The Nervous System

What Is the Nervous System?

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:

• Central nervous system (CNS): the brain and spinal cord. This is where information is integrated and most decisions are made.
• Peripheral nervous system (PNS): all the nerves outside the brain and spinal cord. These nerves connect the CNS to limbs, organs, skin, and muscles.

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.

How the Nervous System Is Organized

Central nervous system

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.

Peripheral nervous system

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:

• Somatic nervous system: controls voluntary movement of skeletal muscles and carries sensation (touch, pain, temperature) to the CNS.
• Autonomic nervous system (ANS): runs involuntary functions—heart rate, blood pressure, digestion, breathing. It has two opposing branches: the sympathetic ("fight or flight") system that ramps the body up, and the parasympathetic ("rest and digest") system that calms it down. Balance between them helps maintain homeostasis.

How Neurons Send Signals

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.

Key Organs and Structures

• Brain — integrates sensory input, generates thought and emotion, and commands movement.
• Spinal cord — transmits signals between brain and body; mediates reflexes.
• Neurons — excitable cells that transmit electrical and chemical signals.
• Cranial and spinal nerves — the wiring that links the CNS to the entire body.

Common Conditions

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:

• Stroke: a sudden interruption of blood flow to the brain. According to the U.S. Centers for Disease Control and Prevention, more than 795,000 people in the U.S. have a stroke each year—about one every 40 seconds—and stroke is a leading cause of long-term disability and death.
• Alzheimer's disease and other dementias: progressive loss of memory and thinking. The Alzheimer's Association estimates that Alzheimer's affects about 1 in 9 adults aged 65 and older (roughly 11%).
• Parkinson's disease: a movement disorder caused by loss of dopamine-producing neurons.
• Epilepsy: recurrent seizures from abnormal electrical activity.
• Migraine: a common, often disabling neurological headache disorder affecting a large share of adults.
• Multiple sclerosis: an immune attack on myelin in the CNS.
• Peripheral neuropathy: nerve damage (often from diabetes) causing numbness, tingling, or pain.

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.

How to Keep Your Nervous System Healthy

Brain and nerve health are strongly tied to overall cardiovascular and metabolic health. Evidence-supported habits include:

• Stay physically active. Regular aerobic exercise supports brain blood flow and is associated with lower dementia and stroke risk.
• Control blood pressure, blood sugar, and cholesterol. These protect the small vessels feeding the brain and nerves.
• Don't smoke and limit alcohol. Both damage blood vessels and nerve tissue.
• Prioritize sleep. Sleep supports memory consolidation and brain waste clearance.
• Eat a balanced, plant-forward diet. Patterns like the Mediterranean diet are linked to better brain aging.
• Stay mentally and socially engaged. Learning and connection support cognitive resilience.
• Protect your head. Wear seatbelts and helmets to prevent traumatic brain and spinal injury.

*This article is for general education and is not medical advice. Talk to a qualified healthcare professional about your individual health.*