2 1 Electrochemical Gradients We ll start learning about animal physiology with neurons and muscles as these two types of cells are found only in animals and they work together to form the foundation of all physiological systems Nerves are bundles of specialized cells called neurons These can vary widely in structure and thus can have highly specialized functions and location in the body as you can see in the figure below but they all communicate with other cells in the same basic way This mode of communication is what makes neurons unique cells and allows animals to move quickly and to coordinate their movements Neurons also have the same basic parts or neural anatomy These are labeled in the cartoon of a stereotypical neuron below beginning with the dendrites Remember that all cells are surrounded by a phospholipid bilayer membrane and filled with an aqueous cytoplasm but their underlying cytoskeletons can confer a variety of cellular shapes structures and movements Neuron cytoskeletons and membranes have extensive projections that are specialized to receive signals from other cells then send that signal at remarkable speeds up to 100 m sec often over long distances The dendrites Greek tree of the neuron receive signals usually from many other neurons These signals travel to the soma Greek body then are summed up at the axon hillock From there they travel down the axon to the axon terminal where the signal gets transmitted to the neuron s target Notice that the nucleus of the neuron is located in the soma so DNA replication and transcription occur in the soma of the neuron How do neurons send signals that can travel 100 meters in a single second Their cytoskeletal structures do not fully explain this remarkable adaptation of signal conduction speed They are not pushing chemical signals like hormones or proteins through the fluid filled tunnels of their axons at 100 meters per second Neurons do not use chemical signals to reach these speeds they use electrical signals All cells have an electrical charge an imbalance of positive and negative ions between their inside and outside Similarly recall that all cells maintain an imbalance of protons inside their mitochondrial matrix and just outside it in the intermembrane space this gradient forms the proton motive force that drives ATP synthase at the end of the electron transport chain of cellular respiration Electrical gradients have potential energy or the ability to do work The mitochondria stores this potential energy as chemical energy in the phosphate bonds of ATP Neurons on the other hand convert this potential energy into kinetic energy Neurons have specialized membrane properties and proteins that allow a local change in electrical charge to spread from one end of the neuron to the other At rest when the neuron is not sending or receiving signals the inside of a neuron is approximately 70 millivolts mV more negative then the extracellular fluid This difference in charge has potential energy to do work thus we say that the membrane potential at rest resting membrane potential is 70 mV Voltage or a separation in charge may be used synonymously with membrane potential in this context The voltage of the neuron membrane at rest is approximately 70 mV It is conventional that membrane potential is the inside compared to the outside not the other way around There are many large impermeable molecules inside the cell that are negatively charged such as proteins and DNA but there are also lots of ions that move back and forth across the membrane It is the movement of these permeable solutes that we are most concerned with If the membrane is freely permeable to a solute it will randomly diffuse back and forth The net movement will be from areas of high to low concentration However recall that charged particles cannot diffuse freely across the membrane without a protein channel By putting channels specific to particular ions in its membrane the cell is able to control its membrane potential Further it can use ATP to pump ions against their gradients active transport actively generating an electrical potential The passive movement of charged particles across the membrane assuming it is permeable to that particle is determined by its electrochemical gradient This is a combination of its chemical gradient concentration and electrical gradient charge on either side of the membrane For any given particle these two gradients could be in the same direction or in opposing directions The net direction that a charged particle will flow is a balance of these two forces Follow through the steps below to see how an electrical gradient can be generated If this creates an imbalance of charge across the membrane it will also result in a voltage or membrane potential In the example above the cell ended up with a slight positive charge outside relative to inside There is a slight electrical gradient that will drive cations into the cell until it reaches electrical equilibrium Conversely anions will be attracted to the outside of the cell by the same electrical gradient You can think of this in terms of high to low concentration or opposites attract Walk through the example below which uses a real cation K a hypothetical anion A and K channels in a membrane to illustrate how a chemical gradient can result in an electrical gradient where it is more positive outside than inside At the end enough K had exited the cell to make it more positive outside than inside If this electrical gradient were the only force acting on K this cation would diffuse inward However it is not There is still more K inside the cell than out so there is still a chemical gradient driving it outward At some point these two forces balance each other out The voltage at which this equilibrium state occurs is called the equilibrium potential Keq We can calculate exactly what membrane voltage Vm will result in no net movement for any given ion as long as we have the charge and starting internal and external concentrations of that ion This membrane voltage is the Keq for that ion at those starting concentrations This is given by the Nernst equation Vm RT zF ln Xout Xin R universal gas constant T Absolute Temperature Kelvin z charge of the ion F Faraday s constant Xout ion concentration outside the cell Xin ion concentration inside the cell If we assume 37 C this simplifies to Vm 61 mV z log Xout Xin You should memorize this simplified version of the Nernst equation and know how to use it
View Full Document
Unlocking...