Question

In what ways are autorhythmic fibers similar to and different from contractile fibers?

Answer 1

Autorhythmic fibers are self excitable cells that can repeatedly generate action potential and trigger heart contractions without any stimulus from nervous system. They have unstable RMP, which is the reason they achieve threshold potential on their own from RMP.

They are slow response action potential as there depolarization is caused by calcium channels whose recovery is time dependent and not voltage dependent.

Example: SA node, AV node

There are three phases in generation of Action potential:

Phase 0: Calcium influx

Phase 3: Potassium efflux

Phase 4: Leaky potassium channels, Funny ( Na+/K+) channels and transient Calcium channel bring RMP back to threshold on its own.

Contractile fibres perfrom contraction only if stimulated by nervous system. They have a stable RMP, so they require stimulus to achieve threshold potential.

Example: Non-automatic myocardial fibers, smooth muscles, skeletal muscles

There are 5 phases:

Phase 0: Na+ influx through voltage gated Na+ channels on stimulus.

Phase1: Transient rectifier K+ channel opens, K+ efflux

Phase 2: This is plateau phase as K+ efflux is balanced by slow opening Ca++ channels which causes Ca++ influx.

Phase 3: calcium and transient rectifier k+ channel closes. Delayed rectifier K+ channel causes repolarizatin.

Phase 4: RMP is stable and is maintained.

The similarities are that they both lead to contraction of muscles and are mediated by various ion channels.