CJH332H1 Lecture Notes - Lecture 5: Voltage-Gated Potassium Channel, Voltage Clamp, Resting Potential
Lecture 5: The Cellular Dissection (Resting Vs. Action Potentials)
Action Potentials
• Hodgkin-Huxley model of the Action Potential and how they were able to figure this out
• Re-visiting leak channels and voltage-gated (voltage-dependent) channels
• Basic review of the action potential and the movement of ions
• What causes the refractory period
• Structure-function of voltage-dependent ion channels
• Mental health models
Resting membrane potential – mainly K+
• In 1902, Bernstein postulated that the membrane potential
(Vm) was essentially due to potassium equilibrium potential
• There is always high concentration of K+ inside than outside
• Changing extracellular concentration of K+ - by increasing it,
he was able to depolarize the membrane (450mM = membrane potential to 0)
• He tested his hypothesis by varying the concentration of potassium in the bath and found predictable changes in
the equilibrium potential and that in turn regulated the membrane potential
• Not the case for Na+ i erae, e do’t hae uh Na leak channels – not much difference)
Background to Action Potentials
• AP is an explosive change in electrical activity (sudden change in membrane potential)
• APs are transported along an excitable cell such as a nerve or muscle cell
• An AP induced by a stimulus
- Electrical change, Chemical (taste, drugs, smell, neurotransmitters)
- Mechanical (touch, pressure, sound) , Light (vision, photo-detection)
- Temperature (hot and cold receptors)
• Current clamp:
- Control current injected into neuron (can be 0) and measure membrane potential change (keep current
steady)
- i.e. inject positive current into the neuron and watch it fire AP
• Voltage clamp:
- Cotrol euro’s membrane potential and measure the currents that flow across its membrane
- i.e. step the neuron to a depolarized potential and measure voltage-gated currents (keep membrane
potential steady)
Membrane responses – part 1
• If we use current-clamp mode to observe what happens with the membrane
potential, inject current (hyperpolarizing currents)
• Measure what is the change in the membrane potential? – it hyperpolarizes
• Linear relationship of V = IR
- Bigger current, bigger membrane potential change
find more resources at oneclass.com
find more resources at oneclass.com
Membrane responses – part 2
• Inject a positive current → positive deflection
• Current-clamp mode (current is held steady, measure change in voltage)
• Also a linear relationship (all due to leak channels)
• Slow steady increase
Explain this using linear relationships?
• Keep repeating the steps, there is a massive change in the membrane potential
• Passing the threshold value, action potential occurs – the same size!
Action Potential general properties
• APs results from sudden, large increases in Na+ permeability
• This is a voltage gated channel (Na+ and K+)
- Rise and falling phase due to changes in different voltage gated channels open/close
• Chagig extraellular Na+ oetratio is’t goig to allo AP ut Na+ is ioled i proess easure peak
• Driving force (equilibrium potential for Na) is close to peak of AP
• Increases in + charge on the inner surface of the membrane drives Vm towards ENa – rising phase of the AP
• Cell resting at -65mV will reach above +40mV at peak of the AP
Driving force
• We are always interested in the difference between resting membrane potential and equilibrium potential for
any particular ion
- At rest, resting membrane potential is due to changes in permeability in K+ while AP is due to Na+
Threshold for generating an AP
• The threshold is the critical value of Vm at which an AP impulse is generated
• Threshold is the point at which iNa+ = -iK+ (flow of sodium equal to flow of potassium in opposite direction/out)
• As soon as iNa+ exceeds -iK+ (threshold), positive feedback sets in, and an action potential is initiated
Action Potential
• When threshold is reached, we can now start
opening voltage gated channels
• Falling phase of the AP is also called
repolarization
• Repolarization results from a large increase in K+
permeability and loss of + charge on the inner
surface of the membrane
• Vm returns towards EK
Ionic basis of Action Potential
• Ion channels underlying AP generation are
voltage-dependent
• Depolarization increases the membrane conductance to Na+ initially and to K+ after a delay
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Lecture 5: the cellular dissection (resting vs. action potentials) Background to action potentials: ap is an explosive change in electrical activity (sudden change in membrane potential, aps are transported along an excitable cell such as a nerve or muscle cell, an ap induced by a stimulus. Mechanical (touch, pressure, sound) , light (vision, photo-detection) Temperature (hot and cold receptors: current clamp: Control current injected into neuron (can be 0) and measure membrane potential change (keep current steady) i. e. inject positive current into the neuron and watch it fire ap: voltage clamp: Co(cid:374)trol (cid:374)euro(cid:374)"s membrane potential and measure the currents that flow across its membrane i. e. step the neuron to a depolarized potential and measure voltage-gated currents (keep membrane potential steady) Inject a positive current positive deflection: current-clamp mode (current is held steady, measure change in voltage, also a linear relationship (all due to leak channels, slow steady increase.
