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PS363 (29)


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Steve Risavy

 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 neuronal membrane contribute to the resting and action potentials by forming a barrier to water-soluble ions and in water itself o The Phospholipid Bilayer  The main chemical building blocks of cell membranes are phospholipids  A phospholipid has a polar phosphate group attached to one end of the molecule  Thus phospholipids are said to have a polar “head” that is hydrophilic and a non-polar “tail” that is hydrophobic  The neuronal membrane consists of a sheet of phospholipids, two molecules thick  Hydrophilic heads face each other  This stable arrangement is called a phospholipid Bilayer and it effectively isolates the cytosol of the neuron from the extracellular fluid  Protein o The type and distribution of protein molecules distinguish neurons from other types of cells (e.g. enzymes, cytoskeleton, and receptors) o Protein Stucture  All amino acids have a central carbon atom (alpha carbon) which is covalently bonded to four molecular groups – a hydrogen, carboxyl and amino groups and also a variable R group  The differences between amino acids result from differences in the size and nature of these R groups  The properties of the R groups determine the chemical relationships in which each amino acid can take part in  Amino acids assemble into a chain connected by peptide bonds, which join the amino group of one amino acid to the carboxyl group of the next  The four levels of protein structure are:  Primary: a chain in which the amino acids are linked together by peptide bonds  Secondary: the linear chain may coil up into an alpha helix  Tertiary: Interactions among the R groups can cause the molecule to change its 3D conformation even further and assume a globular shape  Quaternary: different polypeptides chains can bond together to form a larger molecule o Each polypeptide contributing to the protein’s structure is called a subunit Channel Proteins  Ion channels are made from just these sorts of membrane-spanning protein molecules.  Typically, a functional channel across the membrane requires that 4-6 similar protein molecules assemble to form a pore between them  One important property of most ion channels, specified by the diameter of the pore and the nature of the R groups lining it, is ion selectivity.  Another important property is gating. o Channels can be opened and closed by changes in the local microenvironment of the membrane o Ion Pumps  Other membrane-spanning proteins come together to form ion pumps  Ion pumps are enzymes that use energy released by the breakdown of ATP to transport certain ions across the membrane The Movement of Ions  Ionic movements through channels are influenced by two factors: diffusion and electricity  Diffusion o Ions and molecules dissolved in water are in constant motion. o This temperature-dependent, random movement will tend to distribute the ions evenly throughout the solution o There will be a net movement of ions from regions of high concentration to regions of low concentration  diffusion o Diffusion will cause ions to be pushed through channels in the membrane  For example, NaCl is dissolved in fluid on one side of a permeable membrane  The Na and Cl ions will cross until they are evenly distributed in the solutions on both sides  The difference in concentration on both sides is called a concentration gradient  Thus, it is said that ions flow down a concentration gradient o Driving ions across the membrane by diffusion happens when: 1) The membrane possess channels permeable to the ions, and 2) There is a concentration gradient across the membrane  Electricity o Another way to induce a net movement of ions in a solution is to use an electrical field o Since opposite charges attract and like charges repel, there will be a net
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