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BIOL 107 (27)


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BIOL 107
Kara Somerville

Introduction  The neuron solves the problem of conducting information over a distance by using electrical signals that sweep along the axon.  Axons act like telephone wires, however, the type of signal used by the neuron is constrained by the special environment of the nervous system  Electrical charge in the cytosol of the axon is carried by electrically charge atoms (ions) instead of free electrons o This makes the cytosol far less conductive o Also, the axon is not especially well insulated and is bathed in salty extracellular fluid, which conducts electricity  Fortunately, the axonal membrane has properties that enable it to conduct a special type of signal, action potential, that overcomes these biological constraints  Action potentials do not diminish over distance; they are signals of fixed size and duration o Information is encoded in the frequency of action potentials of individual neurons, as well as in the distribution and number of neurons firing action potentials in a given nerve  Cells capable of generating and conducting action potentials are said to have excitable membrane. Thus the “action” in action potentials occurs at the cell membrane  When a cell is not generating impulse, it is said to be at rest o In resting, the cytosol along the inside surface of the membrane has a negative electrical charge compared to the outside o This difference in electrical charge across the membrane is called the resting membrane potential  The action potential is simply a brief reversal of this condition and for an instant, the inside of the membrane becomes positively charge with respect to the outside The Cast of Chemicals  Cytosol and Extracellular Fluid o Water is the main ingredient of the fluid inside the neuron, the cytosol, and the extracellular o Electrically charged atoms are dissolved in this water  They are responsible for the resting and action potentials o Water  The most important property of the water molecule is its uneven distribution of electrical charge. Because of this property, the oxygen atoms in water acquire a net negative charge  Water is said to be a polar molecule, held together by polar covalent bonds o Ions  Atoms or molecules that have a net electrical charge are known as ions  Ions are held together by ionic bonds  Ions with a net positive charge are called cations  Ions with a negative charge are called anions  Ions are the major charge carriers involved in the conduction of electricity in biological systems  The Phospholipid Membrane o Substances with uneven electrical charges will dissolve in water  These substances are hydrophilic o Compounds whose atoms are bonded by non-polar covalent bonds have no basis for chemical interactions with water  Such compounds will not dissolve in water and are said to be hydrophobic  One familiar example are lipids which are important to the structure of cell membranes  The lipids of the
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