In higher animals, including humans, the brain and the nervous system are physically connected. They are both composed of specialized cells called nerve cells, or neurons. Technically, the brain is part of the nervous system, along with the spinal cord and individual nerve cells distributed throughout the body. In medicine and biology, however, the brain and the nervous system are often considered separate organs, and both represent specialized fields of study. Together these organs are responsible for thought, sensation, and responses to outside stimuli.
The brain and the nervous system are both made entirely of neurons. Unlike other cells, which are microscopically small, individual neurons can be as long as 3 feet (1 meter) in humans. In larger animals, they can be even longer; in giraffes, for example, a single neuron can reach 15 feet (5 meters). In the body, neurons stretch to all extremities, providing sensation, pain, and touch. In the brain, billions of neurons are tightly grouped into a complex network capable of highly advanced processes.
Neurons use chemical and electrical signals to communicate with each other and with other cells. These signals are conveyed at very high speeds through the brain and the nervous system. If, for example, a person stubs his toe, nerve cells in the foot automatically register pain as well as other sensations, such as a sudden cease in motion and the general shape of the object that was struck. All these sensations are conveyed up the neurons in the legs to the spinal cord and then to the brain, usually in less than a single second. The brain, in turn, directs instinctive responses, such as withdrawing the foot from the injuring object, as well as voluntary responses, like a shout of pain.
Both the brain and the nervous system are composed of neurons, but in the brain these cells have an incredibly complex organization and specialized function. Axons, the communicating bodies of neurons, form a vast network in the core of the brain, sometimes called the white matter. Communicating through connections called synapses, axons and neurons control thought, emotion, and all other brain functions, voluntary or otherwise. In the human brain, these synapses number in the trillions and are capable of processing millions of functions in any given second.
The brain and the spinal cord together are called the central nervous system, because they are responsible for processing the majority of nerve functions. The spinal cord and the connected nerve cells throughout the body are called the nervous system, and this may or may not include the brain, depending on the focus of a given study. In this regard, the relationship between the brain and the nervous system is one of constant, interconnected activity. This activity is responsible for all activities of the organism and indeed for the very functions that constitute life.