Physio - AP Propagation

Question 1

What are the 2 types of changes in membrane potential?

Answer 1

Graded potential

• Subthreshold
• Decays as a function of time & distance
• Does NOT lead to AP

Action Potential

• Depolarization to above the threshold potential
  
  • Threshold potential = membrane depolarized to an extent that further depolarization becomes regenerative
• Regenerative

Question 2

What is a graded potential?

Answer 2

• Local depolarization
  
  • Cell body or muscle fiber
  • Inadequate to reach threshold potential
  • Exponential decay as a fxn of time & distance from site of depolarization
    
    • Repolarization also a fxn of time

Question 3

What is an action potential?

Answer 3

Basics:

• Reversal of transmembrane potential
• Drastic changes in Na+ & K+ permeabilities of the plasma membrane
• Time-dependent changes in membrane permeability to Na+ and K+
  
  • Translates = change in membrane conductance for these 2 ions
• Specific sequence for movement (flow) of Na+ and K+ ions across the cell membrane

Different Types of APs:

• Shape of an AP does NOT change
• Membrane properties of different excitable cells –> APs w/ diff profiles
  
  • Present of ion channels (K & Ca)
  • Different dynamics for K+ channels
  • Leaky ion channels lead to spontaneous depolarization

Question 4

What is the importance of

Voltage Dependent Changes of Na+ permeability?

Answer 4

• Threshold voltage: Activation!
  
  • membrane protential (-70 to -50 mV)
  • conformational change in a membrane protein opens Na+ channels
  • increases permeability to Na+

Question 5

What is the depolarization stage?

Answer 5

• Influx of Na+ down its electrochemical gradient
  
  • Inside of the cell = (-) relative to outside
  • [Na+] outside the cell = GREAT than inside
  • Cell transmembrane potental approaches the Na+ equilibrium potential
• Note:
  
  • [Na] and reversal potential leads to the rush of Na into the cell
  • All or nothing

Question 6

What is the relation between Voltage and Time Dependent Changes in Na+ Permeability and Inactivation/Repolarization?

Answer 6

• Inactivation:
  
  • Inactivation begins soon after Na+ channel opens
  • Closing of the channel = slower time course than opening the channel
    
    • Once inactivation process starts, channel cannot be activated again until after the transmembrane potential has returned nearly to normal RMP
• Repolarization:
  
  • Increase in membrane permeability to K+
  • K+ moves in accordance to its electrochemical gradient
    
    • Inside the cell = (+) = during depolarization
    • [K+] inside cell >> [K+] outside
    • Efflux of K+ driven by the K+ reversal potential
  • Hyperpolarization
    
    • movement of K+ out of the cell leads to negative value lower than RMP

Question 7

What are the Voltage and Time Dependent Changes in K+ Permeability?

Answer 7

Basics:

• K+ channel is activated slowly as the cell depolarizes
  
  • ie: -90 –> 0mV
• K+ is opening at the time when Na+ channel is closing
  
  • ie: Na+ inactivation
• Efflux of K+ ions = major contributor to restoration of the RMP

Note:

• No inactivation gates for K!
  
  • Only when RMP is reached, they start to close

Question 8

Describe the changes in membrane conductances for Membrane Potential, Na+, and K+

Answer 8

Question 9

What do the Na+ and K+ currents look like during the AP?

Answer 9

Question 10

What are the 2 Refractory Periods of an AP?

Answer 10

• Absolute refractory period
  
  • AP cannot be evoked by a stimulus regardless of the stimulus strength
  • Na channels = inactivated
  • Cell is too positive
• Relative refractory period
  
  • Suprathreshold stimulus required to elicit another AP
  • Some Na+ channels returned to the resting state
  • K channels = still open
    
    • outward movement of K

Question 11

How is the Propagation of an Action Potential achieved?

Answer 11

Basics:

• AP can be propagated in either direction along an axon
• Movement of AP is dependent on the progressive depolarization along the length of the axon
• Progressive depolarization = function of “local circuit” currents

Notes:

• current is moving outward —> partial depolarization of the membrane
  
  • only moves in 1 direction (towards cells w/ RMP)
  • cant move back on itself b/c of Na inactivation gates

Question 12

AP Propagation in an Unmyelinated Axon

Answer 12

Basics:

• Events that occur in continuous propagation

Process:

1. AP develops at initial segment
   
   • transmembrane potential = becomes depolarized
2. Na+ ions entering spread away from open channel
   
   • depolarization brings membrane to threshold
3. Current causes AP to occur in adjacent section
   
   • original section gets repolarized
4. Current moves forward b/c previous section is in absolute refractory period of repolarization

Diameter:

• Unmyelinated = smaller diameter
• Slower conduction
  
  • ex: fibers carrying pain & temp sensations

Question 13

How is AP Propagation in a Myelinated Axon achieved?

Answer 13

Basics:

• Events that occur in saltatory propagation
• Absence of Na+ and K+ channels in myelin area
  
  • myelin sheath = insulator
  • Prevents outward movement of current
• High [Na+] and [K+] channels in nodes (Ranvier)
• Current flow in the interior of the axon “jumps” from one node to the next

Process:

1. AP occurs in initial segment
2. Local current produces a graded depolarization
   
   • brings the axolemma at the next node to threshold
3. AP develops at second node
4. Local current produces a graded depolarization that brings the next axolemma node to threshold

Diameter:

• Myelinated axon = LARGER diameter
  
  • fibers from muscles/golgi tendon organs
  • fibers conducting curde touch, temp, and pain

Note:

• Allows for SUPER FAST conduction

Question 14

What is a Compounded Action Potential?

Answer 14

• Compound Action Potential
  
  • Bundle of axons (has both myelinated + unmyelinated)
    
    • shape = determined by the extracellular current of each individual axon
    • Higher peak = faster it is moving down axon
      
      • unmyelinated = slower
      • myelinated = faster