The human nervous system has two main parts: the central nervous system, which includes the brain and spinal cord, and the peripheral nervous system, which includes every nerve branching out from there to the rest of your body. Together, these two divisions contain somewhere between 61 and 99 billion neurons in the brain alone, plus over 500 million more lining your digestive tract. Here’s how all the pieces fit together.
Central Nervous System: Brain and Spinal Cord
The central nervous system (CNS) is the command center. Your brain processes information, stores memories, generates thoughts, and coordinates movement. Your spinal cord serves as the main highway between your brain and the rest of your body, carrying signals in both directions. Nerves branch out from the spinal cord at regular intervals, reaching your organs, limbs, fingers, and toes.
Three layers of protective membranes called meninges wrap around both the brain and spinal cord. The outermost layer, the dura mater, sits closest to the skull. The middle layer is the arachnoid mater, and the innermost layer, the pia mater, rests directly against brain and spinal cord tissue. Cerebrospinal fluid flows between these layers, cushioning the CNS against physical impact.
Peripheral Nervous System: Everything Outside the CNS
The peripheral nervous system (PNS) is the massive network of nerves that connects your brain and spinal cord to the rest of your body. It splits into two functional divisions: the somatic nervous system and the autonomic nervous system. Each one handles a different type of job.
Somatic Nervous System
The somatic nervous system controls conscious, voluntary activities. It includes the nerves running to your skin and skeletal muscles. When you decide to pick up a cup of coffee, reach for a doorknob, or kick a soccer ball, the somatic system carries those commands from your brain to the muscles that make it happen. It also carries sensory information back, like the feeling of a hot mug against your palm.
Autonomic Nervous System
The autonomic nervous system handles everything you don’t consciously control: heart rate, breathing rate, blood pressure, body temperature, and digestion. It runs automatically and continuously, keeping your internal environment stable. This system connects the CNS to organs like the heart, lungs, stomach, and intestines, sending motor signals to smooth muscle, cardiac muscle, and glands.
The autonomic system breaks down further into three branches:
- Sympathetic nervous system: This activates your “fight or flight” response. It ramps up body processes during stress or danger, increasing heart rate, dilating airways, and redirecting blood flow to muscles.
- Parasympathetic nervous system: This does the opposite. It manages “rest and digest” functions, slowing your heart rate, stimulating digestion, and conserving energy when you’re calm and safe.
- Enteric nervous system: Sometimes called the “second brain,” this branch manages digestion independently. It contains over 500 million neurons embedded in the walls of your gastrointestinal tract. Beyond just moving food along, the enteric system controls fluid secretion, regulates blood flow to the gut, influences your intestinal immune system, and helps repair the gut lining.
The Three Types of Neurons
Regardless of where they sit in the nervous system, neurons fall into three basic categories based on the direction they carry signals.
Sensory neurons detect information from the environment and send it toward the brain and spinal cord. When you touch a hot surface, sensory neurons in your fingertips fire and relay that signal inward. Motor neurons do the reverse. They carry commands outward from the spinal cord to muscles, glands, and organs, directly controlling movement and bodily functions. Interneurons sit in between, connecting sensory and motor neurons within the brain and spinal cord. They also connect to each other, forming circuits that range from simple reflexes to enormously complex processing networks.
Glial Cells: The Support Network
Neurons get most of the attention, but they couldn’t function without glial cells, the support staff of the nervous system. Glial cells outnumber neurons in some brain regions and perform several essential jobs.
Oligodendrocytes produce myelin, a fatty insulating layer that wraps around nerve fibers. Myelin allows electrical signals to travel quickly and efficiently between nerve cells. As a signal travels along a nerve fiber, it jumps between gaps in the myelin coating, getting recharged at each gap so the signal stays strong over long distances. Without myelin, nerve signals would slow dramatically and lose strength before reaching their destination.
Astrocytes maintain the chemical balance around neurons, regulating water and ion levels. They also contribute to the blood-brain barrier, which filters what substances can pass from your bloodstream into brain tissue. Microglia act as the nervous system’s immune cells, clearing out damaged cells and debris. They also play a role in pruning unnecessary connections between neurons during development. A fourth type, called NG2 cells, continuously generate new oligodendrocytes throughout your life and even form functional connections with neurons.
How Signals Travel Through the System
Nerve signals are electrical impulses. Within a single neuron, the signal travels along the nerve fiber (the axon) as an electrical charge. When it reaches the end of that neuron, the signal triggers the release of chemical messengers that cross a tiny gap to the next neuron. That next neuron picks up the chemical signal, converts it back to an electrical impulse, and the process repeats. This chain reaction can move a signal from your toe to your brain and back in a fraction of a second.
Myelin is what makes this speed possible. In myelinated nerves, the electrical signal doesn’t travel smoothly down the entire length of the fiber. Instead, it jumps from one gap in the myelin to the next, getting a fresh burst of energy at each stop. This jumping pattern is far faster than continuous conduction and is the reason you can pull your hand away from a flame almost instantly.
How the Parts Work Together
None of these divisions operate in isolation. When you step on a sharp rock, sensory neurons in your foot send a signal through peripheral nerves to your spinal cord (part of the CNS). Interneurons in the spinal cord can trigger a reflex, causing motor neurons to contract your leg muscles before the pain signal even reaches your brain. Meanwhile, your autonomic system may spike your heart rate slightly as part of a stress response. All of this happens in milliseconds, coordinated across every division of the nervous system simultaneously.
The enteric nervous system adds another layer of independence. While it communicates with the brain through the autonomic system, it can also coordinate digestion on its own, managing the rhythmic contractions that push food through your intestines, adjusting fluid secretion, and responding to local conditions without waiting for instructions from the brain.