Chapter 14 Flashcards
Define motor unit as it applies to skeletal muscle. Does cardiac muscle have motor units? Explain.
Motor Unit - a somatic motor neuron and all skeletal muscles it innervates.
Cardiac muscles DO NOT have motor units. They communicate contraction through intercalated discs.
Where does the electrical stimulus come from that stimulates muscle contraction in cardiac muscle cells?
the SA Node
Describe the anatomy of a cardiac muscle cell and explain how its structure directly relates to the regulation of its contraction.
The muscle cell is striated due to lots of actin and myosin which bind together causing the cells to contract. It’s branched form allows it to connect with three or four other cells at a time for faster communication. The intercalated disks form tight junctions between the cells so that they cannot separate under the strain of pumping blood and so that electrochemical signals can be passed quickly from cells to cell. The passage of signals from cell to cell allows cardiac muscle tissue to contract very quickly in a wave-like pattern to effectively pump blood throughout the body.
What roles does the nervous system have in regulating cardiac muscle cell contraction?
The heart is not dependent on the nervous system for control as it has it’s own pacemaker that sets its contraction rhythm. The pacemaker cells, however, normally receive inputs from the nervous system to increase or decrease the heart rate depending on the body’s needs. However, in the absence of nervous system stimulation, the pacemaker cells can produce a regular heart rhythm.
Explain the mechanism by which cardiac muscle cells relax. Be sure to name the proteins (and ions) involved, and their activities.
Opening of voltage gated potassium channels releases potassium out of the cell and causes repolarization at which time the channels reset and the heart relaxes. A long refractory period prevents tetanus.
Explain how the nervous system and endocrine systems are able to cause a cardiac muscle cell to generate different amounts of force. Be sure to include the signaling molecules, proteins, and ions involved, and their activities in your explanation.
The sympathetic Nervous system generally releases norepinephrine while the adrenal medulla releases both epinephrine and norepinephrine. These catecholamines bind to beta 1 receptors that activates cAMP second messenger system and uses adenylate cyclase to open the voltage gated calcium channel allowing Ca+ to enter the SR from the ECF. The more Ca+ available means more active crossbridge cycles and greater force of contraction.
Compare and contrast the heart and skeletal muscles in terms of: 1)how the amount of force generated is controlled, and 2.) how the nervous system initiates contraction.
- Amount of force generated: Skeletal - all/max force; Cardiac - graded depending on the amount of Ca+ available.
- How nervous system initiates contraction: Skeletal - all or none; Carciac - release of epi/norepi
About how long is the refractory period in a myocardial contractile cell action potential? Why is this significant?
about 250 msec.
Significant because it prevents tetanus and it lasts almost as long as the entire muscle twitch
List the phases of the cardiac cycle in order.
- Atrial and ventricular diastole
- Atrial systole
- Isovolumetric contraction
- ventricular ejection
- Isovolumetric relaxation
For each phase of the cardiac cycle describe the primary events that occur. (focus on 1. which chambers in systole or diastole, 2. pressure changes, 3. valves opening and/or closing, 4. flow of blood)
Left Side:
- Atrial and ventricular diastole - some pressure in LA and lower pressure in LV; Aortic valve closed and left AV valve open; (passive filling) blood flows into the LV from the LA filling to about 80%
- Atrial Systole - pressure increases in the LA pushing blood from LA to the LV filling it to EDV (adding another about 20%)
- Isovolumetric Contraction - Ventricles enter systole; pressure rises in the ventricles; AV valves close so all four valves are closed; no filling/no ejection
- Ventricular ejection - when ventricular pressure is higher than aortic pressure then aortic valve is pushed open and the ventricle ejects 1 stroke volume (about 70mL); blood flows from LV into the aorta.
- Isovolumetric relaxation - ventricles relax; pressure drops and when ventricular pressure is lower than aortic the aortic valves close so all 4 valves are closed again.
Define EDV.
End Diastolic Volume - the volume of blood in a ventricle right before it contracts (fullness)
Define ESV.
End Systolic Volume - the volume in ventricle at end of systole
What is the major difference between the right and left sides of the heart?
Right ventricle generates a lower pressure.
LV=120mmHg/RV=25mmHg
Define Stroke Volume. What is a typical volume of SV?
The volume of blood ejected by a ventricle in 1 systole. Typical is about 70mL per ventricle per systole
Define Cardiac Output in words (not mathematical equation). What is a typical CO volume?
the volume of blood ejected by a ventricle in 1 minute.
typical is 5L/min.
What is the mathematical equation that describes CO?
CO = HR x SV
cardiac output = heart rate x stroke volume
Explain how the parasympathetic NS is able to adjust HR. Be sure to include the signaling molecules, proteins and ions involved, and their activities in your explanation.
The parasympathetic neurotransmitter acetylcholine (ACh) slows heart rate. ACh activates muscarinic cholinergic receptors that influence K+ and Ca2+ channels in the pacemaker cell. K+ permeability increases, hyperpolarizing the cell so that the pacemaker potential begins at a more negative value and Ca+2 permeability of the pacemaker decreases which slows the rate at which the pacemaker potential depolarizes. This combination causes the cell to take longer to reach threshold, delaying the AP and slowing the HR.
Explain how the sympathetic NS and endocrine system is able to adjust HR. Be sure to include the signaling molecules, proteins and ions involved, and their activities in your explanation.
Sympathetic stimulation of pacemaker cells speeds up heart rate. Epinephrine and norepinephrine is released and binds to a beta 1 adrenergic receptor which uses a cAMP second messenger system to alter the transport properties of the ion channels. This causes the HCN channel to stay open creating an influx of Na+ and it reaches threshold sooner.
Explain the relationship between EDV and SV. What is the mechanism (what is happening to the sarcomeres)?
EDV - ESV = SV
EDV is the amount of blood in the heart after filling and SV is the amount pumped out into the body.
The sarcomeres are contracting.
Describe why the Frank-Starling law of the heart is critical to normal heart function.
It means that within physiological limits, the heart pumps all the blood that returns to it. It shows the relationship between the filling of the heart, EDV, stretch, contractility, and SV and how they are all proportional to each other.
Define venous return.
volume of blood returning to the heart (right side) from veins (systemic veins)
List the three basic ways that venous return is increased, and explain how and when each mechanism operates.
- Skeletal muscle pump (exercise)
- Respiratory pump (inspiration)
- Sympathetic nervous system (vasoconstriction)
Describe the general effect that increases or decreases in blood volume have on blood pressure. Describe the effect of vasoconstriction and vasodilation have on blood pressure.
Increase in blood volume = Increase in blood pressure
Decrease in blood volume = Decrease in blood pressure
Vasodilation = decrease in blood pressure
Vasoconstriction = increase in blood pressure
Explain why blood flows from the left ventricle to the right ventricle through the systemic circuit. Why does it have this direction of flow?
The LV has more pressure to push the blood through the body with more force to make it through the body and back to the heart while the RV has less pressure so is better equipped to pump blood through the more delicate pulmonary system.
Explain how pressure affects flow. (if deltaP increases what happens to flow?)
Blood flows from high to low pressure. no deltaP = no flow. If deltaP increases = flow increases.
Define resistance as it applies to a vessel or vessels. Explain how resistance affects flow. (if resistance increases what happens to flow?)
Resistance - Anything that restricts/limits or reduces flow
High resistance = low flow
Low resistance = high flow
List the 3 factors which influence the resistance through vessels. For each factor explain the mechanism by which it affects resistance. Which one of these three has the largest impact on resistance? which of these three can readily be adjusted by the body?
- Radius of vessel. MOST IMPORTANT and readily adjusted - Small increase in radius has exponential impact on resistance and flow (vasodialation)
- Length of a vessel - greater length = greater resistance
- Viscosity - greater viscosity = greater resistance
List the 4 factors included in Poiseulle’s law that determine the flow through a vessel. for each factor state how it affects flow in simple terms.
- deltaP (pressure difference) - deltaP increases = flow increases
- vessel radius - radius increases = flow increases
- length - length increases = flow decreases
- Resistance - resistance increase = flow decreases
