Parts of the nervous system, the system of the body that oversees the function of all other systems, include its main organ, the brain; its vessels, the spinal cord and peripheral nerves; and its cells, known as neurons, which transmit the electrical signals that are nerve impulses. The nervous system can be split into two subsystems: the central nervous system includes the brain and spinal cord as well as the retina of the eyes, and the peripheral nervous system includes all the nerves that exit the spinal cord to travel throughout the body, transmitting signals between the body and the brain such as motor and sensory impulses. Beyond this, the peripheral system features a subdivision known as the autonomic nervous system, which controls involuntary visceral functions, such as heart rate, salivating, and perspiring.
Of the various parts of the nervous system, the brain can be thought of as the control center. Occupying much of the head in humans and protected by the skull in all vertebrates, it is where nerve impulses originate, in the case of voluntary functions such as picking up a book. It is also where they are interpreted, as when one senses that a plate is hot, and recognizing this, the brain tells one to snatch her hand away. As a component of the central nervous system, which coordinates all bodily function and behavior, it receives information from the peripheral nervous system about what is happening both inside and outside of the body and reacts accordingly.
The brain does this by transmitting and receiving nerve impulses via the spinal cord, the part of the central nervous system that channels them to and from the peripheral nervous system. Protected by the vertebral column, the spinal cord is a thick bundle of nerves and the glial cells that form a protective enclosure around them, much like the rubber casing on an electric cable. It is among the components of the nervous system that are responsible for the transmission of nerve impulses, meaning that it both sends information from the brain to the peripheral nervous system and receives information from the peripheral nervous system about stimuli in the body or in the environment. This is not the only function of the spinal cord, however. It is involved in dictating certain motor reflexes, or involuntary movements of muscles in response to a particular stimulus.
Emanating from the spinal cord are the spinal nerves, a major component of the peripheral nervous system. A paired system of nerves, there are 31 pairs of spinal nerves in total, with one pair exiting either side of the spinal column for each vertebral segment and each one branching into smaller nerves that reach all areas of the body. These parts of the nervous system carry three kinds of signals between the body and the brain: motor, sensory, and autonomic. Motor signals travel from the brain out to the body’s muscles, sensory signals come from receptors in the body and bring information to the brain about such external stimuli as temperature and pain, and autonomic signals maintain communication with the brain about whether all bodily systems are working correctly. They are transmitted along chains of individual nerve cells called neurons, the parts of the nervous system that physically carry these electrical impulses from the brain to the body and vice versa.