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Chapter 3

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Kinesiology & Health Science
KINE 2011
Olivier Birot

Chapter 3 – The Central Nervous System Membrane Potential - The plasma membrane is polarized electrically (possesses a membrane potential) o Allows for cellular communication in nervous tissue and muscle Separation of Opposite Charges - Membrane potential: o A difference in the relative number of cations and anions in the ICF and the ECF  Opposite charges attract, like charges repel o A separation of charges across the membrane  Work is performed to separate opposite charges  When oppositely charged particles have been separated, the electrical force of attraction between them can be harnessed to perform work when the charges are permitted to come back together  Therefore separated charges have the potential to do work o Measured in millivolts (potential is too small to be measured in volts) o Determination of membrane potential by unequal distribution of + and – charges across the membrane:  When + and – charges are equally balanced across the membrane, no membrane potential exists  When opposite charges are separated across the membrane, a membrane potential exists  Unbalanced charges responsible for the potential accumulate in a thin layer along opposite surfaces of the membrane (along the ICF and ECF) o Account for a small fraction of the total number of charges particles; the vast majority of the fluid in the ICF and ECF is electrically neutral  *Membrane itself is not charged o Magnitude is proportional to the number of opposite charges separated Concentration and Permeability of Ions - All cells have a membrane potential - Excitable cells (nerve/muscle cells) can produce rapid and transient (temporary) changes in their resting potential when excited - Resting membrane potential – the constant membrane potential present in cells of nonexcitable tissues and those of excitable tissues at rest + + o Ions primarily responsible: sodium (Na ); p-tassium (K ); and large, negatively charged intercellular proteins (anions; A )  [Na ] is much higher in the ECF, [K ] is much higher in the ICF  Na concentration gradient will always be inward +  K concentration gradient w+ll alwa+s be outward  Electrical gradient for Na and K will always be to the negatively charged side of the membrane since both are cations  The amount of Na outside the cell and K inside the cell is equal -  A are only found inside the cell since the plasma membrane is impermeable to them + +  Na and K can passively cross the membrane through specific protein channels (no energy required)  Leak channels: are always open; allow for unregulated leakage of a chosen ion across the membrane o Are selective about what ions go through based on pore diameter  Gated channels: possess a gate that can be opened or closed depending on conformational changes of the proteins that form the channel o Voltage-gated channels – respond to changes in the membrane potential o Chemically-gated channels – respond to the binding of a specific chemical messenger on a specific membrane receptor in close association with the chemical o Mechanically-gated channels – respond to stretching/other mechanical deformations  Ex: baroreceptors detect changes in blood pressure o Thermally-gated channels – respond to changes in temp. o “Channel gating” – process of opening/closing the gate + + - Effect of +he Na -+ pump on membrane potential o Na and K are transported across the plasma membrane by:  Active transport (Na -K pump) –  Pumps 3 Na out for every 2 K pumped in o Both are positive so this generates a membrane potential with the outside becoming relatively more positive than the inside  Passive diffusion (conc. gradients) – + +  Na -K pump is indirectly involved in this process o Is essential in maintaining the concentration gradients directly responsible for the ion movements that generate most of the potential + - How K crosses the plasma membrane + o The concentration gradient for K tends to push this ion out of the cell o The outside of the cell becomes more + as the positively charged K ions move to the outside down their concentration gradient - o The membrane is impermeable to A . The inside of the cell becomes more – as the positively charges K ions move out, leaving behind the negatively charged A - o The
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