DIVISIONS OF THE NERVOUS SYSTEM
Learn the hierarchies of nerve structures that keep our bodies running and safe from harm
When we think of the nervous system, our thoughts immediately go to the brain. The brain is a hive of neuronal activity, with billions of interwoven neurons firing to preserve and recall memories, coordinate thoughts and speech and plan future actions. Along with the spinal cord, the bone-clad parts of our nervous system are called the central nervous system. The majority of our neurons are shielded behind protective fluid and bone, where they receive signals from and dictate to organs around the body. However, the signals sent from the central nervous system must have some means of reaching their target organs. For that they need to connect to nerves that stretch from the central nervous system all the way to the extremities of the body. This second network of nerves is called the peripheral nervous system. Together the central and peripheral form the major divisions of the nervous system.
The peripheral nervous system is responsible for many functions, and as such has numerous subdivisions that specialise in different tasks. The sensory, or afferent division receives signals from the periphery and carries these into the central nervous system. The motor, or efferent division transmits signals for actions outwards from the central nervous system to the peripheral organs and muscles. These motor functions come in two forms: somatic and autonomic. Somatic functions are perhaps the easiest concept of the nervous system to grasp, as these dictate all of our voluntary actions, such as choosing to pick up a cup or jump on the bed.
Not all somatic motor functions are voluntary, however. Some are automatic, preprogrammed responses built into our bodies that help us cope with danger, known as somatic reflexes. You’ll notice such a reflex when you accidentally touch a hot stove, step on a sharp object or something flies towards your eye – your body reacts before you’re aware of it. Your hand pulls away, you hop onto the other foot or your eyelid slams shut. This is all the work of somatic reflexes, which can act incredibly quickly as they do not require voluntary input from the brain. Such reflexes can come in different flavours – pulling your hand away from danger is known as a flexor or withdrawal reflex, whereas stepping on a sharp object initiates a crossed-extensor reflex. This latter reflex automatically triggers multiple motor functions: as one leg retracts the other leg simultaneously expands and becomes more stable, preventing us from falling over.
The innate, hard-wired reflex responses of our peripheral nervous system help keep us safe from danger, but they are not the only automatic functions performed by the
Did you know? Around 80 per cent of the brain’s volume is filled with cells
peripheral nervous system. When actions are not somatic, they are autonomic, which means they operate independently of conscious thought. Such processes include your heartbeat, the churning of food in the digestive tract by contracting muscles and respiration. While our brain can assume control of a few of these processes – think of holding your breath – autonomic functions will continue to operate even when we fall asleep or if we are knocked unconscious.
The processes that we cannot control, however, are by no means unchanging. Instead the organs under the control of the autonomic nervous system are regulated by a balance between the sympathetic and parasympathetic nervous systems. Depending on stimuli, these systems increase or decrease activity of our internal organs, helping to ensure our body is always ready to respond to the challenge at hand.