The brain is the center of all human thought, feeling, emotion, movement, and touch, among other facilities. It consists of the prominent cerebrum, the cerebellum, and the medulla oblongata. The cerebral cortex, or outer layer, has specialized areas for sight, hearing, touch, smell, taste, and so on.
The basic cell of the brain is the neuron, which monitors information coming in to it and directs an appropriate response to a muscle or to another neuron. Each neuron is connected to other neurons through axons, which carry information away from a neuron, and dendrites, which carry information to the neuron. Thus, an axon from one neuron will end at a dendrite of another. The very tiny space between the two nerve endings is a synapse. The message is passed across the synapse by the release of certain chemical "messengers," from the axon which cross the space and occupy receptor areas in the dendrite. These chemicals are called neurotransmitters. Thus neurons are in constant electrical contact with other neurons, receiving and passing on information at the rate of billions of reactions a second.
Neuronal connections are established early in life and remain intact throughout one's lifetime. An interruption of those connections because of a stroke or accident results in their permanent loss. Sometimes, with great effort, alternative pathways or connections can be established to restore function to that area, but the original connection will remain lost.