Topic 98- Membrane potential, local response and action potential, propagation of the action potential

Question 1

Words to include

Answer 1

Membrane potential

• ​Potential difference
• Cell membrane
• Ion gradient
• Na⁺/K⁺ ATPase pump
• Electrogenic pump
  
  • Concentration difference
• Equilibrium potential
• K⁺ channels
• Na⁺ channels
• Resting membrane potential
• Goldman-Hodgkin-Katz equation
  
  • E_m: membrane potential
  • P: permability
  • Concentration
  • Permabilities

Local response

• Membrane potential
• Depolarization
• Hyperpolarization
  
  • Polarization ↑
  • Action potential (ø)
• Local response
  
  • Threshold potential (ø)
• Action potential
  
  • Threshold potential
• Membrane
  
  • Resistance
  • Capacitance
• Sub-threshold potential

Action potential

• All-or-none law
  
  • Threshold potential
  • Complete response
• Voltage-dependent ion channels
  
  • TTX (tetrodotoxin)
    
    • Voltage-dependent Na⁺ channels
    • Voltage-dependent TTX sensitive sodium channel
  • TEA (tetraethylammonium)
    
    • Voltage-dependent K⁺ channels
    • Voltage dependent TEA sensitive potassium channel
• Phases
  
  • Stimulation (phase 0)
  • Threshold potential (phase 1)
  • Overshoot (phase 2)
  • Plateu (phase 3)
  • Repolarization (phase 4)
  • Posthyperpolarization (phase 5)
    
    • ATP-ase pump

Propagation of AP

• Non-myelinated sheets
• Myelinated sheets
  
  • Lipoprotein membranes
  • Nodes of Raniver
    
    • TTX-sensitive sodium channels
    • Internodiums (ø)
  • Internodal part
  • Electrical current

Question 2

Topics to include in the essay

Answer 2

1. Membrane potential
2. Local response and action potential
   
   • Phases of action potential
3. Propagation of the action potential

Question 3

Membrane potential

Answer 3

• The potential difference across a living cell membrane
• The basis of maintaining membrane potential is the ion gradient formed between the two sides
  
  • Maintained by the sodium potassium ATPase pumps
• The equilibrium potential drives K⁺ out of the cell and Na+ from EC into it
• In the plasma membrane, there are much more K⁺ channels open in resting state, than leak Na⁺ channels. Therefore, the resting membrane potential is the result of slow outflow of K⁺
• Na⁺/K⁺- ATPase pump:
  
  • 3 Na⁺ → EC space
  • IC space ← 2K⁺
• Electrogenic pump: More positive ions are pumped out than in
  
  • Task: maintain the concentration difference
• The presence of all ions together determined the final value of the membrane potential. This can be calculated using the Goldman-Hodgkin-Katz equation:
  
  • E^m: membrane potential
  • P: permability
• A transient change of the membrane potential may occur in two ways according to the GHK equation:
  
  1. Change in the concentrations
  2. Change in the permeabilities
• In the nature, only the change in permeability constants are used for signaling
• The membrane potential may be changed artificially in two directions:
  
  • Depolarization: giving positive charge to IC space to reduce the membrane potential
  • Hyperpolarization: giving negative potential to IC space to increase the polarization
    
    • Never evokes an AP
    • Hyperpolarizing stimuli inhibits the receptor

Question 4

Define local response

Answer 4

If the depolarization does not reach threshold potential, the potential change will only be conducted a few mm with decreasing intensity.

Question 5

Define action potential

Answer 5

If depolarization does reach the threshold potential, it evokes an „all-or-none” response with strictly amplitud

Question 6

Action potential

Define all-or-none law

Answer 6

All-or-none law: strength by which a nerve fiber responds to a stimulus is independent of the strength of the stimulus. If the stimulus exceeds the threshold potential, the nerve or fiber will give a complete response; otherwise, there is no response

Question 7

Action potential

Generation of action potential

Answer 7

• Generation of AP depends on voltage gated ion channels
  
  • ​These channels may be blocked by:
    
    • TTX (tetrodotoxin)
      
      • ​Specifically blocks voltage-dependent TTX sensitive Na⁺ channels
    • TEA (tetraethylammoium)
      
      • ​Specifically blocks voltage-dependent TEA sensitive K⁺ channels

Question 8

Action potential

Phases of action potential

Answer 8

• Stimulation - Phase 0

​

• Threshold potential - Phase 1
  
  • Voltage dependent Na⁺ sodium channel is opening
• Overshoot - Phase 2
  
  • Na⁺ channel inactivates
• Plateu - Phase 3
  
  • Slow Ca²⁺ influx

​

• Repolarization - Phase 4
  
  • K⁺ efflux

​

• Posthyperpolarization - Phase 5
  
  • Later: resting potential
  • Voltage dependent Na⁺ and K⁺ channels are closing
  • The ATP-ase pump restores concentration gradient

Question 9

Propagation of the action potential

Answer 9

Propagation of action potential in naked fibers

• Action potential propagates step-by-step in the fibers with non-myelinated sheets
• A channel activates the next one directly next to it
• This type of conduction is much slower than in myelinated sheets

Propagation of action potential in myelinated fibers

• In the myelinated fibers segments surrounded by several hundreds of lipoprotein membranes
• High resistance interchange with nodes of Ranvier
• Fast conduction is made possible by the fact that only the nodes of Ranvier contain the TTX-sensitive sodium channels but not the internodiums
  
  • In the segment between two nodes of Ranvier it is unnecessary for the channels to activate and inactivate consecutively because this internodal part operates in a large distance as an “electrical cable”, it means it conducts the current electrically and therefore very quickly
• The conduction velocity is proportional to the diameter of the myelin sheet

Figure: propagation of AP in myelinated fibers