Physiology practical VI.
Action and membrane potentials 1906 – Bernstein: membrane theory i. Electrolyte concentration is higher inside the cell ii. Resting membrane is selectively permeabile for cations (+), so the membrane potential (-ve inside) is driven by concentration gradient. iii. During action potential, the membrane becomes temporaly premeabile for cations and anions, so they neutralise each other, and membrane potential becomes approx. zero
Resting potential Action potential Resting potential +ve excel
-ve intracel
membrane theory Membrane structures responsible for membrane potential
Ion channels
Size
Electric charge
Gates
Ions channels „sensors“
Voltage-gated
Second messenger-gated
Ligand-gated Potentials
Diffusion potential
Ion diffusion and permeable membrane
Magnitute depends on concentration gradient
Electrochemical equilibrium
Chemical and electrical forces are equal and opposite
100 55
10 55 100 100 10 10 Inside and outside concentration of main ions
Equilibrium Ion Extracellular Intracellular potential
Natrium 145 mM 10 mM 65 mV Na+
Potassium 4 mM 135 mM -85 mV K+
Calcium 2 mM 10-4 mM 120 mV Ca2+ Chloride 100 mM 5 mM -90 mV Cl- RT [K] [K] E ln o 0.0615log o K nF [K] 10 [K] Nernst equation: i i
RT PK [K]o PNa [Na]o PCl [Cl]i E ln F PK [K]i PNa [Na]i PCl [Cl]o Resting membrane potential
Potential difference across membrane of excitable cells
Between action potentials
Diffusion potentials responsible
Membrane is more permeable for Cl- & K+ at rest
Resting potential -70 - -80 mV, i.e. intra- is more negative than extra-cellular environment
Na+/K+ ATPase
3 Na+ out, 2 K+ in
Significance: K+ concentration gradient Action potential - terminology
Depolarization Process of making membrane potential interior less negative (positive) Hyperpolarization Process of making the membrane potential more negative Inward current The flow of positive charge into the cell Depolarization of membrane potential Outward current The flow of positive charge out of the cell Hyperpolarization of membrane potential Threshold potential Membrane potential at which occurrence of action potential is inevitable Overshoot Portion of action potential where the membrane potential is positive Undershoot (hyperpolarizing afterpotential) Portion of action potential where membrane potential is more negative than in rest Action potential
Basic mechanism for transmission of information
overshoot
hyperpolarization/ undershoot Action potential
hyperpolarization/undershoot Refractory phase
Cell sensitivity to other stimulation is decreased for some period of time
Two phases Absolute Complete cell loss of sensitivity to stimulus From the begining of the AP to the „end“ of repolarization
Relative Stronger than threshold stimulus will create action potential Frequency of Action potential
The number of APs that are created for a period of time
Subthreshold stimuls – Creates no AP Threshold stimulus - Creates AP Submaximal stimulus – Creates more AP Maximal stimulus – Creates maximal No of AP Supramaximal stimulus – Creates maximal No of AP Action potential: phenomenology Advantage of impulse conductance by AP Reliable • Tension treshold for activation • Once triggered, self-limiting
Self-propagating • Long distances travelled without signal loss
Fast • Impulse travels at speed of 120 m/s
Specific • Signal is transvered from one place to other without mixing Characteristics of AP
Stereotypical size and shape
Each AP identical for a cell type
Propagation
AP at one site causes depolarization at adjacent sites
Nondecremental
All or none response
Either does or does not occur The rheobase is defined as the lowest stimulus amplitude eliciting an action potential using long stimulus durations (stimulus duration is also called impulse width).
Chronaxy - stimulus duration at the point where the threshold amplitude is two times the Rheobase. Electrical stimuli with the duration of the Chronaxy are very efficient (at relatively low amplitudes) to elicit action potentials. If the stimulus duration is too short, the stimulus may not elicit an action potential at any amplitude (e.g. pain fibers are not excited with stimulus durations of 0.1 ms).
Practicals
Sumation of action potential and stimuli intensity
Chronaxy and rheobase
Absolute and relative refractory period
Hoorweeg-Weiss curve