The human nervous system is built from two major divisions: the central nervous system (the brain and spinal cord) and the peripheral nervous system (every nerve that branches out from them into the body). Within those two divisions sit smaller systems, protective layers, and billions of specialized cells that work together to process information, control movement, and keep organs functioning without conscious thought.
The Central Nervous System
The central nervous system, or CNS, is the command center. It consists of two structures: the brain and the spinal cord.
The brain develops from three primary regions. The forebrain is the largest, responsible for complex thought, sensory processing, and voluntary movement. The midbrain relays signals between higher and lower brain regions and plays a role in vision and hearing. The hindbrain, which includes the brainstem and cerebellum, controls basic life-sustaining functions like breathing, heart rate, and coordination.
The spinal cord extends from the base of the skull down to roughly the first or second lumbar vertebra in the lower back. It serves as a two-way highway between the brain and the rest of the body, carrying sensory information up and motor commands down. The cord is divided into four regions: cervical (neck), thoracic (mid-back), lumbar (lower back), and sacral (base of the spine). In total, 31 pairs of spinal nerves branch off from these regions, each pair serving a specific area of the body. Eight pairs exit the cervical region, twelve from the thoracic, five from the lumbar, five from the sacral, and one from the coccygeal (tailbone) area.
Protective Layers and Fluid
The brain and spinal cord are too vital to sit unprotected inside bone. Three membrane layers called meninges wrap around them. The outermost layer, the dura mater, is thick, dense, and inelastic, forming a tough shield just inside the skull. Beneath it lies the arachnoid mater, a thin, web-like membrane that bridges across the brain’s folds and plays a role in circulating cerebrospinal fluid. The innermost layer, the pia mater, hugs every contour and groove of the brain’s surface directly.
Cerebrospinal fluid fills the space between the arachnoid and pia layers. It cushions the brain and spinal cord from physical impact, delivers nutrients, and carries away waste products. Think of it as a shock absorber and cleaning system in one.
The Blood-Brain Barrier
The brain also has a chemical gatekeeper: the blood-brain barrier. Unlike blood vessels elsewhere in the body, the capillaries in the brain are lined with endothelial cells that have no tiny pores and are sealed together by extremely tight junctions. This prevents most substances circulating in the blood from freely entering brain tissue. Wrapped around these capillaries are pericytes, which help maintain structural integrity, and astrocyte end-feet, extensions from star-shaped support cells that reinforce the barrier. The result is a highly selective filter that lets in essentials like oxygen and glucose while blocking most pathogens and toxins.
The Peripheral Nervous System
Everything outside the brain and spinal cord belongs to the peripheral nervous system. It includes 12 pairs of cranial nerves (which exit directly from the brain) and 31 pairs of spinal nerves. The peripheral nervous system splits into two functional branches: somatic and autonomic.
The somatic nervous system handles voluntary movement and conscious sensation. When you decide to pick up a cup or feel the texture of fabric, somatic nerves carry those signals. The autonomic nervous system, by contrast, manages functions you don’t consciously control: heart rate, digestion, pupil dilation, and gland secretion.
Sympathetic and Parasympathetic Divisions
The autonomic nervous system further divides into two branches that often work in opposition. The sympathetic branch activates the body’s “fight or flight” response. Its nerve cell bodies sit in the lateral horns of the spinal cord, and its fibers exit through the thoracic and upper lumbar spinal nerves. Sympathetic signals travel through a chain of relay stations called paravertebral ganglia, which run alongside the spinal column. Because these relay stations are close to the spinal cord but far from target organs, the outgoing fibers tend to be long. At their endpoints, sympathetic nerves release norepinephrine, the chemical messenger that speeds up the heart, dilates airways, and redirects blood flow to muscles.
The parasympathetic branch does the opposite, promoting “rest and digest” functions. Its fibers leave the CNS through four cranial nerves and through spinal nerves in the sacral region. Unlike sympathetic ganglia, parasympathetic relay stations sit close to or directly on the walls of target organs, so the outgoing fibers are very short. Parasympathetic nerve endings release acetylcholine, which slows the heart, stimulates digestion, and constricts the pupils.
The Enteric Nervous System
Buried in the walls of the gastrointestinal tract is a third division sometimes called the “second brain.” The enteric nervous system consists of two networks of nerve clusters. The myenteric plexus sits between the muscle layers of the gut wall and drives the rhythmic contractions (peristalsis) that push food through the digestive tract. The submucosal plexus, positioned closer to the inner lining, regulates enzyme secretion and blood flow to the intestines. While the enteric nervous system receives input from the autonomic nervous system, it can operate independently, coordinating digestion on its own.
Neurons: The Signaling Cells
The fundamental working unit of the nervous system is the neuron. Each neuron has three main parts. The cell body (soma) contains the nucleus and the machinery that keeps the cell alive. Branching off the cell body are dendrites, thin extensions that pick up electrical signals from neighboring cells. A single long projection called the axon carries the signal away from the cell body toward its target, whether that’s another neuron, a muscle fiber, or a gland.
The human brain contains roughly 67 to 86 billion neurons. Speed matters in a system this large, so many axons are wrapped in a fatty insulating layer called a myelin sheath. In the CNS, cells called oligodendrocytes produce this myelin, with a single oligodendrocyte wrapping segments of multiple axons at once. In the peripheral nervous system, Schwann cells do the same job, but each Schwann cell insulates only one axon. Myelin prevents the electrical signal from dissipating and dramatically increases transmission speed. When myelin breaks down, as it does in diseases like multiple sclerosis, the result can be paralysis, loss of coordination, or seizures.
Glial Cells: The Support Network
Neurons get most of the attention, but they’re roughly matched in number by non-neuronal cells. The glia-to-neuron ratio in the human brain is close to 1:1, with the total number of glial cells falling below 85 billion (a figure that also includes blood vessel cells). Glial cells don’t transmit electrical signals, but they perform critical support functions.
Astrocytes are star-shaped cells with numerous branching processes. Their foot-like extensions help form the blood-brain barrier, and they regulate the chemical environment around neurons by absorbing excess signaling molecules and potassium ions. They also supply neurons with the raw materials needed to produce energy. Microglia are the brain’s immune cells. Derived from the same lineage as white blood cells, they patrol the CNS, engulfing dead cells, debris, and invading organisms. When tissue is damaged, microglia multiply and cluster around the injury site.
Synapses: Where Signals Pass Between Cells
Neurons communicate at junctions called synapses. In a chemical synapse, which is the most common type, the signal-sending neuron’s axon ends at a terminal that sits less than 50 nanometers from the receiving cell’s membrane. When an electrical impulse reaches this terminal, it triggers the release of chemical messengers (neurotransmitters) into the narrow gap, called the synaptic cleft. These molecules drift across the cleft and bind to receptors on the receiving cell, either exciting it to fire its own signal or inhibiting it from doing so.
Electrical synapses work differently. Instead of a chemical intermediary, two neurons are connected almost directly through protein channels called connexins. These channels allow electrical current to pass straight from one cell to the next, making electrical synapses faster but less flexible than their chemical counterparts. Electrical synapses appear in circuits where speed and synchronization matter most, such as certain reflexes and rhythmic processes in the brain.