Like any organisation, your success depends on communication between your individual members. Ina sense, you actually are this communication, since it is the magic that makes you a single, clever creature. Your built-in communications network, known as the nervous system, perceives the outside world, keeps all body parts working in harmony, and forms the thoughts and memories that make you unique.
The nervous system comprises hundreds of billions of specialised cells called neurons. A typical neuron consists of a compact cell body, protruding filaments called dendrites, and a long single fibre called an axon. The axon can transmit signals to other neurons and to muscle cells, while the dendrite can receive signals from other neurons and sensory cells. A neuron’s axon may extend across the brain or body and branch off hundreds of times.
When something excites a neuron, the cell body will send an electrical charge down the length of an axon, triggering axon terminals to release chemicals called neurotransmitters. These neurotransmitters can travel to receptors on dendrites of an adjoining neuron, across a small gap called a synapse. Depending on the type of neurotransmitter and receptor, the signal may excite the adjoining neuron to fire an electrical charge down its own axon, or the signal may inhibit the neuron from firing. The complex connections and signal patterns among the hundreds of billions of neurons in your brain form thoughts, memories and all other mental activities.
Similarly, axons that extend out from your brain and spinal column into your body can release neurotransmitters to trigger muscle movement and organ activity. This is how your brain controls the rest of your body. Neurons also carry signals from the body back to the brain. You perceive sights, sounds, smells and taste when sensory cells in your eyes, mouth, nose and ears excite nearby neurons. The neurons send an electrical signal up to the brain, which interprets them. Sensory neurons near your skin and other parts of the body fire an electric signal in response to pressure, which your brain perceives as the sense of touch.
How does our automatic nervous system keep us running?
The automatic nervous system (ANS) works behind the scenes to keep your body running smoothly. The ANS is part of your peripheral nervous system, made up of sensory nerve fibres that constantly relay information about the state of your body and the motor nerves that relay commands from the brain and spinal cord to various glands, the involuntary smooth muscles in organs and blood vessels, and the cardiac muscles that control your heart.
The ANS’s chief function is homeostasis -adjusting bodily processes to maintain internal stability. The ANS does this through two opposing, yet complementary sub-systems: the sympathetic division and parasympathetic division. The sympathetic division is like the accelerator in your car. The motor neurons excite your body, by increasing your heart rate and producing stress hormones, among other things. The parasympathetic division is like the brakes. The motor neurons can relax your body, by doing things like decreasing heart rate, constricting the trachea and bronchial tubes, and relaxing the bladder sphincter.
Why is the funny bone not at all funny?
Most of the larger nerves in your body are insulated by muscle, bones and tissue. The big exception is the ulnar nerve, which runs down your arm, by way of your elbow. The nerve carries motor commands to your ring and pinkie fingers and relays sensory information back to the central nervous system. If you bang your elbow, the humerus bone bumps the nerve, jarring the axons inside, which your brain interprets as a tingling sensation.
What do nerves do?
Your neurons are fragile cells, and for the most part can’t be replaced if they get damaged. So, instead of bare axons running through your body, we have nerves.
A nerve is like an electrical cable, housing thousands (or millions) of axons in a protective sheath of tissue. Nerves extend out from your brain and spinal column, known as the central nervous system, to the rest of your body. There are 12 pairs of nerves extending from the brain and 31 pairs extending from the spinal cord. The nerves branch off in your body, forming a network called the peripheral nervous system.
The afferent division of the peripheral nervous system relays signals from sensory neurons back to the central nervous system, while the efferent division relays instructions from the central nervous system to muscles and glands. Most nerves carry both types of signals.
2. Facial nerve – Branching sensory fibres run to the taste buds and the front of the tongue, while motor nerves connected to your salivary glands and muscles form facial expressions.
3. Vagus nerve – A critical nerve running from the brain to the neck, throat, chest and abdomen, the vagus is key to controlling your heart rate, swallowing, digestion and respiration.
4. Ganglion – Bundles of tightly neurons that serve as key connection hubs in the body’s complex network of nerves.
5. Spinal cord – A bundle of long axons that run from the brain to the lower spinal column, forming the key connection between the brain and body.
6. Radial nerve – A nerve that carries muscle motor commands that move your elbow, wrist and fingers.
7. Ulnar nerve – A key nerve involved in bending your fingers and wrist.
8. Lumbar plexus – A plexus is a point where many spinal nerves intersect. The lumbar plexus is the meeting point for nerves controlling the abdomen, lower back, and legs.
9. Femoral nerve – A key nerve with many smaller nerve branches that carry motor commands to leg muscles, as well as sensory information from the thigh and lower leg.
10. Sciatic nerve – Your longest nerve, which carries motor signals to muscles that bend your leg.
11. Common peroneal nerve – Connects to muscles in your lower leg, which lift your foot.