Cardiac Muscle Physiology

I. Cardiac MUSCLE cells: Stable RMP, not self-excitable. Cardiac muscle cells operate ONLY via twitches – they can never reach tetany, unlike skeletal muscle.

RECALL: Skeletal muscle – timing of twitch:

Cardiac muscle – timing of twitch:

Heartbeats are just one twitch after another. How to make contractions stronger? Increase calcium levels – cardiac muscle contractions are graded (similar to smooth muscle), and when there’s more calcium available, the contraction is stronger.

Skeletal: at optimal length, at tetany; variation in contraction = recruitment of motor units

Cardiac: entire muscle contracts (no motor units), cannot reach tetany variation in contraction = 1) increased stretch towards optimal length

2) graded contractions (the more Ca2+i, the stronger the contraction)

2. Cardiac PACEMAKER cells: unstable membrane potentials that continually drift towards threshold (no RMP); because of this, they are “self-excitable”:

Neurotransmitters from the ANS simply affect the RATE of depolarization: