CARDIAC PACEMAKER POTENTIALS

·         Seen in Pacemaker tissue a.k.a. Slow Response Fibres

o   SA node

o   AV node

o   His-Purkinje system

o   Coronary sinus

o   Pulmonary veins

·         Features

o   Action potentials in Pacemaker tissue is mainly due to Ca²⁺, with little contribution from Na⁺ (Therefore, there is no sharp/rapid depolarization spike in initial part of action potential)

o   In partially depolarized condition (i.e. -60mV) voltage-gated Na⁺ channels are in their inactive state; therefore do not take part in the action potential.

o   Characteristic feature- ‘Prepotential’ a.k.a. ‘Pacemaker potential/current’

o   Atrial & ventricular muscle fibres do not have prepotentials; therefore they do not discharge automatically. (But, may discharge when injured/ abnormal).

o   RMP of pacemaker tissue (-60mV) is relatively more positive than that of atrial & ventricular muscle.

·         Two types of Ca²⁺ channels present

o   Ca_T (Transient) channels- opening mainly responsible for prepotential

o   Ca_L (Long-acting) channels- responsible for upstroke/phase 0

·         Phases-

o   Phase 0- Upstroke/Impulse (due to slow Ca²⁺ current)

o   Phase 1- Repolarization (due to delayed rectifier K⁺ current)

o   Phase 2- Prepotential

§  Due to inward Ca²⁺ (major), inward Na⁺ (minor) & outward K⁺ (minor) current

· Main phase affected by autonomic nervous system.

§  Rise in intracellular Ca²⁺ mainly occurs due to-

·         Influx of Ca²⁺ via Ca²⁺ channels

·         Local Ca²⁺ release from sarcoplasmic reticulum (Ca-sparks)

•      Regulation-

o   ANS control

§  Parasympathetic Nervous System

§  Sympathetic nervous system

o   Temperature- Increased temperature causes increased firing