Untitled Quiz

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 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

(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

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.

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

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)

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.

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

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

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

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

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: 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

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

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.

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