Neuromuscular Junctions

What is a neuromuscular junction?

The junction between a motor neurone and a skeletal muscle fibre

Why are neuromuscular junctions distributed along the muscle length?

To ensure muscle fibres contract simultaneously for maximum power

What is a motor unit?

A motor neurone and all the muscle fibres it supplies

How is the force of muscle contraction controlled?

By varying the number of motor units stimulated

How is a strong muscle contraction produced?

By stimulating many motor units

How is a weak muscle contraction produced?

By stimulating a few motor units

What happens first when an action potential reaches a neuromuscular junction?

Voltage gated calcium ion channels open

What happens when calcium ions enter the neurone at a neuromuscular junction?

Acetylcholine vesicles release their contents into the synaptic cleft

How does acetylcholine reach the sarcolemma?

It diffuses across the synaptic cleft

What happens when acetylcholine binds to receptors on the sarcolemma?

Sodium ion channels open

What is the effect of sodium ions entering the muscle fibre?

The sarcolemma depolarises

What enzyme breaks down acetylcholine?

Acetylcholinesterase

What does acetylcholinesterase break acetylcholine into?

Choline and ethanoic acid

Why must acetylcholine be broken down?

To prevent continuous stimulation of the muscle fibre

What happens to choline and ethanoic acid after acetylcholine is broken down?

They return to the synaptic knob and are used to reform acetylcholine

What provides energy for the reformation of acetylcholine?

ATP supplied by mitochondria in the neurone

How does depolarisation spread deep into a muscle fibre?

Through transverse tubules

What is the function of the sarcoplasmic reticulum in muscle contraction?

It stores and releases calcium ions

What happens when an action potential stimulates the sarcoplasmic reticulum?

Calcium ion channels open and calcium ions enter the sarcoplasm

What triggers muscle contraction inside the muscle fibre?

Calcium ions released into the sarcoplasm

How do calcium ions initiate the sliding filament mechanism?

They bind to troponin and move tropomyosin away from actin binding sites