The nervous system is built around two primary organs, the brain and the spinal cord, supported by an extensive network of nerves that reach every part of your body. Together, these structures form two major divisions: the central nervous system and the peripheral nervous system. A third, often overlooked division sits inside your digestive tract and operates with surprising independence.
The Two Divisions of the Nervous System
Everything in the nervous system falls into one of two categories. The central nervous system (CNS) includes the brain and spinal cord, both protected by bone. The peripheral nervous system (PNS) includes every nerve and sensory structure outside the brain and spinal cord, from the 12 pairs of cranial nerves attached to your brain to the tiny nerve endings at the tips of your fingers.
These two divisions work as a loop. Sensory nerves in the peripheral system collect information from your environment, things you see, hear, touch, taste, and smell, and deliver it to the brain and spinal cord. The central nervous system processes that information and sends commands back out through motor nerves, telling muscles to move or organs to adjust their activity.
The Brain
The brain is the command center of the entire nervous system. It weighs about three pounds and contains roughly 86 billion nerve cells, yet it consumes about 20% of your body’s energy at rest. Its outer layer, the cerebrum, is divided into four lobes on each side, and each lobe handles different types of work.
The frontal lobes sit behind your forehead and control thinking, planning, problem-solving, short-term memory, and voluntary movement. The parietal lobes, positioned behind the frontal lobes near the top of the head, interpret sensory information like taste, texture, and temperature. The occipital lobes, at the back of the skull, process visual images from your eyes and match them against stored memories so you can recognize what you’re seeing. The temporal lobes, on either side of your head near your ears, handle smell, taste, and sound processing, and they play a major role in storing memories.
Beneath the cerebrum sit structures that handle functions you rarely think about consciously. The brainstem controls breathing, heart rate, blood pressure, and sleep cycles. The cerebellum coordinates balance and fine motor movements like writing or catching a ball. The hypothalamus regulates body temperature, hunger, thirst, and hormone release.
The Spinal Cord
The spinal cord is a long, thin bundle of nerve tissue that runs from the base of the brain down through the vertebral column. It has 31 segments, and each segment gives rise to a pair of spinal nerves that branch out to serve a specific region of the body. These 31 pairs of spinal nerves are the main communication highway between the brain and everything below the neck.
The spinal cord does more than relay messages. It can process certain responses on its own through reflexes. When you touch a hot surface, sensory nerves send a signal to the spinal cord, which fires back a command to pull your hand away before the pain signal even reaches your brain. This shortcut shaves critical milliseconds off your reaction time.
Nerves of the Peripheral System
The peripheral nervous system contains two main groups of nerves. The 12 pairs of cranial nerves connect directly to the brain (11 of those 12 pairs belong to the peripheral system; one pair, the optic nerves, is technically part of the central nervous system). These cranial nerves control functions concentrated in the head and neck: eye movement, facial expressions, hearing, taste, and swallowing. One cranial nerve, the vagus nerve, extends far below the head to influence heart rate, digestion, and other organ functions deep in the chest and abdomen.
The 31 pairs of spinal nerves branch off the spinal cord at each vertebral level and divide into progressively smaller nerves that spread throughout your body. Each nerve is a bundle of nerve fibers, long cell extensions called axons twisted together like wires in a cable. By the time these nerves reach their endpoints, they’re fine enough to sit just beneath the surface of your skin or at the tips of your toes.
Peripheral nerves come in three functional types. Sensory nerves carry information to the brain and spinal cord. Motor nerves carry commands from the brain to muscles. Autonomic nerves control the automatic functions of internal organs, things like digestion, blood vessel diameter, and sweat production.
Somatic vs. Autonomic: Two Operating Modes
Within the peripheral nervous system, two subsystems divide responsibilities by conscious control. The somatic nervous system handles everything you can deliberately sense and do: feeling the texture of fabric, deciding to raise your arm, noticing pain. You’re aware of its activity and can direct it.
The autonomic nervous system runs behind the scenes without your conscious input. It keeps you alive by managing heart rate, blood pressure, breathing rate, digestion, pupil size, and dozens of other processes. It further splits into two branches that act as a gas pedal and a brake. The sympathetic branch accelerates your body during stress or physical activity. The parasympathetic branch slows things down during rest and recovery, promoting digestion and tissue repair.
Sensory Organs
Your eyes, ears, nose, tongue, and skin are often grouped with the nervous system because they contain specialized nerve receptors that serve as the system’s input devices. Each one converts a type of environmental energy, light, sound waves, chemical molecules, pressure, into electrical signals that travel along sensory nerves to the brain.
These organs don’t just passively receive information. The retina at the back of your eye, for example, contains over 100 million photoreceptor cells and performs significant processing before signals ever leave for the brain. The inner ear converts sound vibrations into nerve impulses and simultaneously monitors head position for balance. Receptors embedded throughout your skin detect pressure, temperature, vibration, and pain at varying depths, giving the brain a detailed, constantly updated map of your physical environment.
The Enteric Nervous System
A third division of the nervous system lives inside the walls of your digestive tract and is sometimes called the “second brain.” The enteric nervous system is a complex network of neurons and supporting cells embedded in the gut wall, stretching from the esophagus to the rectum. It contains an estimated 100 million nerve cells, more than the spinal cord.
What makes this system remarkable is its independence. While it communicates with the brain through the vagus nerve, it can coordinate digestion entirely on its own, managing the rhythmic muscle contractions that move food through the intestines, regulating secretions, and monitoring the gut’s chemical environment. Enteric neurons work alongside immune cells and hormone-producing cells in the gut lining to control digestion with precise spatial and temporal coordination. This is why digestive processes continue even when the vagus nerve is severed, and why gut health can influence mood and immune function through the signals this system sends back to the brain.