Bioelectrical phenomena Bioelectrical phenomena in nervous cellsin nervous cells
Measurement of the Measurement of the membranemembrane potentialpotential of of the nerve fiber usingthe nerve fiber using a microelectrodea microelectrode
Basic ConceptsForces that determine ionic movement
Electrostatic forcesOpposite charges attractIdentical charges repel
Concentration forcesDiffusion – movement of ions through
semipermeable membraneOsmosis – movement of water from region of
high concentration to low
BasicBasic mechanismsmechanisms of transport of transport
Gating of Gating of protein protein channelschannels provides a provides a means of means of controlling ion controlling ion permeability of permeability of thethe channels.channels.
I. Membrane Resting Potential
A constant potential difference across the resting cell membrane
Cell’s ability to fire an action potential is due to the cell’s ability to maintain the cellular resting potential at approximately –70 mV (-.07 volt)
The basic signaling properties of neurons are determined by changes in the resting potential
Concept of Resting Potential (RP)Concept of Resting Potential (RP) A potential difference across the cell A potential difference across the cell
membrane at the rest stage or when the membrane at the rest stage or when the cell is not stimulated.cell is not stimulated.
Property:Property: It is constant or stableIt is constant or stable It is negative inside relative to the outsideIt is negative inside relative to the outside Resting potentials are different in Resting potentials are different in
different cells.different cells.
Resting Membrane Potential
Na+ and Cl- are more concentrated outside the cell
K+ and organic anions (organic acids and proteins) are more concentrated inside.
Active TransportActive Transport
Movement of molecules and ions against Movement of molecules and ions against their concentration gradients.their concentration gradients.
– From lower to higher concentrations.From lower to higher concentrations. Requires ATP.Requires ATP. 2 Types of Active Transport:2 Types of Active Transport:
– PrimaryPrimary
– SecondarySecondary
Primary Active TransportPrimary Active Transport
ATP directly required ATP directly required for the function of the for the function of the carriers.carriers.
Molecule or ion binds Molecule or ion binds to carrier site.to carrier site.
Binding stimulates Binding stimulates phosphorylation phosphorylation (breakdown of ATP).(breakdown of ATP).
Conformational Conformational change moves change moves molecule to other molecule to other side of membrane.side of membrane.
P
Extracellular fluid1. Three sodium ions (Na+) and adenosine triphosphate (ATP) bind to the carrier protein.
2. The ATP breaks down to adenosine diphosphate (ADP) and a phosphate (P) and releases energy.3. The carrier protein changes shape, and the Na+ are transported across the membrane.
4. The Na+ diffuse away from the carrier protein.
5. Two potassium ions (K+) bind to the carrier protein.
6. The phosphate is released.
7. The carrier protein changes shape, transporting K+ across the membrane, and the K+ diffuse away from the carrier protein. The carrier protein can again bind to Na+ and ATP.
Cytoplasm Na+
ATP
K+
Breakdown of ATP(releases energy)
Carrier protein changesshape (requires energy) ADP
1
3
2
Carrier protein resumesoriginal shape
Na+
Na+ K+
P
K+
45
6
7
ATP binding site
Carrier protein
Intracellular vs extracellular ion concentrations
Ion Intracellular Extracellular
Na+ 5-15 mM 145 mMK+ 140 mM 5 mMMg2+ 0.5 mM 1-2 mMCa2+ 10-7 mM 1-2 mMH+ 10-7.2 M (pH 7.2) 10-7.4 M (pH 7.4)
Cl- 5-15 mM 110 mM
Resting Membrane Potential
Potassium ions, concentrated inside the cell tend to move outward down their concentration gradient through nongated potassium channels
But the relative excess of negative charge inside the membrane tend to push potassium ions out of the cell
Resting Membrane Potential
• But what about sodium?• Electrostatic and Chemical forces act together on
Na+ ions to drive them into the cell • The Na+ channel close during the resting state
Na+ is more concentrated outside than inside and therefore tends to flow into the cell down its concentration gradient
Na+ is driven into the cell by the electrical potential difference across the membrane.
Resting Potential
Postulated Postulated mechanismmechanism of the of the sodium-potassium pumpsodium-potassium pump
The formation of resting potential The formation of resting potential depends on:depends on:
Concentration difference of KConcentration difference of K++ across across the membranethe membrane
Permeability of NaPermeability of Na++ and K and K++ during the during the resting stateresting state
NaNa++-K-K++ pump pump
Factors that affect resting Factors that affect resting potentialpotential
Difference of KDifference of K++ ion concentration ion concentration across the membraneacross the membrane
Permeability of the membrane to NaPermeability of the membrane to Na++ and Kand K++..
Action of NaAction of Na++ pump pump
Basic Electrophysiological Basic Electrophysiological Terms I:Terms I:
PolarizationPolarization: a state in which membrane is : a state in which membrane is polarized at rest, negative inside and positive polarized at rest, negative inside and positive outside.outside.
DepolarizationDepolarization: the membrane potential : the membrane potential becomes less negative than the resting potential becomes less negative than the resting potential (close to zero). (close to zero).
HyperpolarizationHyperpolarization: the membrane potential is : the membrane potential is more negative than the resting level.more negative than the resting level.
Basic Electrophysiological Terms I:Basic Electrophysiological Terms I:
ReverspolarizationReverspolarization: a reversal of membrane : a reversal of membrane potential polarity. potential polarity.
The inside of a cell becomes positive relative to the The inside of a cell becomes positive relative to the outside.outside.
RepolarizationRepolarization: restoration of normal polarization : restoration of normal polarization state of membrane. state of membrane.
a process in which the membrane potential returns a process in which the membrane potential returns toward from depolarized level to the normal resting toward from depolarized level to the normal resting membrane value.membrane value.
Effect of stimuli of increasing Effect of stimuli of increasing voltages to elicit an voltages to elicit an actionaction potentialpotential
Typical action potentialTypical action potential
II Action Potential
Successive Stages:
(1) Resting Stage
(2) Depolarization stage
(3) Repolarization stage
(4) After-potential stage
(1)
(2) (3)
(4)
The Action PotentialThe Action Potential
Components/characteristicsComponents/characteristics
– RMPRMP
– Depolarizing stimulusDepolarizing stimulus
– ThresholdThreshold
– Rapid NaRapid Na++ entry entry (depolarization)(depolarization)
– IsopotentialIsopotential
– OvershootOvershoot
– Repolarization (KRepolarization (K++ moves out)moves out)
– Undershoot (after-Undershoot (after-hyperpolarization)hyperpolarization)
– Absolute refractory Absolute refractory periodperiod
– Relative refractory Relative refractory periodperiod
PropagationPropagation of of action action potentials in potentials in both directions both directions along a along a conductiveconductive fiberfiber
SaltatorySaltatory conductionconduction along a myelinated axon. along a myelinated axon. Flow of electricalFlow of electrical current from node to node is current from node to node is
illustrated by the arrows.illustrated by the arrows.
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