Your nervous system controls virtually every function in your body, from the obvious ones like moving your legs and feeling pain to the invisible ones like keeping your heart beating, regulating your body temperature, and digesting your food. It’s the command and communication network that connects your brain and spinal cord to every organ, muscle, and patch of skin you have. The human brain alone contains roughly 20 billion neurons in its outer layer, each forming an average of 7,000 connections to other neurons, creating a web of communication dense enough to manage all of these tasks simultaneously.
Voluntary Movement
The most obvious thing your nervous system controls is how you move. Every time you walk, type, throw a ball, or even blink on purpose, your brain sends electrical signals down your spinal cord and out to motor neurons that connect directly to your skeletal muscles. These signals travel fast. The largest nerve fibers carry impulses at 80 to 120 meters per second, which is roughly 270 to 430 kilometers per hour.
At the point where a motor neuron meets a muscle fiber, the electrical signal triggers the release of a chemical messenger called acetylcholine. This chemical crosses the tiny gap between the nerve and muscle, binds to receptors on the muscle cell, and causes the muscle to contract. The entire chain, from thought to movement, happens so quickly it feels instantaneous. Your brain’s motor regions plan and coordinate these signals so that dozens of muscles fire in the right sequence, letting you do everything from threading a needle to sprinting.
Reflexes: Movement Without the Brain
Not all movement requires your brain’s involvement. Reflexes are controlled at the level of the spinal cord, which is why they happen before you’re even consciously aware of what triggered them. When you touch a hot stove, a sensory receptor in your fingertip sends a signal to your spinal cord. There, the signal connects (sometimes through a single synapse) to a motor neuron that fires back to your arm muscles, pulling your hand away. Your brain finds out about the pain a fraction of a second later.
A reflex arc has five components: the receptor that detects the stimulus, a sensory neuron that carries the signal inward, an integration center in the spinal cord, a motor neuron that carries the response outward, and the muscle or gland that acts on it. This shortcut exists because waiting for the brain to process the information and send a response back down would take too long when you’re in danger.
Heart Rate, Breathing, and Blood Pressure
Your nervous system keeps you alive without any conscious effort on your part. A region deep in the brainstem called the medulla contains specialized clusters of neurons that generate the rhythm of your breathing. One group drives inhalation while another drives active exhalation, working together to maintain a steady breathing pattern even while you sleep. Separate clusters in the same area regulate your heart rate and blood pressure by controlling how fast your heart beats and how wide or narrow your blood vessels are at any given moment.
These brainstem centers also contain chemical sensors that monitor carbon dioxide levels in your blood. When CO2 rises, they increase your breathing rate automatically. You can override this system briefly (holding your breath, for example), but the brainstem will eventually force you to breathe again.
The Fight-or-Flight and Rest-and-Digest Systems
The autonomic nervous system handles the body processes you don’t consciously think about, and it works through two branches that balance each other. The sympathetic branch activates your body during stress or danger. It speeds up your heart, widens your pupils to let in more light, redirects blood flow to your muscles, and slows digestion. This is the classic fight-or-flight response.
The parasympathetic branch does the opposite. It slows your heart rate, narrows your pupils, stimulates digestion, and promotes recovery. These two systems are constantly adjusting against each other, creating a dynamic balance. After a stressful event, your parasympathetic system gradually brings everything back to baseline. This balance is also how your body manages blood pressure moment to moment: if you stand up suddenly, your sympathetic system quickly tightens blood vessels to keep blood flowing to your brain.
Four of your twelve cranial nerves carry autonomic fibers. These nerves control pupil size, tear production, saliva, nasal mucus, and the function of organs throughout your chest and abdomen.
Body Temperature and Hormones
A small structure near the base of your brain called the hypothalamus acts as your body’s thermostat. It monitors your core temperature and triggers responses to keep it stable: shivering and constricting blood vessels when you’re cold, sweating and dilating blood vessels when you’re hot. This is one example of homeostasis, the nervous system’s broader job of keeping your internal environment within a livable range.
The hypothalamus also serves as the bridge between your nervous system and your hormonal (endocrine) system. It controls the pituitary gland, a pea-sized structure just below it, which in turn regulates the adrenal glands, thyroid, ovaries, and testes. Through this chain, your nervous system influences growth, metabolism, stress hormones, reproductive function, and much more. When you feel stressed and your body floods with cortisol, that cascade started with your hypothalamus telling your pituitary gland to send the signal.
Sensory Processing
Everything you see, hear, taste, and feel passes through your nervous system before you become aware of it. Sensory receptors throughout your body convert physical stimuli (light, sound waves, pressure, temperature) into electrical signals. These signals travel along sensory nerves to your brain, where a structure called the thalamus acts as a relay station. Nearly all sensory information (smell is the exception) passes through the thalamus before reaching the parts of your brain that interpret it.
The thalamus doesn’t just pass information along. It filters and prioritizes it, helping your brain decide what to pay attention to out of the enormous flood of sensory data arriving every second. Specialized clusters within the thalamus handle different types of input: some process visual information and send it to the visual cortex, others handle sound and route it to the auditory cortex, and still others relay touch and body-position signals from your limbs, trunk, head, and face to the areas that map physical sensation.
Thinking, Emotions, and Self-Control
The front part of your brain, just behind your forehead, manages the cognitive functions that make you distinctly human. This prefrontal cortex handles planning, decision-making, problem-solving, and working memory (the ability to hold and manipulate information in your mind for short periods). It’s also where impulse control lives. When you resist the urge to check your phone during a conversation or stop yourself from saying something you’d regret, your prefrontal cortex is doing that work.
Different sections of this region specialize in different tasks. Some help you stay focused by filtering out distractions. Others regulate emotions, allowing you to feel empathy, evaluate experiences as positive or negative, and adjust your behavior when circumstances change. The prefrontal cortex also learns from past experiences, which is how you develop better judgment over time rather than simply reacting on impulse to every situation.
Digestion
Your gut has its own dedicated nervous system called the enteric nervous system, a complex network of neurons and supporting cells embedded in the walls of your digestive tract. This system coordinates the muscle contractions that push food through your stomach and intestines, regulates the release of digestive enzymes, and manages the absorption of nutrients. It integrates signals from immune cells, hormone-producing cells, and other specialized cells in the gut lining to fine-tune digestion in real time.
The enteric nervous system can operate independently from the brain, which is why it’s sometimes called the “second brain.” But it also communicates with the central nervous system through the vagus nerve, which is the longest cranial nerve and a major highway of the parasympathetic system. This connection is why stress can cause nausea or upset your stomach, and why gut problems can sometimes influence mood.