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2225L_HR_Introduction_920.docHR, Page 2
Heart Rate (HR) Introduction
How does heart rate respond to a stimulus?
At the end of this Introduction, you should also know:
A. How does a reflex loop signal changes in heart rate?
B. How does ion diffusion across membranes affect heart rate?
Figure description: The stimulus is a tiger’s roar. The receptor/sensor is hair cells in the ear. The afferent pathway is sensory neurons in the cochlear nerve. The integrating center is the brain in the CNS. The efferent pathway is the autonomic sympathetic neurons. The effectors/target cells are cardiac autorhythmic cells in the heart. The response is increased heart rate.
Textbook:
· 8th edition: pages 181-186, 446, 450-451, 455-459, 464-466 , 481-482
· 7th edition: pages 182-187, 447, 453-454, 457-460, 466-468, 482-483
· 6th edition: pages 192-196, 475, 481, 486-489, 495-497, 513-514
· 5th edition: pages 196-202, 481, 485-487, 491-494 & Figure 14-23, 516-517
· 4th edition: pages 191-197, 470, 474-476, 480-484, 489-490, 504-506
Clinical Application:
A patient went to the emergency department with chest pains. Before she even completed registration, she was taken to an examination room. Twelve electrodes with leads where quickly put in place to perform an electrocardiogram (ECG).
Study Questions:
(These questions are included to help your learning and do not need to be turned in. Quiz questions may be based on these.)
(1) From previous experiences, what do you already know about heart rate and electrocardiograms (ECG, EKG)?
(2) What is the function of the heart?
(3) What are the functions of blood circulation?
(4) Which stimuli can cause heart rate to increase or decrease?
(5) From previous learning, which senses do we have? (Hint: there are more than 5)
Study Tip: Use the reflex loop provided in the run from a tiger worksheet to follow along with the descriptions below. Compare the HR reflex loop to the run from a tiger loop from the first lab.
How does heart rate (HR) relate to a reflex loop?
To prepare for the HR experiment, you will select one stimulus you would like to study and make a prediction about how you think it will affect heart rate (the response). In other words, you will write a hypothesis. This should be something you think would be interesting to study. Since we already know that exercise will increase heart rate, select some stimulus other than exercise (see the assignment for more information).
Then, predict the parts of the reflex loop that will lead to your predicted change in heart rate. Your laboratory team will then discuss the stimulus proposed by each team member, select one stimulus to study, and plan an experiment to test the effects of this stimulus. One of your team members will then perform the experiment. Your team will then analyze and discuss the experimental results. (Each team member will be assigned a letter, and the roles will be assigned and rotate based on these letters.)
What are the functions of the heart and circulatory system?
The main function of the heart is to pump blood to the body. Blood transports nutrients and oxygen (as well as clotting factors, white blood cells, antibodies, etc.).
· Increased heart rate increases blood flow to tissues.
· Decreased heart rate reduces blood flow to tissues.
Body systems respond to various stimuli to increase or decrease activity. For example, the heart responds to stimuli to increase heart rate to pump more blood or decrease heart rate to pump less blood.
How does a reflex loop help us respond to stimuli to change heart rate?
1. A stimulus that signals us to fight or flee is received by our sensory system, and our heart responds by increasing heart rate. For example, a stimulus may be a tiger getting out of a cage at a zoo and roaring behind a crowd.
2. Receptors/sensors detect the stimulus. We have various sensory receptors in our body including eyes, taste buds, sensory neurons, etc. For example, the receptor/sensor organ is the ear. Receptor/sensor cells in the inner ear called hair cells sense this signal.
3. Input/afferent pathways send signals to the central nervous system (CNS). Afferent neurons are also called sensory neurons and are a part of the peripheral nervous system (PNS). For example, the ear uses the cochlear nerve as an afferent pathway to send signals (to the brain).
Study Questions:
(6) Which are other stimuli that can increase or decrease heart rate? For each one, what would be the sensory receptor/sensor? (For example, a test on your desk is sensed by your eye/photoreceptors.)
(7) Which organ is the receptor/sensor in the reflex loop example above? Which cell is the receptor? To which organ system do these belong?
(8) Which two organs are part of the CNS? Which tissues and cells are part of these organs?
(9) What do you predict will happen to signaling in the receptor/sensor and afferent pathway if there is more stimulus?
(10) From previous learning, what do you remember about the CNS and PNS?
4. Integrating centers receive various signals, integrate the information, and “decide” what to do. The integrating center for simple nervous reflex loops is the central nervous system (CNS). The CNS is made up of the brain and spinal cord. For example, the ear sends signals to the brain. For example, the CNS is the integrating center for signals from the ear.
5. Output/efferent pathways send information from the CNS to the body to stimulate an effect. Like afferent neurons, efferent neurons are part of the PNS. Afferent nerves/neurons send signals to the CNS. Efferent nerves/neurons sent signals from the CNS. Memory tip: A comes before E. The efferent PNS has two main divisions:
a. Somatic motor neurons stimulate skeletal muscle contraction (soma means body, motor means move).
b. Autonomic neurons stimulate the other two types of muscle (cardiac and smooth), endocrine and exocrine glands, and adipose tissue. The autonomic nervous system can be further divided into the:
i. parasympathetic nervous system, also called rest or digest. This signals decreases in heart rate and other body responses. Parasympathetic neurons secrete the chemical signal acetylcholine.
ii. sympathetic nervous system, also called fight or flight. This signals increases in heart rate and other body responses. Sympathetic pathways secrete the chemical signals epinephrine and norepinephrine (also called adrenaline and noradrenaline).
· Memory tip: You can remember sympathetic responses by thinking of how your body responds if you needed to run from a tiger. If this happened, people might have sympathy for you.
· For example, hearing a tiger would signal your brain to use your sympathetic nervous system as the efferent pathway.
Study Questions:
(11) In a nervous reflex loop, which organ system is the integrating center? Which two organs are part of this system?
(12) Which two parts of a nervous reflex loop make up the peripheral nervous system? Compare: How are these two parts of the PNS similar and different?
(13) What are the two main divisions of the efferent peripheral nervous system?
(14) If a stimulus changes heart rate, what are the efferent pathway organ, cells, and molecules? (Hint: A nerve is made up of neuron axons surrounded by connective tissue, and an organ is made up of more that one tissue).
(15) Compare: How are somatic and autonomic neurons similar and different?
(16) Compare: How are sympathetic and parasympathetic signaling similar and different?
(17) Think about what happens in your body just before you take an exam. What happens to your heart rate, blood pressure, and pupil size?
6. Targets/effectors receive signals from the efferent neurons to carry out a response. For example, sympathetic neurons release chemicals onto cardiac muscle cells to signal an increase in heart rate. Sympathetic neurons are the efferent neurons and cardiac muscle cells are the target/effector cells. There are two types of cardiac muscle cells:
a. contractile cells. As their name suggests, cardiac contractile cells help the heart contract.
b. autorhythmic/pacemaker cells. As their name suggests, cardiac autorhythmic/pacemaker cells set the pace or the heart and are the cells stimulated by the autonomic nervous system.
i. Acetyl choline secreted from parasympathetic neurons stimulates cholinergic receptors on target/effector cells. Specifically, autorhythmic cells have choline rgic muscarinic receptors.
ii. Epinephrine ( adren aline) and norepinephrine (nor adren aline) secreted from sympathetic neurons stimulate adrenergic receptors on target/effector cells. Specifically, autorhythmic cells have beta one (β1) adren ergic receptors.
7. Responses to efferent neurons will depend on the type of efferent neuron and the type of target/effector cell. For example, in response to a tiger, sympathetic neurons stimulate increased heart rate in cardiac autorhythmic/pacemaker cells. Heart rate is usually reported as beats per minute (BPM).
Study Questions:
(18) If a stimulus changes heart rate, what are the receptor/effector organ, cells, and molecules?
(19) If you saw a tiger, which other responses would you expect? For example, what would happen to blood pressure, radius of airways, and pupil size?
(20) What are the 7 parts of a simple nervous reflex loop? (Hint: see numbers above)
(21) If there is increased epinephrine, what do you predict will happen to receptor binding and heart rate?
(22) If there is increased parasympathetic signaling, what do you predict will happen o secretion of acetylcholine and heart rate?
(23) Compare: How are acetylcholine and epinephrine similar and different?
(24) Compare: How are cholinergic muscarinic and beta one adrenergic receptors similar and different?
(25) What is an ion? What are examples of ions?
Study Tip: Draw a cell and label intracellular and extracellular compartments. Draw ion channels in the cell membrane. Write sodium, calcium and potassium on small pieces of paper (or use different types of household items like paperclips for sodium, etc,). Move ions through ion channels as you read the notes below.
How does ion movement across autorhythmic cell membranes affect heart rate?
Intracellular fluid has different concentrations of ions than extracellular fluid:
· Sodium and calcium ions are higher in extracellular fluid than in intracellular fluid
· Potassium ions are higher in intracellular fluid than in extracellular fluid
Remember that ions are charged particles and have an electrical charge. Since there is an unequal concentration of ions on the two sides of the cell membrane, cells are at a chemical and electrical disequilibrium. These cells are at homeostasis, so this is an example of homeostasis not being the same thing as equilibrium.
There is more negative charge in the intracellular fluid than the extracellular fluid, so we say these cells are polarized. We use the term “polar” when two things are different. Analogy: political parties are often polarized on issues.
Study Questions:
(26) What are other times you have heard/used the terms polarized, polar, or poles?
(27) What do you already know about polar and nonpolar substances? How do these cross membranes?
(28) If a sodium channel opens, will sodium diffuse into or out of a cell?
Chemicals are another example of when we use the term polar. For example, water is made up of two hydrogen atoms and one oxygen atom. When electrons orbit around this molecule, they spend more time around the oxygen, giving this a more negative charge, and the hydrogen atoms have a more positive charge. Therefore, this molecule is polar. Substances that dissolve in water are polar. Fats and lipids are nonpolar.
Ions are polar so are not able to move between the nonpolar lipid tails in the cell membrane. Therefore, ions cannot cross the membrane by simple diffusion. Ions cross membranes by protein-mediated transport through membrane protein transporters called channels.
Some channels always form an open pore in the membrane. Other channels are gated so that they open/close in response to stimuli.
· Voltage-gated channels open/close when there are changes in voltage due to ion movement.
· Chemically-gated channels open/close when chemicals (such as neurotransmitters) bind channels.
· Mechanically-gated channels open/close with mechanical force (such as pressure or temperature).
When an ion channel is stimulated to open, ions diffuse from where their concentration is higher to where their concentration is lower. Sodium and calcium ions are higher outside of cells. Potassium is higher inside of cells.
· When sodium channels open, sodium ions diffuse into cells (from the extracellular fluid to the intracellular fluid). Since sodium ions have positive charges, the intracellular fluid becomes more positive. Since the cell is now less polarized, we say the cells are depolarized.
Study Questions:
(29) Compare: How are voltage-gated, chemically-gated, and mechanically-gated channels similar and different?
(30) If calcium channels open, which way would calcium ions diffuse? Will the inside of the cell become more positive or less positive? Is this depolarization?
(31) If potassium channels open, which way would potassium ions diffuse? Will the inside of the cell become more positive or less positive?
· When potassium channels open, potassium ions diffuse out of cells (from the intracellular fluid to the extracellular fluid). Since potassium ions have positive charges, this causes the intracellular fluid to become less positive. If cells were first depolarized by sodium or calcium diffusion, then potassium ion diffusion causes the cells to become repolarized.
Opening and closing of ion channels in cardiac autorhythmic/pacemaker cells causes these cells to go through cycles of depolarizations and repolarizations. The time it takes to depolarize then repolarize is the length of one heartbeat. The rate at which this cycling occurs determines heart rate.
These changes in ion movement cause voltage-gated channels to open and close during this cycling so that these cells can contract without signals from other cells of the body. This is why they are called autorhythmic cells (auto means self). They are also called pacemaker cells since they set the pace of the heart (the heart rate).
When autonomic neurons release chemical signals onto autorhythmic/pacemaker cells, chemically-gated channels change ion movement across their membranes.
· Sympathetic neurons stimulate more diffusion of sodium and calcium ions across cell membranes causing depolarization. It then takes less time to depolarize then to repolarize in each cardiac cycle. This increases heart rate.
· Parasympathetic neurons increase membrane diffusion of potassium ions and decreased diffusion of calcium ions causing hyperpolarization (more polarized). It then takes more time to fully depolarize then to repolarize in each cardiac cycle. This decreases heart rate.
Study Questions:
(32) In your own words, what do depolarize and repolarize mean? Compare: How are these similar and different?
(33) Do cells depolarize or repolarize when sodium diffuses across the membrane? calcium? potassium?
(34) How does sympathetic stimulation affect autorhythmic/pacemaker cells? Parasympathetic stimulation?
(35) From previous learning, what do you remember about contraction of the heart chambers?
Cardiac muscles cells are connected by gap junctions (Analogy: tunnels between cells). When the intracellular ion concentration changes in one cardiac cell, these ions can move through gap junctions to connected cardiac cells.
This change in ion concentration causes voltage-gated ion channels to open in connected cardiac cells, stimulating more ion movement and changes in polarity. Cardiac muscle cells can pass electrical signals from one cell to another for coordinated contraction of the heart.
These electrical signals stimulate cardiac contractile cells in the atria and ventricles to contract. Contraction of these cells causes contraction of the heart chambers. When a heart chamber contracts, there is increased pressure within the chamber, and blood moves from the chamber into arteries.
Analogy: Think about squeezing the sides of a water bottle. This makes the inside of the bottle contract which increases the pressure in the bottle. If the top is off of the bottle, water will move from the higher-pressure area in the bottle to the lower pressure area outside of the bottle.
Study Questions:
(36) What is the order of electrical conduction through the heart? (Hint: draw it using the paragraphs above)
How do ECG electrodes detect electrical activity of the heart?
The electrocardiogram (ECG or EKG-from the German spelling of Kardio) measures depolarizations and repolarizations of heart cells. As cardiac muscle cells depolarize and repolarize, the changes in polarity travel through muscles and skin and are measured by electrodes on the skin.
Clinical Application:
ECGs are used to help diagnose myocardial infarction (heart attacks), heart block, pericarditis (peri means around, itis means inflammation), arrhythmias (abnormal heart rhythms) including tachycardia (tachy means fast) or bradycardia (brady means slow), or abnormal potassium levels (hyper or hypokalemia).
How will we design an experiment to measure heart rate?
Since you do not have access to ECG, you will measure pulse. Blood is pumped into arteries. The elastic tissue in artery walls
1. allows the radius of arteries to increase as blood moves into it.
2. then “snaps back” to decrease the radius of arteries to push blood forward. (Analogy: a rubber band stretches then “snaps back”)
Blood cannot move backwards because the valves of the heart closed. When we measure pulse, we detect these increases and decreases in the radius of an artery.
You will want to test something that is interesting and not already know. Therefore, do not submit a hypothesis that exercise will increase heart rate.
1. Each person in your team will turn in a hypothesis to test. This will include a stimulus and a prediction of whether the response will be increased or decreased heart rate.
2. Your team will then decide which hypothesis to test and design an experiment to test it.
3. One person in the team will complete the experiment.
4. You and your team will then analyze and discuss the results.
You can measure heart rate with a heart rate monitor, app, by pressing fingers on an artery on the inside of the wrist or at the neck (do not use your thumb since you will also feel the pulse in your thumb). You could also use other heart rate equipment if you have access to it (if it at work, make sure you have permission).
Note: For each experiment, one person is assigned to perform the experiment (see the syllabus). They will need to find a second person to be the “subject” or the “assistant”. The “subject” will focus on the control and stimulus. The “assistant” will time the experiment, switch between controls and stimulus, and count and record heart rate/pulse.
When designing experiments, it is very important to have controls so that the response to a stimulus can be more accurately measured. For example, your team might hypothesize that listening to mindfulness exercises will decrease heart rate. You would need to design an experiment to compare this to when the test subject is not listening to mindfulness exercises. To control this experiment, the subject should be in the same position with eyes closed and headphones on for both conditions.
Study Questions:
(37) When and how is pulse measured clinically? What could cause pulse to increase or decrease?
(38) What are other ways you could control this experiment?
(39) What else do you already know about designing and controlling experiments?
Being able to repeat experiments with the same results is one of the most important parts of science. When designing experiments, scientists usually test the stimulus at least three times. Scientists then repeat the experiment on different days (in this course, you will have a chance to do this in the last experiment in this course). Then, scientists from other laboratories will repeat experiments.
Experiments are better with more subjects, and scientists use statistics to determine what is a representative sample. In this course, we will only use one subject, and this is a major limitation to the experiment. However, this will help you to understand the importance of things such as large clinical trials and being careful not to make conclusions based on one small study.
A problem with sample size is what stimulated the “Anti-vaxxer” movement. There was one small study with 12 children. The research paper was later retracted based on flawed experiments, and the doctor lost his license. Many, many larger studies with tens of thousands of subjects have not found a link between vaccines and autism. See these article links for some excellent graphs and to learn more about these studies and how the “Anti-vaxxer” movement influenced vaccinations in the Minnesota Somali community.
This first source is from the Guardian which is a .com so considered less reliable source. However, there are great graphs showing the number of subjects in the retracted study connecting vaccination and autism compared to the number of subjects in studies that do not show a connection. Note: there might be bias in this source as The Guardian is a left-leaning publication ( https://www.allsides.com/media-bias/media-bias-ratings ).
· https://www.scientificamerican.com/article/why-autism-seems-to-cluster-in-some-immigrant-groups/
This second source is also from a .com publication, but it is a respected source of scientific information. There have been decreased vaccinations in the Somali community in response to fears of vaccine links to autism and early data that there might be increased autism in the Minnesota Somali community. This article discusses the cultural bias in diagnosing autism and addresses the concern within the Minnesota Somali community about an increased number of cases of autism. A study showed that there 1 in 32 Somali children are diagnosed with autism, and this is similar to the 1 in 36 in White children; as noted in this article, there was a small sample size.
When designing experiments, it is also best to alternate between control and stimulus conditions. In this course, we will start with 1 minute of no stimulus followed by 1 minute of stimulus. We will repeat this three times – so there will be a total of 3 minutes of conditions and 3 minutes of stimulus.
Why is this important? If you did 3 minutes of control then 3 minutes of listening to mindfulness, a decrease in heart rate might be in response to sitting still for 6 minutes. If you alternate between control and stimulus, you will have more accurate results and can be more sure that any changes in your results is dependent on your stimulus.
Your team will design an experiment with one subject and one stimulus, trying to control for anything else that might stimulate the subject.
· Since heart rate increases with movement, the subject should be as still as possible while measuring heart rate.
· You will alternate between 1 minute of no stimulus and 1 minute of stimulus for a total of 6 minutes.
· You will then average the three measured heart rates with no stimulus and compare this to the average of the three measured heart rates with stimulus.
Big Picture:
· The body changes heart rate in response to stimuli using simple nervous reflex loops including:
1. stimulus
2. receptors/sensors
3. PNS input/afferent pathway
4. CNS as the integrating center
5. PNS output/efferent pathway: somatic motor or autonomic divisions
6. targets/effectors
7. responses
· Sodium and calcium ions diffuse into cardiac autorhythmic/pacemaker cells causing the inside to become more positive, or depolarize. Potassium diffusion out of cells causing the inside to become less positive, or repolarize. This cycling creates the cardiac cycle (heart beat).
· Sympathetic stimulation of cardiac autorhythmic/pacemaker cells increases heart rate, and parasympathetic stimulation decreases heart rate.
· Ions diffuse through gap junctions to neighboring cells for coordinated contraction of the heart.
· ECG electrodes measure cardiac muscle cell ion movement during depolarization and repolarization.
· Pulse measured changes in artery radius with each contraction of the heart.
· Experiments are controlled to more accurately measure the response to stimulus.
Integration & Application:
(1) If you saw a tiger, what are the parts of the reflex loop? Explain the ion movement/changes in autorhythmic/pacemaker cells in response to this stimulus.
(2) For each part of the reflex loop, what are the organs, cells, molecules, and ions?
Stimulus:
tiger roar
Receptor/Sensor:
ear
Input/afferent pathway:
cochlear nerve
Integrating center:
brain
Output/efferent pathway:
sympathetic nerve
Effectors/target cells:
heart
Response:
increased heart rate
