Question

1. Your pet has a muscle that does not seem to be contracting when stimulated via the motor neuron fibre. Can you explain any possible problems? You should include the steps in neuromuscular junction synaptic transmission and excitation-contraction coupling in your answer.

Answer 1

When Action Potential responsible for muscle Contraction Reaches Pre synaptic Membrane---There is Release of Ca2+ from Voltage gated Ca2+ Channels in Pre Synaptic Membrane---this Causes fusion of Synaptic vesicle with pre synaptic Membrane ---which results in the Release of Neurotansmitter(Acetyl CHoline) into Synaptic Cleft---It Binds to Acetyl Choline receptors in the Post Synaptic Membrane---which leads to Opening of Voltage gated Na+ channels---this results in the Depolarisation in the Post Synaptic Membrane or Sarcolemma---This causes Release of Ca2+ from Sarcoplasmic Reticulum.

In a Relaxed muscle,Tropomyosin molecules masks the Myosin binding Site on actin molecule.But During Stimulus,When action potential Reach the Motor End plate,the released ca2+ Binds to Troponin molecules.This troponin molecules are in contact with Tropomyosin molecule,so Binding of Ca2+ on troponin molecule induces Change in shape of Troponin molecule so that Tropomyosin gets moved from the myosin binding sites on Actin molecule.

This exposes Myosin binding sites on actin ,So that Myosin heads binds to Action------which form a cross bridge --now they contract using ATP as energy---ATP is re-synthesised ,actin and myosin maintain their binding state--During this the actin filaments are pulled over myosin filament which causes muscle contraction.

So the Problem that makes the pet not contracting may be associated with -

1.Pre synaptic calcium channel which release calcium necessary for Synaptic vesicle fusion.

2.Toxins that Inhibits Neurotransmitter or Neurotransmitter release.

3.Anti Acteyl choline receptor antibodies which destroys antibodies so that binding of Neurotransmitter(Acetyl choline) to its receptor is affected.

4.Damage to Post synaptic Voltage gated Na+ channel