Lecture 5

Muscle Fiber Types

 
 

Muscle fibers vary by the myosin

Heavy chain

Light chain

Troponin (isoforms of Tn-I, Tn-C, Tn-t)

Tropomyosin (isoforms)

Myoglobin (like hemoglobin, a singular thing instead of four, holds oxygen in muscle, has iron makes it red)

Varies how many enzymes are set up for aerobic and anaerobic activity

 
 

Classify fibers according to myosin heavy chains

 
 

Different types of fibers:

Type I

• Slow
  
• high aerobic capacity
  
• red – high myoglobin content
  

Type IIA

• Faster
  
• fairly high aerobic capacity
  

Type IIX

• Faster still
  
• Good deal MORE aerobic capacity (used to be called Type IID)
  
• Smaller animals can get an even faster fiber called Type IIB
  

Type IIB

• Fastest greatest anerobic capacity
  
• Called White cell
  
  • Still has myoglobin and is actually light red in color
    
• ONLY PLACE Type IIB fibers: in eye muscle in human
  

 
 

TypeIIX and Type IIB – low aerobic ?????????

 
 

Fibers originally genetically determined when first produced

• however what they become from then on is dependent on the activity of the motor neuron that innervates them
  

   
   

All types can switch back and forth depending on how much activity

I <--> IIA <--> IIX <--> IIB

If we unhook one and attach to a different nerve, will slowly switch to firing rate of new nerve

 

There are also intermediates - when transitioning.

Called IIX, IIB

 
 

If do endurance training, push fibers to Type I (aerobic)

If do strength training, push fibers towards Type IIB or Type IIX in humans

Idleness will also push to Type IIB/Type IIX (aerobic)

 
 

Can use fiber names to name motor units

 
 

Each skeletal muscle is mixture of motor units

Every muscle has types up to IIX, but have different ratios

 
 

During a slow initiated contraction motor unit incorporation is sequential

Slower fibers activated first

During fast contraction motor unit of all types are activated at the same time

 
 

Can monitor electrical event and can produce a single twitch, if let muscle rest for a while and we do a twitch and allow twitch to come all the way back down (completely relaxed), but before it gets to far and then produce a second twitch the second twitch goes up higher, this stair-step effect called:

Treppe

• see it in muscle that is relaxed
  
• Definition comes from Boddich (1881), first to describe treppe in hearts that had been stopped for a while and were restarted and then they would go a greater contraction each time until it got to a certain level
  
• Treppe will go higher and higher until it reaches a certain level and then it will plateau
  
• Classical explanation of treppe: when first open ryanodine receptor have efflux of Ca which will bring Ca off of calsequestrin which will increase amount of Ca, then pump Ca back into sarcoplasmic reticulum which will diffuse over back to terminal cistern and now have greater concentration of Ca for next fire and will come out faster
  

   
   

Summation:

• If fire twitch and before it relaxes, twitch it and will go higher and higher =
  
• Kicking Ca out faster than Ca can come back in, more Ca have out in a muscle up to a certain point will increase the contractability and will get more movement
  

   
   

If fire very fast so the level doesn't come down at all have a tetanus and the twitch flatlines

• Can't stay up there forever, b/c it will run out of ATP and start to see it drop and that line dropping is called fatigue (due to lack of ATP)
  
  • Called Fatigue
    

    Other types of fatigue such as fatigue of neuromuscular junction
    

 
 

When these muscles contract, they make noise

• William Wollatson determined the frequency of this sound
  
  • Got in a carriage with a horse in london with cobblestone street
    
  • Marking system to mark start and 1 second later
    
  • Stick his fingers in his ears - relaxed thumbs, contracted fingers
    
  • Went until pace of horse matched the noise in his ears
    
  • Counted cobblestones - 23
    
  • Late 1800's
    
• 23 Hz sound produced by muscle, actual number is 25 Hz
  

   
   

 
 

Synapses:

-two types of synapses

Electrical (like a gap junction) - quick

Chemical – prolonged, constitutes most synapses we look at, use

• Neurotransmitters
  

   
   

-some synapses both electrical (quick) and chemical (slow)

Both - conjoint

 
 

-space between is the synaptic cleft (20-30nm wide),

• NT cross the cleft and are released by synaptic vesicle cycle
  
• Delay called synaptic delay
  

 
 

synaptic knob

-have endosome, have railway from the cell body b/c NT are made up in the soma, sometimes made up as precursors in the soma and change on the way down

 
 

• Budding
  
  • very end of endosome will pinch off and (called budding)
    
  • called a synaptic vesicle, but is empty: NO NT
    

   
   

• Neurotransmitter Uptake
  
  • NT taken into the synaptic vesicle by active transport (burning ATP)
    

   
   

• Translocation: vesicle going down to active site
  
  • when vesicle full with NT, goes down to active site
    

   
   

1. Docking: proteins on the synaptic vesicle bind to proteins of the neurolemma (plasma membrane of neuron)
   

    
    

2. Priming
   
   1. series of protein bindings and dissociations that form a partial fusion of the synaptic vesicle to the neural lemma
      

   Primed synaptic vesicle waiting there for the completed spinal fusion and will do that in response to electrical stimulation
   

    
    

3. Full Fusion/Exocytosis: release NT into synaptic cleft and will diffuse down to the target tissue
   
   1. Action Potential induced
      
   2. Opens a hole and NT flows into the synapse
      

      -this dumping may be a calcium-calmodulin? regulated process
      

       
       

4. Endocytosis:
   
   1. sarcolemma closes back and synaptic vesicles break away
      

       
       

5. Translocation 2: empty synaptic vesicle binds to the other end of the endosome
   

    
    

6. Endosome Binding
   

    
    

7. Endosome Revamping: vesicle reattaches and gets ready to bind again
   

 
 

PROCESS CAN BE COMPLETED IN ONE MINUTE

 
 

Electrical event causes all of this

• Causes Ca influx
  
• Amount of Ca determined by intensity of electrical impulses
  
• Amount of NT directly proportional to Ca released
  

 
 

-CONTINUE ABOUT Ca Process Later….