The nervous system is typically divided into two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS). However, many anatomy courses break it into three functional divisions: the central nervous system, the somatic nervous system, and the autonomic nervous system. The somatic and autonomic systems are both branches of the peripheral nervous system, but they do such different jobs that they’re often taught as separate parts. Here’s what each one does and how they work together.
The Central Nervous System: Brain and Spinal Cord
The central nervous system is the command center. It consists of just two organs, the brain and the spinal cord, and every decision your body makes runs through one or both of them. The brain contains roughly 86 billion neurons and handles everything from conscious thought and emotion to regulating breathing, digestion, and heart rate. The spinal cord acts as the main highway connecting the brain to the rest of the body, relaying signals in both directions.
Your CNS performs three core functions: receiving sensory information from the body, processing that information, and sending out instructions. When you touch a hot pan, sensory signals race up your spinal cord to your brain, your brain interprets the signal as pain, and it fires back a command telling your hand muscles to pull away. Some of those responses, like reflexes, get processed at the spinal cord level before the signal even reaches the brain, which is why you can jerk your hand back before you consciously feel the burn.
Because the CNS is so critical, it’s heavily protected. The brain sits inside the skull, and the spinal cord runs through a channel in the vertebrae. Both are wrapped in layers of connective tissue called meninges and cushioned by cerebrospinal fluid, which absorbs shock. Conditions that damage the CNS, such as stroke, Alzheimer’s disease, Parkinson’s disease, epilepsy, and spinal cord injuries, tend to have widespread effects on the body because they disrupt the central processing hub itself.
The Somatic Nervous System: Voluntary Control
The somatic nervous system is the part of the peripheral nervous system responsible for things you do on purpose. It connects your brain and spinal cord to your skin, skeletal muscles, and joints through a network of nerves that fan out across your entire body. Twelve pairs of cranial nerves emerge from the base of the brain, and 31 pairs of spinal nerves branch off from the spinal cord, giving you sensation and movement from head to toe.
This system works in two directions. Sensory neurons carry information inward, from receptors in your skin, eyes, ears, and other organs to the CNS. Motor neurons carry commands outward, from the CNS to your muscles. When you decide to pick up a cup of coffee, your brain generates the instruction, your spinal cord relays it, and motor neurons deliver it to the specific muscles in your arm and hand. The speed of those signals varies dramatically depending on the nerve fiber. Touch and position signals travel through large, insulated nerve fibers at 80 to 120 meters per second (up to 268 miles per hour), while pain signals move through smaller, uninsulated fibers at just 0.5 to 2 meters per second, which is why you sometimes feel the pressure of an injury before the pain hits.
Peripheral neuropathy, carpal tunnel syndrome, and Guillain-Barré syndrome are examples of conditions that affect somatic nerves. Because these nerves sit outside the protective bone and fluid of the CNS, they’re more exposed to injury, compression, and damage from conditions like diabetes.
The Autonomic Nervous System: Automatic Functions
The autonomic nervous system handles everything your body does without you thinking about it. It connects the CNS to your internal organs: heart, lungs, stomach, intestines, bladder, and glands. It runs continuously in the background, adjusting your heart rate, blood pressure, digestion, and body temperature to keep you alive and stable.
The autonomic system itself splits into two opposing branches that work like a gas pedal and a brake. The sympathetic branch activates your “fight or flight” response, increasing heart rate, dilating your pupils, and redirecting blood flow to your muscles when you’re stressed or in danger. The parasympathetic branch does the opposite, slowing your heart, stimulating digestion, and promoting rest. These two branches constantly balance each other throughout the day. After a stressful moment passes, your parasympathetic system gradually dials things back down to baseline.
The key distinction from the somatic system is conscious control. You can choose to move your arm, but you can’t willfully speed up your digestion or change how quickly your pupils adjust to light. The autonomic system handles those tasks automatically.
The Enteric Nervous System: A Possible Fourth Division
Some textbooks now recognize a third peripheral division called the enteric nervous system, sometimes nicknamed the “second brain.” This is a dense mesh of more than 100 million nerve cells lining your gastrointestinal tract from esophagus to rectum. That’s more neurons than exist in your spinal cord.
The enteric nervous system controls digestion largely on its own: coordinating the muscle contractions that move food through your gut, triggering enzyme release, and managing blood flow for nutrient absorption. What makes it unusual is its independence. While it communicates with the brain through the vagus nerve, it can operate without direct CNS input, running digestive processes even when those communication lines are disrupted.
Research from Johns Hopkins Medicine has highlighted that the gut-brain connection runs both ways. Irritation in the gastrointestinal system can send signals to the brain that trigger mood changes, which may help explain why conditions like irritable bowel syndrome frequently overlap with anxiety and depression. This bidirectional relationship has opened up treatment approaches where therapies aimed at the gut can improve mental health, and vice versa.
How All Three Systems Work Together
None of these systems operates in isolation. Every action, whether it’s catching a ball or digesting lunch, involves coordination between all of them. A simple example: you see food on the table (sensory neurons in the somatic system carry visual information to the CNS), you decide to reach for it (the CNS processes the decision and sends a motor command through somatic nerves to your arm), and once you start eating, your autonomic and enteric systems take over to manage saliva production, stomach acid release, and the muscular contractions that push food through your digestive tract.
The full pathway for any signal follows a consistent pattern. A stimulus triggers sensory receptors somewhere in the body. Sensory neurons carry that information inward to the spinal cord and brain. The CNS processes it and generates a response. Motor neurons then carry that response outward to muscles or glands. This loop, from sensation to processing to action, repeats billions of times a day and underlies everything from reflexes to complex thought.