Why is my index finger having involuntary spasms?

Muscle/Neuro/Lytes? There are many potential causes tht may be related to muscular, neurologic or electrolyte imbalances but all require physical evaluation.
Multiple causes. Spasms in the index finger can be caused by many things - excessive use can read to cramping of the finger. A low calcium level can also cause muscle spasms. If it is persistent, it would make sense to get an evaluation from your doctor.