Physiology Coloring Book: Panels 29, 32, 33,

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1 ELEC4623/ELEC9734: Semester 2, 2009 Dr Stephen Redmond School of Electrical Engineering & Telecommunications Ph: Rm: 458, ELECENG (G17) Physiology Coloring Book: Panels 29, 32, 33, Session 2, 2009 ELEC4623/ELEC9734 1

2 Biomedical Instrumentation, Measurement and Design ELEC4623/ELEC9734 Lecture 2 The Origin of Biopotentials

3 Overview Background Physiology Excitable Cells Phospholipid molecule Cell membrane Energy gradients Passive and active transport Nernst equation Membrane potentials Nerve cell and impulses Ion channels Cell excitation ti Action potential Refractoriness Conduction and myelination Electrophysiology Autonomic nervous system Cardiac potentials Electrocardiogram Electroneurogram Electrooculogram Electromyogram Electroencephelogram Session 2, 2009 ELEC4623/ELEC9734 3

4 Measurement Modalities We have come a long way since the first physiological measurement ECG recording Session 2, 2009 ELEC4623/ELEC9734 4

5 Phospholipid molecule Polar portion (hydrophilic) Charged group (alcohols, phosphate, glycerol) Nonpolar portion (hydrophobic) Fatty acid chain Session 2, 2009 ELEC4623/ELEC9734 5

6 Phospholipids in water Polar head groups remain in water Nonpolar tails are excluded d Micelles fat absorption in liver Lipid id bilayer the cell membrane Session 2, 2009 ELEC4623/ELEC9734 6

7 Cell membrane Membrane proteins folded so polar parts exposed Receptors for hormones Catalyse specific chemical reactions Links between cells Some proteins traverse entire membrane Transport Session 2, 2009 ELEC4623/ELEC9734 7

8 Energy gradients Energy gradients are forces that generate movements Substances flow down energy gradients The steeper the free energy gradient (the greater the energy differences), the faster the flow (flux) Concentration gradient diffusion Osmotic gradient osmosis Voltage gradient ionic current Pressure gradient - bulk flow Session 2, 2009 ELEC4623/ELEC9734 8

9 Concentration gradient Solutes flow (diffuse) down concentration gradients Stops when concentrations in compartments are equal Process of transporting oxygen and nutrients from capillary blood vessels to tissue cells Session 2, 2009 ELEC4623/ELEC9734 9

10 Osmotic gradient Semipermeable membrane prevents solute from passing but allows water movement Water flows down free energy gradient toward the solute Osmotic flow can be prevented by applying a pressure Responsible for swelling/shrinkage of tissue Session 2, 2009 ELEC4623/ELEC

11 Voltage gradient Ions are solutes that carry electrical charge Ions of like charge repel and unlike attract Session 2, 2009 ELEC4623/ELEC

12 Transport Session 2, 2009 ELEC4623/ELEC

13 Passive transport Solutes move passively down concentration gradient Session 2, 2009 ELEC4623/ELEC

14 Active transport (against concentration gradient) (also co-transport and counter-transport) Energy from phosphorylation of ATP to ADP Session 2, 2009 ELEC4623/ELEC

15 Sodium potassium pump Na pumped out, K pumped in (both against concentration gradient) Energy from phosphorylation of ATP to ADP Provides osmotic stability Provides co-transport (glucose in gut cells) Provides voltage gradient (maintains low Na + concentration inside cell) Session 2, 2009 ELEC4623/ELEC

16 Membrane potentials A. Impermeable membrane separates K + and dcl - at tdifferent concentrations ti B. K + channels introduced into membrane (not Cl - channels though) K + diffuses from left to right down concentration gradient C. Voltage gradient grows until it is able to balance the concentration gradient. K + movement ceases and cell is at Equilibrium (Nernst) potential Session 2, 2009 ELEC4623/ELEC

17 Nernst equation Nernst equation: E ion = - RT ln [ion] i zf [ion] o R = universal gas constant (8.314 J mol -1 K -1 ), T = absolute temperature (in K), z = valence of ion (i.e. Cl - = -1), F = Faraday's constant (96500 C mol -1 valence -1 ) Applies when membrane is totally permeable to specific ion species alone At rest membrane tends to be permeable mainly to K + Membrane potential (V m ) is therefore negative and near E K Session 2, 2009 ELEC4623/ELEC

18 Goldman-Hodgkin-Katz equation V m = -RT ln (P K [K + ] o + P Na [Na + ] o + P Cl [Cl - ] i ) F (P K [K + ] i + P Na [Na + ] i + P Cl [Cl - ] o ) where P K : P Na : P Cl are the relative permeabilities of the ion species e.g. 1.0 : 0.01: 0.1 Session 2, 2009 ELEC4623/ELEC

19 Nerve cell and impulses Nerve cells have short dendritic processes extending from cell body, and a long cylindrical axon Axons transmit signals (nerve impulses to other nerve cells or to effector organs (muscles or glands) Impulses consist of a wave of electrical negativity (as measured on cell surface) that moves along axon Session 2, 2009 ELEC4623/ELEC

20 Nerve impulse Impulses are called action potentials (AP) To produce an AP, need a stimulus that brings cell voltage to a threshold i.e. depolarises membrane (makes voltage inside cell more positive, relative to outside) Occurs under a negative (cathodal) electrode Are all-or-none events once initiated they are always the same size The more impulses per second (higher frequency), the larger the signal Session 2, 2009 ELEC4623/ELEC

21 Ion channels Cell membrane contains separate channels for different ions Many channels contain voltage sensitive gates Na + channel also has a time dependent inactivation gate A. normal resting potential: leaky K + channel and Na-K pump p working B. depolarisation: fast Na + gate opens C. repolarisation: slow Na + gate closes and a slow K + gate opens Session 2, 2009 ELEC4623/ELEC

22 Refractory period Weak stimulus (1, 2) not enough Na + flows in to overcome outflow of K + (caused by the stimulus induced reduction in V m ). NB: as membrane depolarises the K + drive increases as we are further away from E K With stronger stimulus (3, 4, 5), this is overcome and Na + gates open, depolarising membrane more, causing more Na + gates to open Absolute refractory period, no stimulus can cause AP (Na + gates still closed) Relative refractory, can cause another AP but threshold is higher (as voltage sensitive K + gates still open) Session 2, 2009 ELEC4623/ELEC

23 Transmission of impulses Most axons encased in fatty, myelin sheath, broken at Nodes of Ranvier Conserves energy and result in faster conduction as impulse jumps from node to node (saltatory conduction) Without myelin a 1mm diameter nerve would need to be 38 mm to achieve same conduction speed Session 2, 2009 ELEC4623/ELEC

24 Transmission of impulses Current can flow through external medium Electrodes can be used to collect current Session 2, 2009 ELEC4623/ELEC

25 Cardiac electrophysiology Autonomic nervous system Cardiac muscle Anatomy and conduction Electrocardiogram Home telecare Session 2, 2009 ELEC4623/ELEC

26 Autonomic nervous system Session 2, 2009 ELEC4623/ELEC

27 Cardiac muscle Duration of cardiac action potential can be 100 times more prolonged than that of skeletal muscle Long refractory period Plateau sustained by slow Ca ++ entry and slow K + efflux Session 2, 2009 ELEC4623/ELEC

28 Anatomy and conduction Session 2, 2009 ELEC4623/ELEC

29 The electrocardiogram (ECG) Session 2, 2009 ELEC4623/ELEC

30 Some typical ECGs Normal sinus rhythm Tachycardia and bradycardia Heart block Arial fibrillation/flutter Heart block Premature ventricular contractions Ventricular fibrillation Asystole Pacing Session 2, 2009 ELEC4623/ELEC

31 Electroneurogram (ENG) The figure shows a spinal reflex generated by stimulating the posterior tibial nerve (a mixed nerve) Measure potentials in or near axons Can test propagation velocities and reflex arcs Later evoked response (H wave) is from spinal reflex As stimulus increases H wave decreases but M wave increases Session 2, 2009 ELEC4623/ELEC

32 Electromyogram (EMG) We can measure AP from a single motor unit (SMU) of a group of muscle fibres Or can measure it at the skin surface Session 2, 2009 ELEC4623/ELEC

33 Electrooculogram (EOG) There is a steady state potential difference between the cornea and retina The eye acts like a dipole This can be used to track the position/gaze of the eye Used in sleep science to determine rapid eye movement (REM) sleep phase This is achieved by placing electrodes above or lateral to the eye Session 2, 2009 ELEC4623/ELEC

34 Electroencephelogram (EEG) Different areas of the brain govern different functions Can measure single neurons invasively Or superposition of large groups from the scalp Session 2, 2009 ELEC4623/ELEC

35 Electroencephelogram (EEG) When measured on the scalp the EEG is seen to occupy bandsfrom01hzto30hz 0.1 (approx.) Four sub-bands bands have been arbitrarily defined Delta (<3.5 Hz) Theta (4-7 Hz) Alpha (8-13 Hz) Beta (14-30 Hz) Session 2, 2009 ELEC4623/ELEC