The nervous system has two major divisions: the central nervous system (CNS) and the peripheral nervous system (PNS). The central nervous system is your brain and spinal cord. The peripheral nervous system is everything else: the vast network of nerves that branch out from the brain and spinal cord to reach every other part of your body. Within the peripheral nervous system, there are further subdivisions that handle different jobs, from voluntary movement to digestion to your fight-or-flight response.
The Central Nervous System
Your brain and spinal cord form the central nervous system, which acts as the command center for your entire body. The brain handles an enormous range of tasks: regulating thoughts, feelings, and movements, managing learning and communication, and directing organ function like breathing and digestion. When your body needs to act on something, the brain creates a signal, sends it down through the spinal cord, and the spinal cord relays that signal outward to the appropriate nerves and muscles.
The spinal cord isn’t just a passive cable. It serves as a relay station, routing sensory information up to the brain and motor commands back down to the body. In some cases, the spinal cord can trigger rapid reflexes on its own, before a signal even reaches the brain, which is why you pull your hand away from a hot surface before you consciously feel the pain.
Gray Matter and White Matter
The tissue inside the CNS comes in two types. Gray matter is made up of nerve cell bodies, the parts that process information and send short-range messages. White matter consists of long nerve fibers coated in a fatty substance called myelin, which helps signals travel faster over longer distances. In the brain, gray matter sits mostly on the outer surface (the cortex), while white matter fills the interior. In the spinal cord, this arrangement flips: gray matter is on the inside, surrounded by white matter on the outside.
The Peripheral Nervous System
Every nerve outside the brain and spinal cord belongs to the peripheral nervous system. This includes 12 pairs of cranial nerves that emerge directly from the brain or brainstem, handling functions like vision, hearing, smell, and facial movement. It also includes the spinal nerves that branch off from the spinal cord and extend to your trunk, arms, and legs.
The peripheral nervous system splits into two functional divisions: the somatic nervous system and the autonomic nervous system. These handle fundamentally different types of tasks.
The Somatic Nervous System
The somatic nervous system controls your conscious, voluntary actions. When you decide to pick up a glass, throw a ball, or walk across a room, the signals from your brain travel through somatic motor nerves to reach the muscles that carry out those movements.
This system also handles most of your sensory input. All of your senses except vision travel through the somatic nervous system to reach your brain. (Your retina and optic nerve connect directly to the brain, bypassing this route.) Touch, sound, smell, and taste on your head use somatic nerves to reach the brain directly, while touch sensations below your neck travel first to the spinal cord, which relays them upward. The nerves in this system are one-way streets: sensory nerves carry information up to the brain, and motor nerves carry instructions from the brain down to your muscles.
The Autonomic Nervous System
The autonomic nervous system manages everything your body does without you thinking about it. It connects the central nervous system to your internal organs, including the heart, lungs, stomach, and intestines. It runs automatically and continuously, controlling smooth muscle, cardiac muscle, and glands. You don’t decide to speed up your digestion or adjust your heart rate. The autonomic system handles that on its own.
This system divides further into two branches that generally work in opposition to each other: the sympathetic and parasympathetic nervous systems.
Sympathetic Nervous System
The sympathetic nervous system controls your body’s “fight or flight” response. It takes the lead when your safety or survival is at risk, sending signals that put your body’s systems on high alert. When it activates, your heart rate increases, your pupils dilate to let in more light, your airways open wider, and blood flow shifts toward your muscles and away from your digestive system. This is the system that kicks in when you’re startled, under stress, or facing a physical threat.
Parasympathetic Nervous System
The parasympathetic nervous system does roughly the opposite. It promotes “rest and digest” functions, conserving energy and maintaining your body during calm periods. When active, it lowers your heart rate and reduces the pumping force of your heart. It constricts your pupils, stimulates saliva and mucus production, and increases your rate of digestion. It signals your pancreas to release insulin, helping break down sugars into energy your cells can use. It also manages waste removal by relaxing the muscles involved in urination and bowel movements, and plays a role in sexual arousal.
These two branches don’t take turns in a simple on-off pattern. Both are active to some degree at all times, constantly adjusting the balance depending on what your body needs in the moment.
The Enteric Nervous System
There’s one more division that doesn’t fit neatly into the categories above. The enteric nervous system is a mesh of neurons embedded in the walls of your digestive tract, sometimes called the “second brain.” It contains as many neurons as the spinal cord, and what makes it remarkable is its ability to function independently. Even without input from the brain or spinal cord, the enteric nervous system can coordinate digestion on its own, managing the muscular contractions that move food through your gut, regulating enzyme secretion, and monitoring the chemical environment of your intestines.
Some classifications group the enteric nervous system under the autonomic nervous system, since it deals with unconscious processes. But its size and independence have led many researchers to treat it as a distinct third division of the peripheral nervous system.
How the Divisions Work Together
These divisions aren’t isolated systems. They form a continuous, interconnected network. Here’s a practical example: you see a ball flying toward your face. Your eyes (connected via cranial nerves) send visual data to your brain (CNS). Your brain processes the threat and triggers two responses simultaneously. Your somatic nervous system fires motor signals to your arm muscles so you can raise your hands to block. Your sympathetic nervous system spikes your heart rate and sharpens your focus. Once the ball passes and you’re safe, your parasympathetic system gradually brings your heart rate back down and resumes normal digestion.
In summary, the hierarchy looks like this:
- Central nervous system: brain and spinal cord
- Peripheral nervous system: all nerves outside the CNS, divided into:
- Somatic nervous system: voluntary movement and sensory input
- Autonomic nervous system: unconscious organ control, split into sympathetic (fight or flight) and parasympathetic (rest and digest) branches
- Enteric nervous system: semi-independent nerve network governing digestion
Every sensation you feel, every movement you make, and every heartbeat you never notice runs through this structure. The divisions simply reflect how the body organizes different types of work: conscious versus unconscious, voluntary versus automatic, internal versus external.