Molly Ayala
The human brain is an incredibly complex organ, with about 85 billion neurons and ten quadrillion synapses or connections. These neurons communicate via electrical impulses, which allow the brain to coordinate behaviour, sensation, thoughts, and emotions. This electrical activity can be measured using electrodes inserted into the brain or attached to the scalp, a process known as an electroencephalogram (EEG). EEGs can detect abnormalities in brain waves and are used to diagnose disorders such as epilepsy, Alzheimer's disease, and sleep disorders. Newer techniques, such as using fluorescent molecules and proteins, are also being developed to image and measure electrical activity in the brain, providing a clearer picture of how individual neurons work together. These advancements are helping researchers understand the complex electrical activity that underlies our thoughts, behaviours, and perceptions.
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What You'll Learn
Electrical brain stimulation
The brain functions by transmitting and receiving tiny electrical signals between nerve cells. Brain cells, or neurons, communicate via rapid electrical impulses, allowing the brain to coordinate behaviour, sensation, thoughts, and emotions.
There are several types of electrical brain stimulation therapies, including:
- Electroconvulsive therapy (ECT): ECT involves using an electric current flowing through electrodes placed at specific locations on the scalp to induce a brief seizure while a patient is under anaesthesia. ECT has been used since the 1930s and is highly effective for severe depression that has not responded to standard therapies. It uses higher levels of electrical current than transcranial direct current stimulation (tDCS). Unilateral ECT, which involves placing an electrode on only one side of the head, typically the right side, has a lower risk of memory loss.
- Transcranial magnetic stimulation (TMS): TMS uses a device worn over the forehead to stimulate specific areas of the brain by changing nearby magnetic fields. TMS is cleared by the FDA to treat depression and obsessive-compulsive disorder.
- Repetitive transcranial magnetic stimulation (rTMS): rTMS is a non-invasive therapy that uses an electromagnet to stimulate the brain using repeated low-intensity pulses. It is FDA-cleared for treatment-resistant depression, OCD, migraines, anxiety with depression, and smoking dependence.
- Deep brain stimulation (DBS): DBS involves surgically implanting electrodes in specific areas of the brain to generate electrical pulses. DBS is used to treat conditions such as Parkinson's disease, epilepsy, or tremors that do not improve with medications.
The effectiveness of electrical brain stimulation therapies varies, and research is ongoing to determine the best use of these therapies and their effectiveness in treating various disorders and conditions. While there is hype around tDCS for an expanding list of conditions, the current evidence does not support the claims made by commercial devices for home use.
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Electroencephalography (EEG)
EEG involves placing electrodes along the scalp to detect tiny electrical charges that result from the activity of brain cells. The electrodes are small metal disks with thin wires that record the electrical activity. The recordings are then interpreted by a clinical neurophysiologist or neurologist, who examines the basic waveform as well as brief bursts of energy and responses to stimuli.
EEG is used to evaluate several types of brain disorders, including epilepsy, brain lesions, Alzheimer's disease, certain psychoses, and sleep disorders like narcolepsy. It can also be used to determine the overall electrical activity of the brain, such as in cases of trauma, drug intoxication, or brain damage. Additionally, EEG can be used to monitor blood flow in the brain or neck blood vessels during surgery.
The electrical activity monitored by EEG originates in neurons in the underlying brain tissue. However, the recordings made by the electrodes on the surface of the scalp vary depending on their orientation and distance from the source of the activity. The value recorded can also be distorted by intermediary tissues and bones, which act like resistors and capacitors in an electrical circuit. Therefore, EEG poorly measures neural activity that occurs below the upper layers of the brain.
EEG activity reflects the summation of the synchronous activity of thousands or millions of neurons with similar spatial orientation. Pyramidal neurons of the cortex are believed to produce the most EEG signal due to their alignment and synchronous firing. The frequency bands observed in EEG range from 1 to 30 Hz and are subdivided into alpha, beta, delta, and theta waves, each associated with different states of brain functioning.
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Neurons and electrical activity
The brain is composed of billions of neurons, which are a type of electrically excitable cell that transmit information through electrical and chemical signals. Neurons communicate via rapid electrical impulses, which allow the brain to coordinate behaviour, sensation, thoughts, and emotion.
Neurons conduct electrical impulses using the Action Potential. This phenomenon is generated through the flow of positively charged ions across the neuronal membrane. Neurons maintain different concentrations of certain ions (charged atoms) across their cell membranes. There is a high concentration of sodium ions present outside the neuron, and a high concentration of potassium ions inside. The neuronal membrane also contains specialised proteins called channels, which form pores in the membrane that are selectively permeable to particular ions.
The sodium channels in the neuronal membrane open in response to a small depolarisation of the membrane potential. When an action potential depolarises the membrane, the leading edge activates other adjacent sodium channels, creating another spike of depolarisation. This leads to a wave of depolarisation spreading from the point of initiation. However, action potentials move in one direction because sodium channels have a refractory period following activation, during which they cannot open again.
The electrical activity of neurons can be measured by inserting an electrode into the brain, but this technique is labour-intensive and only allows researchers to record activity from one neuron at a time. A new technique, developed by MIT, uses a light-sensitive protein that can be embedded into neuron membranes, where it emits a fluorescent signal that indicates how much voltage a particular cell is experiencing. This method allows scientists to study how neurons behave, millisecond by millisecond, as the brain performs a particular function.
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Measuring electrical activity
Brain cells function using rapid electrical impulses, which form the basis of our thoughts, behaviour, and perception of the world. Measuring this electrical activity is challenging, and no single technology is sufficient. Therefore, scientists use a variety of tools, including functional magnetic resonance imaging (fMRI), electroencephalography (EEG), electrocorticography (ECoG), and imaging with light-sensitive proteins.
FMRI is the most well-known technology for recording neural activity, but it does not directly record neuron activity. Instead, it measures blood flow in the brain, which is a proxy for neuron activation. It provides detailed information about where and to what extent different functions are localised within the brain.
EEG is another widely used technique that directly records the brain's electrical activity via electrodes placed on the scalp. It detects abnormalities in brain waves and can be used to evaluate trauma, drug intoxication, brain death, and overall electrical activity. The test is safe, non-invasive, and causes no discomfort. However, it has poor spatial resolution and cannot identify specific locations in the brain.
ECoG is similar to EEG but requires the insertion of electrodes under the scalp through surgery, making it more invasive. It provides improved localisation of the activity source and records higher-frequency electrical activity.
More recently, researchers have developed light-sensitive proteins that can be embedded into neuron membranes. These proteins emit fluorescent signals that indicate the voltage a particular cell is experiencing, allowing for the study of neuron behaviour on a millisecond-by-millisecond basis. This technique has been successfully used in mouse brain tissue, zebrafish larvae, and the worm Caenorhabditis elegans.
Overall, each of these methods contributes to our understanding of the electrical activity in the brain, providing insights into the complex workings of the human mind.
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Brain cell activity imaging
Brain cells function using rapid electrical impulses, which form the basis of our thoughts, behaviour, and perception of the world. For a long time, it was challenging for scientists to observe how individual neurons work together in larger circuits. Traditionally, scientists have used electrodes inserted into the brain to measure electrical activity, but this method is time-consuming and labour-intensive.
However, recent breakthroughs in brain cell activity imaging have provided a clearer picture of brain cell activity. Researchers at Boston University and the Massachusetts Institute of Technology (MIT) have developed a new technique using a voltage-sensing molecule that fluoresces when brain cells are electrically active. This method has allowed researchers to observe the activity of many more individual neurons than previously possible in the brains of mice.
Another technique, called calcium imaging, allows for dense sampling of neural electrical activity but measures calcium, which is an indirect and slow measure. To address this, researchers at MIT have developed a light-sensitive protein that can be embedded in neuron membranes. This protein emits a fluorescent signal that indicates the voltage a particular cell is experiencing, allowing scientists to study how neurons behave millisecond by millisecond as the brain performs a specific function.
Furthermore, genetically-encoded fluorescent sensors derived from proteins have been developed for various aspects of neuronal activity. These sensors can be used with microscopy techniques to capture activity-dependent fluorescence from proteins expressed in neurons of transgenic animals. This approach has been successful in imaging whole-brain activity to understand behaviour, particularly in small organisms such as Caenorhabditis elegans, Drosophila melanogaster, and Danio rerio.
Overall, these advancements in brain cell activity imaging techniques have provided valuable insights into the understanding of brain function and behaviour.
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Frequently asked questions
Electrical activity in the brain refers to the flow of electrical charges and chemical ions that allow neurons to communicate with each other. This electrochemical activity is what enables the brain to coordinate thoughts, behaviour, sensation, and emotion.
Electrical activity in the brain can be measured through a test called an electroencephalogram (EEG). During an EEG, electrodes are attached to the scalp to detect tiny electrical charges that result from the activity of brain cells.
An EEG test can detect abnormalities in brain waves and electrical activity. It is often used to diagnose brain disorders such as epilepsy, Alzheimer's disease, psychoses, and sleep disorders like narcolepsy. EEG can also be used to evaluate trauma, drug intoxication, or brain damage in comatose patients.
