Your nervous system is your body’s communication network. It collects information from the world around you and from inside your body, processes that information, and sends out instructions that control everything from your heartbeat to your ability to read this sentence. It has two main parts: the central nervous system (your brain and spinal cord) and the peripheral nervous system (the vast web of nerves that branches out to every other part of your body).
The Two Main Divisions
The central nervous system, or CNS, is the command center. Your brain interprets sensory information, stores memories, generates thoughts, and coordinates movement. Your spinal cord serves as the main highway between the brain and the rest of the body, carrying signals in both directions. It also handles some fast, automatic responses on its own, like pulling your hand away from a hot surface before your brain even registers pain.
The peripheral nervous system, or PNS, is everything else. Nerves branch out from the spinal cord and reach your organs, arms, legs, fingers, and toes. Some of these nerves carry information inward (telling your brain what your skin feels, what your eyes see, or how full your stomach is), while others carry commands outward (telling muscles to contract or glands to release hormones).
How Nerve Cells Work
The basic working unit of the nervous system is the neuron, a cell specialized for sending electrical signals. Neurons have a cell body, branching extensions called dendrites that receive incoming signals, and a long fiber called an axon that transmits signals to the next cell in line. The system also contains glial cells, which don’t carry signals themselves but support, nourish, and protect neurons. Glial cells far outnumber neurons.
At rest, the inside of a neuron holds a slight negative electrical charge compared to the outside, roughly -70 millivolts. When a neuron receives enough stimulation from neighboring cells, its charge shifts to about -50 millivolts, crossing a threshold that triggers an action potential. This is a rapid electrical pulse that travels down the axon, and it’s the fundamental unit of communication between nerve cells.
When that electrical pulse reaches the end of the axon, it can’t simply jump to the next neuron. There’s a tiny gap, about 20 to 40 nanometers wide, called a synapse. Instead of leaping across, the signal converts to a chemical one: the neuron releases molecules called neurotransmitters into the gap. These molecules drift across, attach to receptors on the receiving neuron, and trigger a new electrical change there. The signal effectively switches from electrical to chemical and back to electrical again, thousands of times per second across billions of connections.
The speed of these signals varies dramatically depending on the type of nerve fiber. Small, uninsulated fibers conduct signals at less than one meter per second. The largest motor fibers, which are wrapped in an insulating layer and connect to skeletal muscles, can transmit at 80 to 120 meters per second. That’s over 250 miles per hour.
The Autonomic Nervous System
Many of your body’s most critical functions happen without any conscious effort. Your heart beats, your lungs expand, your stomach digests food, and your pupils adjust to light, all automatically. The part of the peripheral nervous system that handles this is the autonomic nervous system, and it has two opposing branches that work like a gas pedal and a brake.
The sympathetic branch is your “fight or flight” system. When you perceive a threat, it increases your heart rate and breathing, redirects blood flow away from digestion and toward your skeletal muscles, and dilates your pupils. Your digestive system essentially shuts down so your blood can carry oxygen to the muscles that might need to run or fight. The parasympathetic branch does the opposite, often called “rest and digest.” It slows the heart rate, stimulates digestion, and constricts the pupils. These two branches constantly adjust your internal state based on what you’re doing and what’s happening around you.
Your Gut Has Its Own Nervous System
There’s a third division that often surprises people. The enteric nervous system is a network of more than 100 million nerve cells lining your gastrointestinal tract from esophagus to rectum. It’s sometimes called “the second brain” because it can operate with a degree of independence, coordinating the complex muscular contractions and chemical secretions needed for digestion without waiting for instructions from your brain.
The enteric nervous system and the brain communicate constantly, though. Researchers at Johns Hopkins Medicine have found evidence that irritation in the gastrointestinal system can send signals to the brain that trigger mood changes. This gut-brain connection may explain why digestive problems so often accompany anxiety and depression, and why psychological therapies like cognitive behavioral therapy can sometimes improve gut symptoms. The communication runs both ways: stress in your brain affects your gut, and trouble in your gut affects your brain.
How the Nervous System Protects Itself
Because the brain and spinal cord are so vital, the body layers multiple defenses around them. The skull and vertebral column provide a hard outer shell. Beneath that, three membranes called meninges wrap the brain and spinal cord. Cerebrospinal fluid cushions these structures, absorbing shocks that could otherwise cause damage.
There’s also a chemical defense. The blood-brain barrier is formed by the cells lining the brain’s smallest blood vessels, which are packed so tightly together that most substances in the blood can’t pass through. Only small molecules, fat-soluble molecules, and certain gases can slip through freely. Larger molecules the brain needs, like glucose, get in through specialized transporter proteins that act like doors opening only for specific molecules. This barrier keeps out most toxins and pathogens circulating in the blood while also maintaining stable levels of hormones, nutrients, and water in the brain. Even small fluctuations in that environment could disrupt normal function.
What Can Go Wrong
Because the nervous system is so complex and so central to every function in the body, it’s vulnerable to a wide range of disorders. These generally fall into a few broad categories.
Neurodegenerative diseases involve the progressive loss of nerve cells. Alzheimer’s disease, Parkinson’s disease, and ALS (amyotrophic lateral sclerosis) are among the most well known. Each targets different populations of neurons: Alzheimer’s erodes memory and cognition, Parkinson’s disrupts movement control, and ALS destroys the motor neurons that let you move voluntarily.
Demyelinating diseases damage the insulating layer that wraps many nerve fibers, slowing or blocking signal transmission. Multiple sclerosis is the most common example, and it can cause symptoms ranging from numbness and vision problems to difficulty walking, depending on which nerves lose their insulation.
Vascular disorders affect the blood supply to the nervous system. Stroke, the most familiar, occurs when blood flow to part of the brain is cut off, either by a clot or a burst blood vessel. Without oxygen, the affected neurons begin to die within minutes. Transient ischemic attacks, sometimes called mini-strokes, involve temporary blockages that resolve on their own but serve as warning signs.
Other conditions affecting the nervous system include epilepsy (abnormal electrical activity causing seizures), peripheral neuropathy (damage to nerves outside the brain and spinal cord, often from diabetes), and infections like meningitis that inflame the protective membranes around the brain.