Skeletal Muscle Activity: Contraction Chapter 6 Part 2A

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 Ca2+ levels Neuromuscular Junction Location where motor neuron stimulates muscle contractions Spinal cord Motor neuron cell body Muscle Nerve Motor unit 1 unit 2 fibers neuron axon neuromuscular junctions

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

Labeling the NMJ

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

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

Sliding Filament Theory Chapter 6 Sliding Filament Theory

Sliding Filament Model of 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 A Z H

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

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.

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, Ca2+ is pumped back into the SR and contraction ends

1 2 Action potential is propagated along the sarcolemma and down the T tubules. 1 Steps in E-C Coupling: Sarcolemma Voltage-sensitive tubule protein T tubule Ca2+ release channel 2 Calcium ions are released. Terminal cisterna of SR Ca2+

Troponin Tropomyosin blocking active sites Myosin Actin Ca2+ The aftermath

3 Troponin Tropomyosin blocking active sites Myosin Actin Active sites exposed and ready for myosin binding Ca2+ Calcium binds to troponin and removes the blocking action of tropomyosin. The aftermath 3

3 4 Troponin Tropomyosin blocking active sites Myosin Actin Active sites exposed and ready for myosin binding Ca2+ cross bridge Calcium binds to troponin and removes the blocking action of tropomyosin. Contraction begins The aftermath 3 4

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. Ca2+ Myosin cross bridge Thick filament Thin filament ADP Pi 1

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

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

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 Pi 4