teachmephysiology.com
Open in
urlscan Pro
2606:4700:3037::ac43:c32f
Public Scan
URL:
https://teachmephysiology.com/cardiovascular-system/cardiac-cycle-2/action-potential-in-cardiac-pacemaker-cells/
Submission: On March 28 via manual from NL — Scanned from NL
Submission: On March 28 via manual from NL — Scanned from NL
Form analysis 6 forms found in the DOM
GET
<form action="" method="get" class="report-form">
<div class="h1 text-white text-center pb-3">Report question</div>
<div class="form-group">
<textarea name="report" placeholder="Comments on question*" required=""></textarea>
</div>
<div class="form-group">
<button class="btn btn-primary btn-block"><i class="material-icons">thumb_up</i>Submit</button>
</div>
</form>GET
<form action="" method="get" class="rate-form">
<div class="h1 text-white text-center pb-3">Rate this article</div>
<div class="form-group">
<textarea name="feedback" placeholder="Leave your feedback..."></textarea>
</div>
<div class="form-group">
<select name="rating" class="invisible position-absolute">
<option value="">Not selected</option>
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
</select>
<div class="rating text-center text-primary">
<div class="rate" data-value="1">
<i class="material-icons not-selected">star_border</i>
<i class="material-icons selected">star</i>
</div>
<div class="rate" data-value="2">
<i class="material-icons not-selected">star_border</i>
<i class="material-icons selected">star</i>
</div>
<div class="rate" data-value="3">
<i class="material-icons not-selected">star_border</i>
<i class="material-icons selected">star</i>
</div>
<div class="rate" data-value="4">
<i class="material-icons not-selected">star_border</i>
<i class="material-icons selected">star</i>
</div>
<div class="rate" data-value="5">
<i class="material-icons not-selected">star_border</i>
<i class="material-icons selected">star</i>
</div>
</div>
</div>
<div class="form-group">
<div id="recaptcha" class="g-recaptcha" data-sitekey="6LfjBZ0UAAAAAOAFx4v-rxAqVA0nuW03imrSUqjE" data-callback="rateSubmit" data-size="invisible">
<div class="grecaptcha-badge" data-style="bottomright"
style="width: 256px; height: 60px; position: fixed; visibility: hidden; display: block; transition: right 0.3s ease 0s; bottom: 14px; right: -186px; box-shadow: gray 0px 0px 5px; border-radius: 2px; overflow: hidden;">
<div class="grecaptcha-logo"><iframe title="reCAPTCHA"
src="https://www.google.com/recaptcha/api2/anchor?ar=1&k=6LfjBZ0UAAAAAOAFx4v-rxAqVA0nuW03imrSUqjE&co=aHR0cHM6Ly90ZWFjaG1lcGh5c2lvbG9neS5jb206NDQz&hl=nl&v=2uoiJ4hP3NUoP9v_eBNfU6CR&size=invisible&cb=dqggmsm8gvc0"
width="256" height="60" role="presentation" name="a-sko98dre696k" frameborder="0" scrolling="no" sandbox="allow-forms allow-popups allow-same-origin allow-scripts allow-top-navigation allow-modals allow-popups-to-escape-sandbox"></iframe>
</div>
<div class="grecaptcha-error"></div><textarea id="g-recaptcha-response" name="g-recaptcha-response" class="g-recaptcha-response"
style="width: 250px; height: 40px; border: 1px solid rgb(193, 193, 193); margin: 10px 25px; padding: 0px; resize: none; display: none;"></textarea>
</div><iframe style="display: none;"></iframe>
</div>
<button class="btn btn-primary btn-block"><i class="material-icons">thumb_up</i>Submit</button>
</div>
</form>GET
<form action="" method="get" class="edit-article-form">
<div class="h1 text-white text-center pb-3">Edit this article</div>
<div class="text-white text-center mb-5">
<p>Found an error? Is our article missing some key information? Make the changes yourself here!</p>
<p>Once you've finished editing, click 'Submit for Review', and your changes will be reviewed by our team before publishing on the site.</p>
</div>
<div class="form-group text-center">
<button class="btn btn-primary modal-close" data-dismiss="modal" aria-label="Close"><i class="material-icons">reply</i>Go back</button>
<button class="btn btn-white"><i class="material-icons">edit</i>Edit this article</button>
</div>
</form>GET https://teachmephysiology.com
<form action="https://teachmephysiology.com" method="get" class="search-form">
<div class="form-group">
<input name="s" type="text" placeholder="Type here...">
<button class="btn btn-white icon-only"><i class="material-icons">search</i></button>
</div>
</form>GET
<form action="" method="get" class="rate-form">
<div class="h1 text-white text-center pb-3">Rate this article</div>
<div class="form-group">
<textarea name="feedback" placeholder="Leave your feedback..."></textarea>
</div>
<div class="form-group">
<select name="rating" class="invisible position-absolute">
<option value="">Not selected</option>
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
</select>
<div class="rating text-center text-primary">
<div class="rate" data-value="1">
<i class="material-icons not-selected">star_border</i>
<i class="material-icons selected">star</i>
</div>
<div class="rate" data-value="2">
<i class="material-icons not-selected">star_border</i>
<i class="material-icons selected">star</i>
</div>
<div class="rate" data-value="3">
<i class="material-icons not-selected">star_border</i>
<i class="material-icons selected">star</i>
</div>
<div class="rate" data-value="4">
<i class="material-icons not-selected">star_border</i>
<i class="material-icons selected">star</i>
</div>
<div class="rate" data-value="5">
<i class="material-icons not-selected">star_border</i>
<i class="material-icons selected">star</i>
</div>
</div>
</div>
<div class="form-group">
<button class="btn btn-primary btn-block"><i class="material-icons">thumb_up</i>Submit</button>
</div>
</form>GET
<form action="" method="get" class="edit-article-form">
<div class="h1 text-white text-center pb-3">Edit this article</div>
<div class="text-white text-center">
<p>Found an error? Is our article missing some key information? Make the changes yourself here!</p>
<p>Once you've finished editing, click 'Submit for Review', and your changes will be reviewed by our team before publishing on the site.</p>
</div>
<div class="form-group text-center text-white my-5">
<div class="custom-control custom-checkbox">
<input type="checkbox" class="custom-control-input" id="dont-ask-again">
<label class="custom-control-label" for="dont-ask-again">Don't ask me again</label>
</div>
</div>
<div class="form-group text-center">
<button class="btn btn-primary modal-close" data-dismiss="modal" aria-label="Close"><i class="material-icons">reply</i>Go back</button>
<button class="btn btn-white"><i class="material-icons">edit</i>Edit this article</button>
</div>
</form>Text Content
TeachMe
Physiology
Part of the TeachMe Series
* Subjects
* Biochemistry
* Growth & Death
* Apoptosis
* Cell Cycle
* DNA Replication
* Meiosis
* Mitosis
* Molecules and Signalling
* Active Transport
* Cellular Receptors
* Diffusion
* Endocytosis and Exocytosis
* Enzyme Inhibition
* Enzyme Kinetics
* G-proteins
* Osmosis
* Protein Synthesis
* Protein Structure
* Transcription of DNA
* Translation of DNA
* ATP Production
* Anaerobic Respiration
* Electron Transport Chain
* Gluconeogenesis
* Glycolysis
* TCA Cycle
* Electrolytes
* Calcium Regulation
* External Balance of Potassium
* Internal Balance of Potassium
* Sodium Regulation
* Histology
* Cell Structures
* Cell Membrane
* Endoplasmic Reticulum
* Golgi Apparatus
* Lysosomes
* Mitochondria
* Nucleus
* Tissue Structure
* Blood Vessels
* Bones
* Cartilage
* Cellular Adaptations
* Epithelial Cells
* Muscle Histology
* Skin
* Structure of Glands
* Cardiovascular
* Cardiac Output
* Control of Stroke Volume
* Control of Heart Rate
* Cardiac Cycle
* Action Potential in Ventricular Cells
* Cardiac Cycle
* Cardiac Pacemaker Cells
* Conduction System
* Contraction of Cardiac Muscle
* Circulation
* Control of Blood Pressure
* Capillary Exchange
* Flow Within The Cardiovascular System
* Regulation of Peripheral Blood Flow
* Venous Return
* Special Circulations
* Cardiac Muscle
* Cerebral
* Cutaneous
* Hepatic Circulation
* Pulmonary
* Skeletal Muscle
* Respiratory
* Pulmonary Ventilation
* Airway Resistance
* Lung Volumes
* Mechanics of Breathing
* Gas Exchange
* Gas Exchange
* Oxygen Transport in The Blood
* Transport of Carbon Dioxide in the Blood
* Ventilation-Perfusion Matching
* Regulation of Respiration
* Chemoreceptors
* Cough Reflex
* Neural Control of Ventilation
* Respiratory Regulation of Acid-Base Balance
* Responses of The Respiratory System to Stress
* Gastrointestinal
* Mouth
* Regulation of Saliva
* Secretion of Saliva
* Swallowing
* Stomach
* Gastric Acid Production
* Appetite
* Gastric Mucus Production
* Small Intestine
* Digestion and Absorption
* Histology and Cellular Function of the Small Intestine
* Large Intestine
* Absorption in the Large Intestine
* Defecation
* Large Intestinal Motility
* Liver
* Bilirubin Metabolism
* Metabolic Functions of the Liver
* Storage Functions of the Liver
* Vitamins
* Vitamin K
* Other
* Bile Production
* Function of The Spleen
* Exocrine Pancreas
* Somatostatin
* Urinary
* Nephron
* Glomerulus
* Proximal Convoluted Tubule
* Loop of Henle
* Distal Convoluted Tubule and Collecting Duct
* Micturition
* Storage Phase of Micturition
* Voiding Phase of Micturition
* Regulation
* Antidiuretic Hormone
* Renin-Angiotensin-Aldosterone System
* Urinary Regulation of Acid-Base Balance
* Water Filtration and Reabsorption
* Reproductive
* Embryology
* Development of the Reproductive System
* Gametogenesis
* Hormones and Regulation
* Gonadotropins and the Hypothalamic Pituitary Axis
* Puberty
* Menstrual Cycle
* Menopause
* Fetal Physiology
* Placental Development
* Fetal Circulation
* Pregnancy
* Coitus
* Conception
* Labour
* Lactation
* Maternal Adaptations in Pregnancy
* Neurology
* Components
* Cells of the Nervous System
* Central Nervous System
* Cerebrospinal Fluid
* Neurotransmitters
* Peripheral Nervous System
* Synapses
* Action Potential
* Excitatory and Inhibitory Synaptic Signalling
* Myelin
* Resting Membrane Potential
* Synaptic Plasticity
* Synaptic Transmission
* Sensory System
* Ascending Tracts
* Auditory Pathway
* Consciousness and Sleep
* Modalities of Sensation
* Pain Pathways
* Sensory Acuity
* Visual Pathway
* Motor System
* Descending Tracts
* Lower Motor Neurones
* Muscle Stretch Reflex
* Upper Motor Neurones
* Ocular Physiology
* Aqueous Humour
* Ocular Accommodation
* The Retina
* Endocrine
* Thyroid and Parathyroid Glands
* Thyroid Gland
* Parathyroid Glands
* Adrenal Glands
* Adrenal Medulla
* Zona Glomerulosa
* Zona Fasciculata
* Zona Reticularis
* The Pancreas
* Endocrine Pancreas
* Insulin
* Glucagon
* Hypothalamus and Pituitary
* The Hypothalamus
* Anterior Pituitary
* Posterior Pituitary
* Immunology/Haematology
* Cells of the Immune System
* B Cells
* Phagocytes
* T Cells
* White Blood Cells – Summary
* Innate Immune System
* Barriers to Infection
* Cytokines
* Infection Recognition Molecules
* Phagocytosis
* The Complement System
* Adaptive Immune System
* Antibodies
* Antigen Processing and Presentation
* Primary and Secondary Immune Responses
* T Cell Memory
* Immune Responses
* Acute Inflammation
* Autoimmunity
* Chronic Inflammation
* Hypersensitivity Reactions
* Immunodeficiency
* Types of Immunity
* Infections
* Antibiotics
* Pathogens
* Viral Infection
* Haematology
* Blood Groups
* Coagulation
* Erythropoiesis
* Iron Metabolism
* Mononuclear Phagocyte System
* Platelets
* Quizzes
* Contact Us
search menuclose
* Biochemistry
* Growth & Death
* Apoptosis
* Cell Cycle
* DNA Replication
* Meiosis
* Mitosis
* Molecules and Signalling
* Active Transport
* Cellular Receptors
* Diffusion
* Endocytosis and Exocytosis
* Enzyme Inhibition
* Enzyme Kinetics
* G-proteins
* Osmosis
* Protein Synthesis
* Protein Structure
* Transcription of DNA
* Translation of DNA
* ATP Production
* Anaerobic Respiration
* Electron Transport Chain
* Gluconeogenesis
* Glycolysis
* TCA Cycle
* Electrolytes
* Calcium Regulation
* External Balance of Potassium
* Internal Balance of Potassium
* Sodium Regulation
* Histology
* Cell Structures
* Cell Membrane
* Endoplasmic Reticulum
* Golgi Apparatus
* Lysosomes
* Mitochondria
* Nucleus
* Tissue Structure
* Blood Vessels
* Bones
* Cartilage
* Cellular Adaptations
* Epithelial Cells
* Muscle Histology
* Skin
* Structure of Glands
* Cardiovascular
* Cardiac Output
* Control of Stroke Volume
* Control of Heart Rate
* Cardiac Cycle
* Action Potential in Ventricular Cells
* Cardiac Cycle
* Cardiac Pacemaker Cells
* Conduction System
* Contraction of Cardiac Muscle
* Circulation
* Control of Blood Pressure
* Capillary Exchange
* Flow Within The Cardiovascular System
* Regulation of Peripheral Blood Flow
* Venous Return
* Special Circulations
* Cardiac Muscle
* Cerebral
* Cutaneous
* Hepatic Circulation
* Pulmonary
* Skeletal Muscle
* Respiratory
* Pulmonary Ventilation
* Airway Resistance
* Lung Volumes
* Mechanics of Breathing
* Gas Exchange
* Gas Exchange
* Oxygen Transport in The Blood
* Transport of Carbon Dioxide in the Blood
* Ventilation-Perfusion Matching
* Regulation of Respiration
* Chemoreceptors
* Cough Reflex
* Neural Control of Ventilation
* Respiratory Regulation of Acid-Base Balance
* Responses of The Respiratory System to Stress
* Gastrointestinal
* Mouth
* Regulation of Saliva
* Secretion of Saliva
* Swallowing
* Stomach
* Gastric Acid Production
* Appetite
* Gastric Mucus Production
* Small Intestine
* Digestion and Absorption
* Histology and Cellular Function of the Small Intestine
* Large Intestine
* Absorption in the Large Intestine
* Defecation
* Large Intestinal Motility
* Liver
* Bilirubin Metabolism
* Metabolic Functions of the Liver
* Storage Functions of the Liver
* Vitamins
* Vitamin K
* Other
* Bile Production
* Function of The Spleen
* Exocrine Pancreas
* Somatostatin
* Urinary
* Nephron
* Glomerulus
* Proximal Convoluted Tubule
* Loop of Henle
* Distal Convoluted Tubule and Collecting Duct
* Micturition
* Storage Phase of Micturition
* Voiding Phase of Micturition
* Regulation
* Antidiuretic Hormone
* Renin-Angiotensin-Aldosterone System
* Urinary Regulation of Acid-Base Balance
* Water Filtration and Reabsorption
* Reproductive
* Embryology
* Development of the Reproductive System
* Gametogenesis
* Hormones and Regulation
* Gonadotropins and the Hypothalamic Pituitary Axis
* Puberty
* Menstrual Cycle
* Menopause
* Fetal Physiology
* Placental Development
* Fetal Circulation
* Pregnancy
* Coitus
* Conception
* Labour
* Lactation
* Maternal Adaptations in Pregnancy
* Neurology
* Components
* Cells of the Nervous System
* Central Nervous System
* Cerebrospinal Fluid
* Neurotransmitters
* Peripheral Nervous System
* Synapses
* Action Potential
* Excitatory and Inhibitory Synaptic Signalling
* Myelin
* Resting Membrane Potential
* Synaptic Plasticity
* Synaptic Transmission
* Sensory System
* Ascending Tracts
* Auditory Pathway
* Consciousness and Sleep
* Modalities of Sensation
* Pain Pathways
* Sensory Acuity
* Visual Pathway
* Motor System
* Descending Tracts
* Lower Motor Neurones
* Muscle Stretch Reflex
* Upper Motor Neurones
* Ocular Physiology
* Aqueous Humour
* Ocular Accommodation
* The Retina
* Endocrine
* Thyroid and Parathyroid Glands
* Thyroid Gland
* Parathyroid Glands
* Adrenal Glands
* Adrenal Medulla
* Zona Glomerulosa
* Zona Fasciculata
* Zona Reticularis
* The Pancreas
* Endocrine Pancreas
* Insulin
* Glucagon
* Hypothalamus and Pituitary
* The Hypothalamus
* Anterior Pituitary
* Posterior Pituitary
* Immunology/Haematology
* Cells of the Immune System
* B Cells
* Phagocytes
* T Cells
* White Blood Cells – Summary
* Innate Immune System
* Barriers to Infection
* Cytokines
* Infection Recognition Molecules
* Phagocytosis
* The Complement System
* Adaptive Immune System
* Antibodies
* Antigen Processing and Presentation
* Primary and Secondary Immune Responses
* T Cell Memory
* Immune Responses
* Acute Inflammation
* Autoimmunity
* Chronic Inflammation
* Hypersensitivity Reactions
* Immunodeficiency
* Types of Immunity
* Infections
* Antibiotics
* Pathogens
* Viral Infection
* Haematology
* Blood Groups
* Coagulation
* Erythropoiesis
* Iron Metabolism
* Mononuclear Phagocyte System
* Platelets
THE ACTION POTENTIAL IN CARDIAC PACEMAKER CELLS
1. Home
2. Cardiovascular
3. Cardiac Cycle
4. The Action Potential in Cardiac Pacemaker Cells
star star star star star
based on 29 ratings
Original Author(s): Chloe Hill
Last updated: 6th February 2021
Revisions: 24
Original Author(s): Chloe Hill
Last updated: 6th February 2021
Revisions: 24
format_list_bulletedContents add remove
* 1 Pacemaker Cells
* 2 Action potential in SA node
* 2.1 Phase 4 – Pacemaker potential
* 2.2 Phase 0 – Depolarisation
* 2.3 Phase 3 – Repolarisation
* 3 Control by the Autonomic Nervous System
* 4 Clinical Relevance – Arrhythmias
In the heart, electrical impulses are generated by specialised pacemaker
cells and spread across the myocardium in order to produce a coordinated
contraction in systole.
The action potential generated is generated by a change in the potential
difference between the inside and the outside of the cell. The particular action
potential generated by cardiac pacemaker cells is very different from that of
ventricular myocardial cells. In this article, we will discuss cardiac pacemaker
cells and the action potential they generate in more detail.
PACEMAKER CELLS
Cardiac pacemaker cells are mostly found in the sinoatrial (SA) node, which is
situated in the upper part of the wall of the right atrium. These cells have
natural automaticity, meaning they can generate their own action potentials.
By Madhero88 (original files); Angelito7 (this SVG version); [CC BY-SA 3.0
(https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
Fig 1.0 – The conduction system of the heart.
The atrioventricular (AV) node and the Purkinje fibres also have cells capable
of pacemaker activity, however, their natural rate is much slower than the SA
node, so they are normally overridden.
ACTION POTENTIAL IN SA NODE
The action potential in the SA node occurs in three phases which are discussed
below.
PHASE 4 – PACEMAKER POTENTIAL
The pacemaker potential occurs at the end of one action potential and just
before the start of the next. It is the slow depolarisation of the pacemaker
cells e.g. cells of the sinoatrial node, towards the membrane potential
threshold. This is sometimes referred to as the ‘funny’ current, or If.
The pacemaker potential is achieved by activation of hyperpolarisation activated
cyclic nucleotide gated channels (HCN channels). These allow Na+ entry into the
cells, enabling slow depolarisation. These channels are activated when the
membrane potential is lower than -50mV. Once the membrane potential gets
depolarised to reach the threshold, an action potential can be fired.
PHASE 0 – DEPOLARISATION
Once the HCN channels have brought the membrane potential to around -40mV,
voltage-gated calcium channels open. This allows an influx of Ca2+ which
produces a faster rate of depolarisation to reach a positive membrane potential
(responsible for the upstroke of the action potential). HCN channels then start
to inactivate. At the peak of the action potential, Ca2+ channels inactivate,
and K+ channels open.
PHASE 3 – REPOLARISATION
Once the Ca2+ channels inactivate, and the K+ channels open, there is an efflux
of K+ ions out of the cells. This results in the repolarisation of the membrane,
which is seen as the downstroke of the action potential.
Unlike the ventricular action potential, the opening of Ca2+ channels is not
sustained, and there is no ‘plateau’ stage. Therefore, the action potential is
triangular in shape. After the action potential, repolarisation must occur and
the membrane potential must reach negative values. This allows the HCN channels
to be reactivated again, enabling another action potential to be generated
(phase 4).
By Pacemaker_potential.svg: Diberri derivative work: Silvia3
(Pacemaker_potential.svg) [CC-BY-SA-3.0
(http://creativecommons.org/licenses/by-sa/3.0/) or GFDL
(http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons
Fig 2 – Diagram showing the action potential in cardiac pacemaker cells and the
main ion movements at each stage.
CONTROL BY THE AUTONOMIC NERVOUS SYSTEM
The autonomic nervous system (ANS) alters the slope of the pacemaker potential,
in order to alter heart rate.
Heart rate is affected by both the parasympathetic and sympathetic branches of
the ANS, which innervate both the SA and AV nodes.
* Parasympathetic activity is mediated via acetylcholine acting
on M2 muscarinic receptors at the SA node. This lengthens the interval
between pacemaker potentials, hence slowing heart rate.
* Sympathetic activity is mediated via noradrenaline acting on
B1 adrenoceptors. This shortens the interval between impulses by making the
pacemaker potential steeper, hence increasing the heart rate.
If all autonomic inputs are blocked, the intrinsic heart rate is about 100 beats
per minute (bpm). The normal resting rate of about 60bpm is produced because the
parasympathetic system dominates at rest. Initial increases in heart rate are
brought about by a reduction in the parasympathetic outflow. Increasing
sympathetic outflow allows for further increases in heart rate.
CLINICAL RELEVANCE - ARRHYTHMIAS
Disturbance to the natural pacemaker activity of the heart can lead to
arrhythmias i.e. a heartbeat with an irregular rate and/or rhythm.
Causes of arrhythmias include:
* Ectopic Pacemaker Activity: This is when another area of the myocardium
becomes spontaneously active and its depolarisations dominate over the SA
node. A latent pacemaker region can become activated due to ischaemic damage.
* After-Depolarisations: This is when abnormal depolarisations follow the
action potential – thought to be caused by high intracellular Ca2+.
* Re-entry loop: This occurs when the normal spread of excitation across the
heart is disrupted due to a damaged area. When the conduction damage is
incomplete, it allows the impulse to spread the wrong way through the damaged
area and create a circle of excitation. Multiple small re-entry loops can
occur in the atria, leading to atrial fibrillation.
printPrint this Article
star_borderRate this Article
Quiz
The Action Potential in Cardiac Pacemaker Cells
Results
Question 1 of 5
Where are the pacemaker cells that normally control the heart rate located?
Right atrium
Left atrium
Right ventricle
Septum
Skip Submit
check_circle
Correct
cancel
Incorrect
Next
Report question
Well done!
You scored:
33%
Skipped: 2/5
Try again to score 100%. Use the information in this article to help you with
the answers.
Retake Quiz
close
Report question
thumb_upSubmit
bookmarksRecommended reading
1. GDMT in HFrEF: Recap of HFSA Data and Recommendations
ReachMD
2. Free CME: Strategies to Maximize GDMT & Optimize RAASi Dosing
ReachMD
1. Strategies & Clinical Techniques to Maximize GDMT and Optimize RAASi Dosing
for HFrEF
ReachMD
2. Global Heart Failure Academy: Key Perspectives on the Evolving Role of
Precision Medicine in HFrEF
ReachMD
Powered by
* Privacy policy
* Do not sell my personal information
* Google Analytics settings
I consent to the use of Google Analytics and related cookies across the TrendMD
network (widget, website, blog). Learn more
Yes No
In the heart, electrical impulses are generated by specialised pacemaker
cells and spread across the myocardium in order to produce a coordinated
contraction in systole.
The action potential generated is generated by a change in the potential
difference between the inside and the outside of the cell. The particular action
potential generated by cardiac pacemaker cells is very different from that of
ventricular myocardial cells. In this article, we will discuss cardiac pacemaker
cells and the action potential they generate in more detail.
PACEMAKER CELLS
Cardiac pacemaker cells are mostly found in the sinoatrial (SA) node, which is
situated in the upper part of the wall of the right atrium. These cells have
natural automaticity, meaning they can generate their own action potentials.
[caption id="attachment_14135" align="aligncenter" width="800"] Fig 1.0 - The
conduction system of the heart.[/caption]
The atrioventricular (AV) node and the Purkinje fibres also have cells capable
of pacemaker activity, however, their natural rate is much slower than the SA
node, so they are normally overridden.
ACTION POTENTIAL IN SA NODE
The action potential in the SA node occurs in three phases which are discussed
below.
PHASE 4 - PACEMAKER POTENTIAL
The pacemaker potential occurs at the end of one action potential and just
before the start of the next. It is the slow depolarisation of the pacemaker
cells e.g. cells of the sinoatrial node, towards the membrane potential
threshold. This is sometimes referred to as the 'funny’ current, or If.
The pacemaker potential is achieved by activation of hyperpolarisation activated
cyclic nucleotide gated channels (HCN channels). These allow Na+ entry into the
cells, enabling slow depolarisation. These channels are activated when the
membrane potential is lower than -50mV. Once the membrane potential gets
depolarised to reach the threshold, an action potential can be fired.
PHASE 0 - DEPOLARISATION
Once the HCN channels have brought the membrane potential to around -40mV,
voltage-gated calcium channels open. This allows an influx of Ca2+ which
produces a faster rate of depolarisation to reach a positive membrane potential
(responsible for the upstroke of the action potential). HCN channels then start
to inactivate. At the peak of the action potential, Ca2+ channels inactivate,
and K+ channels open.
PHASE 3 - REPOLARISATION
Once the Ca2+ channels inactivate, and the K+ channels open, there is an efflux
of K+ ions out of the cells. This results in the repolarisation of the membrane,
which is seen as the downstroke of the action potential.
Unlike the ventricular action potential, the opening of Ca2+ channels is not
sustained, and there is no ‘plateau’ stage. Therefore, the action potential is
triangular in shape. After the action potential, repolarisation must occur and
the membrane potential must reach negative values. This allows the HCN channels
to be reactivated again, enabling another action potential to be generated
(phase 4).
[caption id="attachment_14134" align="aligncenter" width="960"] Fig 2 - Diagram
showing the action potential in cardiac pacemaker cells and the main ion
movements at each stage.[/caption]
CONTROL BY THE AUTONOMIC NERVOUS SYSTEM
The autonomic nervous system (ANS) alters the slope of the pacemaker potential,
in order to alter heart rate.
Heart rate is affected by both the parasympathetic and sympathetic branches of
the ANS, which innervate both the SA and AV nodes.
* Parasympathetic activity is mediated via acetylcholine acting
on M2 muscarinic receptors at the SA node. This lengthens the interval
between pacemaker potentials, hence slowing heart rate.
* Sympathetic activity is mediated via noradrenaline acting on
B1 adrenoceptors. This shortens the interval between impulses by making the
pacemaker potential steeper, hence increasing the heart rate.
If all autonomic inputs are blocked, the intrinsic heart rate is about 100 beats
per minute (bpm). The normal resting rate of about 60bpm is produced because the
parasympathetic system dominates at rest. Initial increases in heart rate are
brought about by a reduction in the parasympathetic outflow. Increasing
sympathetic outflow allows for further increases in heart rate.
[start-clinical]
CLINICAL RELEVANCE - ARRHYTHMIAS
Disturbance to the natural pacemaker activity of the heart can lead to
arrhythmias i.e. a heartbeat with an irregular rate and/or rhythm.
Causes of arrhythmias include:
* Ectopic Pacemaker Activity: This is when another area of the myocardium
becomes spontaneously active and its depolarisations dominate over the SA
node. A latent pacemaker region can become activated due to ischaemic damage.
* After-Depolarisations: This is when abnormal depolarisations follow the
action potential – thought to be caused by high intracellular Ca2+.
* Re-entry loop: This occurs when the normal spread of excitation across the
heart is disrupted due to a damaged area. When the conduction damage is
incomplete, it allows the impulse to spread the wrong way through the damaged
area and create a circle of excitation. Multiple small re-entry loops can
occur in the atria, leading to atrial fibrillation.
[end-clinical]
close
Rate this article
Not selected 1 2 3 4 5
star_border star
star_border star
star_border star
star_border star
star_border star
thumb_upSubmit
close
Edit this article
Found an error? Is our article missing some key information? Make the changes
yourself here!
Once you've finished editing, click 'Submit for Review', and your changes will
be reviewed by our team before publishing on the site.
replyGo back editEdit this article
TeachMePhysiology
Part of the TeachMe Series
The medical information on this site is provided as an information resource
only, and is not to be used or relied on for any diagnostic or treatment
purposes. This information is intended for medical education, and does not
create any doctor-patient relationship, and should not be used as a substitute
for professional diagnosis and treatment.
By visiting this site you agree to the foregoing terms and conditions. If you do
not agree to the foregoing terms and conditions, you should not enter this site.
*
*
© TeachMe Series 2022 | Registered in England & Wales
* Privacy Policy
close
search
close
Rate this article
Not selected 1 2 3 4 5
star_border star
star_border star
star_border star
star_border star
star_border star
thumb_upSubmit
close
Edit this article
Found an error? Is our article missing some key information? Make the changes
yourself here!
Once you've finished editing, click 'Submit for Review', and your changes will
be reviewed by our team before publishing on the site.
Don't ask me again
replyGo back editEdit this article
TeachMePhysiology
This website uses cookies.
We use cookies to improve your experience on our site and to show you relevant
advertising. To find out more, read our privacy policy.
Accept
Manage consent
Close
PRIVACY OVERVIEW
This website uses cookies to improve your experience while you navigate through
the website. Out of these, the cookies that are categorized as necessary are
stored on your browser as they are essential for the working of basic
functionalities of the ...
Necessary
Necessary
Always Enabled
Necessary cookies are absolutely essential for the website to function properly.
These cookies ensure basic functionalities and security features of the website,
anonymously.
CookieDurationDescriptioncookielawinfo-checkbox-analytics11 monthsThis cookie is
set by GDPR Cookie Consent plugin. The cookie is used to store the user consent
for the cookies in the category "Analytics".cookielawinfo-checkbox-functional11
monthsThe cookie is set by GDPR cookie consent to record the user consent for
the cookies in the category "Functional".cookielawinfo-checkbox-necessary11
monthsThis cookie is set by GDPR Cookie Consent plugin. The cookies is used to
store the user consent for the cookies in the category
"Necessary".cookielawinfo-checkbox-others11 monthsThis cookie is set by GDPR
Cookie Consent plugin. The cookie is used to store the user consent for the
cookies in the category "Other.cookielawinfo-checkbox-performance11 monthsThis
cookie is set by GDPR Cookie Consent plugin. The cookie is used to store the
user consent for the cookies in the category
"Performance".viewed_cookie_policy11 monthsThe cookie is set by the GDPR Cookie
Consent plugin and is used to store whether or not user has consented to the use
of cookies. It does not store any personal data.
Functional
Functional
Functional cookies help to perform certain functionalities like sharing the
content of the website on social media platforms, collect feedbacks, and other
third-party features.
Performance
Performance
Performance cookies are used to understand and analyze the key performance
indexes of the website which helps in delivering a better user experience for
the visitors.
Analytics
Analytics
Analytical cookies are used to understand how visitors interact with the
website. These cookies help provide information on metrics the number of
visitors, bounce rate, traffic source, etc.
Advertisement
Advertisement
Advertisement cookies are used to provide visitors with relevant ads and
marketing campaigns. These cookies track visitors across websites and collect
information to provide customized ads.
Others
Others
Other uncategorized cookies are those that are being analyzed and have not been
classified into a category as yet.
SAVE & ACCEPT