Humans have the enzyme hSMUG1, which removes any U residues that occur in single-stranded DNA during replication or transcription.  Two other human DNA glycosylases, TDG and MBD4, remove either U or T residues paired with G, generated by deamination of cytosine or 5-methylcytosine, respectively.

TLS Online TPP Program

#Id: 5258

#Unit 7. System Physiology – Animal

Conducting cells are specialized muscle cells which function to rapidly spread action potentials over the entire myocardium.

TLS Online TPP Program

#Id: 5259

#Unit 7. System Physiology – Animal

The action potential spreads throughout the myocardium in the following sequence:

SA node  Atrial internodal tracts and atria  AV node  Bundle of His  Purkinje system  Ventricles

TLS Online TPP Program

#Id: 5260

#Unit 7. System Physiology – Animal

Myocardial fibers have a resting membrane potential of approximately –90 mV.

TLS Online TPP Program

#Id: 5261

#Unit 7. System Physiology – Animal

The individual fibers are separated by membranes, but depolarization spreads radially (syncytium) because of the presence of gap junctions.

TLS Online TPP Program

#Id: 5262

#Unit 7. System Physiology – Animal

In cardiac muscle, the action potential is caused by opening of three types of channels: 

Voltage activated fast sodium channels 

L-type calcium channels (slow calcium channels or calcium-sodium channels)

Voltage activated potassium ion channels

TLS Online TPP Program

#Id: 5263

#Unit 7. System Physiology – Animal

Phase 0 (depolarization), fast sodium channels open.