What are the properties of the excitability of a muscle cell, muscle fiber?

Muscle fibers are able to contract under the action of impulses sent by the brain. These pulses are inherently very weak electrical discharges. Muscle fibers contract from these discharges. The heart is an organ made up of muscle tissue, but the heart does not contract from brain impulses. The impulses are created directly in the heart. If a person is electrocuted, the muscles contract under the influence of electricity. Various involuntary movements, convulsions and so on begin.