Untitled Quiz

Action Potential

• Lecture delivered by Prof. Sahar El Agaty, Professor of Physiology
• Part of a Biophysics-Physiology course, lecture 5
• Intended learning outcomes include discussing resting membrane potential, demonstrating the ionic basis of action potential, and differentiating between action potential and local potential

Membrane Potential

• Membrane potential is the difference in the relative number of cations and anions in the intracellular fluid (ICF) and extracellular fluid (ECF)
• Inside of the cell has excess negative charge (blue circles) and outside has excess positive charge (red circles) at rest.
• Magnitude depends on the number of opposite charges separated
• Greater the number of charges, larger the potential

Membrane Potential

• (a) Membrane has no potential: Equal separated charges, electrically neutral on both sides
• (b) Membrane has potential: Separated charges forming a layer along plasma membrane, unequal charges on both sides
• (c) Separated charges responsible for potential
• (d) Separated charges forming a layer along plasma membrane

Membrane potential

• The magnitude of the potential depends on the number of opposite charges separated.
• The greater the number of separated charges, the larger the potential.
• In the image, membrane B has more potential than A and less potential than C.

Membrane Potential and Excitable Tissues

• All cells have resting membrane potential produced by accumulation of positive charges outside and negative charges inside the cell membrane.
• Nerve and muscle cells are excitable tissues
• Respond to stimuli by producing rapid, transient changes in membrane potential (action potential)
• These brief fluctuations act as electrical signals

Membrane Potential and Excitable Tissues

• Electric events are rapid (measured in milliseconds).
• Small (measured in millivolts).
• Can be measured using 2 electrodes: one outside and one inside (monophasic action potential)
• Two electrodes on the outside (biphasic action potential)

Properties of Excitable Tissues

• They have high transmembrane potential.
• Their cell membranes contain ion channels (voltage-gated e.g., Na+, K+, and Ca2+).
• Also ligand-gated channels
• Passage of ions in and out produces electric response upon stimulation.

Resting Membrane Potential (RMP)

• Voltage difference across cell membrane of excitable tissues (nerve and muscles) during resting state.
• Inside of the membrane is more negative than outside
• Neurons: typically around -70 mV.
• Ions directly responsible for generating RMP: Na+ and K+
• Presence of large, negatively charged (anionic) intracellular proteins is also important
• Other ions (calcium, magnesium, chloride) don't significantly contribute

Causes of Resting Membrane Potential (RMP)

• Passive forces
  • Because of high intracellular levels of K+,
  • Concentration gradient of K+ ions favors K+ passive movement out of the cell (efflux)
  • High extracellular level of Na+ favors passive movement of Na+ into the cell (influx)
• Active force
  • Na+-K+ pump actively pumps leaked Na+ out and leaked K+ in
  • Transfers 3 Na+ ions out and 2 K+ ions in
  • Maintains negativity of RMP

Causes of Resting Membrane Potential (RMP)

• Permeability to K+ (50-70 times greater than Na+)
• Permeability to intracellular proteins (negatively charged proteins trapped inside the cell)

Action Potential of a Neuron

• Transient reversal of membrane polarity produced by stimulation of excitable tissues.
• Threshold stimulus triggers a reduction in membrane negativity.
• Recorded using galvanometer and amplifier via 2 microelectrodes.
• (Monophasic) Action potential recorded.
• Change in behavior of Na+ and K+ channels leads to a change in conductance

Phases of Action Potential

• Latent period: time interval between stimulus application and start of depolarization, time required for stimulus conduction to the recording electrode
• Depolarization phase: Na+ influx, reverses membrane polarity, triggers opening of more voltage gated Na + channels. Sets up positive feed-back loop of increasing depolarization. Membrane potential reaches +35 mv, repolarizing relative to exterior (limits Na+ influx)
• Repolarization phase: Na+ channels close, K+ channels open, K+ efflux, membrane potential returns to resting state (RMP), a period of after-hyperpolarization follows.

Excitability and Refractory Periods

• Excitability: Ability to respond to stimulus, altered during action potential
• Absolute refractory period: Zero excitability, lasts through depolarization and most of repolarization.
• Relative refractory period: Partial recovery of excitability, lasts through rest of repolarization. Nerve can respond, but a stronger stimulus is needed

Action Potential Propagation

• Unmyelinated nerve: Local current flow, positive charges flow into the negative area, depolarizing adjacent areas sequentially. Repolarization follows, propagation is in one direction.
• Myelinated nerve (Saltatory Conduction): Positive charge jumps between nodes of Ranvier due to insulation of myelin, faster than unmyelinated conduction

Characteristics of Action Potential

• Followed by ARP and RRP, thus not summated
• A subthreshold stimulus produces no action potential, while a threshold stimulus creates a maximal action potential, and a suprathreshold stimulus produces the same action potential.
• Action potentials are propagated along the nerve at a constant intensity.

Local Potential (Graded Potential)

• Subthreshold stimulus produces a localized depolarizing potential change, local potential, rising and fading with time.
• It is non-propagated and follows all-or-none law (can be added/summed) receptors, synapses, motor-end plates
• Threshold stimulus results in action potential