The human nervous system is divided into two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS includes the brain and spinal cord. The PNS consists of every nerve and neural structure outside of those two organs. Together, they receive sensory information from your environment, process it, and coordinate your body’s response.
The Central Nervous System
The central nervous system is the command center. Your brain handles everything from conscious thought and memory to regulating breathing and body temperature. The spinal cord serves as the main highway connecting the brain to the rest of the body, relaying signals in both directions and also coordinating some rapid responses on its own.
The human brain contains roughly 86 billion neurons and around 40 to 50 billion supporting cells called glia, which help with insulation, nutrient delivery, and waste removal. For decades, textbooks claimed the brain had 100 billion neurons and ten times as many glial cells, but modern counting methods have revised both numbers downward.
Because the CNS is so critical, it has layers of physical protection that the rest of the nervous system lacks. The brain sits inside the skull, and the spinal cord runs through the vertebral column. Beneath the bone, three membranes called the meninges wrap around both structures. Between those layers, cerebrospinal fluid circulates continuously, cushioning the brain and spinal cord from impact while also delivering nutrients and clearing waste. The brain also has a blood-brain barrier, a selective filter that prevents many toxins and pathogens in the bloodstream from reaching brain tissue.
The Peripheral Nervous System
The peripheral nervous system is everything outside the brain and spinal cord: the nerves branching through your limbs, torso, and organs. It acts as the messenger network, carrying sensory information inward to the CNS and motor commands outward to muscles and glands. The PNS itself splits into two functional branches.
The somatic nervous system handles voluntary actions and conscious sensation. When you decide to pick up a cup or feel heat on your skin, somatic nerves are doing the work. The autonomic nervous system runs processes you don’t consciously control: heart rate, digestion, blood pressure, and breathing rate. Both branches operate simultaneously, but they govern very different territory.
Sympathetic, Parasympathetic, and Enteric Divisions
The autonomic nervous system breaks down further into three divisions, each with a distinct job.
The sympathetic division triggers the “fight or flight” response. When you encounter a threat or stressor, sympathetic activation raises your heart rate and blood pressure, releases stored energy into the bloodstream, and slows digestion. Blood vessels in your extremities constrict, redirecting flow toward muscles and vital organs. This is the system that prepares your body for immediate physical action.
The parasympathetic division does the opposite, promoting “rest and digest” functions. It slows the heart, stimulates saliva production, and restores normal digestive activity. The vagus nerve, the longest cranial nerve in the body, is the primary carrier of parasympathetic signals. It originates in the brainstem and extends all the way to the gastrointestinal tract, regulating heart rate, lung function, and gut motility along the way.
The enteric nervous system is sometimes called the “second brain.” It’s a dense network of over 100 million neurons embedded in the walls of the digestive tract, more neurons than exist in all other peripheral nerve clusters combined. It independently regulates muscle contractions that move food through the intestines, controls the secretion of digestive enzymes, and manages fluid absorption. While the enteric system can operate on its own, it stays in constant two-way communication with the brain through the vagus nerve, forming what researchers call the gut-brain axis.
How the Two Systems Work Together
The CNS and PNS are not independent. They form a continuous loop. Sensory neurons in the PNS detect a stimulus, like touching a hot stove, and send that signal toward the spinal cord or brain. The CNS processes the information and sends a motor command back through peripheral nerves to produce a response, such as pulling your hand away.
Some responses bypass the brain entirely. In a reflex arc, a sensory neuron delivers its signal to the spinal cord, which immediately sends a motor command back without waiting for the brain to weigh in. This is why you jerk your hand away from heat before you consciously register pain. The brain gets notified afterward. More complex reflexes can involve additional relay neurons in the spinal cord or brainstem, allowing the response to factor in other sensory input or the current state of the tissue.
Signal speed varies dramatically depending on the type of nerve fiber involved. Large, insulated (myelinated) motor and sensory fibers conduct signals at 50 to 70 meters per second in humans, fast enough to travel the length of your arm in a fraction of a second. Smaller myelinated fibers may conduct as slowly as 12 meters per second. Uninsulated fibers, which carry signals like dull pain or temperature, move at roughly 2 meters per second.
Nerve Regeneration: A Key Difference
One of the most important practical differences between the CNS and PNS is their ability to heal after injury. Peripheral nerves can regenerate. After damage, the disconnected portion of a peripheral nerve breaks down in an organized process, immune cells clear the debris relatively quickly, and the surviving nerve fiber activates a suite of growth-promoting genes that guide regrowth. This is why sensation and movement can return after a cut or crush injury to a limb nerve, though recovery is often slow and sometimes incomplete.
The central nervous system is far less capable of self-repair. CNS tissue contains proteins in its insulation layer that actively inhibit regrowth. Scar tissue forms at the injury site and creates a physical and chemical barrier. Debris clearance is slower than in the PNS. And critically, injured CNS neurons fail to turn on the same regeneration genes that peripheral neurons activate so readily. This is why spinal cord injuries and brain damage tend to cause permanent deficits.
Conditions That Affect Each System
Diseases and injuries can target one system or both. CNS conditions include stroke, epilepsy, Alzheimer’s disease, Parkinson’s disease, and multiple sclerosis. Infections like meningitis and encephalitis attack the protective membranes or tissue of the brain and spinal cord. Tumors and traumatic injuries to the brain or spine also fall under CNS disorders.
PNS conditions include peripheral neuropathy, a common complication of diabetes that damages nerves in the hands and feet, and Guillain-Barré syndrome, in which the immune system attacks peripheral nerves and can cause temporary paralysis. Carpal tunnel syndrome, where a nerve in the wrist is compressed, is another familiar peripheral nerve problem. Bell’s palsy, which causes sudden weakness in the muscles on one side of the face, also involves peripheral nerve damage.
Some conditions blur the line. ALS (amyotrophic lateral sclerosis) destroys motor neurons in both the brain and the peripheral nerves that control muscles. Multiple sclerosis primarily targets CNS insulation but can produce symptoms that feel like peripheral nerve problems, such as numbness and tingling in the extremities.