The human nervous system consists of two main parts: the central nervous system (brain and spinal cord) and the peripheral nervous system (every nerve that branches out from there into the rest of your body). Together, these structures form a communication network built from roughly 86 billion neurons in the brain alone, plus billions more throughout the body. Here’s how each part is organized and what it does.
The Central Nervous System
The central nervous system, or CNS, is the command center. It has two components: the brain and the spinal cord. Your brain reads incoming signals from nerves across the body and uses that information to regulate how you think, move, and feel. It processes everything from the smell of coffee to the decision to pull your hand away from a hot stove.
The spinal cord is the main highway connecting your brain to the rest of your body. It runs from the base of the skull down to roughly the first or second lumbar vertebra in your lower back. Despite making up only about 2% of the entire CNS, it performs critical jobs: carrying sensory information upward to the brain, relaying motor commands downward to your muscles, and even handling some reflexes on its own before the brain gets involved. A cross-section of the spinal cord reveals white matter on the outside (bundles of nerve fibers carrying signals up and down) and gray matter on the inside (clusters of neuron cell bodies doing local processing).
The Peripheral Nervous System
Everything outside the brain and spinal cord belongs to the peripheral nervous system, or PNS. This includes 12 pairs of cranial nerves that emerge directly from the brain, 31 pairs of spinal nerves that branch off the spinal cord, and all the smaller nerve branches that extend to your skin, muscles, and organs. The PNS splits into two functional divisions: somatic and autonomic.
Somatic Nervous System
The somatic nervous system handles conscious, voluntary activity. It consists of nerves running to your skin and skeletal muscles, letting you feel a tap on your shoulder and then turn your head in response. Anytime you deliberately move a muscle or register a touch, temperature, or pain sensation on your body’s surface, somatic nerves are carrying those signals.
Autonomic Nervous System
The autonomic nervous system connects the CNS to your internal organs: heart, lungs, stomach, intestines, bladder, and more. It runs automatically and continuously, without conscious effort, controlling smooth muscle, cardiac muscle, and glands. You don’t have to think about digesting lunch or keeping your heart beating. The autonomic system divides further into two branches that work as a balancing act.
The sympathetic branch triggers your “fight or flight” response. It raises your heart rate, dilates your pupils, redirects blood flow to your muscles, and generally puts your body on high alert when you face a threat. Staying in this state too long can strain your body’s systems.
The parasympathetic branch does the opposite, returning your body to rest mode. It slows your heart rate, constricts your pupils, ramps up digestion, triggers saliva production, and tells your pancreas to release insulin so your cells can use the sugars from your food. It also manages waste removal by relaxing the muscles that control urination and bowel movements. Think of the sympathetic system as the gas pedal and the parasympathetic system as the brake.
The Enteric Nervous System
Sometimes called the “second brain,” the enteric nervous system is a dense network of more than 100 million nerve cells embedded in two thin layers lining your gastrointestinal tract, from esophagus to rectum. It can coordinate digestion largely on its own, managing the contractions that move food through your gut and the release of digestive enzymes. While it communicates with the brain through the autonomic system, it has enough independent circuitry to function even when those connections are disrupted.
Neurons and Supporting Cells
The nervous system is built from two broad categories of cells. Neurons are the ones that actually transmit electrical signals. Each neuron has dendrites (branch-like extensions that receive incoming signals), a cell body, and an axon (a long fiber that sends signals onward to the next cell). The human brain contains about 86 billion neurons.
Glial cells are the support staff. They don’t carry nerve impulses themselves, but they nourish, protect, and insulate neurons. The brain contains roughly as many glial cells as neurons, though the ratio varies dramatically by region. In parts of the cortex, you may find two or three glial cells per neuron. In the cerebellum (the region at the back of the brain that coordinates movement), neurons vastly outnumber glial cells. Unlike neurons, glial cells can divide and reproduce, which is one reason brain tumors often originate from glial tissue rather than neurons.
How Nerves Are Structured
A single nerve in your arm or leg isn’t one lone fiber. It’s a cable-like structure with three layers of connective tissue holding it together. The innermost layer, the endoneurium, wraps around individual axons. The middle layer, the perineurium, bundles groups of axons into units called fascicles. The outermost layer, the epineurium, sheathes the entire nerve. This layered design protects delicate nerve fibers from compression and stretching as you move.
Many axons are coated in myelin, a fatty insulating layer that dramatically speeds up signal transmission. The fastest myelinated fibers conduct impulses at 70 to 120 meters per second, roughly 270 miles per hour. Unmyelinated fibers, like the small ones that carry dull pain and temperature sensations, transmit at just 0.5 to 2 meters per second. That speed difference is why you feel a sharp touch instantly but a burning pain seems to build over a moment.
The 12 Cranial Nerves
Twelve pairs of cranial nerves connect the brain directly to structures in your head, neck, and torso without passing through the spinal cord. Each pair has a specialized job:
- Olfactory (I): smell
- Optic (II): vision
- Oculomotor (III): eyelid movement, pupil size, and most eye movement
- Trochlear (IV): a single eye muscle that helps you look downward and inward
- Trigeminal (V): facial sensation and chewing muscles
- Abducens (VI): moves the eye outward
- Facial (VII): facial expressions
- Vestibulocochlear (VIII): hearing and balance
- Glossopharyngeal (IX): swallowing and taste
- Vagus (X): heart rate, digestion, and voice box control
- Accessory (XI): head turning and shoulder shrugging muscles
- Hypoglossal (XII): tongue movement
The vagus nerve is the longest cranial nerve and plays a central role in the parasympathetic system, reaching all the way from the brainstem down to the intestines. It’s the main line of communication between your brain and your gut.
How It All Works Together
No part of the nervous system operates in isolation. When you step on something sharp, sensory neurons in your foot fire a signal through a peripheral nerve, up through the spinal cord, and into the brain, where you consciously register pain. But even before the brain gets the message, the spinal cord has already triggered a reflex to pull your foot away. Meanwhile, your autonomic system bumps up your heart rate slightly in response to the surprise, and the parasympathetic branch brings it back down once the threat passes.
This layered design, with a central processor, a fast relay in the spinal cord, voluntary control over muscles, and automatic regulation of organs, allows the nervous system to manage everything from breathing in your sleep to solving a math problem, all simultaneously.