Chapter 4&5.pdf

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Zachariah Campbell

Chapter 4 Activity of NeuronsThe Neurons StructureNeurons are the information conducting units of the nervous system A neuron has many characteristics in common with other cells in the body but it also has special characteristics that allow it to send electrical impulses by using changes in chemical charges on its cell membrane Perhaps the most prominent distinguishing features are the dendrites whose presence greatly increases the cells surface area The dendrites surface area is further increased by many branches and by many small protrusions called dendritic spines that cover each branchA neuron may have from 1 to 20 dendrites each of which may have one or many branches and the spines on the branches may number in the many thousands Because dendrites collect information from other cells their surface areas determine how much information a neuron can gather Because the dendritic spines are the points of communication between neurons the many thousands of 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 hillock meanslittle hill Figure 41D The axon may have branches called axon collaterals which usually emerge from it at right angles Toward its end the axon may divide into a number of smaller branches called teleodendriaend branches At the end of each teleodendrion is a knob called an end foot or terminal buttonsee Figure 41B The terminal button sits very close to a dendritic spine on another neuron although it does not touch that spine see Figure 41C Thisalmost connection consisting of the surface of the axons end foot the corresponding surface of the neighbouring dendritic spine and the space between the two is the synapse The released chemical a neurotransmitter carries the message across the synapse to influence the electrical activity of the receiving cell or target to excite it or inhibit it and pass the message along The Cell As a FactoryThe cell is a miniature factory with departments that cooperate to make ship and export proteins including enzymes hormones and antibodies and they form the principa components of all cells as well A factory has outer walls that separate it from the rest of the world and discourage unwanted intruders a cells outer cell membrane separates it from its surroundings and allows it to regulate the materials that enter and leave its domain Un assisted very few substances can enter leave a cell because the cell membrane presents an almost impenetrable barrier Proteins embedded in the cell membrane serve as the factorys gates allowing some substances to leave or enter and denying passage to the restThe nucleus like executive office of a factory houses that blueprints genes and chromosomeswhere the cells proteins are stored and copied when needed copies are sent to the factory floor the part of the cell called the endoplasmic reticulumER The ER an extension of the nuclear membrane is where the cells protein products are assembled in accordance with the genes instructions The finished products are packed in a membrane and addressed in the Golgi bodies which then pass them along to the cells transportation network a system of tubules that carries the packaged proteins to their final destinations much like the factorys interior system of conveyor belts and forklifts Microfilaments constitute the cells structural framework microtubules contract and aid in the cells movements Two other components of the cellular factory are important for our consideration mitochondria are the cells power plants that supply its energy needs whereas lysosomes are saclike vesicles that not only transport incoming supplies but also move and store wastes Interestingly more lysosomes are found in old cells than in young ones Cells apparently have trouble disposing of their garbage just as we do The Cell Membrane Barrier and GatekeeperNeurons and glia are tightly packed together in the brain but like all cells they are separated and cushioned by extracellular fluid This fluid is composed mainly of water in which salts and many other chemical substances are dissolved Fluid is found inside a cell as well This intracellular fluid or cytoplasm also is made up mainly of water with dissolved salts and other chemicals 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 StructureThe cell membrane encases a cell and separates the intracellular from the extracellular fluid allowing the cell to function as an independent unit The special double layer structure of the membrane makes this separation possible The membrane bilayer also regulates the movement of substances into and out of the cell For example if too much water enters a cell the cell can burst and if too much water leaves the cell can shrivel The cell membrane helps ensure that neither happens The membrane bilayer is composed of a special kind of molecule called a phospholipid The name comes from the molecules structure which features ahead that contains the element phosphorusP and twotails that are lipid or fat molecules The head has a slight positive charge in one location and a slight negative charge in another The tails consist of hydrogen and carbon atoms bound tightly to one another making them electrically neutral
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