Unit V: MovementMuscle Contraction - Part I

Chapter 9 – pg 286-291

Electrophysiology of Muscle TissueReview

• Resting, excitable cell – polarized

• Resting muscle cell: excess Na+ outside, K+ and anions inside

• Resting Membrane Potential

• Stimulation of muscle cell – Na+ enters, then K+ leaves

• Action Potential

• spreads along the sarcolemmaMotor neuron

Axon

Motor end plate

Neuromuscularjunction

Myofibril

Muscle fiber

Excitation of a Muscle FiberSteps 1 & 2

1. Nerve signal opens voltage-gated calcium channels. 2. Calcium stimulates exocytosis of synaptic vesicles containing ACh = ACh release into synaptic cleft.

3. Binding of ACh to receptor proteins

4. opens Na+ and K+ channels resulting in reversed polarity forming an end-plate potential (EPP).

Excitation of a Muscle FiberSteps 3 & 4

Excitation of a Muscle FiberSteps 5

5. Voltage change in end-plate region (EPP) opens nearby voltage-gated ion channels producing an action potential

Excitation-Contraction CouplingSteps 6 & 7

6. Action potential spreading over sarcolemma enters T tubules 7. voltage-gated channels open in T tubules causing calcium gates to open in SR

Excitation-Contraction CouplingSteps 8 & 9

8. Calcium released by SR binds to troponin.

9. Troponin-tropomyosin complex changes shape and exposes active sites on actin.

Resting Sarcomere

Myosin head

Troponin

ActinTropomyosin

ContractionSteps 10 & 11

10. Myosin ATPase in myosin head hydrolyzes an ATP molecule, activating the head and “cocking” it in an extended position.

11. It binds to actin active site forming a cross-bridge.

ContractionSteps 12 & 13

12. Power stroke = myosin head releasesADP and phosphate as it flexes pulling the thin filament past the thick

13. Recovery stroke =with the binding of more ATP, the myosin head extends to attach to a new active site

Contracted Sarcomere

ContractionSliding Filament Theory

http://www.youtube.com/watch?v=WRxsOMenNQM

• Thick and thin filaments slide past one another• They DO NOT become shorter!• Half of myosin heads bound to thin filaments at a time

Sarcomere at restZ line Z lineH band

I band A band I band A band

Z line Z lineH band

Sarcomere contraction and filament sliding

RelaxationSteps 14 & 15

14. Nerve stimulation ceases 15. Acetylcholinesterase removes ACh from receptors. Stimulation of the muscle cell ceases.

RelaxationSteps 16

16. Active transport needed to pump calcium back into SR to bind to calsequestrin.

RelaxationSteps 17 & 18

17. Loss of calcium from troponin. 18. Moves troponin-tropomyosin complex over active sites. Muscle fiber returns to its resting length.

Rigor Mortis

• Hardening of muscles, stiffening of body

• Time period: 3 – 60 hours after death

• No ATP being produced!

Myasthenia Gravis

• Women between ages 20-40

• Autoimmune disease

– Antibodies clump ACh receptors together

– Muscle fibers less sensitive to ACh

• Treatment

– Cholinesterase inhibitors

– Immunosuppresive agents

– Removal of thymus