10.5: Generation and Transmission of Cardiac Impulses
- Page ID
- 121307
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Specialized cardiac cells generate and transmit electrical impulses that coordinate the heart’s rhythmic contractions.
- Differentiate between myocardial contractile and conducting cells and their roles in the heartbeat.
- Explain what the term "pacemaker cells" means.
- Trace the cardiac conduction pathway from the SA node to the Purkinje fibers.
- Summarize how the nervous and endocrine systems influence heart beats.
Intrinsic Regulation: Pacemaker Cells
Cardiac muscle cells are unique muscle cells found only in the heart, and they play a crucial role in keeping the heart beating and blood circulating throughout the body.
There are two major types of cardiac muscle cells:
- Myocardial contractile cells: they make up most of the heart and generate the force for pumping blood.
- Myocardial conducting cells: these cells account for only about 1% of the heart’s cells, but they are specialized to generate and rapidly transmit electrical impulses, rather like a built-in neuron network allowing the heart to synchronize contractions. Within the group of myocardial conducting cells is a very important subset known as autorhytmic or pacemaker cells.
So, while contractile cells focus on contraction, conducting and pacemaker cells are designed to start and propagate action potentials that travel through the heart and trigger those contractions.
The main cluster of pacemaker cells is found in the sinoatrial (SA) node, the “natural pacemaker” of the heart located in the right atrium. Pacemaker cells can generate their own electrical impulses without any external stimulus — this property is called “autorhythmicity”. These impulses set the pace for the heart’s rhythm by being the fastest cells in the heart and drive the timing for all other cells. If any part of the system is damaged or blocked, other conducting cells can act as backup pacemakers
As you have already learned, normal cardiac rhythm is established by the sinoatrial (SA) node, which is why it is called the heart’s natural pacemaker. It generates rhythmic electrical impulses that spread through the atria, triggering their contraction. The signal then travels to the atrioventricular (AV) node, where it briefly pauses before continuing through the heart’s specialized conduction pathways:
- Atrioventricular (AV) bundle, or bundle of His
- Right and left bundle branches
- Purkinje fibers
Working together, the SA node, AV node, and these pathways form the heart’s conduction system — an intrinsic wiring network that ensures each heartbeat occurs in a precise, coordinated sequence.
Cardiac Conduction System
Sinoatrial (SA) Node
The specialized myocardial conducting cells located in the upper posterior wall of the right atrium, near the opening of the superior vena cava make up the SA node. Because it has the fastest natural rate of depolarization, the SA node acts as the heart’s pacemaker, initiating each heartbeat and setting the sinus rhythm, or normal electrical pattern.
From the SA node, the electrical impulse spreads across the atrial muscle cells and toward the atrioventricular (AV) node through what is known as internodal pathways, allowing both atria to contract at nearly the same time. This synchronized atrial contraction ensures efficient filling of the ventricles before they contract. The impulse takes about 50 milliseconds (ms) to travel from the SA node to the AV node.
Atrioventricular (AV) Node
The AV node is another cluster of specialized myocardial conducting cells located in the lower part of the right atrium, within the atrioventricular septum. The fibrous skeleton of the heart — a dense connective tissue framework that surrounds the valves — acts as an electrical insulator, preventing impulses from passing directly from the atria to the ventricles. The only route for the signal to continue is through the AV node.
When the impulse reaches the AV node, it slows down briefly before moving on to the AV bundle. This delay, lasting about 100 milliseconds (ms), occurs because the cells in the AV node are smaller and conduct signals more slowly. The pause is essential — it gives the atria time to complete their contraction and push blood into the ventricles before ventricular contraction begins.
Atrioventricular Bundle (Bundle of His), Bundle Branches, and Purkinje Fibers
The AV bundle, also called the bundle of His, arises from the AV node and passes through the interventricular septum before dividing into the right and left bundle branches. The right branch conducts impulses to the right ventricle, while the thicker left branch supplies the left ventricle. Because the left ventricle is much larger and more muscular, its bundle branch is correspondingly broader. Both branches extend toward the apex of the heart, where they connect with the Purkinje fibers. The signal travels through this portion of the pathway in about 25 milliseconds (ms).
The Purkinje fibers are specialized conducting cells that deliver the electrical impulse rapidly to the ventricular myocardial cells. These fibers branch extensively through the ventricular walls, beginning at the apex and spreading upward toward the atrioventricular septum and base of the heart. Their very fast conduction speed allows the impulse to reach all ventricular muscle cells in roughly 75 ms. Contraction begins at the apex and moves upward — much like squeezing toothpaste from the bottom — so blood is efficiently pushed out of the ventricles into the aorta and pulmonary trunk. From the initial depolarization at the SA node , the entire process takes about 225 ms.
From the moment the SA node fires to the full activation of the ventricles, only about 225 milliseconds pass — less than one-quarter of a second. In that blink of time, the electrical signal travels through every chamber of the heart, coordinating the precise sequence of contractions that keep blood flowing smoothly. In other words, your heart’s entire electrical “conversation” for each beat happens faster than you can snap your fingers.
Extrinsic Regulation: Nervous and Hormonal Control of the Heartbeat
The sinoatrial (SA) node normally generates electrical impulses about 80 to 100 times per minute, but your resting heart rate is usually slower — around 60 to 80 beats per minute — because the parasympathetic nervous system continuously keeps the heart rate in check.
The autonomic nervous system (ANS) constantly adjusts heart activity to match the body’s needs:
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Sympathetic stimulation (the “fight or flight” response) speeds up the rate of depolarization in the SA node, causing the heart to beat faster and with greater force.
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Parasympathetic stimulation (the “rest and digest” response) slows the depolarization rate, reducing heart rate and conserving energy when the body is relaxed.
Hormones can also influence the heart’s rhythm. During times of stress or excitement, the adrenal glands release epinephrine (adrenaline) into the bloodstream. This hormone has effects similar to sympathetic stimulation — they increase the heart rate, strengthen contractions, and help the body respond quickly to challenges.
Additionally, thyroid hormones, which regulate metabolism, can also increase the heart rate over the long term if present at higher levels.
Together, the nervous and endocrine systems act as a finely tuned control system, allowing the heart to accelerate when you sprint up stairs and slowing it when you settle into taking a nap.