The major organs of the nervous system are the brain and spinal cord, which together form the central nervous system, and the nerves that branch out from them to reach every part of your body, forming the peripheral nervous system. These organs work as a unified communication network: the brain and spinal cord receive and process information, then send instructions outward through nerves to control everything from deliberate movements to automatic functions like breathing and digestion.
The Brain
The brain is the command center of the entire nervous system. It regulates your thoughts, feelings, and movements while also managing organ functions you never consciously think about, like digestion and breathing. It has three major regions, each with distinct responsibilities.
The cerebrum is the largest part, making up most of what you see when you picture a brain. It interprets all five senses and handles conscious actions that require thinking: speech, memory, behavior, personality, reasoning, and judgment. The cerebrum is divided into left and right hemispheres, each containing four lobes that specialize in different tasks.
The cerebellum sits at the back and bottom of the brain. It maintains your balance, posture, coordination, and fine motor skills. When you catch a ball, ride a bike, or thread a needle, your cerebellum is doing much of the precision work behind those movements.
The brainstem connects the brain to the spinal cord and regulates automatic body functions you don’t consciously control: heart rate, breathing, sleep and wake cycles, and swallowing. It serves as the relay station for signals traveling between the brain and the rest of the body.
The Spinal Cord
The spinal cord is a cylindrical column of nervous tissue running from the base of the brainstem down through your vertebral column. It measures 40 to 50 centimeters long and only about 1 to 1.5 centimeters in diameter, yet it carries virtually all the signals traveling between your brain and body. When your brain creates a message, it sends that signal to the spinal cord, which then relays it outward through peripheral nerves to complete an action.
The cord is divided into 31 segments across four regions: cervical (neck), thoracic (mid-back), lumbar (lower back), and sacral (base of the spine). Each segment gives rise to a pair of spinal nerves. Two areas are noticeably thicker than the rest. The cervical enlargement, between the third cervical and first thoracic segments, handles the heavy nerve traffic going to and from your arms and hands. The lumbar enlargement serves the legs.
If you sliced the spinal cord crosswise, you’d see white matter on the outside and a butterfly-shaped core of gray matter on the inside. The gray matter contains the cell bodies of neurons and is where initial processing of sensory information happens. It also houses motor neurons that send signals out to muscles for both voluntary movements and involuntary reflexes, like yanking your hand away from a hot surface. The white matter consists of long nerve fibers coated in a fatty insulating substance called myelin, bundled into pathways that carry signals up to the brain and back down again. The spinal cord also contains neurons that manage autonomic control for most of your internal organs.
Nerves of the Peripheral Nervous System
Nerves are the cable-like structures that extend from the brain and spinal cord to every corner of the body. They fall into two main categories.
Twelve pairs of cranial nerves originate directly in the brain and extend through the face, head, and neck. Some are purely sensory (carrying information about smell or vision to the brain), some are purely motor (controlling muscles in the face or eyes), and some do both. They’re responsible for functions like making facial expressions, moving your eyes, and processing smells.
Thirty-one pairs of spinal nerves branch out from the spinal cord. These nerves can carry sensory information (like pain or temperature from your skin, joints, and muscles) up to the spinal cord and brain, or deliver motor commands from the brain down to your muscles. They also control many reflexes.
Along these nerve pathways sit clusters of nerve cell bodies called ganglia, which act as relay and processing stations outside the brain and spinal cord.
Functional Divisions Within the System
The peripheral nervous system splits into two functional divisions that use many of the same nerves but control very different things. Your somatic nervous system handles everything you consciously sense and do: feeling a texture, lifting your arm, or speaking. Your autonomic nervous system works without you thinking about it, running the behind-the-scenes processes that keep you alive, like regulating heart rate, blood pressure, and digestion. Some functions cross the line between the two. Breathing, for example, happens automatically most of the time, but you can also take deliberate control of when you inhale and exhale.
The autonomic nervous system itself has two branches that act as counterweights. The sympathetic branch speeds things up during stress or activity (the “fight or flight” response), while the parasympathetic branch slows things down during rest and recovery.
The Enteric Nervous System
Your digestive tract contains its own extensive nerve network, often called the body’s “second brain.” This enteric nervous system uses the same chemical messengers and cell types as the brain to regulate digestion and alert the central nervous system when something is wrong in the gut. It operates with a surprising degree of independence. While it communicates with the brain through the autonomic nervous system, it can coordinate the complex muscular contractions of digestion largely on its own. The trillions of bacteria in the gut also interact with this network, creating a communication loop between your microbiome, your enteric nervous system, and your brain.
Cells That Build These Organs
Every organ in the nervous system is built from two broad categories of cells: neurons and glial cells. Neurons are the signaling cells, transmitting electrical impulses across the system. Glial cells outnumber neurons and serve as the essential support staff.
In the brain and spinal cord, astrocytes (star-shaped cells) maintain the chemical environment around neurons by regulating neurotransmitter levels and providing metabolic support. Oligodendrocytes wrap a fatty insulating layer around long nerve fibers, which speeds up signal transmission. Microglia act as the brain’s immune cells, identifying threats and clearing away dead cells. Ependymal cells line the inner cavities of the brain and spinal cord and produce cerebrospinal fluid.
In the peripheral nervous system, Schwann cells perform the same insulating role that oligodendrocytes perform centrally. Satellite cells surround nerve cell bodies in ganglia and help regulate their chemical environment. The digestive tract even has its own specialized type, enteric glial cells, supporting the nerves embedded in the gut wall.
How the Brain and Spinal Cord Are Protected
Because the brain and spinal cord are irreplaceable and difficult to repair, the body wraps them in multiple layers of protection. Both sit inside bone: the skull encases the brain, and the vertebral column surrounds the spinal cord. Beneath the bone, three layers of membrane called meninges provide additional shielding. The outermost layer, the dura mater, is a tough covering closest to the bone. The middle layer, the arachnoid mater, is more delicate. The innermost layer, the pia mater, clings directly to the surface of the brain and spinal cord tissue.
Between the arachnoid and pia layers is a fluid-filled space containing cerebrospinal fluid. This clear liquid cushions the brain and spinal cord, absorbing shocks so that the soft nervous tissue doesn’t jostle against the surrounding bone. Cerebrospinal fluid also carries nutrients and removes waste products from the central nervous system.
Sense Organs as Extensions of the System
The eyes, ears, nose, and tongue are not classified as organs of the nervous system in the traditional sense, but they contain specialized nervous tissue that is functionally part of it. The retina at the back of the eye, for instance, is a layer of neural tissue that converts light into electrical signals. The inner ear contains the cochlea for hearing and the vestibule for balance, both of which relay neural signals to the brainstem through the vestibulocochlear nerve. General sensory receptors for touch, temperature, and pain are distributed throughout the skin and internal organs, feeding information into the nervous system through peripheral nerves. These sensory structures are where the nervous system first makes contact with the outside world.