2_Nervous

2_Nervous - Neurons and the nervous system Human...

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Neurons and the nervous system
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Human development from infancy to adulthood involves dramatic anatomical and physiological changes. In addition, our daily lives involve complex physiological and behavioral responses to various stimuli. The question that we wish to address is: how are these anatomical, physiological, and behavioral changes and responses regulated or controlled?
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Regulation requires information. In humans, information is encoded as electrical signals in the nervous system and as chemical signals in the endocrine system. We begin by discussing the mechanisms by which electrical signals are generated and propagated through the nervous system. In other words, we focus on the details of how the nervous system works at the cellular level.
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The principal cells of the nervous system are neurons. A key feature of neurons is that their plasma membranes can generate and propagate electrical signals. As illustrated in Figure 44.3 in the text, neurons have four regions: the cell body, dendrites, axon, and axon terminals.
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The cell body contains the nucleus and most of the organelles. The dendrites are bush like projections that bring information from other neurons or sensory cells to the cell body
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The axon terminals, which are swellings at the tips of the axon, are involved in communicating the information in electrical signals to other neurons, muscle cells, or gland cells.
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Where an axon terminal comes close to another cell, the membranes of both cells are modified to form a synapse. As we’ll see later, information transfer across the synapse usually involves chemical signals. The human brain may contain 100 billion neurons, and each neuron may make synapses with 1,000 or more other neurons. This large number of synapses helps to explain the brain’s exceptional ability to process complex information.
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The nervous system also contains gliel cells. Glial cells serve many important “housekeeping” functions, such as supplying the neurons with nutrients and main training the proper ionic environment around the neurons. However, glial cells do not generate or propagate action potentials.
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Ion pumps and channels The plasma membrane of a neuron contains protein molecules that act as pumps to move ions against their concentration gradients. The key pump is the sodium-potassium pump, which moves sodium (Na + ) ions from inside to outside the neuron, and moves potassium (K + ) ions from outside to inside. The pump expels about 3 Na + ions for every 2 K + ions that it brings in.
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channels allowing ions to pass through by diffusion. The channels are selective, in that they may allow only a single type of ion to pass through by diffusion. Some channels are always open.
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2_Nervous - Neurons and the nervous system Human...

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