L18. Anatomy and Electrical Properties of the Heart Flashcards
(15 cards)
Blood flow path through cardiovascular system?
Right side of the heart –> pulmonary trunk (get oxygenated) –> left side of the heart –> aorta –> around the body via blood vessels –> vena cava
–> right atrium again
Contractile myocardial cells?
- Arranged in layers and are closely associated with each other at the intercalated disc, and form the bulk of the atria and ventricles
- Unlike skeletal muscle cells, all the myocardial cells contract with every beat of the heart
- Gap junctions, which have low electrical resistance, are present in the intercalated disc
Gap junctions?
- Allow the small passage of ions and small molecules of up to 1KDa
- Filaments of actin and myosin are arranged similarly to skeletal muscle
–> striated appearance
Nodal tissue (SA and AV node)?
- 1% of the cardiac cells
- Contains small round cells with no, or little, contractile protein
- Specialised for the generation and conduction of action potentials in the atria
- Gap junctions are present
Conduction of the cardiac action potential?
- Action potentials spontaneously generated in the SA node
- Action potentials can travel from cell to cell by gap junctions - these are organised along preferential pathways, determined by the location of the intercalated discs
- Main conduction pathways comprise groups of specialised cells
- To get to the ventricles, action potential has to pass through the atrioventricular ring via the AV node
Roles of cardiac action potentials?
- SA node generates APs at about 100 - 110 min-1
- AP conducted through atrial muscle at ~0.5 ms-1
- AP conducted slowly through the AV node at ~0.05 ms-1; this provides a delay which permits full depolarisation and contraction of the atria before the ventricles are depolarised
- AP conducted rapidly through the bundle of His, bundle branches and Purkinjie system at ~5.0 ms-1 to the ventricular myocardium where it then spreads at ~0.5 ms-1; this allows a fairly synchronous depolarisation and contraction of all regions of the ventricles
- The rapidly conducting system comprises modified myocardial cells called Purkinjie fibres
Resting potential of cardiac tissue?
- The resting membrane potential (RMP) of nodal and myocardial cells depends upon a high resting permeability to K+ (Pk)
- Pk is lower in nodal tissue than myocardial cells and the RMP is less negative
- The RMP of the SA nodal cells is unstable
Pacemaker cells?
- Pk gradually reduces, and this combined with an increasing permeability to Na+ (PNa) and Ca2+ (PCa) causes the gradual decrease in RMP
- This slow depolarisation is called the pacemaker potential
- At threshold voltage, other Ca2+ channels open, and a relatively ‘slow’ AP occurs
- A similar AP occurs in AV nodal cells
- Other cells may spontaneously depolarise –> ectopic pacemakers/arrhythmias
Ventricular cells?
- Pk is higher and hence the RMP is more negative
- Depolarisation phase of AP is due to the opening of the voltage-gated Na+ channels
Excitation contraction coupling?
Calcium entry occurs at the T-tubules - L-type channels –> entry of Ca2+ into the sarcoplasm from outside the cell induces calcium release from sarcoplasmic reticulum –> calcium triggers contraction (via troponin), contraction lasts for as long as the plateau of the AP
Normally about 30% maximal effect
–> if more calcium, can get stronger contraction
Refractory period?
Long absolute and relative refractory periods prevent re-excitation of the heart muscle during most of the contraction period
They also prevent circuitous recycling of the action potential
Tetanus does not normally occur in heart muscle - summation of contractions isn’t possible
ECG?
Is a recording of potential changes at the skin surface that result from the depolarisation and repolarisation of the heart muscle
ECG waves?
P: atrial depolarisation
QRS: ventricular depolarisation
T: ventricular repolarisation
- Depolarisation toward +ve electrode = +ve bump
- Depolarisation away from +ve electrode = -ve bump
- Repolarisation away from +ve electrode = +ve bump
ECG leads?
- Left arm to right arm = lead I
- Left leg to right arm = lead II (classic ECG wave)
- Left leg to left arm = lead III
The AP moves through the AV node at ~0.05ms-1 BECAUSE the AP must move rapidly from the atria to the ventricles
A. If both statements are true, and the second causes the first
B. If both statements are true, but the second does not cause the first
C. If the first is true and the second is false
D. If the first is false and the second is true
E. Both statements are false
C
