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Anatomy & Physiology Park Tudor School Skeletal Muscle Activity: Contraction Chapter 6 Part 2A.

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Presentation on theme: "Anatomy & Physiology Park Tudor School Skeletal Muscle Activity: Contraction Chapter 6 Part 2A."— Presentation transcript:

1 Anatomy & Physiology Park Tudor School Skeletal Muscle Activity: Contraction Chapter 6 Part 2A

2 The Nerve Stimulus & Action Potential Muscle Contraction Requires: – Electrical current propagation – Stimulation by motor neuron  Motor Unit – one neuron and all the skeletal cells it stimulates – Rise in intracellular Ca 2+ levels Neuromuscular Junction – Location where motor neuron stimulates muscle contractions Spinal cord Motor neuron cell body Muscle Nerve Motor unit 1 Motor unit 2 Muscle fibers Motor neuron axon neuromuscular junctions

3 Anatomy of a Neuromuscular Junction Synaptic Cleft – Space between motor neuron and Synaptic Vesicles – Contain a Neurotransmitters Sarcolemma – Motor End Plate  specialized region with many folds  Receives neurotransmitter T Tubule – Carries Action Potential SR – Regulates Calcium Sarcomere – Contracting Unit of Muscle

4 Labeling the NMJ

5 Transmission of Nerve Impulse to Muscle 1.Neurotransmitter— chemical released by nerve upon arrival of nerve impulse – The neurotransmitter for skeletal muscle is acetylcholine (ACh) 2.Acetylcholine attaches to receptors on the sarcolemma 3.Sarcolemma becomes permeable to sodium (Na+) 4.Sodium rushes into the cell generating an action potential

6 Transmission of Nerve Impulse to Muscle Once started, muscle contraction cannot be stopped

7 SLIDING FILAMENT THEORY Chapter 6

8 Sliding Filament Model of Contraction Contraction: – The generation of force – Does not necessarily cause shortening of the fiber During Relaxation: – thin and thick filaments overlap only slightly During contraction: – Myosin heads bind to actin on thin filament – Detach – Bind again to next site on thin filament  Propels the thin filaments toward the M line H zones shorten and disappear, sarcomeres shorten, muscle cells shorten I Fully relaxed sarcomere of a muscle fiber Fully contracted sarcomere of a muscle fiber I A ZZ H IIA ZZ

9 Molecular Participants Myosin – Flexing heads (Cross Bridge)  Provides ‘Power Stroke’ – Flexing rods  Allows Actin binding ATP (Nucleotide) – Transfers energy to myosin and initiates flexing (shape change) Actin – Contain myosin binding sites Tropomyosin – Regulates access to actin’s myosin binding sites Troponin – Exposes actin binding site by moving tropomyosin Calcium Ions – Activate Troponin

10 Myosin & ATP ATP transfers its energy to the myosin head, which in turn energizes the power stroke. ATP disconnects the myosin head from the binding site on actin.

11 Calcium Ions – EC Coupling Role of Calcium in Starting Muscle Contraction (Excitation- Contraction Coupling): – Action potential moves along sarcolemma to the T tubules – Calcium Ions are Released from SR – Calcium Ions then Bind to Troponin – Tropomyosin Moves Away from the Myosin Binding Sites on Actin – When nervous stimulation ceases, Ca 2+ is pumped back into the SR and contraction ends

12 Steps in E-C Coupling: Terminal cisterna of SR Voltage-sensitive tubule protein T tubule Ca 2+ release channel Ca 2+ Sarcolemma Action potential is propagated along the sarcolemma and down the T tubules. Calcium ions are released. 1 2

13 TroponinTropomyosin blocking active sites Myosin Actin Ca 2+ The aftermath

14 TroponinTropomyosin blocking active sites Myosin Actin Active sites exposed and ready for myosin binding Ca 2+ Calcium binds to troponin and removes the blocking action of tropomyosin. The aftermath 3

15 TroponinTropomyosin blocking active sites Myosin Actin Active sites exposed and ready for myosin binding Ca 2+ Myosin cross bridge Calcium binds to troponin and removes the blocking action of tropomyosin. Contraction begins The aftermath 3 4

16 4 Steps of Cross Bridge Cycle Step 1: Cross Bridge Formation – Binding of Myosin to Actin  Tail hinge of the myosin bends and energized myosin head binds to the actin. Actin Cross bridge formation. Ca 2+ Myosin cross bridge Thick filament Thin filament ADP Myosin PiPi 1

17 4 Steps of Cross Bridge Cycle Step 2: Power Stroke of the Cross Bridge – The ADP and Pi are released from the actin. – The activated myosin head tilts backward.  The power stroke occurs as the thin filament is pulled inward toward the center of the sarcomere.  There has been a transfer of energy from the myosin head to the movement of the thin filament. The power (working) stroke. 2

18 4 Steps of Cross Bridge Cycle Step 3: Cross Bridge Detatchment – Disconnecting the Myosin Head  ATP binds to the myosin head, and it disconnects from the actin Cross bridge detachment. ATP 3

19 4 Steps of Cross Bridge Cycle Step 4: Reactivation of the Myosin Head – ATP hydrolysis transfer energy to the myosin head producing ADP + Pi and returns Myosin to energized position. Cocking of myosin head. ATP hydrolysis ADP PiPi 4


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