1. MUSCLE CONTRACTION
© 2016 Paul Billiet ODWS

2. Muscle tissue Fibres Myofibril Muscle Z line Tendon H band A band
Striations
Sarcolemma
Nucleus
Tendon
H band
A band
I band
Sarcomere
© 2016 Paul Billiet ODWS

3. Striated muscle
Striated muscle viewed at x400

4. Striated muscle
Relaxed muscle fibrils showing sarcomeres (Z to Z) and A, H and I bands

5. Muscle contraction observed
ContractionTEM view

6. The sliding filament theory
Myosin
Cross bridges
Z line
Actin
RELAXED
H band
I band
A band
H band and I band disappear when the muscle is fully contracted
CONTRACTED
© 2016 Paul Billiet ODWS

7. The motor end plate
The motor end plate is the terminal button of a motor neurone that makes contact with a muscle cell
The motor end plate releases the neurotransmitter acetylcholine into the synapse
Acetyl choline is broken down in the synaptic cleft by choline esterase enzyme
The products of this breakdown are reabsorbed by the motor end plate.
© 2016 Paul Billiet ODWS

8. The motor end plate (neuromuscular junction)
biology.clc.uc.edu/fankhauser/Labs/Anatomy_...

9. 8. Neurotransmitter destroyed by choline esterase.
Stops signal being perpetuated.
7. Action potential generated which travels across the muscle cell.
© 2016 Paul Billiet ODWS

10. The motor end plate (neuromuscular junction)
An impulse arrives at the motor end plate from the axon of a motor neurone
Acetyl choline is released
Acetyl choline diffuses across the synapse and
Acetyl choline binds with the receptor sites on the sarcolemma (the muscle cell membrane)
The sarcolemma depolarises
An action potential is created (from –90 mV to +40mV) once the threshold of the muscle cell is reached (all-or-nothing response).
© 2016 Paul Billiet ODWS

11. The muscle cell
plasma membrane
© 2016 Paul Billiet ODWS

12. The muscle cell
sarcoplasmic reticulum covers the myofibrils and stores Ca2+
Infoldings from the sarcolemma (T-tubules) connect to sarcoplasmic reticulum
Depolarisation of the sarcolemma leads to depolarisation of the sarcoplasmic reticulum
The depolarised sarcoplasmic reticulum becomes permeable to Ca2+
© 2016 Paul Billiet ODWS

13. Muscle contraction
Ca2+ diffuses into the cytosol (cytoplasm of the muscle fibre) from the sarcoplasmic reticulum
Troponin/tropomyosin protein complex blocks actin filament stops myosin head groups from binding to it
Ca2+ lifts the blockage.
© 2016 Paul Billiet ODWS

14. ATP is hydrolysed myosin reaches out and attaches to actin
ADP is released the myosin head moves
When myosin picks up an ATP molecule myosin detaches itself from actin
Scitable Nature Publishing
© 2016 Paul Billiet ODWS

15. The power stroke Myosin is an actin-activated ATPase
When it binds with actin it hydrolyses ATP to ADP and inorganic Phosphate (Pi)
As ADP & Pi is released the conformation (shape) of the myosin head changes
ATP hydrolysis is coupled to movement at a molecular level
This change pushes the myosin along the actin filament (the “power stroke”)
By 8-10nm
The muscle contracts.
© 2016 Paul Billiet ODWS

16. Muscle relaxation
Contraction continues as long as Ca2+ levels remain high in the cytosol
Ca2+ is rapidly pumped back across the sarcoplasmic reticulum out of the cytosol (this also requires ATP) …
and the muscle relaxes.
© 2016 Paul Billiet ODWS