What Are the Main Parts of the Nervous System?

The nervous system divides into two main parts: the central nervous system, which is the brain and spinal cord, and the peripheral nervous system, which is essentially everything else, including the nerves that branch out to your limbs, organs, and skin. That clean two-part split is where most textbook diagrams stop, but the real picture is richer and stranger. The peripheral side has its own subdivisions that handle voluntary movement, automatic organ regulation, and even an independent network in your gut that operates with surprising autonomy.

The Central Nervous System

Your brain and spinal cord together make up the central nervous system, or CNS. Think of it as the command center. The brain handles everything from conscious thought and memory to regulating body temperature and breathing, while the spinal cord serves as the main highway connecting the brain to the rest of the body. The spinal cord controls the voluntary muscles of your limbs and trunk, receives sensory information from those regions, and also manages much of what happens in the organs and blood vessels of the chest, abdomen, and pelvis.¹

Although the spinal cord is often described as a series of segments, it is actually a continuous cylinder of neural tissue. The “segments” people refer to are defined by where pairs of spinal nerves branch off. These nerve roots bundle together and exit the spine at regular intervals, giving the cord its segmented appearance even though the tissue itself is uninterrupted.¹

The brain itself is enormously complex, but the broad layout is worth knowing. The cerebral cortex, the wrinkled outer layer, handles higher-order functions like language, reasoning, and sensory perception. Beneath it sit structures involved in emotion, memory consolidation, and movement coordination. The brainstem, which connects the brain to the spinal cord, manages many of the automatic processes that keep you alive, like heart rate and breathing rhythm.

The Peripheral Nervous System

Everything outside the brain and spinal cord belongs to the peripheral nervous system (PNS). The PNS is made up of cranial and spinal nerves that carry signals between the CNS and the rest of your body.² Spinal nerves emerge from the spinal cord and serve the trunk, arms, and legs. Cranial nerves emerge directly from the brain and serve the head and face, controlling things like eye movement, facial sensation, hearing, and taste.

Interestingly, whether cranial nerves truly belong to the PNS has been debated for a long time. Some researchers consider them extensions of the central nervous system rather than purely peripheral structures, because several cranial nerves are deeply integrated with brain tissue in ways that spinal nerves are not.² For practical purposes, though, they’re grouped with the PNS in most references.

The PNS itself splits into two functional divisions. The somatic nervous system handles voluntary actions: you decide to pick up a cup, and somatic motor nerves carry that command to your arm muscles, while somatic sensory nerves carry back information about the cup’s weight and temperature. The autonomic nervous system, by contrast, manages the things you don’t consciously control, from your heartbeat and digestion to how dilated your pupils are.

The Autonomic Nervous System and Its Branches

The autonomic nervous system has three branches, not the two that many people learn in school. The sympathetic branch ramps things up during stress or physical activity, accelerating heart rate, widening airways, and diverting blood to muscles. The parasympathetic branch does roughly the opposite, promoting rest, digestion, and energy conservation. These two systems don’t simply toggle on and off. Brain imaging research has shown that sympathetic and parasympathetic activity involves widespread and distinct networks across the cortex, subcortical structures, and cerebellum, all working in parallel rather than as a simple seesaw.³

The third branch, the enteric nervous system, sits in the walls of your gastrointestinal tract. It is sometimes called the “second brain,” and the nickname is more literal than it sounds. The enteric nervous system contains over 500 million neurons organized into layered networks within the bowel wall.⁴ The diversity of cell types and the complexity of the circuits involved allow it to regulate many digestive processes on its own, without waiting for instructions from the brain.⁵

The enteric nervous system does far more than push food along. It controls motility, manages the secretion of fluids into the gut, regulates local blood flow, interacts with immune cells, and communicates with the microbiome. Enteric neurons work alongside glial cells, immune cells, and hormone-releasing cells to integrate a wide range of signals and coordinate outputs that are precisely tuned in both location and timing.⁶ Neuron and glial diversity in the enteric nervous system even resembles what you find in the brain.⁴

Supporting Cells That Keep Neurons Working

Neurons get most of the attention, but they can’t function without glial cells, the support staff of the nervous system. In the CNS, glial cells include astrocytes, oligodendrocytes, and microglia. They shape neural development at every stage, from when neurons are first born and migrate into position through to synapse formation and circuit assembly. In the mature brain, glia continue to influence synaptic communication, plasticity, and overall network activity by constantly monitoring and adjusting neural structure and function.⁷

In the PNS, the star glial cell is the Schwann cell. Schwann cells wrap around peripheral nerve fibers and form myelin, the insulating sheath that allows electrical signals to travel quickly and accurately between the CNS and the rest of the body.⁸ But Schwann cells have a second, remarkable role. When a peripheral nerve is injured, myelinating Schwann cells can essentially revert to a simpler state, becoming repair cells that guide regrowing axons back to their targets. Once regeneration is underway, they redifferentiate and remyelinate the nerve to restore function.⁹ This is one reason peripheral nerve injuries can sometimes heal on their own while damage to the brain or spinal cord is often permanent: the CNS lacks an equivalent self-repair mechanism.

How Neurons Communicate

The fundamental job of the nervous system is to move information, and it does this through electrical and chemical signaling. Within a neuron, signals travel as electrical impulses called action potentials. When an action potential reaches the end of a neuron, it triggers the opening of calcium channels. Calcium floods in and causes the release of chemical messengers called neurotransmitters into the tiny gap between neurons.¹⁰ Those neurotransmitters cross the gap and bind to receptors on the next neuron, either encouraging it to fire its own electrical signal or discouraging it from doing so. This chemical handoff at the synapse is what allows neurons to process and filter information rather than just relay it blindly.

Some pathways skip the brain entirely. A reflex arc, like the knee-jerk reflex, routes a sensory signal directly to the spinal cord and back out to a muscle. In the stretch reflex, sensory neurons detect a change in muscle length and synapse directly onto motor neurons in the spinal cord, which fire back to contract the same muscle.¹¹ The whole loop can happen in tens of milliseconds, well before the brain becomes aware of what happened. This is why you flinch from a hot surface before you consciously feel pain.

The Barriers That Protect the Brain

Because neurons are exquisitely sensitive to changes in their chemical environment, the CNS has evolved elaborate barriers to keep the blood’s fluctuating chemistry from disrupting neural signaling. The blood-brain barrier (BBB) is formed by the cells lining blood vessels in the brain. These cells are connected by exceptionally tight junctions that block most water-soluble molecules from slipping through. The vessels also lack the tiny pores found in blood vessels elsewhere in the body, and their cells have very low rates of the kind of bulk transport that moves molecules across vessel walls in other organs.¹²

This physical barrier is supplemented by active transport systems that selectively shuttle nutrients into the brain while pumping potentially harmful substances back out. A second barrier, formed by cells in the choroid plexus, separates the blood from the cerebrospinal fluid (CSF), the clear liquid that fills the spaces inside and around the brain and spinal cord.¹² CSF does more than just provide a chemical buffer. It also acts as a physical shock absorber, cushioning the brain against sudden head movements and mild impacts that would otherwise cause the brain to collide with the skull.¹³

These barriers are a double-edged sword for medicine. They do an excellent job of protecting the brain, but they also make it difficult to deliver drugs to the CNS. This is one of the major challenges in treating brain cancers, infections, and neurodegenerative diseases: getting a therapeutic molecule past a barrier designed over hundreds of millions of years of evolution to keep foreign molecules out.

How the Nervous System Builds Itself

The cells destined to become the brain and spinal cord start out on the outer surface of an early embryo. They fold inward to form a hollow tube, the neural tube, in a process called neurulation. Signals from surrounding tissues pattern the tube so that the back side (dorsal) receives sensory input and the front side (ventral) sends motor commands.¹⁴ This dorsal-ventral organization persists into adulthood: sensory nerves enter the back of the spinal cord, and motor nerves exit from the front.

In the developing brain, stem cells near the center of the neural tube produce neurons that migrate outward to form an intermediate layer. A subset of dividing cells from that layer migrates even further out to build the cerebral cortex, which ultimately arranges itself into six distinct layers.¹⁴ Each layer has a characteristic cell composition and wiring pattern, which is why damage to different cortical layers can produce very different deficits. Problems during neurulation or cortical migration can lead to a range of developmental conditions, from spina bifida (when the neural tube fails to close properly) to certain forms of epilepsy linked to neurons that ended up in the wrong cortical layer.

Neuroplasticity and Ongoing Change

The nervous system is not a static piece of hardware that gets installed during development and never changes. Neuroplasticity refers to the brain’s ability to reorganize its connections in response to experience, learning, injury, and disease. This includes changes in how strongly one neuron influences another, the formation of entirely new synapses, alterations in neuronal structure, and even the generation of new neurons in certain brain regions.¹⁵

For decades, researchers assumed that significant rewiring was only possible during early childhood, a period of so-called critical windows. Research over the past couple of decades has overturned that assumption. Novel experience, changes in sensory input, and learning new skills are now recognized as ongoing modulators of brain circuitry throughout life.¹⁶ Structural plasticity allows the mature brain to adapt to environmental changes, to learn, and to partially repair itself after lesions or disease.¹⁷ That said, the adult brain’s capacity for repair has real limits, especially in the CNS, as noted earlier with the contrast between peripheral and central nerve injury outcomes.

How the Nervous System Evolved

The complex CNS-PNS layout found in humans is not the only way to wire an animal. Evolutionary analysis suggests that the earliest bilaterally symmetrical animals, living over 500 million years ago, had only a diffuse net of nerve cells rather than a centralized brain. Brains appear to have evolved independently at least four times across the animal kingdom.¹⁸

The simplest animal to have developed something recognizable as a brain is the planarian, a flatworm. Planarians have bilateral symmetry, a head end with a cluster of neurons forming a two-lobed brain, and nerve cells whose structure closely resembles vertebrate neurons, with features like multiple branching processes, dendritic spines, and relatively slow spontaneous electrical activity.¹⁹ Some researchers consider planarians not just the first animals with a brain but a possible ancestor of the vertebrate brain plan itself. Understanding this evolutionary trajectory helps explain why vertebrate nervous systems share so many features: the blueprint is ancient, and natural selection has been refining it for a very long time.

Jellyfish and hydra, by contrast, get by with a nerve net that has no centralized processing at all. Insects use a chain of ganglia along the body with a modest brain up front. Octopuses have large brains but also distribute a huge proportion of their neurons into their arms, which can operate semi-independently. These variations illustrate that the vertebrate model of a dominant brain plus spinal cord plus branching peripheral nerves is just one successful solution to the problem of coordinating a complex body.

1
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