Unit V: Movement
Muscle Contraction - Part I
Chapter 9 – pg 286-291
Electrophysiology of Muscle Tissue
Review
• 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 sarcolemma
Motor neuron
Axon
Neuromuscular
junction
Myofibril
Motor end plate
Excitation of a Muscle Fiber
Steps 1 & 2
1. Nerve signal opens voltage-gated calcium channels.
2. Calcium stimulates exocytosis of synaptic vesicles containing
ACh = ACh release into synaptic cleft.
Excitation of a Muscle Fiber
Steps 3 & 4
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 Fiber
Steps 5
5. Voltage change in end-plate region (EPP) opens nearby voltagegated ion channels producing an action potential
Excitation-Contraction Coupling
Steps 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 Coupling
Steps 8 & 9
Myosin head
Troponin
Resting Sarcomere
Tropomyosin Actin
8. Calcium released by SR binds to troponin.
9. Troponin-tropomyosin complex changes
shape and exposes active sites on actin.
Contraction
Steps 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.
Contraction
Steps 12 & 13
12. Power stroke =
myosin head releases
ADP 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
Contraction
Sliding Filament Theory
• Thick and thin filaments slide past one another
• They DO NOT become shorter!
• Half of myosin heads bound to thin filaments at a time
I band
Z line
I band
A band
H band
Sarcomere at rest
Z line
A band
Z line
H band
Z line
Sarcomere contraction and filament sliding
http://www.yo
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MenNQM
Relaxation
Steps 14 & 15
14. Nerve stimulation ceases
15. Acetylcholinesterase removes ACh from receptors.
Stimulation of the muscle cell ceases.
Relaxation
Steps 16
16. Active transport needed to pump calcium back into SR to
bind to calsequestrin.
Relaxation
Steps 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