The nervous system has two main parts: the central nervous system (CNS), which includes the brain and spinal cord, and the peripheral nervous system (PNS), which includes every nerve that branches out from the brain and spinal cord to the rest of your body. Together, these two divisions detect what’s happening around you, process that information, and coordinate a response, whether that’s pulling your hand off a hot stove or digesting your lunch.
The Central Nervous System
The CNS is the body’s processing center. It takes in signals from your senses, makes sense of them, and sends out instructions. The brain serves as headquarters, while the spinal cord acts as the main highway connecting the brain to the rest of the body.
Three layers of protective membranes called meninges wrap around both the brain and spinal cord. From outermost to innermost, they are the dura mater, arachnoid mater, and pia mater. Between these layers, cerebrospinal fluid cushions the brain and spinal cord, absorbing impacts that might otherwise damage delicate tissue.
Major Regions of the Brain
The brain itself is divided into three main regions, each handling a different category of work.
The cerebrum is the largest part, making up most of what you see when you picture a brain. It handles everything you’re consciously aware of: interpreting your five senses, controlling speech, storing and retrieving memories, shaping your personality and behavior, and powering reasoning and judgment. When you decide to move your arm, plan your day, or recognize a friend’s face, your cerebrum is doing the heavy lifting.
The cerebellum sits at the back of the brain, tucked beneath the cerebrum. It keeps your balance, maintains your posture, and fine-tunes coordinated movement. Catching a ball, walking in a straight line, or threading a needle all depend on the cerebellum smoothing out your motor commands so they’re precise rather than jerky.
The brainstem connects the brain to the spinal cord and manages the automatic functions you never have to think about: heart rate, breathing, swallowing, and your sleep-wake cycle. Because these processes run continuously without conscious effort, damage to the brainstem is particularly life-threatening.
The Spinal Cord and Spinal Nerves
The spinal cord runs from the base of the brainstem down through the protective bony canal of your spine. It carries signals in both directions: sensory information traveling up to the brain, and motor commands traveling down to your muscles and organs.
A total of 31 nerves branch off the spinal cord, 30 of which come in pairs (one on each side). These are grouped by where they exit the spine:
- 8 cervical pairs in the neck, mostly serving the face, head, and upper limbs
- 12 thoracic pairs in the upper back, extending to the chest and abdomen
- 5 lumbar pairs in the lower back, running to the legs and feet
- 5 sacral pairs near the base of the spine, reaching into the pelvis
Some spinal reflexes, like yanking your hand away from pain, happen right at the spinal cord level before the signal even reaches the brain. This shortcut shaves precious milliseconds off your reaction time.
The Peripheral Nervous System
Every nerve outside the brain and spinal cord belongs to the peripheral nervous system. The PNS is split into two functional divisions based on what kind of activity the nerves control.
The somatic nervous system handles conscious, voluntary activity. Its nerves run to your skin and skeletal muscles, carrying sensory information in (the feeling of cold air on your face) and motor commands out (deciding to zip up your jacket). Any movement you deliberately choose to make travels through somatic nerves.
The autonomic nervous system handles unconscious, involuntary activity. Its nerves connect the CNS to internal organs like the heart, stomach, and intestines, keeping them running automatically and continuously without you having to think about it. It sends motor signals to smooth muscle, cardiac muscle, and glands.
Sympathetic vs. Parasympathetic Branches
The autonomic nervous system splits further into two branches that essentially work as counterweights to each other.
The sympathetic branch controls your “fight or flight” response. When your safety is at risk, or even when you’re stressed or startled, it puts your body on high alert: your heart rate climbs, your pupils dilate to let in more light, your airways open wider, and blood flow shifts toward your muscles. It’s your body’s emergency accelerator.
The parasympathetic branch does the opposite, acting as the brake. After a period of stress or danger, it returns your body to baseline. It lowers your heart rate, constricts your pupils, increases digestion, triggers saliva production, and diverts energy toward processing food. It also tells your pancreas to release insulin so your cells can use sugar for fuel, manages waste removal by relaxing the muscles that control urination and bowel movements, and plays a role in sexual arousal. Essentially, this branch governs the “rest and digest” state.
These two branches don’t take turns in a simple on-off fashion. Both are active to some degree at all times, constantly adjusting the balance based on what you’re doing and what’s happening around you.
The Enteric Nervous System
There’s a third division that sometimes gets overlooked. The enteric nervous system is an intricate network of more than 500 million neurons embedded in the walls of your digestive tract. It’s often called the “second brain” because its neuron diversity, supporting cells, and chemical messengers closely resemble what’s found in the CNS.
The enteric nervous system controls the rhythmic contractions that push food through your gut, manages fluid secretion, and regulates blood flow to the intestines. It also interacts with hormone-producing cells in the gut lining, influences how the intestinal lining repairs itself, and modulates the local immune system. While it communicates with the brain through the autonomic nervous system, it can coordinate many of these functions on its own, which is unusual for a peripheral nerve network.
How Nerve Signals Travel
All of this communication depends on neurons, the specialized cells that carry electrical signals. The adult brain alone contains roughly 20 billion neurons in its outer layer, supported by billions of additional cells called glia that insulate, nourish, and protect them.
A signal travels along a neuron as an electrical impulse. When it reaches the end of that neuron, it hits a tiny gap, roughly 20 to 40 nanometers wide, separating it from the next neuron. The electrical signal can’t jump this gap directly. Instead, the first neuron releases chemical messengers into the gap. Those molecules drift across, latch onto receptors on the receiving neuron, and trigger a new electrical signal on the other side. The process converts an electrical signal to a chemical one and back to electrical again, thousands of times per second across trillions of these connections throughout your body.
This chemical handoff is what allows medications, hormones, and even substances like caffeine or alcohol to influence the nervous system. They work by mimicking, blocking, or altering those chemical messengers at the gaps between neurons.