Voltage-gated sodium channels are found throughout the body, concentrated in neurons, heart muscle, skeletal muscle, and even some non-excitable cells like immune cells and brain support cells. Within neurons, they cluster at two key spots: the axon initial segment, where electrical signals are generated, and the nodes of Ranvier, where signals are boosted as they travel along nerve fibers. Nine distinct subtypes (Nav1.1 through Nav1.9) are distributed across different tissues, each tuned to the electrical needs of the cells they serve.
Key Locations Within Neurons
The highest concentration of sodium channels in any neuron sits at the nodes of Ranvier, the tiny gaps between the insulating myelin sheaths that wrap around nerve fibers. Density at these nodes reaches roughly 1,000 to 2,000 channels per square micrometer, compared to just 3 to 4 channels per square micrometer at the axon initial segment. Despite having far fewer channels, the axon initial segment is where action potentials are born. It acts as the neuron’s trigger zone, converting incoming signals into the electrical impulses that shoot down the axon.
These two sites rely on a scaffolding protein called ankyrin-G to hold sodium channels in place, but they assemble in opposite ways. The axon initial segment builds from the inside out: the neuron accumulates ankyrin-G on its own, which then recruits sodium channels to the site. Nodes of Ranvier, by contrast, assemble from the outside in. In the peripheral nervous system, the surrounding Schwann cells direct the process by positioning an adhesion molecule that pulls ankyrin-G (and with it, sodium channels) into the node.
Brain Isoforms: Nav1.1, Nav1.2, and Nav1.6
Four subtypes handle most of the electrical signaling in the central nervous system: Nav1.1, Nav1.2, Nav1.3, and Nav1.6. Nav1.3 is primarily active during embryonic development, while the other three dominate after birth.
Nav1.1 and Nav1.2 divide the brain’s workload in a surprisingly tidy way. Their expression is largely mutually exclusive, meaning the same cell typically expresses one or the other. In the neocortex (the outer layer of the brain responsible for higher-order thinking), Nav1.1 appears in both inhibitory and excitatory neurons, distributed across all cortical layers. In the hippocampus (critical for memory), Nav1.1 is restricted almost entirely to inhibitory neurons, specifically the fast-spiking types that regulate the timing and rhythm of brain circuits.
Nav1.2, meanwhile, is found in about 95% of excitatory neurons, including nearly all excitatory cells in the neocortex and hippocampus. A small fraction also shows up in certain inhibitory neuron subtypes. Nav1.6 is the dominant channel at mature nodes of Ranvier throughout the brain and spinal cord, making it essential for rapid signal conduction along myelinated nerve fibers.
Heart Muscle: Nav1.5
Nav1.5 is the primary sodium channel in the heart and is responsible for the fast electrical upstroke that triggers each heartbeat. Within a heart muscle cell, Nav1.5 channels occupy at least two distinct pools. The first sits at the intercalated discs, the specialized junctions where neighboring heart cells physically and electrically connect. This pool is critical for passing the electrical impulse from one cell to the next, keeping the heartbeat coordinated.
The second pool is spread along the lateral membrane of the cell, including the transverse tubules (deep infoldings of the cell surface that carry electrical signals into the cell’s interior). For years, researchers assumed only the intercalated disc channels mattered for conduction. More recent work shows the lateral membrane pool also contributes to how efficiently the heart conducts electrical signals, and it interacts with a different set of anchoring proteins than the channels at the intercalated discs.
Skeletal Muscle: Nav1.4
Skeletal muscle relies on Nav1.4, which is embedded across the entire surface membrane of muscle fibers. These channels are present at especially high density right beneath the neuromuscular junction, the point where a motor nerve meets the muscle fiber. When a nerve signal arrives, it triggers a small electrical event at the junction. Nav1.4 channels clustered there amplify that signal into a full action potential that races along the muscle fiber and causes contraction.
This synaptic clustering depends on a receptor called MuSK. When MuSK function is disrupted experimentally, Nav1.4 levels at the neuromuscular junction drop by about 27%, while levels elsewhere on the muscle fiber stay normal. That localized reduction is enough to cause measurable problems with muscle excitability, highlighting how important precise channel placement is for normal muscle function.
Pain-Sensing Nerves: Nav1.7, Nav1.8, and Nav1.9
Three subtypes are concentrated in the peripheral nervous system’s pain-sensing neurons (nociceptors), which have their cell bodies in the dorsal root ganglia, small clusters of nerve cells tucked alongside the spinal cord. Nav1.7 is preferentially expressed in these sensory neurons, as well as in sympathetic ganglia and trigeminal ganglia (which serve the face). It accumulates at the nerve endings themselves, right where painful stimuli are first detected, and helps convert those stimuli into electrical signals.
Nav1.8 is even more tissue-specific, found selectively in sensory neurons of the dorsal root and trigeminal ganglia. Together, Nav1.7 and Nav1.8 set the excitability threshold of pain-sensing nerves. Genetic mutations that make Nav1.7 overactive cause severe pain disorders, while mutations that knock it out entirely produce a rare inability to feel pain at all. Nav1.9, also selective to sensory neurons, contributes to the slow, sustained signaling involved in inflammatory pain.
Surprising Locations: Glia and Immune Cells
Sodium channels aren’t limited to cells that fire action potentials. Astrocytes, the star-shaped support cells of the brain, express Nav1.5 (the same subtype found in the heart). In astrocytes, these channels don’t generate electrical impulses. Instead, they appear to help maintain the balance of sodium and potassium ions by providing a route for sodium to enter the cell, which keeps a critical enzyme (the sodium-potassium pump) running.
Microglia, the brain’s resident immune cells, express Nav1.6. Here the channels contribute to functions like engulfing debris (phagocytosis), migrating toward sites of injury, and releasing inflammatory signaling molecules. Sodium channels have also been documented in peripheral immune cells such as macrophages, where they similarly influence migration, phagocytosis, and proliferation. These roles are still being mapped out, but they suggest voltage-gated sodium channels have a much broader biological footprint than their textbook role in nerve and muscle signaling.