CHAPTER 4
Contents • Introduction: Bioelectric Phenomena • Resting Potential • Action Potential • Examples Of Bioelectric Phenomena
Introduction
Monitoring signals, convey useful information abaout the functions they represent.
These signals are the bioelectric potentials associated with nerve conduction, brain activity, heartbeat, muscle activity and etc.
Bioelectric potentials – ionic voltages produces as a result of electrochemical activity of certain special types of cells.
The use of transducers capable of converting ionic potentials into electrical voltages, these monitoring signals can be measured and results displayed in a meaningful way to aid the physician in diagnosis and treatment of various diseases and illnesses.
The study of bio potentials is FUNDAMENTAL to the understanding of Medical Instrumentation.
Origin of Bio potentials
Differences in Amplitude and Spectrum of Various Bio potentials
Resting and Action Potentials ď ˝
The single cell is the unit from which living systems are built.
ď ˝
Its complexity is illustrated by the fact that within its membrane hundreds of chemical reactions, take place, many of which are not understood. V Electrode
Interstitial fluid Cell membrane
ď ˝
Certain types of cells within the body, such as nerve and muscle cells, are encased in a semi permeable membrane that permits some substances to pass through the membrane while others are kept out.
ď ˝
Neither the exact structure of the membrane nor the mechanism by which its permeability controlled is known, but the substances involved have been identified by experimentation.
Resting Potential
Resting Potential in Nerve Cell
Resting Potential Propagation
Electrical Activity of Excitable Cells
Nervous, muscular, or glandular tissue
Resting potential
Electric potential difference between its interior and exterior
Steady value -50 to -100 mV
Cell membrane is 7 - 15 nm thick (lipoprotein complex)
Slightly permeable Na+
Freely permeable to K+ and Cl–
At Rest (Resting Membrane Potential): Potassium gradient moves positive charge from interior to exterior. Membrane acts like a leaky capacitor. An electric field across the capacitor inhibits the outward flow of positive charge. Appendix A.1 (p. 659) An equilibrium potential is established. R = 8.31 J/(mol•K) Gas Constant F = 96500 C/equiv. Faraday’s Constant Resting membrane is effectively a potassium membrane. Nernst Equation:
K 0 RT K 0 E ln 0.0615 log 10 F K i K i
Resting Membrane Potential of Excitable Cells Goldman, Hodgkin, Katz (more accurate):
RT PK K 0 PNa Na0 PCl Cli E ln F PK K i PNa Nai PCl Cl0 PM is the permeability coefficient for ion M [M] is the concentration of ion M in moles/liter
Ion gradients (frog skeletal muscle)
Resting Membrane Potential of Excitable Cells
PNa = 2E-8 cm/s [Na+] 145 mM/l
[Na+] 12 mM/l
PCl = 4E-6 cm/s [Cl–]120 mM/l
[Cl–] 4 mM/l
[K+] 155 mM/l
[K+] 4 mM/l PK = 2E-6 cm/s
Example 4.1 ď ˝
ď ˝
Q: A frog skeletal muscle has the following ion concentrations and permeabilities of the membrane: Ion
Inside (mmol/liter)
Outside (mmol/liter)
Na+
11
146
K+
150
4.35
CL-
5
125
Permeability (cm/s)
Compute the membrane voltage from inside to ouside the cell at 37oC.
Action Potential
Action Potential Propagation
Waveform showing Depolarization & Repolarization in Action Potential
Refractory periods
Absolute refractory period – brief period of time during which the cell cannot respond to any new stimulus. Lasts about 1 ms in nerve cells.
Relative refractory period – following the absolute refractory period, during which another action potential can be triggered, but a much stronger stimulation is required. In nerve cells, it lasts several milliseconds.
This refractory periods are believed to be the result of after-potentials that follow an action potential.
Propagation of Potentials in Nerve Impulse
Electrical Activity of Excitable Cells
End of Chapter 4