Electrical Properties of Cardiac Cells PDF

Electrical Properties of Cardiac Cells by Dr. Greg Szulgit  learning goals: 1. Understand action potentials (APs) in general 2. Understand myogenic APs 3. Understand the conduction of waves of APs 4. Understand the contraction of cardiac muscle cells quic...

Electrical Properties of Cardiac Cells by Dr. Greg Szulgit  learning goals: 1. Understand action potentials (APs) in general 2. Understand myogenic APs 3. Understand the conduction of waves of APs 4. Understand the contraction of cardiac muscle cells quick review of neurons As a student, your first exposure to action potentials (APs) was probably to those in the nervous system.  learning goals: 1. Understand action potentials (APs) in general 2. Understand myogenic APs 3. Understand the conduction of waves of APs 4. Understand the contraction of cardiac muscle cells  hearts are myogenic hearts can beat without neural stimulation https://www.youtube.com/watch?v=lmhBEeEMqYo another example (beware; lots of blood) https://www.youtube.com/watch?v=HYWmYJNg5Jw  The pacemaker cells It starts with the sino-atrial node (SAN) The blue tissue exhibits spontaneous generation of APs  How does this happen? Discuss this with your groups.  Here’s a hint. AP in a pacemaker cell membrane potential (mV) time (ms) Describe this. “funny current” 1979 funny channels  funny channels Funny channels Note: There appear to be no and T-Type ‘regular’ Na+ channels in the calcium channels pacemaker cells in the apex are voltage- of the SAN, so the sodium gated, but they ion influx is due to funny are also opened channels. by cAMP. When this happens, sodium and calcium ions enter cell. take home messages of previous slide: funny channels allow Na+ to enter and are only in pacemaker cells T-type calcium channels allow Ca2+ to enter transiently (quickly) and are only in pacemaker cells L-type calcium channels allow Ca2+ to enter long term and are in pacemaker cells and cardiomyocytes they are voltage-gated they are also activated by cAMP General point for biologists: Neurotransmitters and drugs often work indirectly through G-proteins 1994 Nobel prize in Physiology or Medicine There are many channels affected by G-proteins.  AP in a pacemaker cell membrane potential (mV) time (ms)  hearts are myogenic and here are isolated heart cells (cardiomyocytes) that have been growing and replicating in a pitri dish  learning goals: 1. Understand action potentials (APs) in general 2. Understand myogenic APs 3. Understand the conduction of waves of APs 4. Understand the contraction of cardiac muscle cells  conduction system special fibres that are The contractile made of muscle cells muscle tissue (the wrapped in cells that comprise collagenous tissue* this tissue are (these are generally generally referred to referred to as as cardiomyocytes). pacemaker cells).  cardiac skeleton (aka skeleton of the heart)  the conduction system is also surrounded by fibrous (collagenous) tissue connective *The evidence for tissue around the pacemaker bundles is difficult and contentious. what does the cardiac skeleton do? 1. structural 2. guides and insulates electrical conduction down the specialised muscle cells within 3. causes electrical waves passing through heart muscle to pause between atria and ventricles  T-tubules (an extension of the sarcolemma) membrane (sarcolemma) myofibril sarcomere mitochondria to supply sarcoplasmic reticulum (banded by z-disks) lots of energy (holds lots of Ca++) cardiac muscle cells (cardiomyocytes) are conductive  fluorescent labeling for connexin (the protein that makes gap junctions) Gap junctions are important to keep the APs flowing to neighboring cells.  Here’s what it looks like all together (from a gif on Wikipedia). With waves, timing is everything!  learning goals: 1. Understand action potentials (APs) in general 2. Understand myogenic APs 3. Understand the conduction of waves of APs 4. Understand the contraction of cardiac muscle cells  T-tubules (an extension of the sarcolemma) membrane (sarcolemma) myofibril sarcomere (banded by z-disks) mitochondria to supply sarcoplasmic reticulum lots of energy (holds lots of Ca++) in skeletal muscle  in cardiac muscle Not only is Ca2+ shuttled across the membrane of the sarcoplasmic reticulum, it is also allowed to flow in from the T- tubules across the cell membrane via L-type channels. in cardiac muscle The incoming Ca2+ will activate ryanodine receptors. These are Ca2+ channels that are activated by Ca2+ (so, even more Ca2+ comes out). cardiomyocyte AP Effective Refractory Period. Note that cardiomyocytes cannot be re- stimulated for a long time compared with skeletal muscle.  in cardiac muscle The reason that the potassium permeability looks so complicated is that there are at least different K+ currents. Question: why are the rising slopes different? (f) = “funny” NOT “fast”  learning goals: 1. Understand action potentials (APs) in general 2. Understand myogenic APs 3. Understand the conduction of waves of APs 4. Understand the contraction of cardiac muscle cells  Coda: Let me show you a magic trick. Don’t take things for granted as blasé just because we are used to them. Consciousness is the holy grail of biology. Stay curious!