What Causes Recurring Spasms On The Nape Radiating To The Body?

My husband is 76 years old and has been having these symptoms for about 6 months. It starts in the back of his neck and feel like an electrical shock which jumps down his entire body. It is not really painful just jerks his body quickly which startles him and feel very disturbing. It usually happens when he is laying in bed. He had an MRI on his head and his neck but nothing showed up. I read once that this can sometimes be a lack of Vitamine B12. Have you ever heard of this condition? He is very concerned and afraid it is something serious. Thanks for your time. My e-mail is YYYY@YYYY