The Neuromuscular Junction or end plate - the point where the effector nerve meets a skeletal muscle - many end plates spread throughout a muscle to stimulate a group of fibres called effector (motor) unit to create a rapid and powerful muscle contraction

Skeletal muscle fibre Motor neurone False colour scanning EM of a neuromuscular junction (3600x app.)

Neuromuscular junction – the end plate

-When a nerve impulse is received at the end plate, synaptic vesicles fuse with the end plate membrane and release their acetylcholine. -The transmitter travels across the sarcolemma where it alters its permeability to sodium ions which now rapidly enter, depolarizing the membrane.

-Provided the threshold value is exceeded, an action potential is fired in the muscle fibre and the effector (motor) unit served the end plate contracts. -Acetylcholine then breaks down by enzyme to ensure that the muscle is not overstimulated and the sarcolemma becomes repolarized.

Mechanism of Muscular Contraction - the sliding filament theory -filaments of actin & myosin slide past one another -actin filaments, hence the Z lines to which they are attached, are pulled towards each other, sliding as they do over the myosin filaments. No shortening of either type of filament occurs.

Appearance of a muscle fibre: 1. The isotropic band becomes shorter 2. The anisotropic band does not change in length 3. The Z lines become close together, i.e. the sarcomere shortens 4. The H zone shortens

Relationship of tropomyosin & troponin to the actin filament

Sliding filament mechanism of muscle contraction

Summary of Muscle Contraction: 1. Impulse reaches the neuromuscular junction 2. Synaptic vesicles fuse with end-plate membrane & release a transmitter (e.g. acetylcholine) 3. Acetylcholine depolarizes the sarcolemma 4.Acetylcholine is hydrolysed by acetylcholinesterase 5. Provided the threshold value is exceeded, an action potential is created in the muscle fibre 6. Ca 2+ are released from the T-system & sarcoplasmic reticulum 7. Ca 2+ bind to troponin, changing its shape

8.Troponin displaces tropomyosin which has been blocking the actin filament 9. Myosin heads now become attached to the actin filament 10. The myosin head changes position, causing the actin filaments to slide past the stationary myosin ones 11. An ATP molecule becomes fixed to the myosin head, causing it to become detached from the actin 12. Hydrolysis of ATP provides energy for the myosin had to be ‘cocked’ 13. The myosin head becomes reattached further along the actin filament

14. The muscle contracts by means of this ratchet mechanism 15. The following changes occur: (a) I band shortens; (b) Z lines move closer together (sarcomere shortens); (c) H zone shortens. 16. Ca 2+ are actively absorbed back into the T- system 17. Troponin reverts to its original shape, allowing tropomyosin to again block the actin filament 18. Phosphocreatine is used to regenerate ATP