Electrical Activity of the Heart Flashcards
Describe the T-tubles in cardiac muscle cells compared to skeletal
muscle cells
The t-tubules of myocardial cells are larger than those of skeletal muscle, and they branch inside the myocardial cells.
Describe the sarcoplasmic reticulum in cardiac muscle compared to skeletal muscle and explain why this difference exists
Myocardial sarcoplasmic reticulum is smaller than that of skeletal muscle, reflecting the fact that cardiac muscle
depends in part on extracellular Ca2+ to initiate contraction. - In cardiac muscle Ca2+ from outside the cell regulates contraction.
Describe how Ca2+ can regulate contraction of myocardial cells
If cytosolic Ca2+ concentration decrease then less Ca2+ will enter the cell snd less troponin will be saturated so the force of the contraction will low. vice-versa if cytosolic Ca2+ conc. is high
In terms of action potential, the plate duration is determined by Ca2+
Describe the intercalated disk
Cell junctions that connect neighbouring cardiac muscle cells end-to-end
What are the two components of intercalated disks?
- Desmosomes
- Gap junctions
What is the role of desmosomes?
Physical connection between the cells that keep the muscle cells connected even when action potentials are fired
What is the role of gap junctions?
Electrical connections that allow wave of depolarisation to spread rapidly from cell to cell so that all the cardiac muscle cells contract simultaneously
What feature allows the cardiac muscle to exhibit no tetanus?
The refractory period of the heart muscle is much longer after an action potential has fired. This is due to the the longer period of time that the cardiac action potential lasts for
How long does the action potential of cardiac muscle last for?
250 msec
How long does the action potential for skeletal muscle last for?
2 msec
What are pacemaker cells?
Auto-rhythmic cells that can spontaneously fire action potentials without the need of the nervous system. They usually have a RMP of -60, making them unstable.
Describe how the non-pacemaker action potential is fired?
Resting membrane potential
-high resting PK+
Initial depolarisation
-increase in PNa+ by the opening of Na+ voltage-gated channels
Initial Repolarisation
-Na+ channels close
Plateau
-increase in PCa2+ (L-type), and decrease in PK+
Repolarisation
-decrease in PCa2+, and increase in PK+
P=permeability
Describe how the pacemaker action potential is fired?
Action potential
-increase in PCa2+ (L-type)
Pacemaker potential (= pre-potential) not a simple story! Due to a ... gradual decrease in PK+ early increase in PNa+ (= PF) late increase in PCa2+ (T-type)
Pacemaker explains autorhythmicity - the heart is able to beat even when there is neural input
Modulators of electrical activity - drugs
- Ca2+-channel blockers – decrease force of contraction
- cardiac glycocides – increase force of contraction
Modulators of electrical activity - Temperature
increases ~10 beats/min/ºC
