Download Lecture 10: Cardiac Muscle Cells

Lecture 10: Cardiac Muscle Cells
Reading: OpenStax A&P Text Chapter 19
Cardiovascular system consists of a pump (the heart) which is made of cardiac muscle tissue, and a network of pipes (vessels) that
allow blood to be moved throughout the body. In our exploration of the cardiovascular system, we will look at the anatomy and
physiology of the heart (how does it function as an efficient and effective pump?). Then we’ll take a closer look at the vessels, where
blood pressure is a critical measurement. Finally, we’ll examine blood and its role in capillary exchange and blood clotting.
mm Anatomy of the CV system
The heart consists of 4 chambers (2 atria and 2 ventricles), which are separated by one way valves.
Pulmonary circulation moves blood between the heart and the lungs, while systemic circulation moves blood
between the heart and the body.
Draw a picture of the following anatomical structures:
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•
•
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Right/left ventriclesRight/left atriaPulmonary veins
Pulmonary arteriesAortaVena cava
BodyLungsCapillaries
Other veins (and venules)
Other arteries (and arterioles)
Cardiac muscle cells (aka “myocardium”)
Begin by comparing cardiac muscle cells to skeletal muscle cells…
1. Single nucleus in each cell
2. Branching cells connected by intercalated discs. Intercalated discs include desmosomes (structures that strongly connect cells
together) and gap junctions (which allow depolarizations to spread from cell to cell)
3. Many mitochondria (1/3 of the cell volume is filled by mitochondria)
A. Heart beats 100,000x in 1 day!
B. Heart begins beating after 22 days in the womb! Source: http://www.meddean.luc.edu/lumen/MedEd/GrossAnatomy/
thorax0/heartdev/main_fra.html
4. Smaller SR…what does this mean?
5. Do not require nervous input to contract! (However, nerve input can control heart rate…) See http://www.youtube.com/
watch?v=lmhBEeEMqYo&feature=related for evidence! This should make you wonder about how the message is delivered to
CONTRACT...
6. 2 types of cells: Autorhythmic cells and Contractile cells
Contractile cells
Autorhythmic cells generate the AP, but contractile cells enable the heart to contract. They also have a unique AP graph...
1. Resting membrane potential: -90 mV
2. Action Potential enters contractile cell from adjacent cell through intercalated disks causing voltage gated Na+ (and K+)
channels to open so the cell depolarizes, 20 mV
3. Na+ gates snap shut, fast K+ gates open.
4. Voltage gated Ca2+ gates suddenly open (10% of needed Ca2+ enters this way), fast K+ gates close
A. Ryanodine receptors (RyR) found on the SR bind with the Ca2+ and open, letting more Ca2+ out into the sarcoplasm (90%
of the Ca2+ enters this way)
i. Malfunctioning RyR is correlated with arrhythmias and heart failure!
ii. Ryanodine is also a drug/insecticide that binds with these receptors
B. This surge of Ca2+ is considered a spark, that initiates contraction in thin and thick filaments
C. Ca2+ is removed from the sarcoplasm by the Sodium-Calcium Exchanger
5. Ca2+ gates shut and very slow K+ channels finally open so the cell repolarizes
Note: Cardiac muscle contractions can be “graded” by varying Ca2+ permeability. More Ca2+ causes increased force…less makes
less force. This is different from skeletal muscle...which contracts in an “all or none” manner. Because of this, modifying calcium
permeability can affect the force of contraction in the heart. Applications?
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Autorhythmic cells
AR cells generate periodic action potentials, completely without input from the nervous system! The action potential graph of an AR
cell is pretty unique.
1. AR cells do not have a resting potential…they have the “Pacemaker Potential” that starts at -60 mV and slowly becomes more
positive…
A. Funny Na/K channels (If channels) are leaky and open…Na comes in faster…
B. As the membrane becomes more positive, Ca2+ channels open too!
C. All of this causes the cell to slowly depolarize
2. Threshold is reached at -40mV
A. Ca2+ channels open, and the steep depolarization begins
3. At the peak, Ca2+ gates close, and K+ gates open…leads to repolarization…
4. K+ gates close…
5. Ca+ is pumped back into the ECF by Na/Ca exchanger
A. 1 Ca+ pumped out (against the gradient)
B. 3 Na+ pumped IN (with its gradient)
C. Then the 3 Na+ are used in the Na/K pump
Placement of autorhythmic cells (Electrical conduction system)
AR cells are found in specific areas in the heart. The AP begins in the SA node...and follows this path:
1. SA node (in the top of right atrium), act as the pacemaker of the heart.
2. Internodal pathways
3. AV node (near the base of the right atrium)...the AP slows down slightly here.
4. AV bundle (in the ventricular septum)...aka “bundle of his”
5. R + L bundle branches
6. Purkinje fibers (spread out into the ventricular myocardium from the bottom UP...)
Heart rate
The heart rate is initiated by autorhythmic cells in the SA node, which fire at 90 beats per minute if there is NO nervous input.
1. How could you SLOW DOWN this “default” heart rate? (Increase K+ permeability…Decrease Ca2+ permeability…)
2. How could you SPEED IT UP? (Increased Ca2+ permeability…Increased Na+ permeability…)
Your body regulates HR via NTs and hormones.
Parasympathetic input:
1. Slows HR (can get as low as 30 bpm)
2. Adds ACh!
3. ACh activates muscarinic receptors which increase K+¬ permeability.
Sympathetic input:
1. Speeds it up! (max: 250-300 bpm)
2. Adds catecholamines (Norep and Ep)
3. Increase permeability of Ca2+ and If channels…
Cardiac cycle
The complete sequence of events that include one full heartbeat is the cardiac cycle.
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Lab 10: PhysioEx #6
Questions based on LABORATORY EXERCISE 6 from PhysioEx 9.0 simulation package by Pearson.
Rock the whole thing, doggggs.
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External Brain 10: Cardiac Muscle Function
Study Guide Questions
1. Compare and contrast cardiac muscle tissue and skeletal muscle tissue. (This includes comparing individual cells).
2. Compare and contrast the functions of autorhythmic cells and contractile cells in the heart.
3. Clearly describe the events during an action potential in an autorhythmic cell.
4. Clearly describe the events during an action potential in an contractile cell.
5. Be able to compare and contrast the action potentials in autorhythmic cells, contractile cells, and neurons.
6. Describe the events leading up to contraction in a cardiac contractile cell. (Where does the stimulus to “contract” come
from?)
7. What is the sodium-calcium exchanger? How does it work? What other pump is required in order for it to work? Why is this
other pump required?
8. Be able to clearly describe how heart rate is controlled. Address the roles of the parasympathetic and sympathetic branches
of the ANS and be able to suggest a MECHANISM by which each branch could exert its effect.
9. Be able to draw a graph of an action potential that would result in a fast heart rate and a slow heart rate.
10.Be able to describe the significance of ryanodine receptors found on the SR of cardiac contractile cells.
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