The nervous system, comprised of the brain, spinal cord, nerves, and all the chemical messengers that ensure communication throughout the body, controls and regulates all of the other body systems. The billions of cells that make up the brain are housed in the cerebellum and in two large lobes known as the left and right hemispheres. These hemispheres float in a pool of cerebrospinal fluid and are further safeguarded against outside danger by the skull (cranium) and by the meninges, protective coverings that rest just above the wrinkled layer of brain (the cerebral cortex) nearest the skull. The brain itself sits on a pillar of tissue known as the brain stem. The oldest part of the brain, the brain stem controls the basic functions of the body, including consciousness, heartbeat, blood pressure, and respiration.
The brain stem descends through an opening in the skull and connects to the bundle of nerves known as the spinal cord. Besides being protected by the vertebrae, the spinal cord, like the brain, is covered by meninges and surrounded by cerebrospinal fluid. Two types of nerves are believed to pass between the spinal cord and the brain: large bundled nerve fibers carrying the sense of touch and the smaller bundles carrying sensations of pain.
The brain and the nerves speak to each other, nerve cell by nerve cell, via a distinctive group of chemicals called neurotransmitters. Neurotransmitters are the doorway to a functioning nervous system - they facilitate sensory perception, muscle contraction, emotions, thoughts, and the awareness of pain. Most neurotransmitters carry out multiple tasks throughout the brain and nervous system and can be likened to words whose meanings change to suit the needs of their users.
The nervous system as a whole is comprised of three overlapping systems: the central nervous system (CNS), which includes the brain and spinal cord; the autonomic nervous system (ANS), which controls involuntary functions such as heart rate, digestion, and glandular function; and the peripheral nervous system (PNS), which connects the CNS to all the body tissues and voluntary muscles. Health and healing rely upon the equilibrium of these three interrelated nerve systems and the unimpeded nerve flow that is essential for the proper function of the body's other systems.
Because pairs of spinal nerves exit between the vertebra of the spinal column and extend to every part of the body (muscles, bones, organs, glands, etc.), nerve function can become impeded when the spinal system (including the cranium) and the body's musculoskeletal structures become misaligned due to injury or other causes.