Neuron activity is the process of electrical and chemical signaling that forms the basis of the nervous system. This communication network allows the brain to perceive and process information, enabling responses from simple reflexes to complex thoughts. The brain’s billions of neurons function like an electrical grid, with each one contributing to the overall system.
The Electrical Signal of a Neuron
A neuron’s ability to transmit information begins with its resting state, where its interior is negatively charged relative to the outside at about -70 millivolts (mV). This resting membrane potential is maintained by sodium-potassium pumps, which ensure a higher concentration of potassium ions inside the cell and sodium ions outside.
If a stimulus is strong enough to raise the membrane potential to a threshold of -55 mV, it triggers an action potential. This all-or-nothing electrical impulse causes voltage-gated sodium channels to open, allowing a flood of positive sodium ions into the cell. This influx causes a rapid depolarization, momentarily making the neuron’s interior positive.
This change propagates down the axon, a long, tail-like structure. Following the spike, potassium channels open to allow positive potassium ions to exit, and the sodium channels close. This repolarizes the cell, returning the membrane to its negative resting state so it can fire again.
The entire sequence is incredibly brief. To ensure the signal travels efficiently over long distances, many axons are insulated by a fatty layer called the myelin sheath. This sheath speeds up the propagation of the electrical impulse.
Communication Between Neurons
When an action potential reaches the axon terminal, communication becomes chemical across a microscopic gap called the synaptic cleft. This junction, including the presynaptic terminal, the cleft, and the postsynaptic receiving cell, is known as a synapse.
The action potential’s arrival opens voltage-gated calcium channels, allowing calcium ions into the cell. This influx prompts synaptic vesicles, tiny sacs filled with chemical messengers called neurotransmitters, to fuse with the membrane. The vesicles then release their neurotransmitters into the synaptic cleft.
These neurotransmitter molecules diffuse across the gap and bind to specific receptors on the postsynaptic neuron. This binding initiates a response that depends on the neurotransmitter and its receptor. The effect can be either excitatory or inhibitory.
An excitatory response makes the receiving neuron more likely to fire its own action potential. In contrast, an inhibitory response makes it less likely to fire. A single neuron integrates thousands of these inputs to determine if it will generate an action potential.
From Signals to Functions
The coordinated firing of millions of neurons in specific pathways, or neural circuits, gives rise to human functions. These circuits are structured information pathways that connect different brain regions, allowing them to work in concert. The pattern and timing of activity within these networks translate into a recognizable function.
For example, recognizing a familiar face involves a specific macrocircuit. Visual information travels from the eyes to the primary visual cortex and then to areas in the temporal lobe. The synchronized firing of a population of neurons in this area represents the specific face being seen.
Similarly, recalling a memory involves reactivating a neural network that was strengthened when the memory was formed. Contracting a muscle requires motor neurons in the brain to send signals down the spinal cord to the leg muscles. The strength of the action is determined by the frequency and pattern of the neurons firing.
These functional networks are dynamic and can be strengthened or weakened through experience. Excitatory circuits promote neural activity, while inhibitory circuits provide balance by suppressing it. This interplay allows the brain to process information, make decisions, and execute actions.
How Neuron Activity is Studied
Scientists use non-invasive methods to observe the collective activity of neurons in the human brain. These techniques provide a window into brain function without requiring surgery. Two widely used methods are electroencephalography (EEG) and functional magnetic resonance imaging (fMRI).
Electroencephalography directly measures the brain’s electrical activity. Electrodes on the scalp detect the summed electrical fields from the firing of large populations of neurons. While EEG cannot pinpoint single-neuron activity, it has excellent temporal resolution, measuring changes on a millisecond timescale.
Functional magnetic resonance imaging measures brain activity indirectly by detecting changes in blood flow and oxygen levels. When a brain region is active, it requires more oxygenated blood, which fMRI detects. While fMRI has superior spatial resolution to pinpoint active regions, its temporal resolution is much slower than EEG.
Factors Modifying Neuron Activity
Neuron activity is a dynamic process modified by experiences and behaviors. This adaptability, known as neuroplasticity, is the brain’s ability to reorganize its structure and function. When we learn a new skill, the synaptic connections between the involved neurons are strengthened through repeated activation.
Sleep plays a part in regulating and consolidating neuron activity. During sleep, the brain consolidates memories by replaying and strengthening their associated neural connections. Sleep also performs synaptic pruning, clearing away unnecessary connections to maintain efficiency and make space for new learning.
External substances can also directly alter neuron activity. Caffeine functions by blocking adenosine receptors in the brain. Adenosine is a neurotransmitter that promotes relaxation, so by inhibiting it, caffeine increases alertness and neuronal firing rates.