Final answer:
In cardiac muscle contraction, extracellular calcium is required for contraction, which is different from skeletal muscle contraction where most of the calcium comes from the sarcoplasmic reticulum.
Step-by-step explanation:
Cardiac muscle contraction is unique compared to skeletal muscle contraction in several ways. One of the unique features of cardiac muscle contraction is the sustained depolarization 'plateau' caused by calcium ion (Ca²⁺) entry through voltage-gated calcium channels. This sustained depolarization allows for longer contractions in cardiac muscle compared to skeletal muscle. Another unique aspect of cardiac muscle contraction is that a large percentage of the Ca²⁺ ions responsible for initiating contraction come from outside the cell rather than from the sarcoplasmic reticulum (SR). This is different from skeletal muscle, where most of the Ca²⁺ ions come from the SR. Therefore, the correct answer is 5) extracellular calcium being required for contraction.