The nervous system contains two main organs, the brain and the spinal cord, plus an extensive network of nerves that branch out to every part of your body. Together, these structures collect sensory information, process it, and send out commands that control everything from movement to digestion to emotion. Understanding what each part does helps clarify how they work as a single, coordinated system.
The Two Central Organs: Brain and Spinal Cord
The brain and spinal cord make up the central nervous system (CNS). The brain sits protected inside the skull, while the spinal cord runs through the vertebral canal of the spine. These are the command centers where sensory input gets processed and motor signals originate.
The brain itself has several major regions, each handling different jobs. The outer layer, the cerebral cortex, is the highest level of organization in the nervous system. It’s divided into four lobes. The frontal lobe controls voluntary movement, speech production, personality, and decision-making. The parietal lobe processes touch and spatial awareness. The temporal lobe handles hearing, language comprehension, and memory. The occipital lobe, at the back of the head, is dedicated to vision. Deeper structures beneath the cortex manage things like relaying sensory signals (the thalamus), regulating hunger, thirst, and body temperature (the hypothalamus), and forming memories (the hippocampus).
Below the cerebral cortex sits the brainstem, which connects the brain to the spinal cord. It controls basic life-sustaining functions like breathing, heart rate, and sleep cycles. Attached to the back of the brainstem is the cerebellum, which fine-tunes coordination and balance. The neocortex alone contains roughly 26 billion neurons and 39 billion glial cells, the support cells that insulate, nourish, and protect neurons.
The spinal cord is an elongated cylinder of nerve tissue. Its inner core of gray matter contains neuron cell bodies, while an outer layer of white matter carries bundles of nerve fibers traveling up toward the brain or down toward the body. The spinal cord does more than relay messages. It also processes certain reflexes on its own, letting you pull your hand from a hot surface before the pain signal even reaches your brain.
Protective Layers Around the CNS
Three membranes called the meninges wrap around both the brain and spinal cord. The outermost layer, the dura mater, is tough and sits closest to the skull. The middle layer, the arachnoid mater, is web-like and contains cerebrospinal fluid in the space beneath it. The innermost layer, the pia mater, clings directly to the surface of brain and spinal cord tissue. Cerebrospinal fluid circulates through and around the CNS, cushioning it against impact and carrying away waste products.
Nerves: The Peripheral Network
Everything outside the brain and spinal cord belongs to the peripheral nervous system (PNS). Its main structures are nerves, which are bundles of nerve fibers that carry signals between the CNS and the rest of your body. These nerves fall into two broad categories. Sensory nerves carry information inward, delivering data about touch, temperature, pain, and position to the brain and spinal cord. Motor nerves carry commands outward, telling muscles to contract or glands to release hormones.
Twelve pairs of cranial nerves connect directly to the brain rather than passing through the spinal cord. Each pair serves a specific function. The olfactory nerve provides your sense of smell. The optic nerve carries visual information from the eyes. Others control eye movement, facial expression, hearing, taste, and tongue movement. One cranial nerve, the vagus nerve, extends all the way into the chest and abdomen, influencing heart rate, digestion, and other organ functions.
An additional 31 pairs of spinal nerves branch off the spinal cord and fan out through the torso, arms, and legs. These handle sensation and movement for the trunk and limbs.
Autonomic and Enteric Divisions
Within the peripheral nervous system, the autonomic nervous system controls processes you don’t consciously direct: heart rate, blood pressure, digestion, and pupil dilation. It has two opposing branches. The sympathetic branch accelerates your body during stress or physical activity. The parasympathetic branch slows things down during rest and recovery.
The digestive tract has its own semi-independent nerve network called the enteric nervous system. It’s a dense mesh of neurons and support cells embedded in the walls of the esophagus, stomach, and intestines. This network coordinates the muscle contractions that move food along, regulates secretion of digestive enzymes, and manages blood flow to the gut. The gastrointestinal tract is the most complex organ system in terms of the sheer number of signaling molecules it uses, which is why the enteric nervous system is sometimes called the “second brain.” It can operate on its own without direct input from the brain or spinal cord, though the two systems stay in constant communication through the vagus nerve.
How These Organs Work Together
No part of the nervous system operates in isolation. When you touch something hot, sensory receptors in your skin generate a signal that travels along a peripheral nerve to the spinal cord. The spinal cord triggers an immediate reflex withdrawal while simultaneously sending the signal up to the brain, where the sensation is consciously registered as pain. The brain then coordinates a more complex response: you might run cool water over the burn, remember to avoid that surface, or call out for help. All of this involves the central and peripheral systems acting in concert within fractions of a second.
Motor control works similarly. When you decide to pick up a glass, the frontal lobe initiates the movement, the cerebellum smooths and coordinates it, the brainstem relays the signal to the spinal cord, and spinal nerves deliver the final command to the muscles in your arm and hand. Sensory feedback from your fingers continuously adjusts your grip so you don’t crush the glass or let it slip.
Signs That Something Is Wrong
Because the nervous system touches every part of the body, dysfunction can show up in many ways depending on which organ or nerve is affected. Common warning signs include persistent or sudden-onset headaches, tingling or numbness, muscle weakness, double vision, memory problems, tremors, seizures, loss of coordination, slurred speech, and new difficulty understanding or producing language. Back pain that radiates into the legs, feet, or toes can indicate spinal nerve compression. Muscle wasting, where a muscle visibly shrinks over time, can signal damage to the motor nerves supplying it.
These symptoms vary widely in severity. Some reflect temporary issues like a pinched nerve, while others point to conditions affecting the brain or spinal cord directly. The specific combination of symptoms often tells clinicians which part of the nervous system is involved, since each organ and nerve pathway serves a distinct territory of the body.