[ Pobierz całość w formacie PDF ]
.In the physiological normalsituation, action potentials act as stimuli, although their actual shape maydeviate considerably from the ideal rectangular waveform.Small research subject: Stimuli other than the electric type can elicit a response in excitabletissues.Find out which.Mention three more.Hint: Think in terms of types of energy.Didyou accidentally hit your elbow experimenting a nasty electric sensation? Explain.- Resting membrane potentialMost of the cells of excitable tissues are long and cylindrical in shapesurrounded by a membrane, about 100 Å thick (1 Å = 10 8 cm), whichseparates the intracellular fluid (ICF) and its contents from theextracellular fluid (ECF).When a very small diameter electrode (amicroelectrode, of about 1µm) connected to one of the inputs of a specialhigh impedance amplifier is introduced into the ICF puncturingthe thin cell membrane while the other amplifier terminal is hooked to areturn electrode immersed in the ECF, the recording instrument (a dccoupled oscilloscope) shows a displacement of the base line from the zerolevel to about 80 to 90 mV, assuming that the excitable cell (say,a skeletal muscle one) is alive.This is the resting membrane potentialor the stable electric state, E1, already introduced above.The internalside of the membrane is negative with respect to its external counterpart.This highly summarized description is an experimental fact thatcan be demonstrated in any laboratory of electrophysiology.TheECF contains a high concentration of sodium ions and a low level ofpotassium ions.Conversely, the ICF shows a low level of sodium and ahigh concentration of potassium.Both ions on both sides of the mem-brane are also accompanied by chloride ions.The membrane is relativelypermeable to all these charge carriers; however, it does not permit thepassage of large proteic anions, which abound within the cell.Besides, Chapter 2.Source: Physiological Systems and Levels 57the membrane is a good insulator constituted by oriented proteins andphospholipids, roughly containing one ion per 5,000 water molecules.ECF and ICF, instead, have in the order of one ion per 175 water mole-cules, meaning that these fluids are by far better electrical conductors.Study subject: The student should check in any physiology textbook the values reported forsodium, potassium, chloride and proteic ions in ECF and ICF, in nerve and skeletal musclecells.Via INTERNET, we suggest DEVELOPMENT OF TRANSMENBRANE RESTINGPOTENTIAL, by David L.Atkins, Professor of Biology at George Washington University,atkins@qwis2.circ.qwu.edu, 1998.Calculate also the electric field, in volts/meter, stressingthe cell membrane.Compare it with porcelain.Review also the fluid compartments.Noticethat the ECF faced by the excitable cell membrane is its interstitial fluid part IF, with noproteins.Plasma, instead, the other portion of ECF, contains a large amount of proteins andis exclusively restricted to the cardiovascular system.- Resting potential by the Ionic TheoryOn December 20, 1998, Sir Alan L.Hodgkin died in his home residence of Cambridge,England, at the age of 84.He was 1963 Nobelist in Physiology or Medicine for his analysisof the ionic basis of the action potential, former Master of Trinity College in Cambridge Uni-versity and President of the Royal Society.Co-recipients of the prize were also AndrewFielding Huxley and John Carew Eccles, the latter from Camberra, Australia.However,many others have also contributed to the ionic theory and a few of their names will be men-tioned here (such as Kenneth S.Cole and Bernard Katz).Some classical and enlighteningreferences for the interested student are Hodgkin (1964), Katz (1966), Cole (1982) andGardner (1992).Even though there are still a few who question the validity of the theory,most of the scientific community has accepted it and continues to work in its improvement.Figure 2.18 represents the cell membrane with two vertical lines.On theleft, there is the ICF, i.e., inside the cell, and on the right we have theECF, which coincides here with the interstitial fluid.The former is essen-tially a compartment high in positive potassium and negative proteic ionswhile the latter is characterized mainly by a high concentration of posi-tive sodium ions.Potassium and sodium cannot exist as such just bythemselves because they appear from electrolytic dissociation of chemi-cal compounds, in this case, of KCl and NaCl.Hence, both compart-ments also have their chloride ionic counterparts as the proteic anions inthe ICF have theirs, in such a way that, on both sides, the neutralityprinciple must be met, that is to say,Sum of all positive charges = sum of all negative chargesor 58 Understanding the Human MachineAlgebraic sum of all charges = zero(2.48)The first postulate of the Ionic Theory of Excitation states that, withinthe cell membrane, there is an active ionic pump that extrudes sodiumfrom the cell and carries potassium into it (Figure 2.18).For the time be-ing, let us assume that the ionic exchange is 1:1, or, for each sodium out,one potassium is brought in.It is active because the pump consumes en-ergy supplied by the cell itself.Its slowing down or inactivation tends tolevel concentrations off and, in the end, kills the cell.The amount of evi-dence collected in favor of the Na+ K+ pump over the years in countlesslaboratories all over the world is enormous and overwhelming, indeed;however, perhaps there has not been yet a final conclusive demonstra-tion, so offering one relatively weak side to those questioning the theory.Notice the semantic difference between evidence and demonstration.Infact, other ionic pumps have been described having become a widelySodium backdiffusionNaICFActive[K+]ionic ECF[A-]Pump[Na+]KPotassium backPK >> PNadiffusionEm+Figure 2.18.MEMBRANE RESTING POTENTIAL
[ Pobierz całość w formacie PDF ]