The nervous system consists of two main parts: the central nervous system and the peripheral nervous system:
The brain sends messages through the spinal cord and nerves of the peripheral nervous system to control the movement of the muscles and the function of internal organs.
The basic working unit of the nervous system is a cell called a neuron. The human brain contains about 100 billion neurons. A neuron consists of a cell body containing the nucleus, and special extensions called axons (pronounced AK-sonz) and dendrites (pronounced DEN-drahytz).
Neurons communicate with each other using axons and dendrites. When a neuron receives a message from another neuron, it sends an electrical signal down the length of its axon. At the end of the axon, the electrical signal is converted into a chemical signal, and the axon releases chemical messengers called neurotransmitters (pronounced noor-oh-TRANS-mit-erz).
The neurotransmitters are released into the space between the end of an axon and the tip of a dendrite from another neuron. This space is called a synapse (pronounced SIN-aps). The neurotransmitters travel the short distance through the synapse to the dendrite. The dendrite receives the neurotransmitters and converts them back into an electrical signal. The signal then travels through the neuron, to be converted back into a chemical signal when it gets to neighboring neurons.
Motor neurons transmit messages from the brain to control voluntary movement. Sensory neurons detect incoming light, sound, odor, taste, pressure, and heat and send messages to the brain. Other parts of the nervous system regulate involuntary processes, such as the release of hormones like adrenaline, dilation of the eye in response to light, or regulation of the digestive system, which are involved in the function of the body’s organs and glands.
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