Page 149 - Physiology and Pathophysiology MNU 2024-2025 نظرى

Clinical pharmacy 2024/2025 Level 2 Pharm D Physiology and Pathophysiology (MD303)
Properties of Cardiac Muscle

1-Automaticity

Definition The Ability of Cardiac Muscles to generate action potentials Spontaneously (Without neural stimulation) = self-beat
Peacemaker 1) Primary pacemaker: SA node 2) Latent (Ectopic) pacemakers [AV node, bundle of His, Purkinje
Fibers➔ fire at slower rate than SA node

A) When the SA node drives the heart rate ➔ the latent pacemakers are inactive
Pacemaker Firing Pacemaker Firing
B) Latent pacemaker becomes the pacemaker of the Heart if:
rate rate
 The SA node firing rate ➔ ↓ [due to vagal stimulation] or Stops
SA node 60-100 Bundle of 40
Completely [because SA node is destroyed or suppressed by drug]
His
 If the conduction of impulses from SA node to
AV node 40-60 Purkinje 15-20
the rest of the heart is blocked
fibers
Definition Change of the heart rate by affecting firing of SA node (the effect on Heart rate)
Positive Chronotropic effect Negative Chronotropic effect
Chronotropism
Types ↑ in heart rate (Tachycardia) ➔ ex: ↓ in heart rate (Bradycardia) ➔ ex:
Sympathetic drug parasympathetic drug

2-Conductivity

Ability to conduct impulses throughout the cardiac tissue ➔ (Represented by PR "PR prolonged ➔ heart block” & "PR short
Definition
➔↑Conductivity")

In Purkinje System ➔ to ensure that the ventricles are activated for
Fastest
Types of efficient ejection of blood

conduction Slowest In AV node ➔ allowing enough time for ventricular filling before
ventricular contraction مهم
NB
✓ ↓ in conduction velocity at AV node ➔ Heart block ةمسر اهل
✓ If AV conduction velocity    Ventricular filling. ✓ If AV conduction velocity   Heart block.

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