Which of the labeled structures in the diagram of the heart action potential conduction system consists of specialized cardiac muscle cells that spontaneously create action potentials at a set rate? 
Definitions:
Reinforcement
The increase in reproductive isolation that often occurs over time in a hybrid zone because natural selection strengthens and increases the number of prezygotic barriers between the two species.
Reproductively Isolated
A state in which two populations cannot successfully interbreed with each other under natural conditions, leading to the emergence of distinct species.
Parental Populations
The original groups or populations of organisms from which offspring or new populations are derived, used in genetic and evolutionary studies.
Sexual Selection
A type of natural selection that occurs when individuals of a species vary in their ability to compete for mates; individuals with reproductive advantages are selected over others of the same sex.
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