MgSO4 and loss of deep tendon reflexes

I understand that MgSO4 is a smooth muscle relaxer and that it also causes neuro/muscular blockade ~
Can you please explain the physiology behind how/why increasing levels of serum Magnesium will
diminish/stop deep tendon reflexes.
Is it due to the muscles inability to contract in response to the stimulus because the Magnesium has replaced the calcium in the muscle cell?
Or is it that the nerve pathway is unable to send the stim to the brain to cause the response of the reflex? If so, why?
Thank you very much.