Cardiovascular system revision note -1

Cardiovascular system revision note - introduction

Cardiovascular system composed of

Functions

Pace maker cells

Rhythmically discharging cells
Show prepotentials ( pacemaker potentials)
Membrane potential declines to firing level after each impulse .triggers the next impuls
At peak of each impulse, K+ efflux causes repolarization
K+ efflux decreases , so membrane becomes depolarized , forming first part of prepotential
Ca+2 channels then open
Two types

T- Transient – complete the prepotentials

L- Long Lasting – produces the impulse

Other features

Effect of autonomic nerves system

Parasympathetic

Sympathetic nerves system

Rate of pacemaker cell discharge

Conducting system of heart

Tissue	Location	Rate of discharge	Importance
SA node	Junction of SVC and right Atrium	60- 100 per minute	Referred to as the pacemaker
AV node	Right posteror prtion of I.A septum	40- 60 per minute
Bundle of His	I.V septum ( subendocardium )	20- 40 per minute	Solo connection between atrial and ventricular syncytia
Purkinje fibers	Ventricular myocardium	20 -40 per minute

Electrical Activity of a contractile cell

Initial rapid depolarization and overshoot

Initial rapid repolarization

Plateau

Final repolarization

Due to closure of voltage gated Ca+2 channels and efflux of K+ ions through K+ channels

Resting membrane potential

Other features

Depolarization (duration 2 mS) is followed by a prolonged plateau (200 mS) (contraction time 250 mS)
Importance – cell is refractory to excitation during a contraction. Prevent fatal tetany

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