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Chapter 4

PSYB65 Chapter 4.doc

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Zachariah Campbell

PSYB65 – Chapter 4 The Neuron’s Structure: - Neurons are the information-conducting units of the nervous system o Has special characteristics that allow it to send electrical impulses by using changes in chemical charges on its cell membrane Overview of a Neuron: - The most prominent distinguishing features are the dendrites, whose presence greatly increases the cell’s surface area o The dendrites’ surface area is further increased by many branches and by many small protrusions called dendritic spines that cover each branch o Dendrites collect information from other cells, their surface areas determine how much information a neuron can gather o Dendritic spines are the points of communication between neurons, the spines provide some indication of how much information a neuron may receive - Each neuron has a single axon, extending out of an expansion of the cell body known as the axon hillock o The axon may have branches called axon collaterals, which usually emerge from it at right angles o Axon may divide into a number of smaller branches called teleodendria o At the end of each teleodendrion is a knob called an end foot or terminal button  Sits very close to a dendritic spine on another neuron, although it does not touch that spine  Synpase – the space between the axon’s end foot and the neighboring dendritic spine o The single axon limits the neuron to only one output channel for communication - The dendrites and the axon are simply fluid-filled extensions of the cell body o Information flows from the dendrites to the cell body and axon o At each terminal button, information in the form of a chemical message is released onto a target - A neuron is both an information-collecting and an information-processing device o Receives a great deal of information on its hundreds to thousands of dendritic spines, but it has only one axon  So the message that it sends must be averaged or summarized version of all the incoming signals o Information travels on a flow of electrical current that begins on the dendrites and travels along the axon to the terminals  As the impulse reaches the terminal buttons, they release one or more chemicals • These chemicals called neurotransmitter, carriers the message across the synapse o To influence the electrical activity of the receiving cell or target and pass the message along The Cell as a Factory: - The cell makes, ships and exports proteins, the cell’s products o Proteins are complex organic compounds, including enzymes, hormones and antibodies and forms the principal components of all cells - A cell’s outer cell membrane, separates it from its surroundings and allows it to regulate the materials that enter and leave its domain o It envelops the cell body, dendrites and their spines, axon and its terminals and so forms a boundary around a continuous intracellular compartment - Very few substances can enter or leave a cell because the cell membrane presents as an almost impenetrable barrier o Proteins embedded in the cell membrane serve as the gates allowing certain substances to enter or leave - Nuclear membrane – surrounds the cell’s nucleus o Nucleus – houses the genes and chromosomes, where the cell’s proteins are stored and copied o Copies are then sent to a part of the cell called the endoplasmic reticulum (ER)  ER is an extension of the nuclear membrane where the cell’s protein products are assembled in accordance with the genes’ instructions o Finished products are packed in a membrane and addressed in the Golgi bodies, that pass them along to the cell’s transportation network  The network is a system of tubules that carries the packaged proteins to their final destinations • Microfailments – constitute the cell’s structural framework • Microtubules – contract and aid in the cell’s movement o Two other components are important:  Mitochondria – the cell’s power plants that supply its energy needs  Lysosomes – saclike vesicles that transport incoming supplies and move and stores wastes The Cell Membrane: Barrier and Gatekeeper: - Neurons and glia are tightly packed together in the brain but like all cells, they are separated and cushioned by extracellular fluid o Composed mainly of water in which salts and other chemical substances are dissolved - Fluid is found inside a cell as well as intracellular fluid or cytoplasm o Made up mainly of water with dissolved salts and chemical but the concentrations of dissolved substances inside and outside the cell are very different  This difference helps explain the information-conducting ability of neurons Membrane Structure: - The cell membrane encases a cell and separates the intracellular from the extracellular fluid, allowing the cell to function as an independent unit o Special double-layer structure makes this separation possible, the membrane bilayer - The membrane bilayer regulates the movement of substances into and out of the cell o Also regulates the concentration of salts and chemicals on either side, because precise concentrations of chemicals within a cell are essential to its normal function - Membrane bilayer is composed of a molecule called a phospholipid that features: o A head that contains the element phosphate  Has a slight positive charge in one location and a slight negative charge in another o 2 tails that are lipid molecules  Consist of hydrogen and carbon atoms bound tightly to one another, making them electrically neutral - The differences in the electrical polarity of the head and tails of a phospholipid molecule are the underlying reason why it can form membranes o The head, being polar, is hydrophilic  Attracted to water molecules  Heads of one layer are in contact with the intracellular fluid • The heads of the other layer are in contact with the extracellular fluid o The tails, being nonpolar, is hydrophobic  Have no attraction for water  Tails of both layers point toward the inside of the bilayer, hidden from water How the Cell Membrane Functions: - The cell is impenetrable to intracellular and extracellular water because polar water molecules cannot pass through the hydrophobic tails of the membrane o Only a few small nonpolar molecules such as oxygen can pass - Since the heads of the phospholipid molecules are polar, the cell membrane can also regulate salt concentrations within the cell o Salts are molecules that separate into 2 parts when dissolved in water  One part carrying a positive charge  Other part carries a negative charge • These charged particles are collectively called ions - Proteins embedded in the cell membrane provide one way for substances such as ions to cross the membrane o They act as gates and transportation systems that allow selected substances to pass through the membrane What Do Membrane Proteins Do? - 3 categories of membrane proteins that assist in transporting substances across the membrane o Channels – membrane proteins are shaped in a way that they create channels, or holes  Different prot
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