The Science Behind EEG – How Does Your Brain Create Electricity?

Introduction: Unveiling the Brain’s Electrical Symphony

Electroencephalography (EEG) is a non-invasive neuroimaging technique that measures the electrical activity of the brain. It provides a window into the dynamic and complex workings of the human mind, allowing us to observe the “brainwaves” that reflect underlying neural processes. But how does the brain, an organ composed of biological tissue, generate electricity? This paper will explore the fundamental principles of neurophysiology that make EEG possible, focusing on how neurons, the basic building blocks of the brain, generate and transmit electrical signals. Understanding these principles is crucial for interpreting EEG data and appreciating its significance in neuroscience, medicine, and emerging technologies like brain-computer interfaces. The brain’s electrical activity is not just a byproduct; it is the fundamental language of thought, perception, and action.

Meet Your Brain’s Power Grid: How Neurons Generate Electrical Signals!

What are Neurons, and How Do They Communicate?

The Neuron: The Brain’s Computational Unit

Neurons, or nerve cells, are the fundamental units of the nervous system, responsible for receiving, processing, and transmitting information. They are highly specialized cells, uniquely adapted to communicate rapidly and precisely over long distances. A typical neuron has a distinctive structure, consisting of:

1. Soma (Cell Body): The soma is the metabolic center of the neuron. It contains the nucleus, which houses the neuron’s DNA, and other organelles essential for cellular function. The soma integrates incoming signals and determines whether the neuron will “fire” (generate an action potential).
2. Dendrites: These are branching, tree-like extensions that project from the soma. Dendrites are the primary input zone of the neuron, receiving signals from other neurons. Their surfaces are studded with receptors, specialized protein molecules that bind to neurotransmitters (chemical messengers) released by other neurons. The more dendrites a neuron has, the more connections it can form with other neurons.
3. Axon: The axon is a single, long, slender projection that extends from the soma. It is the primary output zone of the neuron, carrying signals away from the soma to other neurons, muscles, or glands. Axons can vary dramatically in length, from less than a millimeter to over a meter in some cases (e.g., the axons that extend from your spinal cord to your toes).
4. Myelin Sheath: Many axons are covered in a myelin sheath, a fatty, insulating layer formed by glial cells (supporting cells in the nervous system). The myelin sheath acts like the insulation on an electrical wire, preventing signal leakage and significantly speeding up the transmission of electrical signals. Gaps in the myelin sheath, called nodes of Ranvier, play a crucial role in this accelerated signal transmission.
5. Axon Terminals (Synaptic Terminals/Buttons): These are the branched endings of the axon. They form synapses with the dendrites (or sometimes the soma) of other neurons. At the axon terminal, the electrical signal is converted into a chemical signal, triggering the release of neurotransmitters.

Neuron Communication: Electrical and Chemical Signals

Neuronal communication is a remarkably elegant process involving both electrical and chemical signals:

1. Electrical Signaling (Intra-neuronal): Information travels within a neuron as an electrical signal called an action potential. This is a rapid, transient change in the electrical potential (voltage) across the neuron’s cell membrane.
2. Chemical Signaling (Inter-neuronal): Communication between neurons occurs at the synapse, a specialized junction between the axon terminal of one neuron (the presynaptic neuron) and the dendrite (or soma) of another neuron (the postsynaptic neuron). When an action potential reaches the axon terminal, it triggers the release of neurotransmitters into the synaptic cleft,¹ the tiny gap between the neurons.
   • Synaptic Cleft: This is the physical space between the pre- and postsynaptic neurons. It’s incredibly small (about 20-40 nanometers wide), allowing for rapid diffusion of neurotransmitters.
   • Neurotransmitters: These are chemical messengers that transmit signals across the synapse. There are many different types of neurotransmitters, each with specific effects on the postsynaptic neuron. Some are excitatory, increasing the likelihood that the postsynaptic neuron will fire an action potential. Others are inhibitory, decreasing this likelihood. Key examples include:
     • Glutamate: The primary excitatory neurotransmitter in the brain.
     • GABA (gamma-aminobutyric acid): The primary inhibitory neurotransmitter in the brain.²
     • Dopamine: Involved in reward, motivation, movement, and attention.
     • Serotonin: Involved in mood regulation, sleep, appetite, and other functions.
     • Acetylcholine: Important for muscle contraction and also plays a role in learning and memory.
   • Receptors: Neurotransmitters exert their effects by binding to specific receptors on the postsynaptic neuron’s membrane. These receptors are like “locks” that can only be opened by the correct “key” (neurotransmitter). When a neurotransmitter binds to its receptor, it triggers a change in the postsynaptic neuron, either making it more or less likely to fire an action potential.
   • Reuptake: reabsorption of a neurotransmitter by a neurotransmitter transporter

The combined effect of all the excitatory and inhibitory inputs received by a neuron at any given moment determines whether it will generate its own action potential, thus continuing the chain of communication. This intricate interplay of electrical and chemical signaling allows for incredibly complex information processing within the brain.

The Role of Ions and Electrical Charges in Brain Function

The Neuron at Rest: The Resting Membrane Potential

To understand how neurons generate electrical signals, we must first grasp the concept of the resting membrane potential. When a neuron is not actively transmitting a signal, it is said to be “at rest.” However, this “rest” is not a state of inactivity; it’s a dynamic state of readiness, maintained by a difference in electrical charge between the inside and the outside of the neuron’s cell membrane.

• Ions: Ions are atoms or molecules that have gained or lost electrons, giving them a net electrical charge. The key ions involved in neuronal signaling are:
  • Sodium Ions (Na+): Positively charged (cations). Higher concentration outside the neuron.
  • Potassium Ions (K+): Positively charged (cations). Higher concentration inside the neuron.
  • Chloride Ions (Cl-): Negatively charged (anions). Higher concentration outside the neuron.
  • Organic Anions (A-): Large, negatively charged molecules (primarily proteins) that are trapped inside the neuron.
• The Cell Membrane: The neuron’s cell membrane is a phospholipid bilayer, a double layer of fat molecules that acts as a barrier, separating the inside of the neuron from the outside. This membrane is selectively permeable, meaning it allows some substances to pass through more easily than others. This selectivity is crucial for maintaining the resting potential.
• Ion Channels: Embedded within the cell membrane are ion channels, specialized protein structures that form pores through the membrane. These channels can be:
  • Leak Channels: Always open, allowing specific ions to passively diffuse across the membrane down their concentration gradients (from areas of high concentration to areas of low concentration).
  • Gated Channels: Can open and close in response to specific stimuli. These are crucial for generating action potentials. Types of gated channels include:
    • Voltage-gated channels: Open or close in response to changes in the membrane potential (voltage).
    • Ligand-gated channels: Open or close in response to the binding of a specific molecule (e.g., a neurotransmitter).
    • Mechanically-gated channels: Open or close in response to mechanical stimulation (e.g., pressure or stretch).
• The Sodium-Potassium Pump: This is an active transport protein embedded in the cell membrane. It uses energy (in the form of ATP – adenosine triphosphate) to pump sodium ions (Na+) out of the neuron and potassium ions (K+) into the neuron, against their concentration gradients. This pump is essential for maintaining the ionic imbalances that create the resting potential. It pumps three sodium ions out for every two potassium ions it pumps in, contributing to the net negative charge inside the neuron.

Due to the combined effects of: (1) the selective permeability of the membrane (particularly the presence of leak channels that are more permeable to K+ than Na+), (2) the action of the sodium-potassium pump, and (3) the presence of trapped organic anions inside the cell, the inside of the neuron is negatively charged relative to the outside. This difference in charge is the resting membrane potential, and it typically measures around -70 millivolts (mV). This negative potential represents stored electrical energy, ready to be released when the neuron is stimulated.

How Action Potentials Create Measurable Brainwaves

The Action Potential: A Rapid Electrical Signal

The action potential is a rapid, transient, and self-propagating change in the membrane potential. It’s the fundamental unit of electrical signaling in neurons, allowing them to transmit information quickly over long distances. Here’s a step-by-step breakdown of the process:

1. Resting Potential (-70 mV): As described above, the neuron is at rest, with a negative charge inside relative to the outside.
2. Depolarization: The neuron receives excitatory input from other neurons. Neurotransmitters bind to receptors on the dendrites, opening ligand-gated ion channels. If enough excitatory neurotransmitters (like glutamate) bind, they cause an influx of positive ions (mainly Na+) into the neuron. This influx of positive charge makes the inside of the neuron less negative, a process called depolarization.
3. Threshold: If the depolarization reaches a critical threshold (typically around -55 mV), a dramatic change occurs: voltage-gated sodium channels open. These channels are sensitive to changes in membrane potential.
4. Rising Phase (Rapid Depolarization): With the voltage-gated sodium channels open, there is a massive influx of Na+ ions into the neuron, driven by both the concentration gradient (higher Na+ outside) and the electrical gradient (positive ions attracted to the negative interior). This rapid influx of positive charge causes the membrane potential to reverse, becoming briefly positive inside relative to the outside (reaching around +40 mV). This is the defining characteristic of the action potential.
5. Falling Phase (Repolarization): Almost immediately after the sodium channels open, they begin to inactivate (close). Simultaneously, voltage-gated potassium channels open. These channels are slower to open than the sodium channels. With the potassium channels open, positively charged potassium ions (K+) flow out of the neuron, driven by both the concentration gradient (higher K+ inside) and the now-positive electrical gradient. This outflow of positive charge causes the membrane potential to return towards its negative resting value, a process called repolarization.
6. Hyperpolarization (Undershoot): The voltage-gated potassium channels stay open slightly longer than necessary, causing the membrane potential to briefly become even more negative than the resting potential (e.g., -80 mV). This is called hyperpolarization or the undershoot.
7. Return to Resting Potential: The sodium-potassium pump actively transports Na+ ions out of the neuron and K+ ions back into the neuron, restoring the original ionic concentrations and returning the membrane potential to its resting value of -70 mV. The neuron is now ready to fire another action potential.

The All-or-None Law

The action potential follows the all-or-none law. This means that if the depolarization reaches the threshold, an action potential will be generated, and it will always be of the same size and shape, regardless of the strength of the initial stimulus. If the depolarization does not reach the threshold, no action potential will occur. It’s like firing a gun: either the trigger is pulled hard enough to fire the bullet, or it isn’t. There’s no “half-fired” bullet. The intensity of a signal is encoded not by the size of the action potential, but by the frequency of action potentials (how many action potentials occur per unit of time) and the number of neurons firing.

Propagation of the Action Potential: The Traveling Wave

The action potential doesn’t just occur at one point on the neuron; it travels down the axon like a wave. This propagation is crucial for long-distance communication. Here’s how it works:

1. Local Depolarization: The initial depolarization at the axon hillock (where the axon joins the soma) triggers the opening of voltage-gated sodium channels in that region.
2. Spread of Depolarization: The influx of Na+ ions at one point on the axon causes a local depolarization that spreads to the adjacent region of the axon membrane.
3. Triggering of Adjacent Channels: This local depolarization triggers the opening of voltage-gated sodium channels in the adjacent region, initiating a new action potential there.
4. Unidirectional Propagation: The action potential travels in only one direction (down the axon) because the sodium channels that have just fired are in a refractory period, a brief period during which they cannot be opened again. This prevents the action potential from traveling backward.
5. Saltatory Conduction (in Myelinated Axons): In myelinated axons, the action potential “jumps” from one node of Ranvier (gap in the myelin sheath) to the next. This is called saltatory conduction. Because the ionic currents only flow across the membrane at the nodes, the action potential travels much faster than in unmyelinated axons. This is analogous to skipping instead of walking – you cover more ground in less time.

From Action Potentials to Brainwaves: Synchronization is Key

A single action potential in a single neuron produces an extremely weak electrical field, far too small to be detected by EEG electrodes on the scalp. EEG detects the synchronized activity of large populations of neurons. This synchronization is crucial for generating a measurable signal.

• Postsynaptic Potentials (PSPs): When neurotransmitters bind to receptors on the dendrites of a postsynaptic neuron, they cause small, local changes in the membrane potential called postsynaptic potentials (PSPs). These PSPs can be:
  • Excitatory Postsynaptic Potentials (EPSPs): Depolarize the membrane, making the neuron more likely to fire an action potential.³
  • Inhibitory Postsynaptic Potentials (IPSPs): Hyperpolarize the membrane, making the neuron less likely to fire an action potential.
• Summation: PSPs are graded potentials, meaning their size varies depending on the strength of the stimulus (e.g., the amount of neurotransmitter released). They also summate, or add together, both spatially (from multiple synapses) and temporally (over time). If the sum of EPSPs reaching the axon hillock is strong enough to reach the threshold, an action potential is triggered.
• Synchronous Firing: For EEG to detect a signal, thousands of neurons must be firing in a relatively synchronized manner. This means that their PSPs are occurring at roughly the same time, creating a larger, measurable electrical field. The degree of synchrony is influenced by many factors, including:
  • The type of cognitive activity: Different cognitive tasks (e.g., focusing attention, relaxing, sleeping) involve different patterns of neuronal activity.
  • The level of arousal: Alertness, drowsiness, and sleep are associated with different brainwave patterns.
  • Intrinsic brain rhythms: The brain has natural oscillatory activity, even in the absence of external stimuli. These rhythms are generated by complex interactions between different brain regions and neuronal populations.
  • External Stimuli

Why EEG Can Detect These Tiny Electrical Signals

EEG is able to detect the brain’s tiny electrical signals due to a combination of factors:

1. Summation of Postsynaptic Potentials: EEG primarily reflects the summed electrical activity of postsynaptic potentials (PSPs), not action potentials directly. PSPs are slower and longer-lasting than action potentials, making them more likely to summate and produce a detectable signal. Action potentials are too brief and localized to generate a large enough field to be detected at the scalp.
2. Spatial Summation: Thousands of neurons, particularly pyramidal neurons in the cerebral cortex, are oriented in a parallel fashion. When these neurons are activated synchronously, their electrical fields add together constructively (spatial summation). This creates a larger, more detectable signal at the scalp. The parallel orientation of the dendrites of pyramidal neurons is particularly important for generating the EEG signal.
3. Volume Conduction: The electrical signals generated by neurons are not confined to the neurons themselves. They spread through the surrounding brain tissue, cerebrospinal fluid (CSF), skull, and scalp before reaching the EEG electrodes. This phenomenon is known as volume conduction. While the signal is attenuated (weakened) as it travels through these tissues, it is still detectable with sensitive EEG equipment.
4. Electrode Placement: EEG electrodes are strategically placed on the scalp to pick up signals from different brain regions. The 10-20 system is a standardized international system for electrode placement, ensuring consistency across different studies and individuals. The placement of electrodes determines which brain regions are most strongly represented in the EEG recording.
5. Signal Amplification and Filtering: EEG signals are extremely small (measured in microvolts, µV). EEG amplifiers are used to boost the signal strength, making it large enough to be recorded and analyzed. EEG systems also use filters to remove unwanted noise and artifacts (e.g., electrical interference from the environment, muscle activity).
6. Reference Electrode: Because voltage is a differential measure, EEG recordings require a reference electrode. The choice of reference electrode can influence the recorded signal.

In essence, EEG provides a macroscopic view of the brain’s electrical activity, reflecting