The nervous system is made up of two major divisions: the central nervous system (your brain and spinal cord) and the peripheral nervous system (every nerve that branches out from there into the rest of your body). Together, these two divisions contain billions of specialized cells that carry electrical and chemical signals to control everything from your heartbeat to your ability to read this sentence. Beyond that basic split, the system includes protective structures, support cells, and a surprisingly independent network in your gut.
The Central Nervous System
The central nervous system consists of the brain and the spinal cord. It’s the command center. Every sensation you experience, every decision you make, and every movement you initiate passes through it.
The brain itself is organized into three main regions. The forebrain is the largest and includes the cerebral cortex, the wrinkled outer layer responsible for thinking, memory, language, and sensory processing. The cortex is divided into two hemispheres, each with four lobes: frontal, parietal, temporal, and occipital. Deeper inside the forebrain sit smaller structures like the thalamus (a relay station for sensory signals) and the hypothalamus (which regulates body temperature, hunger, and hormones).
The midbrain sits below the forebrain and serves as a bridge connecting it to the hindbrain. It’s the top portion of the brainstem, which links the brain to the spinal cord. The hindbrain, at the base of the skull, contains the rest of the brainstem (the pons and medulla oblongata) plus the cerebellum, a dense ball of tissue at the back that coordinates balance and fine motor control.
The spinal cord extends down from the brainstem through the vertebral column. It carries signals between the brain and the rest of the body and also handles certain reflexes on its own, without waiting for instructions from the brain.
The Peripheral Nervous System
Everything outside the brain and spinal cord belongs to the peripheral nervous system. This includes 12 pairs of cranial nerves that emerge directly from the brain and 31 pairs of spinal nerves that branch off the spinal cord. These nerves reach every organ, muscle, and patch of skin in your body.
The peripheral system breaks down further into two functional branches. The somatic nervous system handles voluntary movement, letting you consciously control your muscles. The autonomic nervous system manages the things you don’t think about: heart rate, digestion, breathing, and blood pressure. The autonomic system itself has two opposing arms. The sympathetic division ramps your body up during stress or danger, increasing heart rate and redirecting blood flow to muscles. The parasympathetic division does the opposite, slowing things down and promoting rest, digestion, and recovery. These two arms work in constant balance to keep your body functioning smoothly.
The Enteric Nervous System
Your digestive tract has its own semi-independent network called the enteric nervous system, sometimes nicknamed the “second brain.” It contains between 200 and 600 million neurons embedded in the walls of the gut, spanning from the upper esophagus all the way to the internal anal sphincter. This network can coordinate digestion largely on its own, managing the rhythmic contractions that move food through your intestines without needing constant input from the brain. It communicates with the central nervous system but doesn’t depend on it for basic operations.
Neurons: The Signaling Cells
The fundamental working unit of the nervous system is the neuron. There are three main types. Sensory neurons detect information from the environment, such as temperature, pressure, light, and sound, and relay it inward toward the brain and spinal cord. Motor neurons carry signals outward from the spinal cord to muscles, glands, and organs, telling them what to do. Interneurons sit between the two, connecting sensory and motor neurons and forming the complex circuits that allow for processing, decision-making, and reflexes.
Structurally, most neurons share a few key parts. Dendrites are branching extensions that receive incoming signals from other neurons. The cell body processes those signals. And the axon is a long, cable-like projection that carries the signal away from the cell body toward the next neuron or target tissue. Motor neurons typically have one axon and several dendrites. Sensory neurons have a slightly different layout, with a single axon that splits into two branches.
How Neurons Communicate
Neurons don’t physically touch each other. They pass signals across a tiny gap called a synaptic junction, less than 40 nanometers wide. When an electrical signal reaches the end of an axon, it triggers small sacs called synaptic vesicles to release chemical messengers known as neurotransmitters into that gap. These chemicals drift across, land on specific receptors on the next cell, and either excite it or quiet it down. A typical neuron has somewhere between 1,000 and 10,000 of these synaptic connections.
Scientists have identified at least 100 different neurotransmitters, grouped into categories like amino acids, monoamines, and peptides. You’ve likely heard of some of the well-known ones: serotonin, dopamine, and acetylcholine all fall into this group. Each neurotransmitter has a specific role, and imbalances in these chemicals are linked to conditions ranging from depression to Parkinson’s disease.
Glial Cells: The Support Network
Neurons get most of the attention, but they couldn’t function without glial cells, the support staff of the nervous system. Glia outnumber neurons and perform a wide range of essential jobs.
Oligodendrocytes (in the brain and spinal cord) and Schwann cells (in the peripheral nerves) wrap axons in a fatty substance called myelin. This insulating layer works much like the coating around an electrical wire, allowing signals to travel faster and more efficiently. Myelin gives white matter its characteristic color. When myelin breaks down, as in multiple sclerosis, nerve signaling slows dramatically.
Astrocytes are star-shaped cells that maintain the chemical environment around neurons. They regulate neurotransmitter levels at synapses, control ion concentrations, and supply metabolic fuel. They can even sense neural activity and release molecules that influence how neurons fire, making them active participants in brain signaling rather than passive bystanders.
Microglia act as the brain’s immune system. They patrol for damage and infection, clearing away dead cells and toxic substances. They also play a role in development by pruning unnecessary connections between neurons, helping to refine brain circuits. Other glial types include ependymal cells, which line the brain’s internal cavities and help produce cerebrospinal fluid, and satellite cells in the peripheral system, which regulate the chemical environment around nerve cell clusters.
Protective Structures
The central nervous system is too important to leave unguarded. Three membrane layers called meninges wrap around the brain and spinal cord for protection. The outermost layer, the dura mater, sits closest to the skull and is tough and durable. The middle layer, the arachnoid mater, is a web-like membrane. The innermost layer, the pia mater, clings directly to the surface of the brain tissue.
Between the arachnoid and pia layers sits the subarachnoid space, filled with cerebrospinal fluid. This fluid cushions the brain and spinal cord against impacts, essentially allowing the brain to float inside the skull rather than resting its full weight against bone.
The blood-brain barrier adds another layer of defense. Unlike blood vessels elsewhere in the body, the capillaries in the brain have endothelial cells sealed together by continuous tight junctions, with no gaps for molecules to slip through. Pericytes and the foot-like extensions of astrocytes reinforce this barrier from the outside. The result is a highly selective filter that lets essential nutrients in while blocking most toxins, pathogens, and large molecules from reaching delicate brain tissue. This same barrier is one reason certain medications have difficulty reaching the brain.