Electrophysiology: what it is and how it is researched
Electrophysiology is the analysis and study of 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 electrophysiology is and what the main techniques for recording electrical activity consist of.
What is electrophysiology?
Electrophysiology is the science that studies the electrical properties of an organism’s cells and biological tissue . Although the best known study is that related to the heart apparatus, measurements (such as the change in voltage or electrical current) can also be recorded on other types of body structures, such as muscles or the brain, using electrodes that measure electrical activity.
In the mid-19th century, Italian physicist Carlo Matteuci was one of the first scientists to study electrical currents in pigeons. In 1893, the Swiss physiologist Wilhelm His, famous as the founder of histology and inventor of the microtome (an instrument that allows biological tissue to be sectioned for microscopic analysis), made new findings in the field of cardiac electrophysiology. And already in 1932, Holzmann and Scherf, discovered and invented the electrocardiogram.
At present, neuroscience is nourished by research and advances in new electrophysiological techniques that allow micro (from a simple ion channel) and macro (to the entire brain) analysis of brain structures.
The advances in the knowledge of the functioning of behaviour 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, the aim is to explore the correlations between certain areas of the brain and higher cognitive functions or certain behaviours, which is why the techniques for recording electrical activity used in electrophysiology are so important.
The electrical properties of cells
In electrophysiology, when we speak of the study of electrical properties we refer to the analysis of ion flow (an atom or a group of atoms with an electrical charge, which can be positive or cation, and negative or anion) and the state of rest and activity of excitable cells (neurons, heart cells, etc.).
The excitability of a cell is a property that allows them to respond actively to the application of a stimulus, that is, any energy 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 capacity to change their electrical potential to transmit this nerve impulse , through the axon, to other neurons.
The membrane that covers the cell regulates the passage of ions from the outside to the inside, as they contain different concentrations of ions. All cells have a difference in potential between the inside and outside of the cell, called membrane potential, which is due to the existence of ion concentration gradients on both sides of the membrane, as well as differences in the relative permeability of the cell membrane to the different ions present.
In addition, 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: short and wide ranging (like action potentials), responsible for transmitting information quickly and over long distances; slower and lower voltage, with an integrative function; and low voltage (like synaptic potentials), which originate through synaptic action.
Types of electrophysiological readings
The recording of electrical activity can occur in different biological tissues and cells, as well as with different electrophysiological techniques.
The most common electrophysiological records include: electrocardiogram, electroencephalography and electromyography. Below, we explain in more detail what each of these consists of.
1. Electrocardiogram
The electrocardiogram (ECG) is an electrophysiology technique that records the electrical activity of the heart by studying voltage changes over a period of time (usually no longer than 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 recorded on the ECG can be seen in the form of a trace that has different waves that correspond to the path of the electrical impulses through the different structures of the heart apparatus.
This test is essential for the study of heart problems such as arrhythmias, heart disease or acute episodes of coronary disease , such as myocardial infarction.
An ECG is performed as follows:
- The patient lies down and electrodes are placed on his arms, legs and chest. Sometimes it is necessary to clean or shave the area.
- The electrocardiograph wires are connected to the subject’s skin by 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 still and a normal breathing rhythm.
2. Electroencephalogram
An electroencephalogram (EEG) is an electrophysiology technique that allows the detection and recording of the electrical activity of the brain , through small electrodes fixed on the person’s scalp. 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 can be diagnosed that suggest diseases such as epilepsy, encephalopathies, narcolepsy, dementias or neurodegenerative diseases. Furthermore, the EEG also allows us to identify the normal and pathological rhythms of brain activity, as well as the waves that we usually have both in the waking and the sleeping states: alpha, beta, delta, theta and gamma.
This test is also frequently used in sleep phase studies (polysomnography), to detect possible abnormalities in the records of rapid eye movement (REM) and normal sleep cycles (NREM), as well as to detect other possible sleep disorders.
The EEG takes approximately 30 minutes and can be performed in a hospital or a neurophysiology unit. To perform it, the patient sits in a chair and the electrodes (15-25 sensors) are attached to the scalp, using a hair gel so that the electrical activity is properly recorded. And while the person is relaxed, the test is performed.
3. Electromyogram
The electromyogram (EMG) is a procedure that is 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.
EMG requires electrodes to be placed on the muscles, either at rest or during exercise. A small needle is inserted to detect the muscle response, which can sometimes be uncomfortable for the patient.
The only complication of this test is that it causes a small amount of bleeding at the site of the electrode insertion, so patients with coagulation disorders or those on anticoagulant treatment should be considered.
Another electrophysiology technique that sometimes accompanies EMG is electroneurography, which studies the speed of conduction of impulses through the nerves . To do this, a nerve is stimulated with low intensity electrical impulses, by means of sensors placed on the skin that pick up the response of other sensors located at a distance, and thus record the time it takes for the response to be produced in the conduction from one side to the other.
Bibliographic references:
- Gilman, S and Winans, S. (1989). Principles of Clinical Neuroanatomy and Neurophysiology. Second edition. Editorial Manual Moderno. Mexico.
- Schmidt, R. F., Dudel, J., Jaenig, W., & Zimmermann, M. (2012). Fundamentals of neurophysiology. Springer Science & Business Media.