The nervous system controls everything you do. Every thought, every movement, every heartbeat, and every breath depends on this network of roughly 86 billion nerve cells working together to send signals throughout your body. It has two main divisions: the central nervous system (your brain and spinal cord) and the peripheral nervous system (the nerves branching out to every other part of your body). Together, they process information from the world around you, coordinate your responses, and keep your internal organs running without you ever having to think about it.
How the Nervous System Sends Signals
Nerve cells communicate through a combination of electrical and chemical signals. Inside each cell, charged particles create a rapid electrical impulse that travels from one end to the other. The fastest of these signals move at 80 to 120 meters per second, which is roughly 270 to 430 kilometers per hour. That speed is what lets you pull your hand off a hot stove before you consciously register the pain.
When an electrical signal reaches the end of a nerve cell, it has to cross a tiny gap, called a synapse, to reach the next cell. That gap is only about 20 to 50 nanometers wide, far too small to see. The sending cell releases chemical messengers that float across the gap and dock onto specialized proteins on the receiving cell. This triggers a new electrical signal in the next nerve cell, and the chain continues. Your brain contains trillions of these connections, allowing enormous networks of cells to coordinate everything from language to balance to emotion.
What Different Parts of the Brain Do
The brain is divided into four major lobes, each handling different tasks. The frontal lobe sits behind your forehead and manages planning, reasoning, emotional regulation, problem solving, and voluntary movement. When you decide to pick up a glass of water, the frontal lobe initiates that action.
The parietal lobe, located at the top and toward the back of the head, integrates touch, temperature, pressure, and pain. It lets you tell the difference between two objects pressing against your skin at nearby points, rather than perceiving them as one. The temporal lobe, on each side of the head, handles hearing, language recognition, and memory. It contains a structure called the hippocampus, which is essential for forming new memories and for learning. The occipital lobe, at the very back of the head, is devoted to vision, processing everything from depth and distance to the identity of objects and faces.
Voluntary vs. Involuntary Control
Not everything the nervous system does requires your conscious input. The peripheral nervous system splits into two branches that handle fundamentally different jobs.
The somatic nervous system controls voluntary movement. It connects your brain and spinal cord to your skeletal muscles, and it’s responsible for everything you can consciously influence: walking, typing, speaking, turning your head. This system uses a direct, single-cell relay from the spinal cord or brainstem to the muscle.
The autonomic nervous system, by contrast, manages everything you don’t have to think about. Heart rate, digestion, blood pressure, pupil dilation, sweating. It operates through a two-cell relay, with an extra nerve cell between the spinal cord and the target organ, which is part of why these processes feel removed from conscious control. You don’t decide to speed up your heart when you’re frightened. The autonomic system does it for you.
Reflexes: When the Spinal Cord Acts Alone
Some responses are too urgent to wait for the brain. Reflexes are processed directly in the spinal cord, shaving precious fractions of a second off your reaction time. When you touch something painfully hot, specialized pain receptors fire off a signal that travels to the spinal cord. There, it connects with motor nerve cells that immediately contract the muscles needed to pull your hand away, all within about half a second.
At the same time, the spinal cord handles a surprising amount of coordination. If you yank one foot off a sharp object, the opposite leg automatically stiffens to support your weight. This crossed-extension reflex keeps you from falling. Only after all of this has happened does the pain signal finally reach your brain, and you become consciously aware of what just occurred. Reflex arcs are an evolutionary shortcut: act first, process later.
How Your Body Stays in Balance
The nervous system doesn’t work in isolation. Deep inside the brain, a small structure called the hypothalamus serves as a bridge between the nervous system and the hormone-producing endocrine system. It maintains homeostasis, the stable internal conditions your body needs to survive. Body temperature, blood pressure, hunger, thirst, mood, sleep, and sex drive all fall under its influence.
The hypothalamus manages some of these directly through the autonomic nervous system. Your heart rate and blood pressure, for example, are adjusted moment to moment without any hormonal involvement. For longer-term regulation, it releases hormones or signals other glands to release them. One example is a hormone that controls how much water your kidneys retain. When this system malfunctions, conditions like excessive thirst and urination can result. Another example involves fullness signals during eating. In rare inherited conditions where the hypothalamus can’t recognize that you’re full, the constant urge to eat creates a serious risk of obesity.
How You Sense the World
Every piece of information from the outside world enters the nervous system through sensory receptors. These specialized structures convert different forms of energy into electrical signals that nerve cells can transmit. Your eyes contain receptors that respond to light by changing the shape of a light-sensitive molecule, triggering a cascade of signals. Your ears rely on tiny hair cells that bend in response to sound waves, opening ion channels that generate electrical impulses.
Taste works through two different mechanisms depending on the flavor. Salty and sour tastes directly open ion channels on receptor cells, while sweet, bitter, and savory (umami) tastes activate a more complex protein-based signaling system. Smell receptors on the cilia inside your nose bind to airborne molecules and generate action potentials that the brain interprets as distinct odors. Touch and stretch receptors throughout your body use channels that open when physically deformed, converting pressure into nerve signals. All of these inputs converge in the brain, where they’re integrated into the seamless experience of being aware of your surroundings.
The Support Cells Behind the Scenes
Nerve cells get most of the attention, but the brain contains roughly 85 billion non-neuronal support cells that are essential to keeping everything running. Astrocytes, the most abundant type, manage blood flow to active brain regions, shuttle energy from the bloodstream to hungry neurons, and clean up excess chemical signals after nerve cells communicate. They also help regulate the fluid balance around cells and control how excitable neurons are. Far from being passive filler, astrocytes actively participate in learning, memory, and the development of new connections between nerve cells. When the brain is injured, astrocytes form a protective scar that limits damage from inflammation.
Microglia serve as the brain’s immune system. They patrol for damage, clear away dead cells and debris, and release signaling molecules that coordinate the brain’s response to infection or injury. They also play a surprising role in normal brain development, pruning unnecessary connections between nerve cells to refine and sharpen neural circuits. This dual role, as both immune defender and circuit sculptor, makes them critical to brain health throughout life.