Nervous Tissue 2009 - 1 Nervous Tissue 2009 I Nervous...

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1 Nervous Tissue 2009 I. Nervous tissue is by far the most complicated of our tissues. How neurons and their electrical and chemical interactions result in thoughts, perceptions, dreams that make the “self” remains a mystery. Traditionally, it has been divided into the Central Nervous System ( CNS ) and the Peripheral Nervous System ( PNS ) but both are intimately connected and intertwined. Cells of nervous tissues come in great variety but can be divided into neurons and glial cells. Neurons function in either electrical or chemical communication. Glial cells are the support cells of nervous tissue. Glial cells outnumber neurons by a 10:1 ratio. There are over 100 billion neurons in human nervous tissue. So, multiply by ten to get a rough estimate of glial cells. A. Neurons consist of a cell body (perikaryon) and two (usually) types of cellular extensions, dendrites and axons. The nucleus is large, euchromatic and contains a prominent nucleolus. The cytoplasm around the nucleus is rich in RER and this RER is called Nissl bodies (so Nissl bodies = the RER of neurons). Several Golgi apparatuses are around the nucleus. (So note that neurons are very active protein factories). Axons are long cytoplasmic extensions from neurons. There is only one axon for each neuron. The diameter of the axon is constant and, except for its terminal end, the axon usually does not branch. The axon arises from perikaryon at a site devoid of Nissl bodies called the axon hillock . Axons are specialized to conduct signals from the neuron to another cell (neuron, muscle, or gland). Axons have a highly organized cytoskeleton of microtubules and intermediate filaments as well as many mitochondria. They do not contain any ribosomes and hence no protein is made in the axon which thus relies on the perikaryon for maintenance. Along the microtubules and filaments there is bidirectional flow of organelles (secretory vesicles, mitochondria) and proteins. Movement from the perikaryon toward the
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2 terminal end of the axon is anterograde flow; flow from the axon toward the perikaryon is retrograde flow. Motor proteins are involved in moving organelles along the microtubules. Know two of these motor proteins: kinesin for anterograde flow; dynein for retrograde flow. Axons may be myelinated to speed signal conduction Dendrites are branching, irregular, tapering cytoplasmic extensions from the perikaryon. Most neurons have numerous dendrites which greatly increase the surface area and allows for many more receptive sites for axons from other neurons. (The dendrites of one neuron may receive information from the axons of hundreds of thousands of other neurons. Dendrites are specialized to receive and integrate signals from other neurons. As the dendrites arborize many small mushroom shaped structures are found on the branches. These are called dendrite spines and the site for many synapses; and may play a role in learning and memory. Dendrites are not myelinated. Close to the perikaryon, the cytoplasm is similar to the perikaryon in that ribosomes are
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This note was uploaded on 11/08/2011 for the course SCIENCE 3254 taught by Professor Dr.shoupe during the Spring '11 term at Fort Valley State University .

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Nervous Tissue 2009 - 1 Nervous Tissue 2009 I Nervous...

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