Properties of the living organism. Interaction between living organism and the environment. Processing informations. Definitions

Transcription

1 thermodynamics material energy Interaction between living organism and the environment Open system: free material and energy exchange. Processing informations information processing answer Properties of the living organism Separation from the enviroment: Changing in the enviroment: accomodation Strictly controlled energy and material transport. Condition: information from the environment, right and fast processing and adequate response. Definitions stimulus: any effects on the organism (signal and noise) outer stimulus from the enviroment (e.g. light, sound etc.) inner stimulus: from the organism (glucose concentration, ph of the blood etc.) 1

2 Simple responses in plants Fast motion in plants phototropism geotropism Sensitive plant Simple responses in animals Animals and the human beings insulin production Motion (requires fast processes) Chemical system : hormones melanin production More complex and faster system: nerves and muscles 2

3 Resting potential Membranes in the cell cell Role of the cell : separation and controlled interaction to the enviroment Inner s: formation of intracellular spaces (compartments). Several basic processes take place on the. observation In resting state about -30 és -90 mv voltage may be measured between the extraand intracellular space. electrodes extracellular space Diffusion of the ions Typical ion distributions The ion concentrations are different on the two sides of the. observation Intracellular space (mm/l) Intracellular space Intensive quantity: chemical potential Diffusion of neutral particles. In the case of charged particles the electric work must be taken into the consideration! Intensive quantity: electrochemical potential Extracellular space(mm/l) z: no. of charges F: Faraday constant. : electric potential Na+ K+ Cl- Squid giant axon Frog muscle Rat muscle Na+ K+ Cl- Squid giant axon equilibrium: 3.8 Frog muscle Rat muscle e 2e e zf Nernst-equation RT c1 ln zf c2 3

4 Diffusion through the Donnan-equilibrium Initial conditions: Use the permeability constant as characteristic quantity! initial state p = D/d - There are non permeable ions. Electric neutrality both sides (the sum of charges is zero) D diffusion constant d thickness of the + c(1) c(2) 0 mobile ions (permeable ), final state equilibrium. equilibrium: Right solution? c(1) = c(2) = 0!!! electric bilayer Ratio of concentrations (extracell./intracell.) Ion Na+ K+ Cl- squid frog rat Calculated potentials on the basis of Nernst-equation for different ions and the measured potential (mv) -potential (meas.) Na+ K+ Cl- Squid giant axon Frog muscle Rat muscle Significant differences between the measured and calculated values! Main difference in the case of Na+. 4

5 Typical values for the heart Calculated potential ion Extracell. space (mm) Intracell. space (mm) ratio (extra/intra) Na K Cl ion Na + K + Cl - Ca 2+ Membrane potential (mv) (37-85) Ca Donnan-equilibrium In the case of the phenomenon described by Donnan constant potential difference may be observed between two sides of the. There are mobile and immobile ions. In the case of equilibrium the electrochemical potential is same. Conclusion On the basis of the measured values there is no Donnan-equilibrium between two sides of the. (The concentration difference of the Na + is too high for example!) The biological system is not in equilibrium! Passive process (diffusion) may change the state to the equilibrium. Active (energy consumption) processes are necessary to keep steady state. 5

6 The role of active transport Charge and material transport exist, the concentration were not constant, e.g. slow inflow of Na+ into the cell. Different, energy consumer mechanisms, socolled pumps ensure the steady state. Na-K pump 3 Na+ ion and 2 K+ exchange extracellular space requires ATP! citoplasm extracellular space (e.g. Na+-K+ pump, Na+-Ca++ etc.) citoplasm Ion flow in the neutral particles J p c J flux p permeability constant c- concentration gradient The basis of the transport-model (single) charged particles F J p c c RT J flux p permeability constant c- concentration gradient F Faraday constant T temperature potential difference R - gas constant The is in rest but there is no equilibrium between two sides. The potential is constant the net ion flow through the is zero. The potential gradient in the is constant d /dx = const. 6

7 Goldman-Hodgkin-Katz (GHK) potential equation condition of steady state: (the net flux is zero) J k 0 k e i p NacNa pk cke pcl ccl RT ln i e F p NacNa pk cki pcl ccl p permeability constant of an ion e extracellular space i intracellular space Electric model of the intracellular space extracellular space The model describing the resting potential and the ion current: Simplified GHK equation (p = relative permeability constant, compared to the K+) e c Ke RT pc Na ln i F pc Na c Ki p (calc.) (mv) (meas.) (mv) Squid giant axon 0, Frog muscle 0, According to the main ions model for resting potential C represents the capacity, R - characterizes the resistance against the flow of the given ion, E voltage source representing the potential 7

8 Changing of the potential The definition of the stimulus: changing of the potential transmits the information. Changing the potential experiment stimulus detection Changing of the resting potential is due to the specific ion flow though the. Depolarization, hyperpolarization Depolarization (example) stimulus hair cells in the ear: response hyperpolarization depolarization Mechanical effect depolarization. exponential increasing and decreasing. 8

9 Hyperpolarization Synapse (example) rod in dark rods in the eye: photochemical effect results the hyperpolarization of the. rod in light vesicules synaptic space postsynaptic presynaptic nerve cell dark current activated transducin closed ion channel A possible mechanism: the released acetyl choline bonding to the receptor opens an ion-channel. Propagation of the changing along the Extension of the electric model: cable model extracellular space place of the local changing exponential decreasing intracellular space R e - longitudinal resistance of the extracellular space. R i - longitudinal resistance of the intracellular space. These elements connect to each other the different parts of the. 9

10 on the basis of the propagation of the changing along the : in relative unit on the basis of the exponential answer of the : (responses according to the distance of the place of the stimulus) m Rm Cm Electric properties: space constant (responses according to the time) Rm Ri Re Rm Ri in relative unit Electric properties: time constant the distance, where the changing decreases by factor e. the time, while the changing decreases or increases by factor e. Propagation of the depolarization Ri Re Processes in nerves and muscles rm (W cm2) ri (W cm2) (ms) diameter ( m) (cm) Squid nerve , ,5 Crawfish nerve ,25 Frog muscle ,2 Both the time constant and the space constant depend on the diameter. The value of the space constant shows that these are local phenomena they are not able to propagate too far. T=0 depolarization threshold a depolarization below the threshold (local response) b depolarization below the threshold (local response) c depolarization above the threshold - action potential 10

11 Ion flow during action potential Action potential 1 voltage sensitive Na+channels the inflow of the Na+is fast at the beginning according to the non-equilibrium state. 2 - voltage sensitive K+-channels 1 g = (1/r) 2 K+ channel conductivity Properties of the action potential depolarization repolarization depolariza tion threshold The first step is fast! (slow, long process is not suitable for fast response.) resting potential negative pot. positive 11

12 Simple calculation Why is it fast? ratio of ion concentration (extra/intracellular space) ion Na+ K+ U (mv) Squid Frog 6.0 0, rat Debye-length and diffusion the ion concentration close to the Let the radius of a cell 20 m! The volume is: ~ l. amount of the K+ : ~ mol. surface of the cell: ~ cm2. capacity of the : ~ F. (specific capacity: ~1 F/cm2) on the basis of resting potential: ~ C ~ mol ion. The changing affects only the small environment of the and transports a small amount of ions. Electrochemical potential (rat muscle) Speed of the diffusion d 3Dt example: D ~ 10-9 m 2/s, t = 0,1 ms d ~ nm (Compare d, the average distance, to the Debye length!) The diffusion transports the ions far from the. e RT ln Na+ c2 zf c1 ena ln ek ln K+ ~ kj/mol ~ kj/mol In rest there is a large thermodynamic force for Na+! K+ ek ln ~ kj/mol After reversing the polarity this force is high for K+! 12

13 Comparison Modified electric model If were equilibrium. extracellular space (Donnan-equilibrium) Large force acts on Naions. Fast passive inflow. No energy consumption. Changing of the potential result the outflow of the K+. Changing of the potential: requires energy and was slower! The trans rseistance is represented by variable resistors, that makes possiblechanging the speed of the ion flow. Propagation of the action potential (AP) t time difference exponential decreasing intracellular space at x the local changing is enough large to produce a new ap. depolarization threshold speed ~ x/ t Advantage Shape is independent from the stimulus: not sensitive to the external effects, noises. It propagates far without any attenuation. Such fast process makes possible fast responses. 13

14 Speed of the propagation Saltatoric propagation space constant depends on: diameter, R m, R i t time difference x larger! Larger speed. r m (W cm 2 ) (ms) diameter ( m) (cm) depolarization threshold (due to the myelin R m is large) Squid Crawfish frog myelin sheath nodes of Ranvier Role of the myelin sheath Speed large space constant: about m/s R m very large, space constant is large too At the nodes of Ranvier: R m ~ 50 W cm 2 about 10 4 Na + -channel/ m cat n. saphaneus the time that is necessary to cover 6 cm 14

15 Speed of the propagation Refracter state fiber diameter ( m) Speed (m/s) a b g d < No sheath < Rm R R i e R R m increasing diameter increasing R m and decreasing R i. i absolute relative absolute: Na-channels are opened, there is no new AP. relative: only larger stimulus is able to produce new AP. Role of the refracter state When is it not true initiated at the end refracter state propagation direction of propagation initiated at the center the refracter state prevents the back propagation of the ap. propagation 15

16 Rectification: synapse synaptic space vesiclues presynaptic nerve postsynaptic neuro-transmitters emitted by the vesicules depolarize the postsynaptic and result action potential after the synapse. The structure makes impossible the back propagation. Unidirectional step! Electric synapse 2-3 nm bidirectional, no rectification. konnexonfehérjék proteins More characteristics for the invertebrates. man: e.g. heart muscle. Not a whitdrawal? remember: speed of the diffusion d 3Dt the diffusion is very fast if the distance is small! the size of the synaptic space is about a few 10 nm! the delay is not more than a few hundred s! Conclusion Developed a fast system based on electric phenomena of the. The charges are ions, so this system is slower than equipment s used by us. The stimulus (signal) is able to propagate far without any attenuation. 16

17 Electric signals on the body surface Source Diagnostics Genesis dipole moment: Electrocardiography (ECG) d = ql (vector quantity) q charge l distance between charges d dipole moment Elementary dipole moments are summed. Electroencephalography (EEG) analogy: geographic map Electromyography (EMG) Electroretinography(ERG) Measurement and its problems electrodes potential difference = voltage Problems: Source is an extended, 3D object. Measured on the nody surface. Noise. 17