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Chpt45___NervousSyst - Chapter 45 The Nervous System The...

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Chapter 45 - The Nervous System The theme of this chapter focuses on how the body receives information, processes and integrates the information, and provides an appropriate response. Nervous tissue (P. 869) and the nervous system are responsible for this task. The Concept Outline (P. 939) and the Concept Review (P.967) provide excellent coverage of the subject. Also, please return to Chapter 42 and read about Nervous Tissue on Page 869 – that is an excellent summary also. Nervous System Organization: The major parts of the nervous system are: the Central Nervous System (CNS) (brain and spinal cord) and the Peripheral Nervous System (PNS) (sensory and motor neurons). There are two types of Motor pathways (somatic and autonomic); and, the Autonomic portion of the nervous system has two components: the sympathetic and the parasympathetic nerves (Fig. 45.3). The functional unit of the nervous system is the nerve cell, or neuron . These cells, and their supporting cells, the neuroglia , make up the nervous system (Fig. 45.3). Supporting cells, or neuroglia, are not directly involved in impulse transmission, but do play extremely important roles in the process. Your book reports neuroglia are ~10X smaller than a neuron and ~10X more abundant than neurons. They supply neurons with nutrients, take away waste products, guide axon migration, and provide immune functions. Two common neuroglia cells are Schwann cells and oligodendrocytes . These cells produce the myelin sheaths that surround the axons of many neurons. The former provides myelin around nerves in the PNS, whereas the latter provide sheathing around neurons in the CNS. Myelin increases the speed of transmission. Nerve Impulses are Carried on the Neuron’s membrane: The communication aspect of neurons is through an “electrochemical process” or “impulse.” Neurotransmission depends on the depolarization of the nerve membrane (Section 45.2). This “depolarization” involves many things, but it is basically the movement of sodium (Na) from the outside of the membrane, across the membrane, into the cytoplasm of the neuron. Potassium (K) is also involved. The movement of Na+ back to the outside of the cell and K+ back to the inside to maintain polarity of the membrane is called the “Sodium/Potassium Pump.” (Figs. 45.6, 45.12). Issues such as “resting potentials, graded potentials, depolarization, hyperpolarization, thresholds, excitatory potentials, and summation” will be discussed in class. However, the material in Section 45.2 should be studied carefully before lecture!!! Much of the energy you expend each day involves maintaining the polarity across the membrane of neurons. Maintaining polarity involves the sodium/potassium pump (Na/K pump). Neurotransmission then is a function of the membrane and not of the cell itself. In fact, a dead neuron will still conduct several thousand impulses before the difference in polarity is exhausted!!!
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