Nervous System
The nervous system is the brain, spinal cord, and nerves that sense the environment, process information, and control the body's responses.
Definition
The nervous system is your body's communication network: your brain, spinal cord, and nerves carry messages that let you feel, think, move, and react. It has a central division (brain and spinal cord) and a peripheral division (the nerves connecting to the rest of the body), built from neurons that carry electrical signals and glial cells that support them; it detects stimuli through sense organs, processes the information, sends signals to muscles and glands, and works with the endocrine system to maintain homeostasis. Neurons transmit information as action potentials, brief reversals of membrane potential from about $-70\ \text{mV}$ to $+30\ \text{mV}$ caused by the sequential opening of voltage-gated Na$^+$ and K$^+$ channels. Signals cross synapses chemically, where neurotransmitters such as glutamate, GABA, acetylcholine, and dopamine bind receptors on the postsynaptic cell, and myelinated axons conduct rapidly by saltatory conduction between nodes of Ranvier. Integration happens as postsynaptic potentials sum at the axon hillock, and synaptic strength changes with use, which is the cellular basis of learning.
Example
If you touch a hot pan, nerves in your finger signal your spinal cord, which sends a signal straight back to pull your hand away before your brain even notices the pain. That kind of reflex arc, sensory neuron to spinal connection to motor neuron, takes about $30$ to $50$ milliseconds for a knee-jerk, and the human brain contains roughly $86$ billion neurons. Conduction velocity ranges from about $0.5\ \text{m/s}$ in thin unmyelinated fibers to about $120\ \text{m/s}$ in large myelinated motor axons, so a signal from the brain to the foot, about $1.5\ \text{m}$, arrives in roughly $15$ to $20$ milliseconds along a fast fiber.
Key Insight
Nerve messages are fast but not instant; signals travel at up to about $120$ meters per second, which is why you feel a stubbed toe a split second after it happens. The nervous system responds in milliseconds while the endocrine system responds in minutes to hours, fast signals using electricity along nerves and slow signals using hormones in the blood. Because the whole system runs on ion gradients maintained by the sodium-potassium pump, the brain consumes about $20$ percent of the body's resting energy despite being only $2$ percent of its mass, much of it spent restoring gradients after every signal.