The human nervous system contains two main organs, the brain and the spinal cord, plus an extensive network of nerves that branch out to every part of the body. Together, these structures are organized into two major divisions: the central nervous system (brain and spinal cord) and the peripheral nervous system (all the nerves outside of them). A lesser-known third division, the enteric nervous system, lines your entire digestive tract and operates with surprising independence.
The Central Nervous System: Brain and Spinal Cord
The central nervous system has three core jobs: receiving sensory information from the body, processing that information, and sending out a response. The brain handles the processing, while the spinal cord serves as the main highway connecting the brain to the rest of the body. Both sit inside protective bone structures (the skull and vertebral column), wrapped in layers of connective tissue called meninges and cushioned by cerebrospinal fluid.
The Brain
The adult human brain weighs roughly 1,300 to 1,400 grams, about three pounds, and takes up around 80% of the space inside your skull. It regulates thoughts, emotions, and voluntary movements like bending your fingers or speaking. It also controls functions you never have to think about, like breathing and digesting food.
The brain’s outer surface, the cortex, is divided into four lobes, each with distinct responsibilities:
- Frontal lobe: Personality, emotions, problem-solving, and controlling movement. This region continues developing until your mid-20s.
- Temporal lobe: Processing hearing and other senses, along with language and reading.
- Parietal lobe: Handling touch, spatial awareness, attention, and language.
- Occipital lobe: Vision, including recognizing shapes and colors.
Deeper structures beneath the cortex manage memory formation, emotional responses, hormone regulation, and coordination of movement. The brainstem, which sits at the base of the brain and connects directly to the spinal cord, controls basic life-sustaining functions like heart rate and breathing.
The Spinal Cord
The spinal cord is a long, thin bundle of nerve tissue that runs from the base of the brain down through the vertebral column. It is divided into 31 segments, each giving rise to a pair of spinal nerves that exit through gaps between the vertebrae. These segments break down into five regions: 8 cervical (neck), 12 thoracic (mid-back), 5 lumbar (lower back), 5 sacral (pelvis), and 1 coccygeal (tailbone).
The spinal cord does more than relay messages. It can process certain reflexes on its own. When you touch a hot surface and yank your hand away before you even feel the pain, that’s a spinal reflex, a loop that bypasses the brain entirely for the sake of speed.
The Peripheral Nervous System: Nerves and Ganglia
Everything outside the brain and spinal cord belongs to the peripheral nervous system. This includes all the nerves that travel from your central nervous system to your face, limbs, organs, and skin. The peripheral nervous system is not a single organ but rather a sprawling web of nerve fibers and small clusters of nerve cell bodies called ganglia.
Cranial Nerves
Twelve pairs of cranial nerves connect directly to the brain rather than the spinal cord. Eleven of the twelve belong to the peripheral nervous system (the remaining one, the optic nerve, is technically part of the central nervous system). These nerves handle the senses of smell, hearing, taste, and touch across your head, face, and neck. Some are purely sensory, some are purely motor, and some carry both types of signals.
A few stand out for the scope of what they do. The olfactory nerve provides your sense of smell. The optic nerve carries visual information from your eyes. The hypoglossal nerve controls tongue movement for speaking, eating, and swallowing. The vagus nerve is particularly notable because it extends far beyond the head, branching down through the neck and connecting to virtually every vital organ from your heart to your colon.
Spinal Nerves
The 31 pairs of spinal nerves emerge from the spinal cord and fan out to supply the trunk, arms, and legs. Each pair corresponds to a specific segment of the spine. These nerves carry sensory information (touch, pain, temperature) from the body back to the spinal cord and brain, and they carry motor commands from the brain out to muscles.
Three Functional Types of Nerves
Peripheral nerves fall into three categories based on what they do:
- Sensory nerves: Carry information about touch, pain, temperature, and body position to your brain and spinal cord.
- Motor nerves: Carry commands from your brain to your muscles, allowing voluntary movement.
- Autonomic nerves: Control automatic functions in your organs, like heart rate, digestion, and blood pressure. These often carry signals in both directions, sending commands out and reporting organ status back.
The autonomic category subdivides further into the sympathetic system (which ramps your body up during stress or danger) and the parasympathetic system (which calms things down for rest and digestion). You never consciously control either one; they run in the background constantly.
The Enteric Nervous System: Your Gut’s Own Brain
Lining the walls of your gastrointestinal tract, from esophagus to rectum, is a network of more than 100 million nerve cells arranged in two thin layers. Scientists call this the enteric nervous system, and it is sometimes referred to as the “second brain.” That nickname is not just marketing. The enteric nervous system can operate independently of the brain and spinal cord, coordinating the complex sequence of muscle contractions, enzyme release, blood flow adjustments, and waste elimination that make up digestion.
Research from Johns Hopkins Medicine highlights that this gut-brain network does more than manage digestion. It communicates constantly with the brain, influencing mood and overall health in ways that are still being mapped out. This two-way communication helps explain why stress can upset your stomach and why digestive problems sometimes accompany anxiety or depression.
How the Organs Work Together
No part of the nervous system works in isolation. The brain interprets sensory input and issues commands. The spinal cord relays those signals and handles fast reflexes. Peripheral nerves deliver information from every corner of the body and carry instructions back out. The enteric nervous system manages digestion locally but stays in constant contact with the brain through the vagus nerve and other pathways.
The result is a single integrated system with billions of nerve cells coordinating everything from a conscious decision to pick up a glass of water, to the unconscious regulation of your heartbeat, to the digestion of your last meal. The “organs” of the nervous system are really just different levels of that same network: a central command center, a communication trunk line, and thousands of branching cables reaching every tissue in the body.