Myocardial cells have long refractory periods in order to ensure each contraction is. This is due to the presence of
channels in these cells that allow theof ions right after
ions have entered.

These contractile elements are virtually identical to skeletal muscle. ... (a) Cardiac muscle cells have myofibrils composed of myofilaments arranged in sarcomeres, ... The refractory period is very long to prevent the possibility of tetany, a ... At this point, the calcium ion channels close and K+ channels open, allowing outflux ..

Explanation:

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the generally accepted parts of modern cell theory include: all known living things are made up of one or more cells. all living cells arise from pre-existing cells by division. the cell is the fundamental unit of structure and function in all living organisms.