Cardiac Electrophysiology, Autonomic Control & Blood Composition

Tetanus Prevention in Cardiac Muscle

• Skeletal muscle can reach tetanus (sustained contraction) → heart must AVOID this.

• Myocardial Contractile Cells (MCCs) action potential:

  • Rapid depolarization: Na^+ influx.

  • Brief early repolarization: small K^+ efflux.

  • Plateau phase: slow Ca^{2+} influx keeps membrane positive.

  • Final repolarization: large K^+ efflux.

• Plateau makes electrical and mechanical events end together → MCCs cannot fire again until relaxation ends → no tetanus.

• Skeletal muscle AP is short; many APs can arrive during one twitch → tetanus possible.

Electrical Activity of Myocardial Autorhythmic Cells (MACs)

• Exhibit continuous cycles of depolarization/repolarization without resting state.

• Pacemaker potential (from \approx -60\,\text{mV} to -40\,\text{mV}):

  • Driven by “funny” current channels (I_f) allowing Na^+ in & K^+ out; Na^+ influx dominates → slow depolarization.

• Threshold ( -40\,\text{mV} ) opens Ca^{2+} channels → rapid depolarization to \approx +20\,\text{mV}.

• Repolarization via K^+ efflux → returns to -60\,\text{mV}; I_f channels reopen.

• Intrinsic rate (no neural input): 80\text{–}100\;\text{cycles·min}^{-1}.

Autonomic Modulation of MACs

• Parasympathetic (vagus, ACh → muscarinic):

  • \uparrow P{K^+} , \downarrow P{Ca^{2+}}.

  • Flattens pacemaker slope → slower reach to threshold.

  • Rate ↓ to 60\text{–}80\;\text{cycles·min}^{-1} (resting heart rate).

• Sympathetic (NE/Epi → \beta_1):

  • \uparrow P{Na^+} , \uparrow P{Ca^{2+}}.

  • Steeper pacemaker slope & faster depolarization peak.

  • Rate ↑ to >100\;\text{cycles·min}^{-1}.

Autonomic Effects on Contractile Cells

• Sympathetic (NE/Epi → \beta_1) ↑ Ca^{2+} handling → stronger ventricular contraction.

• Parasympathetic: no direct effect on MCCs.

Blood Composition Overview

• Total volume: \approx 5\text{–}6\;\text{L}.

• Hematocrit (~45%): red blood cells (erythrocytes) → O2 delivery, CO2 removal.

• Plasma (~55%): water + ions ( Na^+, K^+, Ca^{2+}, Cl^-, Mg^{2+} ), nutrients, gases, wastes; often called serum when clotting factors removed.

• Buffy coat (<1%):

  • Leukocytes (white blood cells) → immune defense.

  • Platelets (cell fragments) → hemostasis.

• Erythrocytes & platelets lack nuclei; only leukocytes are true “cells.”