The nervous system serves as the body’s communication and control network, managing everything from basic reflexes to complex thought. It is broadly divided into the Central Nervous System (CNS), consisting of the brain and spinal cord, and the Peripheral Nervous System (PNS), which includes all the nerves branching out to the rest of the body. This system operates with astonishing scale and speed to orchestrate the human experience.
The Massive Scale of the Nervous System
The average human brain contains an estimated 86 billion neurons, which are the specialized cells that transmit information through electrical and chemical signals. To put this number in perspective, some estimates suggest there are more neurons in a single human brain than there are stars in the Milky Way galaxy.
These neurons are connected through an intricate web of junctions called synapses, which are the points of communication between cells. Each neuron may have thousands of these connections, resulting in a total number of synapses estimated to be in the trillions, perhaps reaching up to 150 trillion connections.
The physical length of the entire nervous system’s wiring, if laid end-to-end, is equally staggering. The total length of all nerve fibers in the human body is widely estimated to be around 45 to 60 miles. These nerves stretch from the brain and spinal cord to the tips of the fingers and toes, forming a complex circuitry that spans the entire body. The longest single nerve, the sciatic nerve, can run from the lower back all the way down to the foot.
Lightning-Fast Communication
Electrical signals, known as action potentials, can travel through the fastest nerve fibers at speeds up to 268 miles per hour (about 120 meters per second). This transmission speed is necessary for immediate responses, such as the reflex action of withdrawing a hand from a hot surface.
This rapid communication is largely due to a fatty insulating layer called the myelin sheath that wraps around many nerve fibers. Myelination works similarly to insulation around an electrical wire, allowing the signal to “jump” along the axon instead of traveling continuously. This process, known as saltatory conduction, significantly increases the speed and efficiency of the neural impulse.
Not all signals travel at this pace, as the speed of a nerve impulse varies greatly depending on the nerve’s function and structure. Pain signals, for instance, are carried by thinner, sometimes unmyelinated fibers that transmit information much more slowly, sometimes as slow as 2 miles per hour. This difference in speed explains why a person might feel a sudden touch instantly but a throbbing pain sensation slightly later. This differential speed allows the body to prioritize immediate actions, like movement, over the slower processing of a sensory experience.
Surprising Facts About Brain Function
Despite making up only about 2% of the body’s total weight, the brain is a metabolic powerhouse. It demands a disproportionate amount of the body’s resources, consuming roughly 20% of the total oxygen and calories used at rest.
This high energy consumption is primarily dedicated to maintaining the electrical activity of neurons and the complex signaling across the trillions of synapses. A significant portion of this energy is spent restoring the electrochemical balance in neurons after they fire.
The brain itself is unable to feel pain. The brain tissue lacks nociceptors, which are the specialized sensory receptors found in the skin and other organs that detect damaging stimuli. This absence of pain receptors means that neurosurgeons can, in some cases, perform surgery on the brain while the patient is awake without causing discomfort to the brain tissue itself.
However, the layers of tissue surrounding the brain, known as the meninges, along with blood vessels and nerves in the head, do contain pain receptors. Headaches and migraines are therefore caused by the irritation or inflammation of these surrounding structures, not the brain tissue itself. The brain’s capacity for information storage is also immense, with estimates suggesting it could store the equivalent of approximately 2.5 petabytes of data.
This vast capacity is attributed to the complex and dynamic nature of synapses, which are thought to be able to store multiple bits of information based on their strength and size. The process of neurogenesis, the creation of new brain cells, continues in certain areas of the adult brain. This occurs particularly in the hippocampus, a region associated with memory and learning. This ongoing neurogenesis demonstrates a continuous capacity for adaptation and change, even late in life.