Your nervous system is a communication network that carries electrical and chemical signals between your brain and every other part of your body. It controls everything you do consciously, like reaching for a cup of coffee, and everything your body handles without your input, like keeping your heart beating at 3 a.m. Understanding how it works starts with its two major divisions and then zooms in on what happens at the cellular level when a signal travels from one place to another.
The Two Main Divisions
The nervous system splits into two parts: the central nervous system and the peripheral nervous system. Your central nervous system is just two organs, your brain and spinal cord. Together they act as the command center, interpreting incoming information and deciding what your body should do next.
Your peripheral nervous system is everything outside the brain and spinal cord. It includes 12 pairs of cranial nerves that connect directly to your brain and 31 pairs of spinal nerves that branch off your spinal cord at each vertebra. These nerves stretch into your face, limbs, organs, and skin. They have two jobs: delivering sensory information from your body up to the brain, and carrying instructions from the brain back out to muscles and organs. One cranial nerve, the vagus nerve, extends all the way from your brainstem down to your colon, connecting to every vital organ along the way.
How Nerve Cells Send Signals
The basic unit of the nervous system is the nerve cell, or neuron. The human brain alone contains roughly 20 billion neurons in the outer layer (the cortex), and each one forms an average of 7,000 connections with other neurons. That adds up to around 0.15 quadrillion connection points, or about a trillion per cubic centimeter of brain tissue. The sheer density of this wiring is what allows you to process language, remember faces, and coordinate movement simultaneously.
Neurons communicate using a combination of electrical impulses and chemical signals. The electrical part works like this: when a neuron is stimulated strongly enough, tiny channels in its membrane snap open and allow charged particles (ions) to rush in. This creates a rapid spike in electrical voltage, from a resting state of about negative 70 millivolts to a peak of roughly positive 40 millivolts. That spike is called an action potential, and it travels down the length of the neuron like a wave. As one section of the membrane fires, it triggers the next section to fire, passing the signal forward.
Not all signals travel at the same speed. Some neurons are wrapped in a fatty insulating layer called myelin, which lets signals skip rapidly along the nerve fiber. The fastest myelinated neurons, the ones carrying touch and body-position information, transmit at 80 to 120 meters per second (up to 268 miles per hour). By contrast, small unmyelinated fibers that carry pain signals move at just 0.5 to 2 meters per second, roughly walking pace. That speed difference is why you feel the impact of stubbing your toe before the sharp pain arrives a moment later.
What Happens at the Gap Between Neurons
Neurons don’t physically touch each other. There’s a tiny gap between them called a synapse, and signals cross it chemically rather than electrically. When an electrical impulse reaches the end of a neuron, it triggers calcium channels to open. Calcium floods into the nerve terminal and interacts with small packets (vesicles) filled with chemical messengers called neurotransmitters. Those packets fuse with the cell membrane and spill their contents into the gap.
The neurotransmitter molecules drift across the synapse and latch onto receptors on the next neuron, either exciting it to fire its own electrical signal or inhibiting it to stay quiet. This process takes only a fraction of a millisecond, and it’s happening at trillions of synapses throughout your nervous system at any given moment. The balance between excitatory and inhibitory signals is what allows your brain to fine-tune everything from muscle coordination to mood.
How Your Body Converts Sensations Into Signals
Before the brain can process anything, raw physical stimuli like light, sound, pressure, or a particular smell need to be converted into electrical signals that neurons can carry. Specialized sensory cells handle this conversion. In your nose, for example, odor molecules land on tiny hair-like structures called cilia that protrude from smell-detecting neurons. This triggers a chain reaction inside the cell that ultimately produces an electrical impulse, which travels along the neuron’s fiber to the brain for interpretation.
Different senses use different conversion strategies. Light-sensitive cells in the eye don’t fire full action potentials the way smell neurons do. Instead, they produce graded electrical changes that adjust neurotransmitter release in a more analog fashion. A single light-activated molecule in one of these cells can trigger a 20-fold amplification cascade within about 50 milliseconds, which is part of why your eyes can detect remarkably faint light. Regardless of the strategy, the end result is the same: a physical event in the outside world becomes an electrical signal your brain can read.
The Autonomic System: Running Things on Autopilot
A large portion of your nervous system operates without any conscious effort on your part. This is the autonomic nervous system, and it has two branches that work in a push-pull balance.
The sympathetic branch handles your “fight or flight” response. When you perceive a threat, or even just feel stressed, it puts your body on high alert: heart rate climbs, pupils dilate, breathing quickens, and blood flow shifts toward your muscles. This system evolved to help you survive immediate danger, but it activates in modern situations too, from a near-miss in traffic to a tense email from your boss.
The parasympathetic branch does the opposite, managing “rest and digest” functions. It slows your heart rate and reduces the pumping force of your heart. It constricts your pupils, increases saliva production, and ramps up digestion so your body can efficiently break down food and absorb nutrients. It signals your pancreas to release insulin, helps manage waste elimination, and plays a role in sexual arousal. When you feel calm after a meal and your eyelids get heavy, that’s your parasympathetic system doing its job.
These two branches don’t take turns. They’re both active all the time, constantly adjusting relative to each other. Your resting heart rate, for instance, reflects a real-time negotiation between sympathetic signals pushing it up and parasympathetic signals pulling it down.
Reflexes: Shortcuts That Skip the Brain
Some responses are too urgent to wait for the brain to weigh in. When you touch a hot stove, you pull your hand back before you’re even consciously aware of pain. This happens through a reflex arc, a streamlined neural circuit with five components: a receptor in the skin that detects the stimulus, a sensory neuron that carries the signal to the spinal cord, an integration center in the spinal cord where the signal is processed (sometimes through a single synapse), a motor neuron that sends a command back out, and the muscle that contracts in response.
Because reflex arcs route through the spinal cord rather than up to the brain’s cortex, they happen automatically and extremely fast. Your brain does get the pain signal eventually, just a fraction of a second later, which is why you feel the burn after your hand is already moving away. Reflexes are also critical for maintaining posture. Your body constantly makes small corrections to keep you upright, and these adjustments run through reflex circuits without requiring your conscious attention.
Support Cells That Keep Neurons Running
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 and handle essential behind-the-scenes work: nourishing neurons, clearing waste, forming the myelin insulation that speeds up signal transmission, and maintaining the chemical environment neurons need to fire properly. Unlike neurons, glial cells can divide and replace themselves, which is one reason brain tumors most often arise from glial tissue rather than from neurons themselves.
The myelin produced by certain glial cells is especially important. When myelin breaks down, as it does in conditions like multiple sclerosis, signals slow dramatically or fail to arrive at all. Symptoms depend on which neurons lose their insulation, ranging from numbness and muscle weakness to vision problems, reflecting just how dependent the system is on that fatty coating to maintain its speed and accuracy.