The nervous system has two main organs: the brain and the spinal cord. Together they form the central nervous system, which acts as the body’s command center. Beyond these two organs, a vast network of nerves branches out to every part of your body, forming the peripheral nervous system. The gut even has its own semi-independent nervous system with enough neurons to earn the nickname “the second brain.”
The Brain
The brain is the largest and most complex organ in the nervous system. It weighs about 1,450 grams (roughly 3.2 pounds) at its peak before age 25, and it contains approximately 86 billion neurons alongside 85 billion supporting cells. Those numbers, confirmed by careful cell-counting studies, are actually lower than the 100 billion figure repeated in many older textbooks.
The brain is divided into several major regions, each responsible for different functions:
- Frontal lobe: Controls personality, emotions, problem solving, and voluntary movement.
- Temporal lobe: Processes hearing, language, and reading.
- Parietal lobe: Handles sensory information, attention, and spatial awareness.
- Occipital lobe: Manages vision, including recognizing shapes and colors.
- Cerebellum: Sits lower in the brain and coordinates movement, balance, and spatial navigation.
- Brainstem: Connects the brain to the spinal cord and controls automatic survival functions like heart rate, breathing, coughing, sneezing, and swallowing.
The brain grows rapidly in early life, increasing from about 397 grams at birth to 1,180 grams by age 6. After peaking in early adulthood, it loses roughly 2 grams per year until age 80, when the decline steepens to about 5 grams per year.
The Spinal Cord
The spinal cord is the second major organ of the central nervous system. It runs from the base of the brainstem down through the spine, serving as the primary communication highway between the brain and the rest of the body. Sensory signals travel up the cord to the brain, while movement commands travel down from the brain to the muscles.
The cord itself is organized with remarkable precision. Sensory-processing neurons sit toward the back, motor neurons sit toward the front, and neurons controlling internal organs occupy the space in between. This layout means that different types of information travel through distinct pathways. Touch and pressure signals, for example, travel up through the back columns of the cord, while pain and temperature signals take a separate route through the side and front columns. Motor commands from the brain travel down through the lateral columns to reach the muscles.
The spinal cord also handles certain tasks without waiting for the brain’s input. Reflexes like pulling your hand away from a hot surface happen at the spinal cord level, allowing your body to react faster than conscious thought would permit.
How the Brain and Spinal Cord Are Protected
Both the brain and spinal cord are wrapped in three layers of protective membranes called the meninges. The outermost layer, the dura mater, sits closest to the skull and vertebrae. The middle layer is the arachnoid mater. The innermost layer, the pia mater, clings directly to the surface of the brain and spinal cord tissue.
Between the arachnoid and pia layers, a space filled with cerebrospinal fluid cushions the brain and spinal cord from impact. This fluid acts as a shock absorber, so a bump to the head doesn’t transmit full force to the delicate neural tissue inside.
Nerves: The Peripheral Network
Outside the brain and spinal cord, the peripheral nervous system connects the central nervous system to every organ, muscle, and patch of skin in your body. It consists of two main groups of nerves.
Twelve pairs of cranial nerves connect directly to the brain (though one pair, the optic nerves, is technically part of the central nervous system). These nerves carry signals from your nose, ears, mouth, and the skin of your face and head. They also control muscles in your face and throat.
Thirty-one pairs of spinal nerves branch off the spinal cord, roughly one pair at each vertebra. These nerves relay sensory information from your trunk, arms, and legs back to the spinal cord, and carry movement commands from the spinal cord out to your muscles.
Ganglia and Sensory Receptors
Scattered along these nerves are clusters of nerve cell bodies called ganglia. Sensory ganglia house two broad categories of neurons: ones that detect sensations you consciously feel (touch, pain, temperature) and ones that monitor your internal environment without you being aware of it, tracking things like blood pressure, gut stretch, and chemical changes in your organs. Ganglia in the head, for instance, relay touch and pain from the face, while ganglia near the throat monitor the heart and digestive tract to keep vital reflexes running.
At the far ends of peripheral nerves, specialized sensory receptors detect specific stimuli. Some respond to pressure, others to temperature, others to chemical signals. Specialized cells in the skin work alongside nerve endings to detect fine touch, while other cell types help sense pain. These receptors convert physical and chemical information from your environment into electrical signals the nervous system can interpret.
Somatic vs. Autonomic Divisions
The peripheral nervous system splits into two functional divisions. Your somatic nervous system handles everything you can consciously sense and control, like feeling the texture of fabric or moving your arm. Your autonomic nervous system runs behind the scenes, managing processes you don’t consciously direct: heart rate, digestion, blood pressure, and temperature regulation.
The line between these two systems isn’t always sharp. Breathing, for example, runs automatically through the autonomic system most of the time, but you can also take deliberate control and decide when to inhale and exhale. Internal organ problems sometimes show up through the somatic system as “referred pain,” where you feel discomfort in a specific area of your skin even though the actual problem is in a nearby organ.
The Enteric Nervous System
Your gut contains its own nervous system, the enteric nervous system, that operates with a surprising degree of independence from the brain. It’s embedded in the walls of the digestive tract and organized into two networks of interconnected nerve clusters. One network sits between the muscle layers of the gut wall and controls the rhythmic contractions that push food along. The other sits closer to the inner lining and helps regulate secretion and blood flow.
The enteric nervous system communicates back and forth with the brain, but it can coordinate digestion largely on its own. The gut is also the body’s largest immune and endocrine organ, and the enteric nervous system integrates signals from immune cells, hormones, and the gut’s massive microbial population. This constant two-way communication between gut and brain is why stress can upset your stomach and why gut problems can affect mood.