The nervous system is the most complex system in the body and regulates hundreds of activities simultaneously. It is the source of our consciousness, intelligence, and creativity and allows us to communicate and experience emotions. It also monitors and controls almost all bodily processes, ranging from automatic functions of which we are largely unconscious, such as breathing and blinking, to complex activities that involve thought and learning, such as playing a musical instrument and riding a bicycle.
In this section
The nervous system has two parts: the central nervous system (CNS), which consists of the brain and spinal cord; and the peripheral nervous system (PNS), which is made up of all the nerves that emerge from the CNS and branch throughout the body. Nerve signals are processed and coordinated by the CNS, while the PNS transmits nerve signals to and from the CNS and other parts of the body.
The nervous system has immediate control over our voluntary actions, such as walking, and our automatic body functions, such as salivation. The longer-term control of automatic body activities, such as maintaining normal blood pressure and temperature, is aided by some of the hormones produced by the endocrine system.
How the nervous system works
Most of the activity of the nervous system is carried out by cells called neurons. Each neuron typically has a long projection known as a nerve fibre (axon), which relays information in the form of electrical signals. When a signal reaches the end of a nerve fibre, it is carried in chemical form to the next neuron or to another kind of cell. In the PNS, nerve fibres form bundles called nerves, and these carry messages concerning the outside world and the inside of the body. For example, information about sound is detected by sensory receptors in the ear and converted into nerve impulses. These signals travel along nerve fibres towards the brain. In response to these nerve impulses, the CNS then transmits signals to the motor nerves in the PNS, which communicate with glands, organs, or muscles to produce an appropriate response to the original stimuli. The CNS also receives a constant stream of data concerning the inner organs and bodily functions, such as blood pressure and body temperature, from other sensory organs throughout the body. This information is monitored continuously and elicits appropriate responses, often without any input from the conscious mind.
Many complex activities and functions, including creativity and logic, involve conscious thought. However, some activities that require the coordination of several physical tasks are regulated unconsciously. For example, your first attempts to ride a bicycle require a conscious effort but when you have learned and memorized the necessary skills you can cycle without conscious thought. However, you still remain conscious of sensory stimuli, such as sight and sound, that aid navigation.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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