The nervous system is organized into two main parts: the central nervous system, which includes the brain and spinal cord, and the peripheral nervous system, a branching network of nerves that extends to every other part of the body. Think of it like a tree. The trunk is your brain and spinal cord, and the branches are the nerves reaching out to your organs, limbs, fingers, and toes. Within this basic framework, several subdivisions handle specific jobs, from conscious movement to digestion to the fight-or-flight response.
Central vs. Peripheral Nervous System
The central nervous system (CNS) is the command center. Your brain interprets signals from nerves throughout the body and decides how to respond, whether that means forming a thought, triggering a movement, or registering pain. The spinal cord serves as the main highway connecting the brain to everything below the neck, bundling millions of nerve fibers into a protected column running through the vertebrae.
The peripheral nervous system (PNS) is everything outside that column. It includes all the nerves that branch off the brain and spinal cord to reach your skin, muscles, and organs. These peripheral nerves carry two types of signals. Sensory nerves bring information inward, from your fingertips or eyes to the brain. Motor nerves carry instructions outward, from the brain to your muscles or glands. When you touch a hot pan, sensory nerves rush that signal up the spinal cord to the brain, the brain decides to pull away, and motor nerves send the command back down to the muscles in your arm.
The Somatic System: Voluntary Control
Within the peripheral nervous system, the somatic nervous system handles everything you do on purpose. It controls skeletal muscles throughout your body, letting you walk, type, chew, and wave. It also funnels sensory information from four of your five senses (smell, sound, taste, and touch) into the brain. Vision is the exception: the retina and optic nerve connect directly to the brain without passing through the somatic system.
The nerves in this system are one-directional. Sensory nerves only carry information up to the brain. Motor nerves only carry commands from the brain down to the muscles. Every time you reach for a cup of coffee, motor signals travel from your brain through the somatic nervous system to coordinate the dozens of muscles involved in that movement.
The Autonomic System: Behind-the-Scenes Control
The autonomic nervous system runs the processes you never have to think about: heart rate, blood pressure, breathing, digestion, and temperature regulation. It has three branches, each with a distinct role.
The sympathetic nervous system activates your body during stress or danger. It’s responsible for the fight-or-flight response, raising your heart rate, dilating your pupils, and redirecting blood flow to your muscles when you need to react quickly.
The parasympathetic nervous system does the opposite. It governs rest-and-digest functions, slowing the heart, stimulating digestion, and conserving energy when you’re calm and safe. These two systems constantly balance each other throughout the day.
The enteric nervous system manages digestion specifically. It contains more than 500 million neurons embedded in the walls of the gut, and its complexity is striking enough that scientists call it the “second brain.” The diversity of its neurons, supporting cells, and chemical messengers closely resembles what you’d find in the brain itself. While it communicates with the CNS, it can coordinate many digestive functions independently.
How the Brain Is Organized
The brain develops from three primary regions in the embryo: the forebrain, midbrain, and hindbrain. These eventually become five distinct subdivisions in the mature brain, each responsible for different functions.
The forebrain is the largest and most complex region. Its outer layer, the cerebral cortex, handles higher-level thinking, language, planning, and conscious perception. Tucked beneath it are structures involved in memory (the hippocampus), emotion (the amygdala), and movement coordination. The forebrain also contains the thalamus, which acts as a relay station routing sensory information to the right part of the cortex, and the hypothalamus, which regulates hunger, thirst, sleep, and hormone release.
The midbrain sits between the forebrain and hindbrain and helps process visual and auditory information. It also plays a role in motor control and pain modulation.
The hindbrain includes two critical structures. The cerebellum coordinates balance and fine motor movements like writing or playing an instrument. The brainstem, which includes the pons and medulla, controls vital automatic functions: breathing, heart rate, swallowing, and the sleep-wake cycle. The medulla is where the brain transitions into the spinal cord.
How the Spinal Cord Is Organized
The spinal cord is divided into four regions, each sending nerve pairs to specific parts of the body. There are 31 total nerve pairs (one nerve on each side of the cord at each level):
- Cervical (8 pairs): Start in the neck and serve mostly the face, head, neck, shoulders, arms, and hands.
- Thoracic (12 pairs): Branch out from the upper back to the chest, upper back, and abdomen.
- Lumbar (5 pairs): Extend from the lower back to the legs and feet.
- Sacral (5 pairs): Emerge near the base of the spine and reach into the pelvis.
This segmented organization is why a spinal cord injury at a specific level affects everything below that point. An injury in the cervical region can affect the arms and legs, while one in the lumbar region may only affect the legs.
Cranial Nerves: Direct Lines From the Brain
Not all peripheral nerves come from the spinal cord. Twelve pairs of cranial nerves emerge directly from the brain or brainstem, mostly serving the head and neck. The first carries your sense of smell. The second provides vision. Others control eye movement, facial sensation, chewing, hearing, taste, swallowing, and tongue movement. The eleventh cranial nerve reaches further down to control shoulder and neck muscles. Some cranial nerves are purely sensory, some purely motor, and several handle both.
How Signals Travel Between Nerve Cells
At the cellular level, the nervous system is built from neurons. Each neuron has three basic parts. Dendrites are branch-like extensions that receive incoming signals. The cell body processes those signals. And the axon is a long fiber that carries the signal outward to the next cell. At the tip of each axon are small button-shaped endings that sit very close to the dendrites of the next neuron, separated by a tiny gap roughly 20 to 40 nanometers wide.
This gap is called a synapse, and signals cross it chemically. When an electrical signal reaches the end of an axon, it triggers the release of chemical messengers stored in tiny vesicles. Those messengers float across the gap and bind to receptors on the next cell, starting a new electrical signal there. This process repeats millions of times per second across billions of connections.
The human brain alone contains tens of billions of neurons. Supporting cells called glia exist in even greater numbers, with roughly 28 billion in male brains and 21 billion in female brains in the outer cortex alone. These glial cells insulate nerve fibers to speed up signal transmission, supply nutrients, clear waste, and help repair damage. Neurons get the attention, but glia make up a substantial portion of the brain’s cellular architecture and are essential for normal function.