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A clear, evidence-based guide to the nervous system: how the brain, spinal cord, and nerves are built, how neurons send signals, the conditions that affect them, and proven ways to protect your brain and nerve health.
Reviewed & updated
The nervous system is the body's command and communication network. It collects information from inside and outside the body, processes that information, and coordinates a response—everything from a heartbeat you never think about to a decision you agonize over. It works through electrical and chemical signals carried by specialized cells called neurons, supported by an even larger population of helper cells called glia.
Anatomically, the nervous system has two main divisions:
The adult human brain contains roughly 86 billion neurons, along with a comparable number of non-neuronal (glial) cells—a figure established by careful cell-counting research that revised the older "100 billion" estimate.
The brain has three broad regions: the cerebrum (thought, perception, voluntary movement, language), the cerebellum (balance and coordination), and the brainstem (automatic functions like breathing, heart rate, and arousal). The spinal cord is a relay highway: it carries signals between the brain and body and controls basic reflexes on its own.
The PNS includes 12 pairs of cranial nerves that emerge from the brain and brainstem (controlling vision, hearing, facial movement, and more) and 31 pairs of spinal nerves that branch from the spinal cord. The spinal nerves are grouped as 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal pair.
The PNS is further split by function:
A neuron has three key parts: a cell body containing the nucleus, branching dendrites that receive incoming signals, and a long axon that carries the outgoing signal. When a neuron is sufficiently stimulated, it fires an electrical impulse called an action potential down its axon.
Where two neurons meet—a synapse—the electrical signal triggers the release of chemical messengers called neurotransmitters (such as dopamine, serotonin, glutamate, GABA, and acetylcholine). These cross the tiny synaptic gap and bind to the next cell, either exciting or inhibiting it. This electrochemical relay is the basis of every thought, movement, and reflex.
Many axons are wrapped in myelin, a fatty insulating sheath that dramatically speeds signal transmission. Myelin is produced by oligodendrocytes in the CNS and Schwann cells in the PNS—two of several glial cell types that insulate, nourish, support, and protect neurons.
Neurological conditions are the leading cause of illness and disability worldwide, according to a 2024 World Health Organization analysis estimating that more than 1 in 3 people are affected at some point. Common examples include:
Warning signs that warrant urgent care include sudden weakness or numbness (especially on one side), trouble speaking, sudden severe headache, vision loss, or loss of balance. For suspected stroke, call 911 immediately—rapid treatment can save brain tissue.
Brain and nerve health are strongly tied to overall cardiovascular and metabolic health. Evidence-supported habits include:
*This article is for general education and is not medical advice. Talk to a qualified healthcare professional about your individual health.*
In-depth, independently reviewed guides to the major organs and structures of this system.
This guide draws on 15 peer-reviewed and authoritative sources (StatPearls, NIH/NHLBI, MedlinePlus, and primary literature).