The nervous system has two major organs: the brain and the spinal cord. Together, these form the central nervous system, which acts as the body’s command center. But the nervous system extends far beyond these two organs, with a vast network of nerves branching out to every corner of your body. Understanding how all these parts work together helps explain how you think, move, feel, and even digest food.
The Brain
The brain is the largest and most complex organ in the nervous system, weighing roughly 1,200 to 1,400 grams in adults (about 3 pounds). It contains approximately 86 billion neurons, plus a roughly equal number of supporting cells that help keep everything running. The brain controls most of the body’s functions, from conscious actions like moving and speaking to the five senses: sight, hearing, touch, taste, and smell.
The biggest structure in the brain is the cerebrum, which is split into left and right hemispheres. The outer surface of the cerebrum is divided into four lobes, each with a primary role:
- Frontal lobe: personality, emotions, problem solving, and controlling movement
- Temporal lobe: hearing, language, and reading
- Parietal lobe: processing touch and other senses, attention, and language
- Occipital lobe: vision, including recognizing shapes and colors
Beneath the cerebrum sit several other critical structures. The thalamus, located near the center of the brain, acts as a relay station that routes sensory and motor information to the right areas of the cortex. It also plays a role in sleep and alertness. The limbic system, tucked under the cortex, processes emotions and drives like hunger and fear. Lower still is the cerebellum, which fine-tunes motor control, coordination, and spatial navigation. And at the very base, the brainstem connects the brain to the spinal cord, managing basic life-sustaining functions like breathing and heart rate.
The Spinal Cord
The spinal cord is essentially an extension of the brain. It’s a column of millions of nerve cells that runs through the center of your spine, carrying electrical signals back and forth between the brain and the rest of the body. Think of it as the main cable connecting your brain to everything below your neck.
The spinal cord is divided into three regions that correspond to sections of the spine: cervical (neck), thoracic (upper back), and lumbar (lower back). It handles three core jobs. First, it relays the brain’s commands for voluntary movement and also transmits signals that control involuntary functions like your heartbeat and breathing. Second, it carries sensory information (pressure, pain, temperature) from your body back up to the brain. Third, it manages certain reflexes entirely on its own, without waiting for the brain’s input. When you pull your hand away from a hot surface before you consciously feel the burn, that’s your spinal cord acting independently.
Peripheral Nerves
Everything outside the brain and spinal cord belongs to the peripheral nervous system. This network of nerves branches out from the brain and spinal cord and reaches every part of your body, all the way to the tips of your fingers and toes.
You have 12 pairs of cranial nerves that connect directly to the brain rather than running through the spinal cord. These handle signals from your face, eyes, ears, nose, and mouth, and they give you the sense of touch across your head and neck. The remaining peripheral nerves extend from the spinal cord and intertwine throughout the rest of the body. Together, these nerves collect sensory information from the outside world and deliver movement commands from the brain to your muscles.
How Signals Travel Between These Organs
Nerve signals are electrical impulses that travel along nerve cells. When a signal reaches the end of one nerve cell, it needs to cross a tiny gap (called a synapse) to reach the next cell. The first cell releases chemical messengers that float across the gap and lock onto the neighboring cell, triggering a new electrical impulse. This handoff takes roughly half a millisecond to one millisecond, which is why nerve responses feel almost instantaneous.
The nervous system uses over 50 different chemical messengers to transmit, amplify, or dampen these signals. Some messengers excite the next nerve cell into firing. Others do the opposite, calming activity down. This balance between excitation and inhibition is how the brain fine-tunes everything from muscle contractions to mood.
The Enteric Nervous System
Your digestive tract has its own extensive nerve network, sometimes called the body’s “second brain.” This enteric nervous system uses the same types of chemical messengers and cells found in the brain. It can regulate digestion largely on its own and also communicates with the central nervous system to flag problems. The trillions of bacteria living in your gut interact with this network, which in turn influences signals sent to the brain. This gut-brain connection helps explain why stress can cause stomach problems and why digestive issues can affect mood.
How the Organs Work as a System
No single organ in the nervous system works in isolation. The brain interprets information and makes decisions, the spinal cord relays those decisions to the body (and handles urgent reflexes on its own), and the peripheral nerves serve as the delivery routes connecting the central nervous system to muscles, skin, and internal organs. The enteric nervous system adds a semi-independent layer of control over digestion. All of these parts communicate through the same basic mechanism: electrical impulses passed along chains of nerve cells, bridged by chemical messengers at every junction.
When any part of this chain is disrupted, whether through injury to the spinal cord, damage to peripheral nerves, or changes in brain structure, the effects can ripple outward. A spinal cord injury, for instance, doesn’t just block movement commands from the brain. It can also cut off sensory feedback from below the injury site and disrupt the involuntary signals that regulate blood pressure and body temperature. The interconnectedness of these organs is what makes the nervous system both remarkably efficient and vulnerable when something goes wrong.