Action Potential of Contractile Fibers

What happens during the "Reaching threshold" phase?

Slow voltage-gated Na⁺ channels open, causing an inflow of Na⁺ that changes the membrane potential from -60 mV to -40 mV.

What occurs during the "Depolarization" phase?

Fast voltage-gated Ca²⁺ channels open, causing an inflow of Ca²⁺ that changes the membrane potential from -40 mV to just above 0 mV.

What happens during the "Repolarization" phase?

Fast voltage-gated Ca²⁺ channels close, voltage-gated K⁺ channels open, allowing K⁺ to exit. The membrane potential returns to the resting membrane potential (RMP) of -60 mV, and K⁺ channels close.

What is the resting membrane potential (RMP) in nodal cells?

The stable electrical charge when the cell is not transmitting an action potential. (-60mV)

What occurs during depolarization in cardiac muscle fibers?

Fast Na⁺ channels open, causing a rapid influx of Na⁺ and a rapid depolarization.

What happens during the plateau phase of the action potential?

- K⁺ channels open, but Ca²⁺ influx through slow channels balances K⁺ efflux.

- Ca²⁺ is released from the sarcoplasmic reticulum, prolonging the contraction.

What occurs during repolarization in cardiac muscle fibers?

K⁺ channels remain open, allowing K⁺ to leave, while Ca²⁺ channels close.

What is the refractory period in cardiac muscle?

A period (0.3 seconds) where the heart cannot respond to a new stimulus, limiting the max heart rate to 200 bpm.

Why is the refractory period of cardiac muscle longer than skeletal muscle?

To prevent tetany (sustained contraction), ensuring the heart can relax and fill between beats.

How does parasympathetic innervation affect heart rate?

Parasympathetic fibers slow the heart rate to 70-75 bpm by affecting the SA node. (0.8s)

How would heart fibers behave without nervous input?

They would fire at about 100 bpm on their own (every 0.6 seconds).

Do autonomic nervous system fibers and hormones initiate the heart rate?

No, they modify the heart rate but do not initiate it.