The human nervous system splits into two major divisions that work as a single coordinated network: the central nervous system (CNS), made up of the brain and spinal cord, and the peripheral nervous system (PNS), the vast web of nerves that branches out to every other part of the body. The basic working unit of both is the neuron, a cell specialized for transmitting electrical and chemical signals. But the structural details of how these divisions are built, how they differ at the cellular level, and how they handle everything from waste clearance to self-repair are more layered than that textbook summary suggests.
What a Neuron Actually Looks Like
A neuron has three main parts: the cell body (soma), dendrites, and an axon. The dendrites are branching extensions that receive incoming signals from other neurons. The soma houses the nucleus and most of the cell’s machinery. The axon is a long, slender projection that carries electrical impulses away from the soma toward the next neuron or target tissue. In some motor neurons, the axon can stretch over a meter from the spinal cord to the foot.
Where the axon begins, right at the junction with the soma, sits a specialized stretch called the axon initial segment. This tiny zone punches above its weight. Recordings from rat cortical neurons and mouse cerebellar cells show that the axon initial segment actually dominates the electrical landscape around the neuron, contributing far more to the extracellular signal than the much larger soma does.1PubMed Central. The Axon Initial Segment is the Dominant Contributor to the Neuron’s Extracellular Electrical Potential Landscape The axon initial segment is where a neuron “decides” whether the incoming signals are strong enough to fire an action potential. It is the trigger zone.
The axon itself is structurally different from the rest of the neuron. Super-resolution microscopy has revealed that the axon’s inner skeleton is organized as a series of ring-like actin structures connected by spectrin filaments held under tension, a pattern not found in the soma or dendrites.2PubMed Central. Modeling of the axon membrane skeleton structure and implications for its mechanical properties This periodic scaffold likely gives axons their mechanical resilience, helping them withstand the stretching and bending that comes with everyday movement.
Glial Cells in the CNS
Neurons get most of the attention, but they are outnumbered and thoroughly supported by glial cells. In the central nervous system, three main types of glia do much of the structural and maintenance work.
Oligodendrocytes are the myelinating cells of the CNS. Each one extends flat, membrane-rich processes that wrap around segments of nearby axons, forming the myelin sheath, an insulating layer that dramatically speeds up electrical conduction.3PubMed Central. Oligodendrocyte, Astrocyte, and Microglia Crosstalk in Myelin Development, Damage, and Repair A single oligodendrocyte can myelinate segments of multiple axons at once, which is a key structural difference from its PNS counterpart.
Astrocytes have star-shaped bodies with numerous fine processes that contact blood vessels, synapses, and other neurons. They regulate the chemical environment around neurons, provide nutrients, and help form the blood-brain barrier. Microglia are the resident immune cells of the brain, constantly surveying for damage or infection. After trauma or in diseases like multiple sclerosis, both astrocytes and microglia undergo significant changes in shape and number, while oligodendrocytes can be destroyed in the lesion areas.4PubMed. Effects of interleukin-1 and tumor necrosis factor-alpha on astrocytes, microglia, oligodendrocytes, and glial precursors in vitro The interplay between all three glial types shapes both healthy brain development and the brain’s response to disease.
How the PNS Differs at the Cellular Level
The peripheral nervous system has its own cast of supporting cells, and the most prominent are Schwann cells. Like oligodendrocytes in the CNS, Schwann cells produce myelin, but they do it one-to-one: each myelinating Schwann cell wraps a single segment of a single axon. Beyond myelination, Schwann cells support fast and accurate communication between the CNS and the rest of the body.5PubMed Central. Beyond Wrapping: Canonical and Noncanonical Functions of Schwann Cells
Not all Schwann cells make myelin, though. The PNS also contains nonmyelinating Schwann cells, sometimes called Remak cells, which bundle groups of small-diameter axons together without wrapping them in myelin sheaths. There are also satellite cells in ganglia that surround neuron cell bodies, terminal Schwann cells at neuromuscular junctions, and enteric glia within the gut wall. All of these arise from neural crest cells during embryonic development.6PubMed Central. Schwann Cell Development and Myelination
Peripheral nerves are also wrapped in layers of connective tissue that serve as barriers and structural scaffolding. These sheaths organize nerve fibers into bundles and regulate what substances can reach the nerve fibers inside, somewhat analogous to the blood-brain barrier in the CNS.7PubMed Central. Barriers of the peripheral nerve
How Signals Travel Along Myelinated Axons
Myelin does more than just insulate. The high conduction speeds achieved in vertebrate axons are a direct consequence of the myelin sheath and the clustered sodium channels found at regularly spaced gaps in it, known as nodes of Ranvier.8Encyclopedia of Life Sciences. Myelin and Action Potential Propagation Instead of the electrical signal crawling along every inch of the axon membrane, it effectively jumps from one node to the next, a process called saltatory conduction.
The mechanics are more intricate than “jumping,” though. Detailed recordings have revealed a nanoscale fluid-filled space between the axon and its myelin wrapping, and the electrical potentials across this space create what researchers describe as a double-cable circuit. Rapid voltage spikes at the nodes travel ahead of slower, attenuated waves running under the myelin in the internodal space.9PubMed Central. Saltatory Conduction along Myelinated Axons Involves a Periaxonal Nanocircuit The result is conduction speeds up to roughly 100 meters per second in the largest myelinated fibers, compared with less than two meters per second in small unmyelinated ones.
What Happens at the Synapse
When an electrical signal reaches the end of an axon, it has to cross a gap to reach the next neuron, muscle cell, or gland. This junction is the synapse, and its structure is remarkably precise. Neural circuits transmit information through synapses, and the efficiency of that transmission depends heavily on the density of so-called active zones in the presynaptic terminal, the spots where neurotransmitter-filled vesicles are released.10PubMed Central. Presynaptic Active Zone Density during Development and Synaptic Plasticity
On the presynaptic side, the active zone is a scaffold loaded with molecules that orchestrate vesicle docking and fusion. On the postsynaptic side sits a dense meshwork of receptors, ion channels, and signaling proteins called the postsynaptic density.11PubMed Central. Ultrastructure of synapses in the mammalian brain Electron microscopy shows that synaptic vesicles in the presynaptic terminal are tethered to a mesh of filamentous proteins that keep them concentrated near the active zone. The binding strength of that mesh can be adjusted, which in turn changes how much neurotransmitter gets released.12PubMed. Membrane and cytoplasmic structure at synaptic junctions in the mammalian central nervous system This structural tunability is one reason synapses are not simple on-off switches: they can be dialed up or down depending on recent activity.
The Internal Highway Inside Axons
Because axons can be extremely long relative to the cell body, neurons face a logistics problem: how do you get proteins, organelles, and signaling molecules from the soma all the way to the axon terminal, and how do you shuttle used or damaged materials back? The answer is axonal transport, powered by molecular motor proteins that walk along microtubule tracks inside the axon.
Two motor families do most of the heavy lifting. Kinesin motors carry cargo in the anterograde direction, away from the cell body toward the axon tip. Dynein motors handle the retrograde direction, hauling material back toward the soma.13PubMed Central. Dynein is the motor for retrograde axonal transport of organelles When researchers injected antibodies that blocked kinesin, both anterograde and retrograde transport of certain cargo was impaired, suggesting the two systems are more interdependent than a simple two-lane highway. Blocking dynein, by contrast, selectively shut down retrograde transport without affecting anterograde movement.14PubMed. Impairment of anterograde and retrograde neurofilament transport after anti-kinesin and anti-dynein antibody microinjection in chicken dorsal root ganglia
Different types of cargo also rely on different kinesin family members. One kinesin variant, for example, is a major contributor to fast anterograde transport of neuropeptide-filled vesicles but contributes little to mitochondrial transport, which depends on a different motor.15PubMed Central. Identification of an axonal kinesin-3 motor for fast anterograde vesicle transport that facilitates retrograde transport of neuropeptides Failures in axonal transport are implicated in neurodegenerative diseases, which makes sense when you consider that a motor neuron’s axon terminal may be more than a meter from its soma and entirely dependent on this supply chain.
The Enteric Nervous System, the Gut’s Own Brain
The traditional CNS-versus-PNS framing leaves out a division that has increasingly earned its own spotlight: the enteric nervous system. This is a complex network of neurons and glia embedded in the walls of the gastrointestinal tract, running from the esophagus to the rectum.16PubMed Central. The enteric nervous system It contains hundreds of millions of neurons, and it can function more or less independently of the CNS, controlling gut motility, secretion, blood flow, and even local immune responses.17PubMed. The enteric nervous system I: organisation and classification
Structurally, the enteric nervous system is organized into two main nerve plexuses sandwiched between the layers of the gut wall. The myenteric plexus sits between the muscle layers and primarily governs movement, while the submucosal plexus regulates secretion and local blood flow. The myenteric plexus forms an enclosed tubular structure partially surrounded by a sheath of supporting cell processes, and it has its own barrier properties that limit what substances from the bloodstream can reach it.18PubMed. Properties of the enteric nervous system: limitation of access of intravascular macromolecules to the myenteric plexus and muscularis externa That structural independence helps explain why the enteric nervous system can keep the gut working even when connections to the brain are severed.
Where CNS and PNS Come From During Development
Despite their structural differences, the CNS and PNS originate from the same embryonic tissue: the neural plate. During early development, the neural plate folds inward to form the neural tube, which expands to become the brain and spinal cord. Cells at the margins of the neural plate peel off and migrate throughout the embryo as neural crest cells. These migrating cells generate the majority of PNS neurons and all of its glia.19PubMed Central. From classical to current: analyzing peripheral nervous system and spinal cord lineage and fate
What makes this remarkable is that individual precursor cells in the neural plate appear to be multipotent, meaning a single cell can give rise to both CNS derivatives (staying in the neural tube) and PNS derivatives (leaving as neural crest).20PubMed. Origins and developmental potential of the neural crest The decision to become a brain neuron or a sensory ganglion cell depends on where a cell ends up and what chemical signals it encounters along the way, not on some pre-assigned identity. Neural crest cells also form the facial skeleton, pigment cells of the skin, and parts of the heart, which is why defects in neural crest migration can produce a surprisingly wide range of developmental disorders affecting structures that seem, at first glance, unrelated to the nervous system.
The Blood-Brain Barrier and Neurovascular Unit
The brain’s internal environment is tightly regulated by a structure called the neurovascular unit. This is not a single cell type but a collaborative assembly of neurons, glial cells, specialized endothelial cells lining brain blood vessels, pericytes (cells that wrap around small vessels), and the surrounding extracellular matrix. Together, these components control the permeability of the blood-brain barrier, which prevents most blood-borne substances from freely entering the brain.21PubMed Central. Research developments in the neurovascular unit and the blood‑brain barrier
Within this unit, the roles are specialized. Astrocytes provide nutrients and help maintain chemical balance. Microglia regulate inflammation. Endothelial cells and pericytes physically strengthen the barrier. The reason this matters for understanding nervous system structure is that the brain’s blood vessels are not just plumbing: they are an integral structural component, and their tight junctions create a barrier far more restrictive than what exists in peripheral capillaries. Disruption of the neurovascular unit is a factor in stroke, Alzheimer’s disease, and multiple sclerosis.
The Glymphatic System and Waste Clearance
The brain also lacks conventional lymphatic vessels in its tissue, which raised a long-standing question about how it clears metabolic waste. The answer, discovered relatively recently, is the glymphatic system, a network of perivascular tunnels formed by astrocyte end-feet that flushes cerebrospinal fluid through brain tissue and carries away soluble waste products.22PubMed Central. The Glymphatic System: A Beginner’s Guide
The glymphatic system acts as a front end for waste clearance, draining into a genuine lymphatic network associated with the membranes covering the brain and with large vessels exiting the skull.23PubMed Central. The Glymphatic System and Waste Clearance with Brain Aging: A Review Glymphatic activity is most efficient during sleep, which is part of why chronic sleep deprivation may be harmful to brain health. Reduced glymphatic function has been linked to the accumulation of proteins associated with neurodegenerative disease, making it a structural feature with significant clinical implications.
Structural Plasticity at the Synapse
Nervous system structure is not static. One of the most vivid examples of ongoing structural change is the remodeling of dendritic spines, the tiny protrusions on dendrites where most excitatory synapses form. Both internal and external stimuli can alter dendritic spine density and shape on the order of minutes, and these structural changes are closely tied to learning, memory, and other cognitive processes.24PubMed Central. Structural and functional plasticity of dendritic spines – root or result of behavior?
When a synapse is strengthened through a process called long-term potentiation, the associated dendritic spine physically enlarges. Simultaneously, a host of structural and signaling proteins rush into the spine, including actin-remodeling molecules and receptor subunits. Some scaffolding proteins, however, remain stable and do not change their amount for at least half an hour after the strengthening event.25Neuron. Structural and Molecular Remodeling of Dendritic Spine Substructures during Long-Term Potentiation The fact that some spine components remodel rapidly while others hold firm gives synapses a mix of flexibility and stability. It is how you can learn something new without erasing what you already know.
Why CNS and PNS Regenerate Differently
One of the most consequential structural differences between the CNS and PNS is their capacity for self-repair. Cut a peripheral nerve and, given the right conditions, the axons can regrow. Schwann cells in the PNS respond to injury by clearing debris and forming a guidance track for regenerating axons. This is why people can regain function after certain nerve injuries in limbs.
The CNS is a different story. Axons in the mature brain and spinal cord generally fail to regenerate after injury, in part because reactive glial cells produce barriers like astrocytic scars and inhibitory molecules, and myelin debris itself contains signals that discourage regrowth.26Cell Metabolism. Glial Metabolic Rewiring Promotes Axon Regeneration and Functional Recovery in the Central Nervous System The very structural features that make the CNS stable and protected under normal conditions become obstacles after damage. Research into overcoming these barriers, including manipulating the metabolism of glial cells, is one of the most active areas in neuroscience.
How Aging Changes Nervous System Structure
Even without disease, normal aging brings measurable structural changes to the nervous system. Brain tissue gradually atrophies, neurotransmitter levels shift, and cellular damage accumulates.27PubMed Central. Normal Aging Induces Changes in the Brain and Neurodegeneration Progress: Review of the Structural, Biochemical, Metabolic, Cellular, and Molecular Changes These changes are milder than what happens in neurodegenerative disease, but they are real and affect cognition, reaction time, and sensory processing.
Glial cells undergo their own age-related changes in both the CNS and PNS. Astrocytes become enlarged and accumulate structural proteins. Oligodendrocytes and Schwann cells show alterations that can degrade the myelin sheath, potentially slowing signal conduction. Microglia become less responsive: their ability to migrate to injury sites, clear debris, and proliferate all decline. In sensory ganglia, gaps appear between the satellite cells that normally enclose neuron cell bodies, leaving neuronal surfaces exposed.28PubMed Central. Quantitative, structural and molecular changes in neuroglia of aging mammals: A review Understanding these structural changes helps explain why older adults are more vulnerable to neurological injury and slower to recover from it.
Imaging the Living Nervous System
Much of what we now know about nervous system structure in living people comes from advances in neuroimaging. One technique that has been particularly revealing for structural questions is diffusion tensor imaging, which tracks the movement of water molecules along white matter tracts in the brain. Because water diffuses preferentially along the length of axon bundles, this approach can map the trajectories of major fiber pathways and estimate how intact and well-connected they are.29PubMed Central. Exploring the Frontiers of Neuroimaging: A Review of Recent Advances in Understanding Brain Functioning and Disorders The metrics derived from these scans are now used in research on brain development, aging, traumatic brain injury, and psychiatric disorders, bridging the gap between microscopic anatomy studied in the lab and the large-scale wiring patterns of an intact human brain.
An Evolutionary Perspective on Centralization
The centralized nervous system we carry around is not the only way to wire an organism. Many invertebrates manage with a diffuse nerve net, a mesh of interconnected neurons spread throughout the body with no clear central hub. Analysis of nervous system complexity across animal groups suggests that diffuse nerve nets likely came first, and that centralized brains evolved independently at least four times across the animal kingdom.30Proceedings of the National Academy of Sciences. Evolution of centralized nervous systems: Two schools of evolutionary thought Whether the last common ancestor of all bilaterally symmetrical animals already had some form of brain, or whether it possessed only a diffuse nerve plexus, remains one of the genuinely unresolved debates in neuroscience. The evidence cuts in different directions depending on whether you look at anatomy or developmental genetics, and researchers have been arguing about it for decades.