The brain and its meninges are contained in a tough, bony protective structure, the skull. The brain communicates with much of the body through nerves that run up and down the spinal cord. Each vertebra forms an opening between it and the vertebrae above and below it. Through this opening, a pair of spinal nerves branch out and carry messages from the spinal cord to the more distant parts of the body. This network of nerves makes up the peripheral nervous system.
Peripheral nerves are actually bundles of single nerve fibers.
The peripheral nervous system also includes nerves that communicate between the brain stem and the body's internal organs. These nerves, called the autonomic nervous system, regulate internal body processes that require no conscious awareness, for example, the rate of heart contractions, the rate of breathing, the amount of stomach acid secreted, and the speed at which food passes through the digestive tract. The nervous system contains 100 billion or more nerve cells that run throughout the body like strings, making connections with the brain, the body, and often with each other. These substances trigger the receptors on the next neuron's dendrite to start up a new electrical current. Different types of nerves use different neurotransmitters to convey messages across the synapses.