Lecture 3

Audio recording started: 12:03 PM Wednesday, January 21, 2009

 
 

Neurons and Muscle Contraction

 
 

• If you monitor the membrane potential between two electrodes with one inside a cell and one outside the cell the potential will be 0
  
  • membrane potential only occurs at the membrane
    
  • caused by movement of ions across membrane
    
  • if the movement of ions stops then the membrane potential will be 0
    
• Potential caused Na/K pumps
  
  • Pumps 3 Na out and 2 K into the cell
    
  • ATP Powered
    
  • Na+ outside, K+ inside
    
  • Enhance potential
    
  • Na leaking in reduces potential
    
• K leak channels
  
  • Allow K to leak out of the cell
    
  • Creating + charge outside and leaving negative charge inside
    

   
   

• Ion Concentrations
  
  • K+
    
    • high [ ] inside the cell
      
    • K+ leak channels act as a major input into membrane potential
      
    • channels are 100x more permeable to K to leaving the cell
      
  • Na+
    
    • high [ ] outside the cell
      
    • Na+ voltage gated channels
      
    • there are some Na+ leak channels that allow for some Na to leak in, reducing potential slightly
      
• Charge
  
  • Outside (+)
    
  • Inside (-)
    
  • Charge inside and outside the membrane is due to concentration gradients
    

    10:1 for Na+
    

    35:1 for K+
    

   
   

• 
  Resting Potential
  
  • average resting potential of a neuron = -70mV (mostly due to K leaking out – there are more pluses going out than are coming in)
    
  • resting membrane potential can range from -40 to -90 mV
    
    • Measured inside vs. outside
      

   
   

• Voltage Gated Ion Channels
  
  • have voltage gated channels at specific voltage that open and allow for transmission of an AP
    
  • usually permeable to (+) ions which can alter the potential
    
  • Chemically gated channels DO exist
    

   
   

• Action Potential
  
  • neurons and muscles use AP as messages
    
  • an electrical stimulus causes hyperpolarization, depolarization (closer to 0) to create an AP
    
  • if reach threshold -55 mV an AP will occur
    
  • if threshold is reached slowly, this will allow the neuron to accommodate ionic changes and no AP will occur
    
  • if there is a rapid depolarization to -55 mV Na+ channels will open and raise the memebrane potential to +35mV
    

1. repolarization occurs when the Na channels close (two gates), K channels open (K channels only have one gate), K channels are slower to open
   
2. hyperpolarization occurs due to slow closing of K channels
   
3. when K channels finally close membrane potential is again achieved
   

 
 

1. 
   Threshold
   
   1. usually -55 mV, but not always
      
   2. a quick 15-30 mV change in membrane potential is needed to initiate an AP
      
   3. accommodation = gradual increase in membrane potential that reaches the threshold and passes it but does not initiate an AP
      

    
    

2. Refractory Periods
   
   1. absolute refractory period = period where where a neuron WILL NOT fire again no matter how big the electrical stimulus (measured in time, msec)
      
   2. relative refractory period = take a strong electrical stimuli to initiate an AP
      
   3. total refractory period = absolute + relative refractory period
      
   4. refractory period of cardiac muscle much longer than skeletal muscle, can keep a skeletal muscle contracted for a long period of time by continuing to supply a continuum of signals
      

       
       

3. Dendrite
   
   1. Don't work with wave propagation
      
   2. These have channels, but they don't have voltage gates, b/c they don't propogate AP
      
   3. These just allow charge to build at Hillock
      

       
       

4. Wave Propagation
   
   
   1. surface of skeletal and cardiac muscle
      
   2. only goes in one direction
      
   3. is the transfer of charge down the axon
      
   4. series of opening/closing Na/K channels
      
   5. slower, covers a longer distance
      
   6. muscles use this ONLY in unmyelinated neurons
      

       
       

5. 
   Axon
   
   1. messages are sent down the axon
      
   2. the start of the electrical signal is at the initial segment (axon hillock)
      
   3. These are what is mylinated
      

       
       

6. Sink Conduction/Electrotonic Conduction
   
   
   1. FASTER than wave propogation, dissipates quickly
      
   2. short distance
      
   3. a positive charge pulls on the negative charge inside the membrane and makes the end slightly positive
      
   4. amplified by the presence of Ca2+
      
   5. transfers the positive charge and causes a dissipation of charge
      
   6. have multiple dendrites feeding into soma and summing charge until hit threshold
      

       
       

7. Myelination – Saltatory Conduction
   
   1. if want to speed up wave propagation have myelination with Nodes of Ranvier
      
      1. Casued by schwann Cells
         
      2. Sodium channels concentrated at nodes of Ranvier
         
      3. Use Sink conduction
         
      4. JUMPING
         
   2. brain is myelinated at 1.5 years
      
   3. fast electrontonic conduction allows for jumping from node to node
      
   4. CNS  oligodendrocytes wrap many axons
      
   5. PNS  schwann cells wrap only ONE axon
      
   6. multipolar axons use the axon hillock to sum all the (+) and (-) input and elicit appropriate responses
      
   7. unipolar and bipolar summation takes place in the initial segment of the axon
      

    
    

8. Definitions
   
   1. tonal = firing all the time
      
   2. phasic = waiting to fire, can only "say" I fired
      

    
    

9. Muscle Contraction in Skeletal Muscle
   
   1. skeletal muscle = muscle fiber that has one motor neuron synapsing on it, this synapse is a chemical synapse
      
   2. acetylcholine (Ach) = neurotransmitter used at synapse to stimulate muscles
      
   3. AP moves 5m/s
      
   4. Ach comes out at synapse at synaptic cleft and causes the opening of gates and the AP travels in one direction
      
   5. Travels into T-Tubule
      
      1. Triggers dihydropyridine receptor
         
         1. Formed in units of 4 (tetrad)
            
         2. Open by Ca and closed by Ca
            
         3. All 4 Sit over EVERY OTHER ryanodine receptor in terminal cistern
            
         4. Electric event triggers increase in cytoplasm potential
            
         5. Triggers every
            
         6. Ca opens _________
            
            1. As it gets higher, the second receptor works only with
               
   6. motor end plate = location of the synapse, chemically gated channels here
      
   7. synapses located ONLY under end plate
      
   8. nicotinic receptor = imitate affects of nicotine
      
   9. Ach signals event across surface
      
   10. positive wave front
       

    
    

   Pasted from <file:///C:\Users\Derek\AppData\Local\Temp\OneNote\0-3\Lecture%202%20Neurons%20and%20Muscle%20Contraction.doc>