It’s all about communication. That’s the job of the body’s nervous system! This week, Tidbits communicates the following facts about the body’s billions of nerve cells.

As the body’s inner means of communication, the nerve cells take in information via the body’s sense of touch, taste, smell, sight, and sound. The brain then interprets these sensory signals, allowing a person’s body to interact with the surrounding environment and prepare for action.

    The nervous system has two parts – the central nervous system and the peripheral nervous system. The CNS includes the nerves in the brain and spinal cord, the main organs of the nervous system, with about 100 billion nerves in the brain and 13.5 million in the spinal cord.    All the body’s other nerves are part of the PNS.

    Two main types of cells make up the system, neurons and glial cells. The neurons communicate with other cells via synapses, membrane-to-membrane junctions that allow signals to pass from one neuron to the next. Neurons are programmed to perform different functions. Sensory neurons deliver electrical signals from the outer parts of the body, such as glands, muscles, and skin, into the central nervous system. Motor neurons do the opposite, carrying signals from the CNS to the outside parts. Receptor neurons sense the environment – light, sound, touch, and chemicals – and convert it into electrochemical energy. Interneurons send messages from one neuron to another.

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Because neurons can’t replace themselves, nerve damage is often permanent. We also lose neurons as we age. By the time we turn 75, 10% of our neurons are gone, resulting in the brain shrinking about 1 to 2 grams every year due to neuron loss.   

    Glial cells provide support and nutrition and participate in signal transmission. Glial cells are from the Greek word for “glue,” and one of their most important functions is to hold neurons in place as well as provide insulation for them, along with removing dead neurons.

The body has both voluntary and involuntary nervous systems. The voluntary, or somatic, system controls things that a person is aware of and can consciously control, such as moving various body parts. The involuntary, or automatic, systems takes care of those things that the body doesn’t consciously control – breathing, regulating the heart rate, metabolism, and other vital body processes. During physical activity, the involuntary system causes the heart to beat harder and faster, and opens up airways for easier breathing. It stimulates digestion, helps the body relax, and detects environmental changes that will impact the body.

    The sciatic nerve is the longest, largest nerve in the body. It starts at the base of the spine, runs through the gluteus muscles, and down the back of the thighs and lower legs. This nerve’s functions are two-fold – it helps the muscles in the body’s legs and feet move, and helps a person feel sensations in the legs. At the nerve’s origin, it’s about ½ an inch (1 cm) wide, but as it extends down the legs, it widens slightly to its thickest point of just under an inch (2 cm) in diameter, about the size of a penny. It splits into two main branches at the knees, with one branch running from the outer part of the knee to the outer part of the foot, and another running down the back of the calf to the heel and sole of the foot. People with sciatica can experience great pain or discomfort as this nerve gets pinched or compressed.