Heart Muscle Flashcards
What are the characteristics of cardiac muscle?j
Branching cells 1-2 nuclei/cell Striated, interconected with intercalated discs Ca++ source= SR and ECF Tropomyosin+ Troponin Involuntary Medium speed contractions Abundant mitochondria No motor United Extensive system of T-tubules
Where does the Ca++ in cardiac muscle come from?
Terminal cisternae contains less ca++, mainly by influx from extracellular space
What is intercalated discs?
Membrane junctions that hold adjacent cells together and transmitts the contraction force to each cell
Why is gap junctions important?
Allow interchange and communication between the sarcoplasm of connected cardiac muscle cells
What are the three major types of cardiac muscle?
Atrial muscle
Ventricular muscle
Specialized excitatory and conductive muscle fibers
What are the two types of cells within the heart?
Autorythmic cells (pacemaker) Contractile cells
What is the features of autorythmic cells?
1% of cardiac cells
Acts as a pacemaker (sets the rhythm of electrical excitation)
Form the conduction system of the heart
SAN-AVN-Bundle of His-R&L bundle branches-Purkinje fibers
AUTORHYHMICITY-EXCITABILITY-CONDUCTIVITY-REFRACTORITY
What are the features of contractile cells?
99% of cardiac cells
Makes the muscular walls of the atrium and ventricles
CONTRACTILITY-EXTENSIBILITY
What does Autorhythmicity mean?
The ability to spontaneously generate and discharge an electrical impulse without external stimulation
What does exitability mean?
The ability to respond to an electrical impulse
What does conductivity mean?
The ability to transmit an electrical impulse from one cell to the next one
What does refractority mean?
The inability of cardiac cell to initiate another AP for some duration of time
What does contractility mean?
The ability of the cell to contract in response to stimulation
What does extensibility mean?
The ability of the cell to stretch
What are the unions of intercalated discs?
Gap junctions (allow free diffusion of ions and AP between adjacent fibers-electrical union) Desmosomes (mechanical union) Fascia adherence (mechanical union)
What is the meaning of functional syncytium?
That the heart are an unique functional unit
What are the parts of the functional syncytium?
Atrial syncytium (the walls of the two atria) Ventricular syncytium (the walls of the two ventricles)
What is the function of having two syncytiums?
The all or None response
Either it contracts or not
Each syncytium contracts individually, never at the same time
No gap junctions between atria and ventricles!!
What is the sequence of the conducting system?
Sinoatrial Node SN Atrioventricular Node AN Bundle of His R&L bundle branches Purkinje fibers
What are the “rhytms” of pacemaker cells?
SA Node- 60-100/min
AV Node and AV bundle- 40-60/min
Bundle branches and Purkinje fibers- 20-40/min
What pacemaker cells acts as the heart pacemaker and why?
SA Node due to having the fastest rate of generating AP
What happens if SA Node fails?
The other pacemaker cells can take over but at a slower rate
Describe the contraction cykle of pacemaker cells
Does not maintain constant RMP
RMP starts at -60mV and gradually depolarizing until -40mV (TP)
Spontaneous depolarization due to slow continous influx of Na+
Depolarization starts with fast Ca++ voltage gated channels open, peaks at -10mV
Repolarization when Ca++ voltage gated channels close and K+ voltage gated channels open and K+ ions rapidly leaves the cell
When MP reaches -60mV a new cykle starts
What are the RMP of cardiac cells?
Atrial muscle cells and His/Purkinje system= -80/-90mV
SA and VA nodes= -65/-50mV
What are the phases of AP in contractile fibers?
Phase 0- rapid depolarization, opening of fast Na+ vgc leading to AP
Phase 1- start of repolarization, rapid closure of Na+ vgc, exit of K+ through cardiac transient outward potassium current channels (CKto) with membrane permiability for K+ five times lower
Phase 2(plateau)- opening of slow Ca++ vgc
Phase 3- repolarization closing of slow Ca++ vgc and exit of K+ via K+ vgc thus the permiability for K+ rapidly increasing
Phase 4- restoring of RMP
Why is repolarization in cardiac cells slower and AP 100 times longer?
Due to slow Ca++ channels and membrane permiability for K+ decreased by five times
Describe the spread of cardiac excitation
Impulse originates at SA Node
AP spreads throughout left and right atria
Impulses passes from atria into ventricles through AV Node
AP is delayed by AV Node and then rapidly travels interventricular septum by bundle of His
Impulse rapidly disperses throughout myocardium by Purkinje fibers
Rest of Ventricular cells activated by spread of impulse via gap-junctions
Describe the effect of the refractory period of cardiac muscle
It has a long redractory period compared to skeletal muscle
The refractory period lasts longer than muscle contraction to prevent tetanus
Heart gets time to relax after each contraction= prevents fatigue
Allows time for the Chambers to fill during diastole before next contraction
