The nervous system is built around two main organs, the brain and the spinal cord, plus a vast network of nerves that branch out to every part of your body. Together, these structures form two major divisions: the central nervous system and the peripheral nervous system. Understanding how they’re organized helps explain how your body senses the world, processes information, and responds to it.
The Central Nervous System: Brain and Spinal Cord
Your central nervous system (CNS) consists of just two organs: the brain and the spinal cord. Despite being only two structures, they serve as the command center for everything you think, feel, and do. The brain regulates thoughts, emotions, and movement. It manages voluntary actions like wiggling your fingers and involuntary ones like breathing and digesting food. The spinal cord acts as the main communication highway, relaying messages between the brain and the rest of the body.
The process works like a relay system. Your brain creates a signal, sends it down the spinal cord, and the spinal cord passes it along to the peripheral nerves that carry out the action. Sensory information travels the same route in reverse: nerves detect something (heat, pressure, pain) and send that signal up through the spinal cord to the brain for processing.
Both organs are heavily protected. The brain sits inside a skull roughly 7mm thick, while the spinal cord runs through a bony canal formed by the vertebrae. Beyond bone, three layers of protective membrane called the meninges wrap around both structures. Between these layers, cerebrospinal fluid cushions the brain and spinal cord against physical impact. There’s also a chemical defense: the blood-brain barrier, a layer of tightly packed cells lining the brain’s blood vessels that blocks pathogens and toxins from reaching brain tissue while still allowing nutrients through.
The brain itself contains roughly 26 billion neurons in its outer layer alone, supported by about 39 billion glial cells that provide structural support, insulation, and nutrient delivery. Newborns already have the same number of neurons as adults in this region, but only about one-sixth of the adult number of glial cells, which continue to multiply during development.
The Peripheral Nervous System: Nerves, Ganglia, and Receptors
Everything outside the brain and spinal cord belongs to the peripheral nervous system (PNS). This includes cranial nerves, spinal nerves, the smaller nerve branches that extend from them, and the junctions where nerves meet muscles. If the CNS is the command center, the PNS is the wiring that connects it to every organ, muscle, and patch of skin in your body.
You have 12 pairs of cranial nerves that emerge directly from the brain. These handle specialized tasks in your head and neck: smell, vision, hearing, facial movement, and taste, among others. You also have 31 pairs of spinal nerves that branch off the spinal cord and fan out through the trunk and limbs. Each spinal nerve carries both sensory fibers (bringing information in) and motor fibers (sending commands out).
Scattered along these nerve pathways are clusters of nerve cell bodies called ganglia. Sensory ganglia sit just outside the spinal cord on each spinal nerve’s posterior root, housing the cell bodies of neurons that detect touch, pain, temperature, and body position. Similar sensory ganglia are also found along several cranial nerves.
At the far ends of these nerve pathways are sensory receptors, specialized structures embedded in your skin, muscles, and organs. Some detect light touch, others respond to pressure or vibration. Pacinian corpuscles, for example, are tuned specifically to detect vibration because they adapt to sustained pressure almost instantly. Inside your muscles, stretch-detecting fibers called muscle spindles monitor how far a muscle is being lengthened, while Golgi tendon organs track how much tension a muscle is generating. These receptors are what allow you to hold a coffee cup without crushing it or dropping it.
The Autonomic Nervous System
Within the peripheral nervous system, a specialized subdivision called the autonomic nervous system controls functions you don’t consciously manage: heart rate, digestion, blood pressure, sweating, and pupil dilation. It has two main branches that generally work in opposition. The sympathetic branch ramps your body up during stress or physical activity, while the parasympathetic branch calms things down during rest and recovery.
The sympathetic branch has its own physical infrastructure. Two chains of ganglia run parallel to the spine, one on each side, extending from the neck down to the lower back. In humans, each chain typically contains 24 ganglia. At the neck level, there are generally three: the superior cervical ganglion at the top, the stellate ganglion at the bottom of the cervical group, and an intermediate ganglion between them. Additional ganglia sit in front of the vertebral column, closer to the organs they control. This network of ganglia allows the sympathetic system to coordinate a rapid, body-wide response when needed.
The Enteric Nervous System
Your digestive tract has its own mesh of nerve cells so extensive it’s sometimes called the “second brain.” The enteric nervous system (ENS) is technically part of the peripheral nervous system, but it can operate with a surprising degree of independence. It coordinates the muscle contractions that move food through your gut, regulates the release of digestive enzymes, and monitors conditions inside the intestines.
Like other parts of the PNS, the enteric nervous system contains both sensory nerves (detecting what’s happening in the gut) and motor nerves (triggering responses). It communicates with the brain through the central nervous system and the sympathetic nervous system, which is why stress can affect digestion and gut discomfort can influence mood.
How These Parts Work Together
No part of the nervous system works in isolation. When you touch a hot surface, receptors in your skin fire a signal through a peripheral nerve. That signal races up a spinal nerve to the spinal cord, which can trigger an immediate withdrawal reflex before the signal even reaches your brain. A fraction of a second later, the brain processes the pain and you become consciously aware of what happened.
When doctors need to evaluate how well these systems are functioning, they use tools matched to each part. An EEG records the brain’s electrical activity to check for seizure disorders. Nerve conduction studies measure how fast and how strongly signals travel through peripheral nerves, revealing damage or disease. EMG tests record electrical activity in muscles to pinpoint whether a problem originates in the nerve or the muscle itself. Brain imaging through MRI or CT scans visualizes the structure of the brain and spinal cord, while a lumbar puncture samples the cerebrospinal fluid surrounding them to check for infection or inflammation.
In short, the nervous system’s organs span from a single three-pound brain down to microscopic receptors in your fingertips. The brain and spinal cord form the central processing core. Cranial and spinal nerves, ganglia, and sensory receptors form the peripheral network. And within that network, the autonomic and enteric systems handle the behind-the-scenes work that keeps your body running without you having to think about it.