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Titin
- -largest protein in the body
- -binds both actin and myosin
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Thin Filaments
- -actin
- -tropomyosin
- -Troponin complex (TnT, TnC, TnI)
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Thick Filaments
-myosin (head, hinge, tail region)
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Myosin Head
- Alkali Light Chain
- Regulatory Light Chain
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Tropomyosin and Troponin
- 1. When no Ca is present tropomyosin blocks myosin binding sites on acting.
2. When Ca enters the cell it binds TnC moving TnI out of the way and moving tropomyosin into the actin groove, freeing the myosin binding sites.
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Muscle Contraction : the Power Stroke
- 1. Attached State
- 2. Released State
- 3. Cocked State
- 4. Cross-bridge State
- 5. Power-stroke State
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Muscle Contraction Step 1
-Attached State to Released State
- When no ATP is bound, myosin is attached to actin in the 'attached state'
- ATP binds myosin head, causing the actin and myosin to dissociate 'released state'
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Muscle Contraction Step 2
-Released State to Cocked State
- ATP is hydrolyzed, causing the myosin heads to return to their resting conformation (45 degree angle to perpendicular to actin)
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Muscle Contraction Step 3
-Cocked State to Cross-Bridge Formation
- Cross-bridge forms between myosin head and the actin at a new position
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Muscle Contraction Step 4
-Cross bridge State to Power Stroke
- ADP and phosphate released the myosin is mused to left and the actin is pushed to the right = power stroke
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Muscle Contraction Step 5
-Power Stroke to Attached State
myosin is now in the attached state again with the myosin heads bound to the actin at a 45 degree angle
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Energy Sources for Muscle Contraction
ATP
- In SKM generated by:
- -glycogen pathway (bulk of synthesis)
- -creatine posphate synthesis (a little)
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Summation
If twitches occur close enough together they can summate, increasing the tension produced for a single fiber
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Unfused Tetnus
There are tiny releases of muscle tension
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Fused tetanus
muscle is always contracted
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Motor Unit Pool
A pool consists of many motor neurons each of which innervates a motor unit within the muscle
-if you need finer muscle control there's a smaller motor unit pool.
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SKM muscle tension can be varied by:
1. Summation of twitches (tetanus)
2. Recruiting additional motor neurons to the pool
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Cardiac Muscle Electrical Properties
- 1. Automaticity (spontaneous depolarization)
- 2. Main pacemaker = SA node
- 3. ACh can affect HR through M2 receptors
- 4. L type Ca channels need physical CONTACT with SR
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Cardiac Muscle Tension
- -no summation
- -contractile force varies by the magnitude of Ca increase, or the sensitivity of contractile proteins to Ca
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Cardiac Muscle Fiber Types
- 1. Atrial
- 2. Ventricular
- 3. Conduction system
- 4. Epicardial
- 5. Endocardial
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Skeletal vs. Cardiac Muscle
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