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True or False
99% of the heart's cells are contractile and do the work of pumping.
1% are the special autorhythmic cells.
True or False
The action potential of a cardiac autorhythmic cell is no different then a neuron.
The action potenital of a cardiac autorhythmic cell is different then a neuron and involves three ions: Na+, K+, and Ca++.
1. Na+ Funny channels open causing the membrane to "drift" towards threshold
2. After threshold hits, Ca++ channels open causing depolarization of he cell membrane.
3. K+ channels open causing repolarization of the cell membrane.
see obj. 11 p. 769
It is not surprising that changes in _____ concentration in the blood can greatly affect the ______ action of the heart. Elevated levels of K+ _______ the heart rate and ________ of the heart.
ion, pumping, decrease, contractibility
When prisoners are put to death by lethal injection, they are given a high dose of K+ which stops the heart.
Why do blood bags used for blood transfusions have expiration dates on them?
If blood sits for too long, red blood cells die and release intracellular K+. Transfusing an old bag of packed red blood cells has the danger of causing a K+ overload.
What are the effects of blood ion concentration on the Heart?
- Increase in K+ slows down the heart
- Increase in Na+ blocks Ca++ from entering the cell slowing down the heart
- Moderate increase in Ca++ speeds up and strengthens the heart.
True or False
The heart needs nervous stimulation to beat.
The heart does not need nervous stimulation to beat.
True or False
The autonomic nervous system can speed up or slow down the rate of the heart and controls the strength of contractions.
The action potential generated by __________ cells travels through the conduction system of the heart and then spreads to the _________ cells of the atria and ________.
autorhythmic, contractile, ventricles
What are the three phases of Cardiac muscle action potential?
- 1. Depolarization
- 2. Plateau
- 3. Repolarization
- see graphics in obj. 13
- Contractile muscle cells have a resting potential of about -90mv
- When an action potential is triggered, voltage-gated fast Na+ channels open which quickly lead to depolarization
- contractile cells are the only cells that sustain a prolonged depolarization phase called a plateau (neurons and skeletal muscle cells don't have this)
- Voltage-gated slow Ca++ channels in the sarcolemma open.
- Ca++ flows into the cytosol triggering the further release of Ca++ from the sarcoplasmic reticulum
- Ca++ ions bind to troponin allowing actin and myosin to bind, sliding past each other and increasing tension causing contraction.
- Voltage-gated K+ channels open causing K+ to rush out of the cell restoring the negative membrane potential.
- Ca++ channels close at the same time leading to repolarization
- The time during which a second action potential cannot be triggered.
- This period is longer than in skeletal muscle, which allows for the ventricles to relax while the atria contract, and for the ventricles to contract while the atria rest.
In _______ muscle, Ca++ prolongs depolarization making the ________ ______ about 200X longer than ________ or nerve action potentials.
cardiac, action potential, skeletal
- disorders of the rhythm of the heart
- are typically either excessively slow (bradyarrhythmias or bradycardias), or overly rapid (tachycardias)
- Drugs used to treat arrhythmias affect either ion channels or neurotransmitter receptors.
What are some drugs used to treat arrhythmias?
- Beta blockers: block norepinephrine neurotransmitters or hormones that speed up the heart rate.
- Calcium channel blockers: slow the flow of calcium into and out of cells
- Quinidine, lidocaine, and others block sodium channels.
Electrocardiogram (ECG or EKG)
a recording of the electrical changes on the surface of the body resulting from the depolarization and repolarization of the myocardium
True or False
by measuring the ECG, we can quantify and correlate the electrical and mechanical activities of the heart.
_________ ECGs show problem within the ________ pathway of the heart. An ECG can show if the _______ is enlarged, if certain regions of the heat are _________, and the cause of chest pain.
Abnormal, conduction, heart, damaged,
An ECG consist of :
- P wave: Atrial depolarizatoin
- QRS complex (wave): Ventricular depolarizatoni
- T wave: Ventricular repolarization
- Atrial repolarization is hidden behind the large QRS complex
- P-Q interval: area between P and Q, measures the time it takes for the atria to depolarize
- S-T segment: time it takes to empty the ventricles before they repolarize (the T wave)
- Q-T interval: time from ventricular depolariztion to the end of ventricular replarization.
See examples of abnormal EKG tracing in objective 14. p.775-776
see obj. 15 p. 777 comparing the cardiac Muscle Action Potential and EKG
The cardiac cycle includes all of the events associated with one heartbeat, including ______ (relaxation phase) and _____ (contraction phase) of both the atria and ventricles.
The build-up of _______ behind the AV valves opens both __________. This is followed by contraction of both ______.
pressure, simultaneously, atria
This is also true if the semilumnar (outflow) valves and the ventricles. they open and contract as pairs.
The Cardiac cycle
- Systole= contraction
- Diastole= relaxation
- Cardiac cycle:
- 1. diastole (entire heart relaxes, atria fill with blood)
- 2. atrial systole (atria contract, atrial "kick")
- 3. ventricular systole (powerful ventricular contraction sends blood from R ventricle to lungs and L ventricle to body.
What are the Individual events of the cardiac cycle?
- 1. Acton potential starts at the SA node causing depolarization of atrium producing P wave.
- 2. Atria contact (atrial systole)
- 3. Action potential pauses at AV node. It then spread to the ventricles causing depolarization (QRS wave)
- 4. Contraction of ventricles. Begins shortly after QRS complex appears and continues during S-T segment
- 5. Repolarization of ventricles (T wave)
- 6. Ventricular diastole begins shortly after T wave begins.
Systole and Diastole
- When ventricles pump, wave of pressure is sent to aorta, then into arteries
- This wave of pressure occurs because of ventricular systole and the pressure developed is systolic pressure
- When ventricles relax (diastole). pressure drops (diastolic pressure)
- see obj. 17 p. 781