MUSCLE CONTRATION

Motor neurones are the neurones that connect the central nervous system (sometimes via intermediate neurones) to the effectors (muscles)

• Stimulus is recieved on presynaptic membrane, depolarisation occurs and an action potential is generated and passed down the neurone.
• As the action potential reaches the presynaptic knob of the neurone, the calcium ion channels open and there is an influx of calcium ions into the presynaptic knob. 
• The calcium ions stimulate the vesticles containing acetylcholine (a neurotransmitter) on the inside of the presynaptic knob to fuse with the presynaptic membrane. This causes the vesticles to release acetylcholine into the synaptic cleft.
• acetylcholine diffuses across the synaptic cleft and binds to the receptor sites on the postsynaptic membrane.
• this causes the ligand gated sodium channels to open, allowing sodium to move into the postsynaptic membrane.
• This depolarises the postsynaptic membrane and once threshold value is exceeded, an action potential is generated and self propagates itself in a wave along the entire postsynaptic membrane. 

• as the action potential reaches the post synaptic membrane, it is passed deep into the muscle fibres via a system of T-tubules.
• These T-tubules pass through the entire sarcoplasm (the cytoplasm of the muscle fibre) and they send the action potential into the sarcoplasmic reticulum.
• the sarcoplasmic reticulum contains calcium ions, and the action potential stimulates it to release these calcium ions into the sarcoplasm.
• when released, the calcium ions bind to troponin, which causes tropomyosin to change shape. Tropomyosin was previously blocking the binding sites on the actin molecule, which are now exposed.
• The myosin heads use an ADP molecule to bind to the binding sites on the actin filament.
• This binding causes the myosin heads to change shape, sliding the actin filament over the myosin filament. ADP is also released from the myosin head.
• ADP molecules bind to each of the myosin heads, causing the myosin filament to become detatched from the actin filament.
• Calcium ions stimulate ATPase to hydrolyse these ATP molecules into ADP and P, releasing energy that is used to move the myosin heads back into their origional position.
• the heads can then combine with ADP again and bind to the actin filament further along and repeat the whole process.
• the process can be repeated up to 100x per second, and calcium and ATP must be present.

• when nervous stimulation ceases, calcium ions use ATP to be actively transported back into the endoplasmic reticulum.
• the readsorbtion of calcium ions allow tropomyosin to block the actin binding sites once more.
• myosin heads cannot bind to actin filaments, no contraction occurs. Muscle is relaxed.

Exam questions often ask the importance of ATP and of Calcium ions

ATP

• used to change the shape of the myosin heads in muscle contraction
• used in the active transport of calcium ions into the endoplasmic reticulum during muscle relaxation

CALCIUM IONS

• used to bind to troponim in order for tropomyosin to change shape and unblock actin binding sites
• used to activate ATPase to hyrolyse ATP into ADP and P for energy (for above reasons.)