Your nervous system is made up of two main divisions: the central nervous system, which includes your brain and spinal cord, and the peripheral nervous system, which includes every nerve that branches out from those two organs to reach the rest of your body. Together, these divisions contain roughly 86 billion neurons, an equal number of supporting cells, protective membranes, fluid-filled spaces, and a network of chemical messengers that keep everything communicating.
The Two Main Divisions
The central nervous system (CNS) is the command center. It consists of just two organs: the brain and the spinal cord. Every decision, memory, sensation, and automatic body function is processed here. The spinal cord is a cylinder-shaped tube of tissue that runs from the brainstem down through the center of the spine, ending in a cone shape in the lower back.
The peripheral nervous system (PNS) is everything outside the brain and spinal cord. It branches outward to reach every part of your body, carrying signals to and from the central nervous system. This network includes 12 pairs of cranial nerves that connect directly to the brain (11 of which belong to the PNS) and 31 pairs of spinal nerves that attach to the spine at roughly the level of each vertebra. Some of these spinal nerve pairs serve the arms and hands, others run to the legs and feet, and still others extend into the pelvis.
How the Peripheral System Breaks Down
The peripheral nervous system splits into two subsystems that handle very different jobs. The somatic nervous system controls muscles you move voluntarily and carries sensory information (touch, temperature, pain) from your body back to the brain. The autonomic nervous system connects your brain to your internal organs and runs processes you don’t consciously control.
The autonomic nervous system itself has three branches:
- Sympathetic nervous system: activates your “fight or flight” response during stress or danger, increasing heart rate, blood pressure, and alertness.
- Parasympathetic nervous system: does the opposite, managing “rest and digest” functions that slow the heart, relax muscles, and promote digestion.
- Enteric nervous system: a dedicated network embedded in the walls of the digestive tract that manages digestion largely on its own.
Neurons: The Signaling Cells
Neurons are the cells that actually carry electrical and chemical signals through the nervous system. Each neuron has three main parts. Dendrites are branch-like extensions that receive incoming signals from other neurons. The cell body (or soma) houses the cell’s DNA and manufactures the proteins the neuron needs to function. The axon is a long, thin fiber that transmits outgoing electrical signals, sometimes over considerable distances, to the next neuron or to a muscle.
When a neuron is active, it generates a brief electrical pulse called an action potential that travels the full length of the axon. When the pulse reaches the end of the axon, it triggers the release of chemical messengers called neurotransmitters into the tiny gap between neurons. Those chemicals cross the gap and bind to the dendrites of the next cell, continuing the signal. This is how every sensation, thought, and movement gets relayed through the body.
Glial Cells: The Support Network
Neurons get most of the attention, but about half the cells in your brain are non-neuronal support cells, primarily glia. These cells don’t carry electrical signals themselves, yet the nervous system couldn’t function without them.
Astrocytes are star-shaped cells that maintain the chemical environment around neurons. They regulate neurotransmitter levels at synapses, control concentrations of ions like potassium, and supply metabolic fuel. They can also sense neural activity and release molecules that modify how synapses behave, making them active participants in brain signaling rather than passive bystanders.
Oligodendrocytes (in the CNS) and Schwann cells (in the PNS) produce myelin, a fatty substance that wraps around axons like insulation around a wire. A single oligodendrocyte can myelinate segments on many different axons, while a single Schwann cell wraps just one segment. Myelin is about 80 percent fat, which gives it excellent insulating properties and allows electrical signals to jump rapidly from one exposed gap to the next along the axon. In the most heavily myelinated fibers, signals travel at 70 to 120 meters per second, roughly the speed of a race car.
Microglia act as the brain’s immune system. They patrol for injury and disease, clear away dead cells, and even prune unnecessary connections between neurons during development. Ependymal cells line the cavities of the brain and spinal cord and produce cerebrospinal fluid. Satellite cells surround nerve cell clusters in the peripheral nervous system and help regulate the local chemical environment. And enteric glial cells support the nerve network in the digestive tract.
Protective Structures
The brain and spinal cord are soft, delicate organs, so the nervous system includes several layers of physical protection. Three membranes called meninges wrap around both structures. The outermost layer, the dura mater, sits closest to the skull and provides a tough shield against injury. 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 and spinal cord tissue.
Between the arachnoid and pia layers sits the subarachnoid space, which is filled with cerebrospinal fluid. This clear fluid acts as a cushion, absorbing shocks that might otherwise damage the brain or spinal cord. It also helps carry away waste products and maintain a stable chemical environment around the central nervous system.
Key Neurotransmitters
The chemical messengers that carry signals between neurons fall into several categories, each serving different roles throughout the nervous system.
Glutamate is the most abundant neurotransmitter in the brain and the primary “go” signal, exciting neurons into action. It plays a central role in thinking, learning, and memory. On the other side, GABA is the brain’s main “stop” signal, calming neural activity. It helps regulate anxiety, concentration, sleep, and seizure risk. In the spinal cord, a similar calming role is played by glycine, which is involved in processing sound, transmitting pain signals, and regulating metabolism.
Serotonin influences mood, sleep, appetite, anxiety, and pain perception. Dopamine drives the brain’s reward system, contributing to feelings of pleasure, motivation, focus, and memory. Norepinephrine raises blood pressure and heart rate while sharpening alertness and decision-making. Its close relative, epinephrine (commonly known as adrenaline), works alongside it to trigger the fight-or-flight response by increasing heart rate, breathing rate, blood sugar, and blood flow to muscles.
Acetylcholine is one of the most versatile neurotransmitters, active in both the central and peripheral nervous systems. It triggers muscle contractions, regulates heart rate and blood pressure through the autonomic nervous system, and supports memory, learning, and sleep. Endorphins are the body’s natural painkillers, reducing pain perception and producing the “feel good” sensation associated with exercise and other rewarding activities. Histamine helps regulate wakefulness, appetite, and motivation.
All of these components, from the large-scale divisions down to individual molecules, work as an integrated system. Neurons generate signals, glial cells support and insulate them, neurotransmitters carry messages across gaps, protective membranes and fluid guard the most vital structures, and the peripheral network ensures those signals reach every organ, muscle, and patch of skin in the body.