The brain processes vast amounts of information through the rapid communication of its fundamental units, the neurons. These specialized cells serve as the nervous system’s core processors, handling sensory input, motor commands, and complex cognitive functions. A neuron “fires” by generating a brief, powerful electrical pulse to transmit a message across its fiber to another cell. This electrochemical signaling occurs on a massive scale, as the human brain contains an estimated 86 billion neurons, each capable of connecting with thousands of others.
The Structure of a Communicating Neuron
A typical neuron possesses three distinct anatomical components that support communication. The cell body, or soma, contains the nucleus and organelles necessary for maintenance, and it integrates all signals received from other neurons. Extending from the soma are the dendrites, branched structures designed to receive signals. Dendrites gather input at synaptic contacts, and the sum of these incoming electrical and chemical messages determines whether the neuron will initiate its own signal. The third component is the axon, a single, long fiber that acts as the output cable, transmitting the electrical impulse (action potential) away from the cell body to the axon terminal.
Generating the Electrical Signal
The “firing” of a neuron is the rapid, temporary reversal of the electrical charge across the cell membrane, known as an action potential. When a neuron is at rest, it maintains a resting membrane potential of about -70 millivolts (mV), kept negative by ion pumps that concentrate sodium ions (Na+) outside and potassium ions (K+) inside the cell. The action potential begins when incoming signals raise the membrane potential toward the threshold voltage, typically -55 mV. If the threshold is reached, voltage-gated sodium channels open, triggering rapid depolarization as positive sodium ions rush into the cell. This influx makes the inside of the neuron momentarily positive, and the electrical spike propagates down the axon, operating on an “all-or-nothing” principle. Sodium channels then close, and voltage-gated potassium channels open, initiating repolarization as potassium ions flow out, returning the membrane potential to a negative state and preparing the neuron to fire again.
Crossing the Gap: Synaptic Transmission
When the electrical signal reaches the axon terminal, it must cross the synaptic cleft to communicate with the next neuron via chemical messengers called neurotransmitters. The action potential’s arrival causes voltage-dependent calcium channels to open, allowing calcium ions to flow into the cell. This influx causes synaptic vesicles, which contain neurotransmitters, to fuse with the presynaptic membrane and release their contents into the cleft via exocytosis. The neurotransmitters diffuse across the gap and bind to specific receptor proteins on the postsynaptic neuron, determining the effect. This effect is either excitatory (e.g., glutamate), causing depolarization and making the neuron more likely to fire, or inhibitory (e.g., Gamma-aminobutyric acid or GABA), causing hyperpolarization and suppressing the signal.
Organizing Information: Neural Circuits and Plasticity
The true power of the brain emerges from billions of cells organized into complex neural circuits. These circuits are functional groups of interconnected neurons that work together to perform specific tasks, such as processing visual input or storing a memory. Information flows through these pathways, constantly shaped by the balance of excitatory and inhibitory inputs. The brain’s ability to adjust and rewire these connections based on experience is known as neural plasticity, which underlies learning and memory. Physical changes associated with plasticity often occur at the synapse, where repeated activity strengthens connections, making signal transmission more efficient. This strengthening, known as long-term potentiation, is a fundamental mechanism for encoding new information.