Sliding Filament Theory
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Slide 1 - Muscle Contraction
- Sarcomeres shorten, distance between z-lines reduced
- Thick and thin myofilaments overlap more during contraction
- 3 stages:
- Neural stimulation
- Muscle cell contraction
- Muscle cell relaxation
Slide 2 - Motor Unit
- Stimulation of a muscle by a nerve impulse (motor nerve) is required before a muscle can shorten
- Neuromuscular junction: point of contact b/w nerve ending and the muscle fiber it innervates.
- Motor unit: motor neuron + muscle cell
Slide 3 - Neuromuscular Junction
- (cell membrane)
Slide 5 - Neural Stimulation
- Motor neuron releases neurotransmitters to stimulate a contraction
- Acetylcholine (Ach) binds to receptors located on sarcolemma (muscle cell membrane)
- Changes transport proteins found in sarcolemma
- Alters transport of ions
- Normally, more Sodium (Na+) ions outside muscle cell, while Potassium (K+) higher inside
- Sodium/Potassium pumps maintain this unequal concentration
- Excitable condition
- When stimulated, ion channels open, depolarizing the cell
- Na+ flows in, K+ out
- sarcoplasmic reticulum releases stored calcium
- Ca2+ travels to sarcomere, initiating muscle contraction phase
Slide 7 - Muscle Contraction Animation
- Click here to view animation
Slide 9 - Sliding filament theory
- In the absence of Calcium ions…
- Troponin (purple M&M) sits on Tropomyosin filament (gray)
- This “protein complex” blocks access to the myosin head’s binding site on actin.
- When Calcium (small specks) is released by the Sarcoplasmic reticulum
- it diffuses into the muscles
- binds to the troponin/tropomyosin complex
- shifting both the troponin and tropomyosin filament
- This reveals the binding sites on actin… myosin now has access!
- (But it still can’t reach)
- Myosin splits ATP and moves into a high-energy state, extending the “arm” of the myosin.
- The head of myosin can now bind to actin
- Forms a cross-bridge between the thick and thin filaments.
Slide 13 - The ATP energy stored by myosin is released
- The myosin molecule relaxes, bending back to its original place…
- Causes rotation of the head, pulling on the actin…
- This leads to the sliding of the filaments.
- This cycle continues until Ca2+ ions gone (and stimulus stops)
Slide 14 - As long as Calcium is present, though…
- New ATP binds to Actin/myosin cross bridge, causing myosin to disconnect from actin.
- Splitting of ATP (turning into ADP + P) leads to re-energizing (extending)/ repositioning of the cross bridge.
Slide 15 - What it REALLY looks like…
Slide 16 - Relaxation Phase
- Complete contraction of muscle cell requires several cycles of neural stimulation and contraction phases
- Ca2+ ions transported back to sarcoplasmic reticulum (req. ATP)
- When the calcium level decreases
- troponin and tropomyosin move back into the blocking position
- thin filament (actin) slides back to the resting state (when ATP binds to myosin head)
- Relaxation phase occurs when no more neural stimulations are stimulating the sarcolemma to release Calcium…
- Na+/K+ pump returns ions to resting state
- Muscle cell remains in contracted, but pliable state
- Must be “stretched” back into position
Slide 18 - Review of the Role of ATP
- ATP transfers its energy to the myosin cross bridge, which in turn energizes the power stroke.
- ATP disconnects the myosin cross bridge from the binding site on actin.
- ATP fuels the pump that actively transports calcium ions back into the sarcoplasmic reticulum.
Slide 19 - Rigor Mortis
- In death…
- Calcium leaks out of sarcoplasmic reticulum into sarcomere
- Causes muscle tension = rigor mortis
- Muscle cell structures start breaking down, causing muscle to loosen (unless body becomes dehydrated)
Slide 20 - Creatine phosphate
- Stores energy in muscle cells
- Collects energy from ATP, stores for long periods of time
- Transfers back to ATP when needed
Slide 21 - Glycogen
- Stored form of glucose
- Energy reserve for muscle action
- Continuous supply needed to produce ATP
Slide 22 - Myoglobin
- Red pigment that stores oxygen for muscle cells
- “Grabs” oxygen from hemoglobin in blood
- High affinity for oxygen
- Allows cells to produce large amounts of ATP