Neurons receive signals primarily through their dendrites, the branch-like extensions that spread outward from the cell body. These structures act as a neuron’s main antenna system, collecting chemical messages from neighboring cells and converting them into electrical activity. But dendrites aren’t the only reception site. Signals can also arrive directly at the cell body and, in some cases, at the axon itself.
Dendrites: The Primary Reception Site
A typical neuron has dozens to thousands of dendrites branching off its cell body, creating an expansive tree-like structure designed to gather incoming signals. These branches are covered in tiny protrusions called dendritic spines, and the vast majority of excitatory connections in the brain form at these spines. Each spine contains a dense collection of chemical receptors, signaling molecules, and protein-building machinery packed into a compartment smaller than a micron across.
Dendritic spines are not static. They grow, change shape, and retract in response to neural activity. A newly formed spine can respond to chemical signals almost immediately after it appears, though its signaling capacity matures over time. Mature spines allow more calcium to flow in during signaling, which strengthens the connection. This constant remodeling is one of the physical bases of learning and memory.
For a long time, dendrites were thought to be passive cables that simply forwarded signals toward the cell body. That turns out to be wrong. Dendrites contain their own voltage-sensitive channels that can amplify weak signals or generate local electrical spikes independently, before the signal ever reaches the cell body. These dendritic spikes help sharpen the timing and reliability of a neuron’s output, essentially giving dendrites their own processing power rather than just relay duty.
How Chemical Signals Cross the Gap
When a signal arrives at a connection point between two neurons (a synapse), the sending neuron releases chemical messengers into a tiny gap between the cells. These molecules drift across and bind to receptors on the receiving neuron’s surface. That binding opens channels in the membrane or triggers internal signaling cascades, converting the chemical message back into an electrical one.
The receiving neuron has two broad categories of receptors that work at very different speeds. The first type is a channel that opens directly when the chemical messenger binds to it. These are the fastest receptors in the brain, responding in less than a millisecond and closing within a few milliseconds. They handle rapid, moment-to-moment communication. The second type doesn’t open a channel directly. Instead, it triggers a chain of molecular events inside the cell that can alter the neuron’s behavior over seconds to minutes. This slower pathway allows for more nuanced, longer-lasting changes in how the neuron responds.
The Cell Body and Axon Also Receive Signals
While dendrites handle the bulk of incoming traffic, the cell body (soma) is another important reception site. Certain inhibitory neurons called basket cells form connections directly onto the cell body of their target, delivering powerful “stop” signals close to where the neuron makes its firing decision. Because the cell body is electrically close to the trigger zone, inhibition delivered here is especially effective at silencing a neuron.
Even the axon, typically thought of as the output cable, can receive signals. Specialized inhibitory neurons called chandelier cells target the initial segment of the axon, the stretch right where it leaves the cell body. This is the exact spot where a neuron decides whether to fire. Inhibition at this location doesn’t just make the neuron less excited; it actually raises the threshold the neuron needs to reach before it can fire at all. About 60% of the inhibitory connections on this part of the axon in mouse visual cortex come from chandelier cells. This axonal inhibition acts almost instantaneously, creating brief but powerful windows where the neuron is much harder to activate.
How a Neuron Combines All Its Inputs
A single neuron can receive thousands of signals simultaneously, some excitatory (pushing it toward firing) and some inhibitory (pushing it away from firing). The neuron adds all of these together in a process called summation. If two excitatory signals arrive at different locations on the dendrites at roughly the same time, their effects combine. Neither one alone might be strong enough to make the neuron fire, but together they can push the voltage past the threshold.
Inhibitory signals subtract from this total. An inhibitory input arriving alongside an excitatory one reduces or cancels the excitatory effect. Whether the neuron ultimately fires depends on the balance: if excitation wins, the neuron sends a signal down its axon. If inhibition wins, the neuron stays silent. This constant, real-time arithmetic is how neurons make decisions, filtering meaningful patterns from background noise across the brain.
Electrical Synapses: A Direct Connection
Not all signal reception involves chemicals. Some neurons are connected by gap junctions, physical protein channels that bridge two cells directly. These allow electrical current to flow straight from one neuron into another without any chemical intermediary. The result is extremely fast transmission, faster than chemical synapses, because there’s no delay for releasing and detecting a chemical messenger.
The trade-off is power. Chemical synapses have built-in amplification: a small amount of released chemical can trigger a large response in the receiving cell. Electrical synapses lack this, so the signal that arrives is only as strong as what the sending neuron pushed through the gap. Electrical synapses are common in circuits that need precise timing and synchronization, such as those coordinating rhythmic activity in the brainstem.
Sensory Neurons: Receiving Signals From the World
Some neurons don’t receive their signals from other neurons at all. Sensory neurons at the body’s periphery detect physical and chemical stimuli directly. Their specialized endings contain channels that open in response to pressure, temperature, or specific chemicals. When skin is pressed, for instance, channels sensitive to mechanical deformation open and allow ions to flow in, generating an electrical signal. Temperature-sensitive channels respond to heat above roughly 43°C (109°F) or to cold. These are the same channels activated by capsaicin in chili peppers, which is why spicy food feels hot even though nothing is actually burning.
Other sensory neurons are tuned to light (in the retina), sound vibrations (in the inner ear), or airborne chemicals (in the nose). In each case, the neuron’s reception site is a specialized structure that converts a specific type of environmental energy into the universal language of the nervous system: electrical signals. During inflammation, chemicals released by damaged tissue can sensitize these endings, lowering the threshold for activation and making normal stimuli feel painful.