Anatomy Physiology Park Tudor School Skeletal Muscle Activity

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

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

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

Labeling the NMJ

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

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

Chapter 6 SLIDING FILAMENT THEORY

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

Molecular Participants • Myosin – Flexing heads (Cross Bridge) • Actin – Contain myosin binding sites § Provides ‘Power Stroke’ – Flexing rods § Allows Actin binding • Tropomyosin – Regulates access to actin’s myosin binding sites • ATP (Nucleotide) – Transfers energy to myosin and initiates flexing (shape change) • 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, Ca 2+ is pumped back into the SR and contraction ends

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

Actin Ca 2+ The aftermath Troponin Tropomyosin blocking active sites Myosin

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

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

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 Ca 2+ Myosin cross bridge Thin filament ADP Pi Thick filament Myosin 1 Cross bridge formation.

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. 2 The power (working) stroke.

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 ATP 3 Cross bridge detachment.

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. ADP ATP Pi hydrolysis 4 Cocking of myosin head.