The human nervous system is divided into two main parts: the central nervous system, which includes the brain and spinal cord, and the peripheral nervous system, made up of nerves that branch out from the spinal cord and brain to reach every part of the body. These two divisions work together through a shared building block, the neuron, but they differ in how they’re protected, organized, and supported.
The Neuron: Basic Building Block
Every function of the nervous system depends on neurons, specialized cells built to carry electrical signals. A typical neuron has three main parts: a cell body (called the soma), dendrites, and an axon. The cell body contains the nucleus and the machinery that keeps the cell alive. Dendrites are branching extensions that receive incoming signals from other neurons. The axon is a single long fiber that carries signals away from the cell body toward the next neuron or a target like a muscle.
Where one neuron meets another, the connection point is called a synapse. When an electrical signal reaches the end of an axon, it triggers the release of chemical messengers that cross the tiny gap and stimulate the next cell. This electrical-to-chemical-to-electrical relay is how information moves through the entire nervous system.
Neurons don’t work alone. They’re supported by glial cells, which outnumber them and handle essential background tasks. In the brain and spinal cord, star-shaped cells called astrocytes help maintain the chemical environment neurons need to fire properly. Oligodendrocytes wrap a fatty insulation called myelin around axons, which speeds up signal transmission. Microglial cells act as the brain’s cleanup crew, clearing away damaged cells and debris. Out in the peripheral nervous system, Schwann cells take over the job of producing myelin around nerve fibers.
The Central Nervous System
The central nervous system is the command center: the brain and the spinal cord. Both are wrapped in three protective membranes called meninges. The outermost layer, the dura mater, sits closest to the skull and vertebrae. Beneath it is the arachnoid mater, a web-like middle layer. The innermost layer, the pia mater, clings directly to the surface of the brain and spinal cord tissue.
The Brain
The brain contains somewhere between 62 and 99 billion neurons, though the commonly cited estimate of 86 billion comes from a small study and remains imprecise. These neurons are organized into distinct regions, each with different structural features and roles.
The largest and most visible part is the cerebrum, which is split into left and right hemispheres. Each hemisphere is divided into four lobes. The frontal lobes, sitting behind the forehead, handle planning, reasoning, and voluntary movement. The parietal lobes, just behind them, process sensory input like touch, temperature, and taste, and also support reading and arithmetic. The occipital lobes, at the very back of the head, are dedicated to vision. The temporal lobes, tucked along the sides, process sound and play a central role in forming and retrieving memories.
Below and behind the cerebrum sits the cerebellum, a densely folded structure that coordinates movement. It’s what allows you to play an instrument or catch a ball with precision. The brainstem connects the brain to the spinal cord and controls vital automatic functions like breathing and heart rate. Its upper portion, the midbrain, helps manage eye movements and certain reflexes.
The Spinal Cord
The spinal cord is a long column of nervous tissue running through the vertebral canal from the base of the brainstem down to the lower back. If you were to slice it crosswise, you’d see a butterfly-shaped core of gray matter surrounded by white matter. Gray matter contains the cell bodies of neurons. Its upper “wings” (dorsal horns) receive sensory information coming in from the body. Its lower wings (ventral horns) contain motor neurons that send commands out to muscles.
The surrounding white matter is made up of bundled axons organized into columns. These axon tracts act as highways, carrying signals up to the brain and back down again. The general layout follows a consistent rule: sensory pathways run along the back of the cord, motor pathways along the front, and pathways controlling internal organs sit in between.
The Peripheral Nervous System
Everything outside the brain and spinal cord belongs to the peripheral nervous system. It includes 12 pairs of cranial nerves that emerge directly from the brain and 31 pairs of spinal nerves that branch off the spinal cord: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. That’s 43 pairs of nerves forming the structural backbone of the peripheral system.
Each peripheral nerve is not a single fiber but a bundle, organized by three layers of connective tissue. Individual axons are wrapped in a delicate layer called the endoneurium. Groups of these wrapped axons are bundled together by the perineurium, a thin but tough sheath. The entire nerve is then cushioned by a thicker outer layer, the epineurium. This layered design protects nerve fibers from compression and stretching during everyday movement.
Somatic vs. Autonomic Divisions
The peripheral nervous system is functionally split into two divisions. The somatic nervous system controls voluntary actions. Its motor neurons run directly from the spinal cord to skeletal muscles in a single uninterrupted path, and its sensory neurons carry information about touch, pain, and body position back to the central nervous system.
The autonomic nervous system handles everything you don’t consciously control: heart rate, digestion, pupil size, blood pressure. Its motor pathways are structurally different from somatic ones because they always involve a two-neuron chain. A preganglionic neuron starts in the brain or spinal cord and connects to a postganglionic neuron in a cluster of nerve cell bodies called a ganglion, which then reaches the target organ.
Where those ganglia sit is the key structural difference between the autonomic system’s two branches. In the sympathetic branch (your “fight or flight” system), ganglia are clustered in a chain running alongside the spinal column, relatively close to the spinal cord. This means the first neuron in the chain is short and the second is long. In the parasympathetic branch (your “rest and digest” system), ganglia sit near or directly on the walls of target organs like the heart, lungs, or intestines. That arrangement flips the proportions: the first neuron is long, the second is short.
The Enteric Nervous System
The gut has its own semi-independent network sometimes called the “second brain.” The enteric nervous system is a mesh of neurons and supporting cells embedded in the walls of the digestive tract, organized into two main layers of interconnected clusters called plexuses. The myenteric plexus sits between the two muscle layers of the gut wall and controls the rhythmic contractions that push food along. The submucosal plexus lies in the connective tissue closer to the inner lining and manages secretion, nutrient absorption, and blood flow.
In humans and other large mammals, the submucosal plexus actually consists of two distinct sublayers, giving the gut an extra degree of control over both its muscular and secretory functions. In the esophagus and stomach, the myenteric plexus dominates, with very few submucosal nerve clusters present. The enteric system can coordinate basic digestive functions on its own, though it stays in constant communication with the central nervous system through the autonomic pathways.
How the Parts Connect
The structural logic of the nervous system is hierarchical. The brain processes, integrates, and initiates. The spinal cord relays signals between the brain and body while also running some reflexes independently. The peripheral nerves carry sensory data inward and motor commands outward. The autonomic and enteric systems keep organs functioning without requiring conscious input.
Sensory information flows inward through the dorsal roots of spinal nerves and cranial nerves, reaching the spinal cord and brain for processing. Motor commands flow outward through ventral roots and cranial nerves to muscles and organs. This two-way traffic, bundled in the same peripheral nerves but carried on separate fibers, is what allows you to simultaneously feel the ground under your feet and decide to take the next step.