The nervous system is built from two main organs, the brain and the spinal cord, plus an enormous network of nerves that branch out to every corner of your body. Together, these structures are organized into two major divisions: the central nervous system and the peripheral nervous system. A third, often overlooked division lives entirely in your gut. Here’s how all of these parts fit together.
The Central Nervous System: Brain and Spinal Cord
The central nervous system consists of just two organs: the brain and the spinal cord. These are the command centers. The brain processes everything from conscious thought and emotion to automatic functions like breathing and heart rate. The spinal cord is a cylindrical column of nervous tissue that runs from the base of the brain down through the vertebral column, carrying signals back and forth between the brain and the rest of the body.
The spinal cord is divided into four regions: cervical (neck), thoracic (upper and mid-back), lumbar (lower back), and sacral (near the tailbone). Across those regions, there are 31 segments, each defined by a pair of nerves that exits the cord and connects to a specific part of the body. That segmental organization is why a spinal injury at a particular level affects sensation and movement below that point but not above it.
One surprising structure also counts as part of the central nervous system: the retina. Though it sits inside the eye, the retina is actually an extension of the brain that developed from the same embryonic tissue. It contains layers of neurons that begin processing visual information before signals ever travel along the optic nerve. Scientists sometimes describe it as an accessible window into the central nervous system for exactly this reason.
The Peripheral Nervous System: Nerves Throughout the Body
Everything outside the brain and spinal cord belongs to the peripheral nervous system. Its main structures are nerves, long cable-like bundles of fibers that carry electrical signals between the central nervous system and the rest of your body. These nerves fall into two groups.
Cranial nerves are a set of 12 pairs that originate directly from the brain. Each pair splits to serve both sides of your head, face, and neck. Some are purely sensory (the olfactory nerve handles smell, the optic nerve handles vision, and the vestibulocochlear nerve handles hearing and balance). Others control movement, like the nerves that let you make facial expressions or move your eyes. Several cranial nerves do both.
Spinal nerves are 31 pairs that branch off the spinal cord, split into 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal pair. These nerves carry sensory information from your skin, joints, and muscles back to the spinal cord, and they deliver motor commands from the brain out to your muscles. They also control many reflexes, like yanking your hand away from a hot surface before you consciously register the pain.
Sensory Organs and Their Nervous System Connection
Your eyes, ears, nose, tongue, and skin aren’t classified as organs of the nervous system, but they contain specialized receptor cells that serve as its front line. Photoreceptors in the retina respond to light and release chemical signals to neighboring neurons, which relay visual data through the optic nerve to the brain. Mechanoreceptors in the inner ear detect sound vibrations and head movement, forming the basis of both hearing and balance. Receptors in the skin detect pressure, temperature, and pain.
All of this sensory information travels to the central nervous system through either cranial nerves or spinal nerves. The cranial nerves handle senses concentrated in the head (sight, smell, hearing, taste), while spinal nerves carry touch and pain signals from the trunk and limbs. Without these receptor cells, the nervous system would have no raw data to work with.
The Autonomic Nervous System: Reaching Internal Organs
The autonomic nervous system is the branch of the peripheral nervous system that connects the brain to your internal organs. It operates largely without conscious effort, managing the behind-the-scenes functions that keep you alive. Four of the 12 cranial nerve pairs carry autonomic fibers, and most of the 31 spinal nerve pairs do as well.
The list of organs and tissues under autonomic control is long. It regulates how fast and hard your heart pumps and how wide your blood vessels are, controlling blood pressure moment to moment. It manages pupil size and the muscles your eyes use to focus. It triggers tear production, controls how much saliva your mouth releases, and governs nasal secretions. In your skin, it activates sweat glands and the tiny muscles that make hair stand on end. It reaches deep into the chest and abdomen to influence the lungs, stomach, intestines, bladder, and reproductive organs.
The autonomic system is further divided into two branches that generally work in opposition. One ramps your body up during stress or activity, increasing heart rate and diverting blood to muscles. The other calms things down during rest, slowing the heart and promoting digestion.
The Enteric Nervous System: A Brain in Your Gut
Embedded in the walls of your gastrointestinal tract is a dense network of neurons and supporting cells called the enteric nervous system. It spans the length of the digestive tract, from the esophagus to the rectum, and it is complex enough that researchers sometimes call it the “second brain.”
The enteric nervous system coordinates the muscular contractions that move food through your intestines, regulates the release of digestive enzymes, and manages blood flow to the gut lining. What makes it unusual is its independence: it can carry out many of these functions on its own, without instructions from the brain or spinal cord. It does communicate with the central nervous system through the vagus nerve and other pathways, but it has enough local circuitry to operate autonomously when those connections are disrupted.
The Cells That Build These Organs
Every organ in the nervous system is made from two basic cell types. Neurons are the signaling cells. They generate electrical impulses and pass them along to other neurons, to muscles, or to glands. Glial cells are the support staff. They insulate neurons, supply them with nutrients, clear away waste, and help maintain the chemical environment neurons need to function. Glial cells far outnumber neurons throughout the nervous system and make up a substantial portion of the brain’s physical mass.
The ratio of these cell types and how they’re arranged is what gives each organ its specific capabilities. The brain’s outer layer is dense with neuron cell bodies (gray matter), while its interior is dominated by the insulated fibers that connect distant regions (white matter). The spinal cord has a similar gray-and-white organization but in a reversed arrangement, with gray matter in the center. Peripheral nerves are almost entirely made of long neuron fibers bundled together like wires in a cable, wrapped in layers of connective tissue for protection.