Trystan Casey
The human brain is an incredibly complex organ, made up of billions of neurons that communicate with each other through electrical impulses. These electrical impulses allow the brain to coordinate behaviour, sensation, thoughts and emotions. The study of the brain's electrical activity is known as electroencephalography (EEG), and it typically involves measuring electrical signals with electrodes placed along the scalp. However, this method only captures the activity of neurons near the electrodes and can be invasive. To address these limitations, researchers have developed new techniques, such as voltage-sensing molecules and light-sensitive proteins, to visualize and measure the electrical activity of individual neurons more effectively. These advancements provide a clearer understanding of how neurons behave and contribute to the overall functioning of the brain.
| Characteristics | Values |
|---|---|
| Neurons in the brain communicate | Via rapid electrical impulses |
| Electrical signals in neurons | Generated by the motion of ions across cell membranes |
| Electrical signals | Used to move information within nerve cells |
| Chemical signals | Used to transfer information between two neighbouring neurons |
| Dendrites and the soma | Responsible for receiving and processing all incoming information |
| Normal electroencephalogram (EEG) | Varies by age |
| Electrical activity in the brain | Can be measured by inserting an electrode into the brain |
| Calcium imaging | Allows dense sampling of electrical activity but measures calcium, which is an indirect and slow measure |
| Fluorescent sensor | Allows imaging of neurons' electrical communication without electrodes |
| Electrical stimulation on the scalp | Can alter the charges in neurons without the need for surgery |
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What You'll Learn
Electroencephalography (EEG)
EEG is used to evaluate several types of brain disorders and detect abnormalities in brain waves. For example, when epilepsy is present, seizure activity will appear as rapid spiking waves on the EEG. People with lesions on their brains, resulting from tumours or strokes, may exhibit very slow EEG waves, depending on the size and location of the lesion. EEG can also be used to diagnose other disorders that influence brain activity, such as Alzheimer's disease, certain psychoses, and the sleep disorder narcolepsy.
The normal EEG varies by age. Prenatal and neonatal EEGs differ from adult EEGs, with fetuses in the third trimester and newborns displaying "discontinuous" and "trace alternant" brain activity patterns. "Discontinuous" electrical activity refers to sharp bursts of electrical activity followed by low-frequency waves, while "trace alternant" electrical activity describes sharp bursts followed by short high-amplitude intervals, indicating quiet sleep in newborns.
EEG results are interpreted by clinical neurophysiologists or neurologists, who visually inspect the waveforms or perform quantitative EEG analysis. The rhythmic activity is divided into bands by frequency, with designations arising from either the distribution of the activity over the scalp or its biological significance. Most of the cerebral signal observed in the scalp EEG falls within the range of 1-20 Hz, with frequencies subdivided into groups: alpha (8-13 Hz), beta (13-30 Hz), delta (0.5-4 Hz), and theta (4-7 Hz).
It is important to note that EEG has limitations. The meninges, cerebrospinal fluid, and skull can obscure the EEG signal, and it is mathematically impossible to reconstruct a unique intracranial current source for a given EEG signal. Therefore, EEG generally should not be used to make claims about global brain activity.
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Electrical impulses
The human brain is an incredibly complex organ, made up of over 86 billion neurons. These neurons communicate with each other via electrical impulses, allowing the brain to coordinate behaviour, sensation, thoughts, and emotions.
Measuring Electrical Activity in the Brain
Traditionally, electrical activity in the brain has been measured by inserting electrodes into the brain. This method is difficult, time-consuming, and typically only allows researchers to record activity from one neuron at a time. More recently, a technique called calcium imaging has been used to measure the electrical activity of neurons. This method is indirect and slow, as it measures calcium rather than electrical activity directly.
New Techniques for Measuring Electrical Activity
Researchers at Boston University and the Massachusetts Institute of Technology (MIT) have developed a new technique that provides the clearest picture yet of brain cell activity. This technique uses a voltage-sensing molecule that fluoresces when brain cells are electrically active. This allows researchers to see the activity of many individual neurons simultaneously.
Another technique, called electroencephalography (EEG), is a non-invasive method that uses electrodes placed along the scalp to record the spontaneous electrical activity of the brain. The EEG can detect bio signals that represent the postsynaptic potentials of pyramidal neurons in the neocortex and allocortex.
The Role of Ions in Electrical Impulses
Neurons generate electrical signals through the motion of ions across cell membranes. When a neuron is at rest, there are more negative ions inside and more positive ions outside, giving the neuron an overall negative charge. When brain activity occurs, positive ions rush into the neuron through channels in the membrane. When the charge gets high enough, the neuron sends a signal to communicate with nearby neurons.
The Impact of Electrical Stimulation on the Brain
Electrical stimulation on the scalp can alter the charges in neurons, affecting their activity. By increasing or decreasing the charge surrounding populations of neurons, scientists can make them more or less likely to send signals. This has implications for our understanding of brain function and for developing treatments for neurological disorders.
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Fluorescent sensors
The brain's neurons communicate via rapid electrical impulses, which allow it to coordinate behaviour, sensation, thoughts, and emotion. Traditionally, scientists have measured these impulses by inserting electrodes into the brain, but this is a difficult, time-consuming, and labour-intensive process.
To overcome these limitations, researchers at MIT have developed a light-sensitive protein that can be embedded into neuron membranes. This protein emits a fluorescent signal that indicates how much voltage a particular cell is experiencing. This technique, called fluorescent imaging, is non-invasive and allows scientists to study how neurons behave millisecond by millisecond as the brain performs a particular function.
One such fluorescent molecule is Archon1, which can be used in conjunction with optogenetic proteins to stimulate neuron activity. Another is SomArchon, which accumulates in the centre of neuron cell bodies, preventing interference from neighbouring neurons.
Other fluorescent sensors include:
- EOS (E [Glutamate] Optical Sensor) – used to report glutamate, the predominant excitatory neurotransmitter in the mammalian brain.
- CNiFERs (cell-based neurotransmitter fluorescent engineered reporters) – used to report the change in intracellular Ca2+ following GPCR activation.
- GRABNE (GPCR activation-based NE) sensors – used to detect NE transmission in both physiological and pathological processes.
- SF-iGluSnFR biosensor – used to detect the release of glutamate at individual synapses.
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Electrode placement
Electroencephalography (EEG):
EEG is a widely used non-invasive technique to record the electrical activity of the brain. The placement of electrodes in EEG follows the International 10-20 system, which ensures consistency in electrode naming across laboratories. In this system, electrodes are placed along the scalp, and 19 recording electrodes (plus ground and reference) are typically used. The number of electrodes can be adjusted based on the specific application and the area of the brain being studied. High-density arrays, containing up to 256 electrodes, can be used for increased spatial resolution. The placement of electrodes on the scalp affects the recorded signals, as the underlying brain structures contribute differently to the EEG.
Deep Brain Stimulation (DBS):
DBS is a surgical procedure where electrodes are implanted deep in the brain to deliver electrical stimulation to specific areas. The placement of electrodes in DBS is highly precise and depends on the symptoms being treated. For example, electrodes can be placed in the subthalamic nucleus (STN) for treating tremors, slowness, and rigidity associated with Parkinson's disease. The thalamus (VIM) is another target area for treating tremors, especially in essential tremor. Globus pallidus (GPi) is targeted for treating dystonia and dyskinesia, in addition to tremor and slowness. The accuracy of electrode placement in DBS is crucial and is confirmed through various tests, such as asking the patient to move their limbs or counting numbers.
Intracortical and Sub-dural Electrodes:
Intracortical electrodes are placed directly within the cortex of the brain, while sub-dural electrodes are placed just below the dura mater, the outermost membrane of the brain. These types of electrodes are used to detect abnormal electrical activity associated with neurological events like epilepsy. By using these electrodes in tandem, it is possible to discriminate and discretize artifacts from epileptiform activity and other severe neurological events.
Electrocorticography:
Electrocorticography involves the surgical placement of electrodes directly on the surface of the cortex, also known as "intracranial EEG." This technique provides a more direct measurement of electrical activity compared to scalp EEG.
Multielectrode Arrays:
Multielectrode arrays allow the monitoring of electrical activity from multiple neurons simultaneously. These arrays are placed in the brain tissue to record activity from multiple neurons in close proximity. However, it is challenging to record the activity of all neurons within a small piece of brain tissue due to their dense packing.
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Calcium imaging
Traditionally, measuring these electrical impulses has been done by inserting electrodes into the brain, but this technique is time-consuming and labour-intensive. Calcium imaging, on the other hand, allows for dense sampling of neural electrical activity. This technique involves genetically engineering neurons to contain a fluorescing molecule, such as Archon1 or SomArchon, that reveals electrical activity. These molecules light up when brain cells are electrically active, allowing researchers to see the activity of individual neurons.
While calcium imaging has provided valuable insights, it is a slow process. Voltage changes in neurons occur on a millisecond timescale, and calcium imaging has not yet been able to capture these rapid fluctuations. Researchers are now exploring voltage imaging as a potential solution to study the brain's electrical activity with higher temporal resolution.
Despite the challenges, calcium imaging remains a valuable tool for studying the brain's electrical activity, particularly in understanding the effects of neuromodulation therapies and the mechanisms of deep brain stimulation (DBS) in animal models of diseases such as Parkinson's disease.
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Frequently asked questions
The brain's electrical input is comprised of neurons that use electrical charges and chemicals called ions to communicate with each other. Neurons have an electrochemical charge that changes depending on whether the neuron is at rest or sending a signal.
When a neuron is at rest, there are more negative ions inside and more positive ions outside of it. When brain activity occurs, positive ions rush in through channels in the neuronal membrane. When the charge gets high enough, the neuron sends a signal to communicate with nearby neurons.
Neurons in the brain communicate via rapid electrical impulses that allow the brain to coordinate behaviour, sensation, thoughts, and emotions.
Scientists who want to study the brain's electrical activity usually measure these signals with electrodes inserted into the brain, but this technique is labour-intensive and time-consuming. A new technique uses a voltage-sensing molecule that fluoresces when brain cells are electrically active, allowing researchers to see the activity of many individual neurons.
