The Nervous System Study Guide (page 2)
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).
From One to Many
Although some reflexes involve very few neurons, most of the complex and versatile behavior of vertebrate animals comes from the fact that neurons form many complex connections with other neurons. This complex webbing of many tiny cells reaches its zenith in the vertebrate brain where billions of neurons each make dozens of connections. Collections of individual neurons become what we call nerves. The vertebrate nervous system is the name we give to this complex networking of neurons into nerves. The vertebrate nervous system is divided into two major parts, the central nervous system and the peripheral nervous system.
The central nervous system consists of the brain and spinal cord (contained within the vertebral column or backbone). The brain is a highly specialized organ where neurons have grouped together into many specific areas, each with a specific function. The brain integrates all the signals in the nervous system and thus controls the body. It also acts as a data storage organ by learning and keeping memories. In the higher mammals it is also the seat of the conscious mind.
The peripheral nervous system consists of nerves that connect the central nervous system to the rest of the body. The peripheral nervous system has nerves that connect the brain to each part of the body, including sensory nerves that bring information to the central nervous system and motor nerves that carry signals away from the brain and to the muscles, glands, or organs. The sensory neurons are, in many cases, specialized and have become part of other organs whose function is to sense the internal and external environment. Examples of such organs would be the eyes, ears, touch receptors, and taste buds. The motor portion of the peripheral nervous system is again further divided into the somatic and autonomic nervous systems.
The somatic nervous system is concerned with motor functions. Its nerves make contact with skeletal muscle and are responsible for controlling movement of all kinds, from fine movements to walking and running.
The autonomic nervous system, also part of the peripheral nervous system, works mostly without our conscious control. It is often responsible for critical life functions such as breathing and heart rate. The autonomic nervous system has two divisions. Nerves from each of these divisions usually make contact with the same organs, but they often have opposite effects. The sympathetic division of the autonomic nervous system is responsible for the fight-or-flight response, so it prepares the body for high-energy, stressful situations. The parasympathetic division of the autonomic nervous system is responsible for rest and digest functions, so it tends to slow down the body.
More on Neurotransmitters
The electrical signal produced in the neuron that travels down the axon does not actually cross the tiny synaptic space between neurons (or their target organs). The nervous system relies upon chemical messengers called neurotransmitters to cross the gap. The use of neurotransmitters allows fine-tuning of the signal because different neurotransmitters can be used for different purposes or in different locations.
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