromyogram

An electromyogram (EMG) is a procedure used to study the electrical activity of muscles and their nerve cells or motor neurons. These neurons transmit the electrical signals that produce muscle activity and contraction.

Electrodes are needed to perform an EMG and are placed on the muscles, either at rest or during exercise. To detect the muscular response it is necessary to insert a small needle, which is why it can sometimes be annoying for the patient.

The only complication of this test is that it causes a little bleeding at the insertion site of the electrode, hence it is necessary to take into account patients with a coagulation disorder or undergoing treatment anticoagulant.

Another electrophysiology technique that sometimes accompanies EMG is electroneurography, which studies the speed of conduction of impulses through nerves. To do this, a nerve is stimulated with low-intensity electrical impulses, using sensors placed on the skin that collect the response from other sensors located at a distance, thus recording how long it takes for the response to occur when driving from one side to the other. other.

Bibliographic references:

• Gilman, S, and Winans, S. (1989). Principles of Clinical Neuroanatomy and Neurophysiology. Second edition. Modern Manual Editorial. Mexico.
• Schmidt, R. F., Dudel, J., Jaenig, W., & Zimmermann, M. (2012). Fundamentals of neurophysiology. Springer Science & Business Media.