The nervous system has two main organs: the brain and the spinal cord. Together, these form the central nervous system, which acts as the body’s command center. Branching out from them is a vast network of nerves that reaches every part of your body, known as the peripheral nervous system. Understanding how these structures work together explains how you think, move, feel, and keep your body running without conscious effort.
The Brain
The brain is the most complex organ in the nervous system and the primary control center for nearly everything your body does. It contains roughly 86 billion nerve cells (neurons), along with a similar number of supporting cells that help neurons function. At a high level, the brain divides into three major parts: the cerebrum, the cerebellum, and the brainstem.
The cerebrum is the largest portion, sitting at the front and top of your skull. It handles the functions most people associate with the brain: thinking, reasoning, problem-solving, speech, emotions, learning, and memory. It also initiates voluntary movement, regulates body temperature, and processes input from your senses like vision, hearing, and touch. The cerebrum’s outer layer, called the cerebral cortex, is made of gray matter packed with neurons. Beneath that lies white matter, which consists of the long cable-like extensions of neurons that carry signals between brain regions.
The cerebellum sits at the back of the head, below the cerebrum and above the brainstem. It’s roughly the size of a fist. Despite being much smaller than the cerebrum, it plays a critical role in coordinating voluntary muscle movements, maintaining posture, and keeping your balance. When you catch a ball or walk across an uneven surface without thinking about it, your cerebellum is doing most of the work.
The brainstem connects the cerebrum to the spinal cord and includes three substructures: the midbrain, the pons, and the medulla. The medulla, located at the very bottom where the brain meets the spinal cord, is essential to survival. It regulates heart rhythm, breathing, blood flow, and oxygen levels. It also controls reflexive actions like sneezing, coughing, swallowing, and vomiting. These are processes your body handles automatically, without any conscious input from you.
The Spinal Cord
The spinal cord is the second major organ of the central nervous system. It’s a cylinder-shaped tube of nervous tissue that runs from the base of the brainstem down through the center of your spine, ending in your lower back in a tapered shape called the conus medullaris. In adults, it measures about 40 to 50 centimeters long and only 1 to 1.5 centimeters in diameter.
Think of the spinal cord as the main communication highway between your brain and the rest of your body. It carries electrical signals in both directions: sensory information travels up from your body to the brain, while motor commands travel down from the brain to your muscles and organs. Without this link, the brain would have no way to receive sensations or send instructions below the neck.
The spinal cord is divided into four regions that correspond to sections of the spine: cervical (neck), thoracic (upper back), lumbar (lower back), and sacral (base of the spine). There are 31 segments in total, each defined by a pair of nerves that branch off the cord and extend outward to serve specific parts of the body. A cross-section of the spinal cord reveals gray matter on the inside, where nerve cell bodies cluster, and white matter on the outside, where bundled nerve fibers carry signals up and down.
The spinal cord also does some processing on its own. Reflexes like pulling your hand away from a hot surface happen at the spinal cord level before the pain signal even reaches your brain. Motor neurons in the spinal cord can activate muscles for both voluntary movements and involuntary reflexes, and the cord also contains neurons that help regulate internal organ function.
How the Brain and Spinal Cord Are Protected
Both the brain and spinal cord are wrapped in three layers of protective tissue called the meninges. The outermost layer, the dura mater, is tough and acts as a shield against injury. The middle layer is the arachnoid mater. The innermost layer, the pia mater, sits directly against the brain and spinal cord tissue. Between the arachnoid and pia layers is a space filled with cerebrospinal fluid, a clear liquid that cushions the brain and spinal cord against impact, absorbing shocks that would otherwise damage these delicate structures.
Peripheral Nerves
Everything outside the brain and spinal cord belongs to the peripheral nervous system. This network is built from nerves, which are bundles of nerve cells with long, arm-like extensions called axons that twist together to form fibers. These nerves are the wiring that connects the central nervous system to every muscle, organ, and sensory receptor in your body.
Peripheral nerves fall into two main categories. Cranial nerves are 12 pairs that connect directly to the brain and serve your face, head, and neck. They handle functions like vision, smell, facial movement, and hearing. Eleven of these 12 pairs are part of the peripheral nervous system (the twelfth, the optic nerve, is sometimes classified differently). Spinal nerves are 31 pairs that branch off the spinal cord, roughly one pair at each vertebra. They fan out to serve the trunk, arms, legs, and internal organs.
Within the peripheral nervous system, nerves carry two types of signals. Sensory (afferent) nerves carry information from the body to the brain, reporting on everything from temperature and pressure to pain and body position. Motor (efferent) nerves carry commands from the brain outward, telling muscles to contract or glands to release hormones.
The Autonomic and Enteric Divisions
Not all of the peripheral nervous system is under your conscious control. The autonomic nervous system manages functions you don’t have to think about: heart rate, digestion, breathing rate, and blood pressure. It operates through two complementary branches. One speeds things up during stress or exertion, while the other slows things down during rest and recovery.
Embedded within the walls of your digestive tract is a specialized network called the enteric nervous system. This is a complex system of neurons and supporting cells that controls digestion largely on its own, integrating signals from multiple cell types to regulate gut movement, secretion, and blood flow with precise timing. It communicates with the brain but can also operate semi-independently, which is why it’s sometimes called the “second brain.” It coordinates the muscular contractions that move food through your intestines, manages the release of digestive enzymes, and helps maintain the gut’s internal balance.
Sensory Receptors as Nervous System Inputs
Your nervous system gathers information about the world through specialized sensory receptors scattered throughout the body. These take several forms. Some are simply the exposed endings of nerve cells embedded in tissue, like pain receptors in your skin. Others are nerve endings wrapped in connective tissue that makes them more sensitive to specific types of pressure or vibration. A third type includes highly specialized receptor cells, like the photoreceptors in your eyes (rod and cone cells), which detect light and convert it into nerve signals passed along to the brain via the optic nerve.
These receptors sit at the boundary between the external world and the nervous system. When a stimulus is strong enough, whether it’s heat on your fingertip or sound waves hitting your inner ear, the receptor generates an electrical signal that travels along sensory nerves to the spinal cord and up to the brain. This is how the nervous system constantly builds and updates its picture of what’s happening both inside and outside your body.