Chapter 3

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Department
Neuroscience
Course
NROB60H3
Professor
Janelle Leboutillier
Semester
Fall

Description
The Neuronal Membrane at Rest o Interneurons -- these neurons connect with the parts of your brain that interpret the signals as being painful. Others connect to the motor neurons that control the leg muscles that withdraw your foot Requires the nervous system to collect, distribute, and integrate information Neurons solve the problems of conducting information over a distance by using electrical signals that sweep along the axon. In this sense, axons act like a telephone wire Electrical charge in the cytosol of the axon is carried by electrically charged atoms (ions) instead of free electrons. This makes cytosol far less conductive than copper wires. Also, the axon is not especially well insulated and is bathed in salty extracellular fluid, which conducts electricity Electrical current passively conducting down the axon would not go very far before it would leak out o The axonal membrane has properties that enable it to conduct special type of signal the nerve impulse, or action potential overcomes these biological constraints Action potentials do not diminish over distance; they are signals of fixed size and duration 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, which include both nerve and muscle cells, are said to have excitable membrane. The action in action potentials occurs at the cell membrane When a cell with excitable membrane is not generating impulses, it is said to be at rest. In resting neuron, the cytosol along the inside surface of the membrane has a negative electrical charge compared to the outside. This difference in electrical charge across the membrane is called the resting membrane potential or resting potential Action potential is simply a brief reversal of this condition, and for an instant about a thousandth of a second the inside of the membrane becomes positively charged with respect to the outside o The resting membrane potential: three main players: 1. The salty fluids on either side of the membrane 2. The membrane itself 3. The proteins that span the membrane Each has certain properties that contribute to establishing the resting potential o Water is the main ingredient of the fluid inside the neuron, the intercellular fluid or cytosol, and the fluid that bathes the neuron, the extracellular fluid. Electrically charged atoms ionsare dissolved in this water, and they are responsible for the resting and action potentials o Most important water molecule property is its uneven distribution of electrical charge. Two hydrogen atoms and the oxygen atom are bonded together covalently, which means they share electrons. The oxygen atom, however, has a greater affinity for electrons than does the hydrogen atoms. As a result, the shared electrons will spend more time associated with the oxygen atom. Therefore, the oxygen atom acquires a net negative charge (because it has extra electrons), and the hydrogen atoms acquire a new positive charge. Thus water is said to be a polar molecule, held together by polar covalent bonds Makes water an effective solvent of other charged or polar molecules; that is, other polar molecules tend to dissolve in water o Atoms or molecules that have a net electrical charge are known as ions. Table salt is a crystal of sodium (Na+) and chloride (Cl-) ions held together by the electrical attraction of oppositely charged atoms This attraction is called an ionic bond Salt dissolves readily in water because the charged portions of the water molecule have a stronger attraction for the ions than they have for each other. As each ion breaks away from the crystal, it is surrounded by a sphere of water molecules Each positively charged ion (Na+) will be covered by water molecules oriented so that the oxygen atom (negative pole) will be facing the ion. Likewise, each negatively charged ions (Cl-) will be surrounded by the hydrogen atoms of the water molecules. These clouds of water that surround each ion are called sphere of hydration, and they effectively insulate ions from one another The electrical charge of an atom depends on the difference between the number of protons and electrons. When the difference is 1, the ion is said to be monovalent (sodium, chloride and potassium); when the difference is 2, the ion is divalent (calcium); Ions with a net positive charge are called cations ions with a negative charge are called anions o Substances with uneven electrical charges will dissolve in water because of the polarity of the water molecule. These substances, including ions and polar molecules, are said to be water-loving or hydrophilic. Compounds whose atoms are bonded by nonpolar covalent bonds have no basis for chemical interactions with water. A nonpolar covalent bond occurs when the shared electrons are distributed evenly in the molecule so that no portion acquires a net electrical charge Such compounds will not dissolve in water and are said to be water-fearing or hydrophobic An example of hydrophobic substance is olive oil, and lipids which are a class of water-insoluble biological molecules important to the structure of cell membranes Lipids of neuronal membrane contribute to the resting and action potentials by forming a barrier to water- soluble ions and, indeed, to water itself o The main chemical building blocks of cells membranes are phospholipids. Like other lipids, phospholipids contain long nonpolar chains of carbon atoms bonded to hydrogen atoms. A phospholipid has a polar phosphate group attached to one end of the molecule. Thus, phospholipids are said to have a polar head (containing phosphate) that is hydrophilic, and a nonpolar tail (containing carbon) that is hydrophobic The hydrophilic heads face the outer and inner watery environments and the hydrophobic tails face each other. This stable arrangement is called a phospholipid bilayer, isolates the cytosol of the neuron from the extracellular fl
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