Physiology 10th Lecture - Cardiovascular System

Cardiovascular system The heart Functional properties of myocardium: 1. Automaticity – spontenous formation of the impulse 2. Rythmicity – regularity of the impulse formation (60-80 per min) 3. Exaitability – accaptation of stimuli and reaction to them 4. Conductivity – conduction of stimuli through the myocardium 5. Contractility – haemodynamic function (to pump the blood) The law “everything or nothing” - after a stimulation by the suprathreshold stimulus the heart reacts by contraction of all cells or does not react when the stimulus was subthreshold. However, the contraction force can be changed. Cardiac conductive system: morphologically & functionally heterogenous Slow (Nodal) cells – low resting potential & amplitude, slow channels and conduction Fast Cells (His-Purkyne system) – high resting potential, fast Na channel and conduction. • Sinoartrial (sinus) node – P cells = normal pacemaker (60 – 80/min) • internodal atrial pathways: Bachmann, Throell, Wenkebach tracts • atrio-ventricular node – AN (upper), N (middle), NH (lower) parts, secondary pacemaker (40 – 60/min), conduction delay --> contraction of the ventricles not immediately after the atria. • Bundle of His – connection between the atria and ventricles, AV node + His bundle = AV junction. • Tawara branches – rights, left anterior, left posterior. • Purkyne cells – the last part, the largest cells, tertiary pacemaker (20-40/min) • transient cells – connection between the CS and working myocardium very sensitive to some pathological changes. Function of the conductive system • connection between the atria and ventricles • formation of the cardiac impulse • synchronized contraction of the whole heart. Cardiac electrophysiology Resting membrane petential (RMP) = the potential difference between the intra and extracellular space (result of displacement of ions). -85 to -100 mV (working myocardium), less in the conductive system. Nerst equation – RMP is the equilibrium potential of K+ RMP – fixed (constant) during the diastole – slow diastolic depolarization (SDD) – prepotential Action potential, in working myocardium – phases 0,1,2,3,4 0 – rapid depolarization (fast inward Na channels) 1 – initial repolarization (outflux of K+) 2 – plateau (delayed repolarization) – influx of Ca & Na through the slow non-specific Ca-Na channels

+ efflux of K+ Importance of plateau: transmission of Ca into myocytes, no tetanic contraction, filling the heart by the blood. 3 – final repolarization (fast K channel – efflux of K+) 4 – psotpotentials (post – depolarization + post-hyperpolarization) + resting potential Pacemaker cells – SDD, no phase 1, only slow channels, slow changes -65 mV – threshold level of MP, -35 mV – critical level of MP voltage dependent channels – level of AP opens or closes the channel Different AP in different parts of the heart. Changes of the MP: Depolarization – phase 0, from the RMP to overshoot (spike) Repolarization – 1,2,3 (from the spike back to the RMP) Transpolarization – during the positive MP (above 0) – a part of de- and re-polarization Hyperpolarization – more negative potential than the normal RMP Excitability of the cardiac muscle Refractory period – period of decreased excitability Absolute RP (effective) – depolarization + transpolarization Relative RP – phase 3 (-65 to -35 mV), AP through the slow channel only, it is smaller, deformed, slower conduction. Supranormal phase – at the end of the AP, between phases 3 and 4, oncreased excitability (lower threshold) Vulnerable period – even weak stimuli can produce an answer, up to VFL. Excitability threshold – very high during ARP, low during SNP, normal during the late diastole. Measuring of the ARP: • extrastimulation (S1,S2) (ARP = period S1-S2) • increasing rate of pulse (ARP = period between 2 impulses) Measuring of VP – one single overthreshold pulse (top of T wave) • inner border (beginning) – shifting of the pulse to the right from QRS • out border (enG) – shifting to the left from the diastole