The Nervous System Study Guide
Interaction with the environment is a unique characteristic of life. The term we use to describe the response of an organism to a change in its surroundings is irritability. You can think of this in terms of a stimulus (meaning a change in the surroundings), which irritates an organism and triggers a response. Both plants and animals can respond to an environmental stimulus. In multicelled animals, we have what is known as a nervous system. Plants do not have nervous systems, but they do have cellular receptors that respond to many environmental stimuli.
Interacting with the Environment
The nervous system of multicelled, vertebrate animals is very sophisticated and able to respond to many external and internal stimuli. Think of a dog who sees a squirrel in the yard. He will receive the visual stimulus through his eyes, which will send a signal to his brain. The brain then remembers that a squirrel is something fun to chase and sends signals to the dog's leg muscles. The dog will run and probably not catch the squirrel, but he will still use his nose to smell the squirrel's scent. After running hard, the dog may be breathing hard and panting, with the breathing muscles also controlled by the nervous system. Having missed the squirrel, the dog may realize he is hungry and the brain will send signals to the stomach and the digestive system. This simple example shows the many necessary interactions the nervous system controls.
Stimulus and Response
In another of the body's cooperative efforts, the nervous system often works in conjunction with the muscular system (as well as all other organ systems). When something causes a change in the environment, the nervous system detects this. Usually, some sort of processing of this information takes place, and a signal is sent out in response. The response is usually to take some action. The action could be the movement of a muscle, the secretion of a substance by a gland, some action in the digestive system, or some regulatory function such as signaling the kidneys to absorb more or less water. The nervous system is built upon cells called neurons.
The functional unit of the nervous system is the neuron, a cell with structures specialized in transmitting electrical impulses. A neuron must be able to receive information from internal or external sources, integrate the signal (especially if from multiple sources), send that signal to other parts of the body that may be far away, and then deliver that message to another neuron, gland, or muscle.
In multicelled vertebrates, neurons have four regions. The dendrites are the tree-branch-like extensions at one end of the neuron that receive signals from other neurons. The cell body of the neuron is where cellular functions take place (just like in any other cell) and where the signal(s) is (are) integrated. The axon is a long extension from the cell body, along which the nerve impulse is sent. Some axons are several feet in length. The final region of the neuron is the synaptic terminal. This is the very end of the axon and consists of several tiny swellings that contain a chemical substance called a neurotransmitter.
A nerve signal reaches the synaptic terminal and causes the neurotransmitter to be released. This chemical messenger then moves across the small space between the neuron and the next neuron (or gland or muscle) called the synapse. Once across the synapse, the neurotransmitter is received by the dendrites of another neuron (or the receptors on a gland or muscle). Thus, a signal is transmitted to another neuron in the chain (or to the gland or muscle where action can take place).
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