·
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 CaT (Transient) channels-
opening mainly responsible for prepotential
o CaL (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
I am a medical resident on cardiology and I also wrote a few words about cardiac pacemakers.
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