Editing T wave (section)

== Cardiac physiology ==
The [[refractory period (physiology)|refractory period]] of cardiac muscle is distinct from that of skeletal muscle. Nerves that innervate skeletal muscle have an extremely short refractory period after being subjected to an [[action potential]] (of the order of 1 ms). This can lead to sustained or [[tetanic contraction]]. In the heart, contractions must be spaced to maintain a rhythm. Unlike in muscle, repolarization occurs at a slow rate (100 ms). This prevents the heart from undergoing sustained contractions because it forces the refractory period and [[cardiac action potential]] firing to be of the same length of time.

Repolarization depends on the charges of ions and their flow across membranes. In skeletal muscle cells, repolarization is simple. First, [[sodium|sodium ions]] flow into the cell to depolarize it and cause skeletal muscle contraction. Once the action potential is over, [[potassium|potassium ions]] flow out of the cell due to increased [[cell membrane]] permeability to those ions. This high permeability contributes to the rapid repolarization of the [[membrane potential]]. This repolarization occurs quickly enough that another action potential can cause depolarization before the last action potential has dissipated. Cardiac muscle differs in that there are more calcium channels that counteract the potassium channels. While potassium quickly flows out of the cell, calcium slowly flows into the cell. This causes the repolarization to occur more slowly, making the refractory period as long as the action potential, preventing sustained contractions.

The T wave is representative of the repolarization of the membrane. In an EKG reading, the T wave is notable because it must be present before the next depolarization. An absent or strangely shaped T wave may signify disruption in repolarization or another segment of the heartbeat.<ref>{{Cite book|title=Human physiology: the mechanisms of body function|author=Raff, Hershel|last2=T.|first2=Strang, Kevin|author3=Vander, Arthur J.|isbn=978-1259294099|oclc=914339346|date = 2015-11-03}}</ref>