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BIO 431 Exam 2 Questions with Correct Answers
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BIO 431 Exam 2 Questions with Correct Answers
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BIO 431 Exam 2 Questions with CorrectAnswers
Location of the heart - Answer--Inside the pericardial sac in the mediastinum
>Mediastinum is separated into the superior mediastinum and the pericardial cavity
-Pericardial sac surrounds the heart. It contains fluid to decrease friction so it can beat
in a frictionless environment.
Two types of cardiac muscle cells - Answer-CONTRACTILE CELLS
-Most cells
-Contractile cells receive their stimulation from these pacemaker cells
AUTORHYTHMIC CELLS (PACEMAKERS)
-Few, smaller, noncontractile
-Spontaneously depolarize - no neural stimulation required for the heart muscle to
contract
-No movement
-Strategically located to help in the initiation and spread of electricity from one cardiac
muscle cell to another
An action potential is initiated in pacemaker cells and spreads VIA GAP JUNCTIONS
from one contractile cell to the next.
Action Potentials in Pacemaker Cells - Answer--Spontaneously depolarize (known as
the pacemaker potential stage)
>Pacemaker potential: -60 mV to -40 mV (threshold)
>-40 mV is where voltage gated channels open
>Na+ influx exceeds K+ efflux
-At threshold: AP initiated
>Ca+ channels (voltage-gated) open → rapid depolarization of pacemaker cells. This
causes repolarization.
-Repolarization
>Ca+ channels close
>K+ channels open: K+ efflux
*Frequency of spontaneous depolarization of these cells establishes heart rate (the
frequency of contraction of the heart)
*NO RESTING MEMBRANE POTENTIAL EVER. No flat line.
Autonomic Influence on Heart Rate (Sympathetic) - Answer--Increases HR
-Catecholamines: NE (neurons + adrenal medulla) + EPI (adrenal medulla) released
from chromaffin cells
,-Catecholamines bind to β1-adrenergic receptors on pacemaker cells
>cAMP 2nd messenger system changes ion channels
>Increased membrane permeability to both Na+ and Ca2+
-More rapid depolarization to threshold to increase rate of action potentials produced
from pacemaker cells
-On the graph: Reach threshold (-40 mV) a lot faster so more APs produced over a
certain period of time
Exact Steps of Action Potentials in Pacemaker Cells - Answer-1. Pacemaker potential:
This slow
depolarization is due to both opening of Na+
channels and closing of K+ channels. Notice
that the membrane potential is never a flat line.
NO RMP!
2. Depolarization: The action potential
begins when the pacemaker potential reaches
threshold. Depolarization is due to Ca2+ influx
through Ca2+ channels.
3. Repolarization is due to Ca2+ channels
inactivating and K+ channels opening. This
allows K+ efflux, which brings the membrane
potential back to its most negative voltage
Autonomic Influence on Heart Rate (Parasympathetic) - Answer--Decreases HR
-Release ACh to bind to muscarinic receptors
>cAMP 2nd messenger system changes ion channels
>Increased K+ permeability and decreased Ca2+ permeability. Higher efflux of K+
(compared to usual) makes it more negative. Hyperpolarizes below -60 mV then comes
back up
-Longer time to depolarize to threshold decreases rate of action potentials from
pacemaker cells.
-On the graph: 2 APs in parasympathetic stimulation compared to the 3 APs without that
influence/normal stimulation
Action Potentials In Contractile Cells - Answer--RMP at -90mV
-Depolarization to threshold at intercalated discs: AP starts at pacemaker cells and will
spread via gap junctions
-Rapid depolarization: Fast voltage-gated Na+ channels open. Causes Na+ influx.
-Slight repolarization: Na+ channels close, stopping depolarization. Some K+ efflux (fast
K+ channels, which is unusual).
-Plateau phase: Slow voltage-gated Ca2+ channels open (Ca2+ influx). K+ efflux
decreases. (This influx of calcium is needed for contraction.)
-Rapid repolarization: Slow Ca2+ channels close. K+ efflux (slow K+ channels).
, Exact Steps of Action Potentials in Contractile Cells - Answer-1. DEPOLARIZATION is
due to Na+ influx
through fast voltage-gated Na+ channels.
A positive feedback cycle rapidly opens
many Na+ channels, reversing the
membrane potential. Channel inactivation
ends this phase.
2. PLATEAU PHASE is due to Ca2+ influx
through slow Ca2+ channels. This keeps
the cell depolarized because most K+
channels are closed.
3. REPOLARIZATION is due to Ca2+channels inactivating and K+ channels
opening. This allows K+ efflux, which
brings the membrane potential back to
its resting voltage.
Cardiac Electrical Events (Compared against Skeletal Electrical Events) - Answer--
Action potentials in cardiac muscle cells (250 msec) are MUCH LONGER than skeletal
muscle cells (5 msec)
>Contraction lasts LONGER as a result
-LONG absolute refractory period in cardiac muscle cells. Absolute refractory period
extends to the end of contraction. This dictates that the heart will be in a relaxed state at
some point to receive blood.
-Summation of contractions is not possible
-No Tetanus!
-No recruitment: All cells contract together (gap junctions), and not as part of individual
motor units
Pumping Activity of the Heart - Answer--When the heart relaxes, blood can flow into the
pump/heart
-When the heart muscle contracts, it pushes the blood out
-The heart cannot stay in a long sustained contraction because it would not be able to
received more blood to be pumped out
-In order for the heart to continue to act like a pump it needs to: relax -> receive ->
contract -> pump out
Cardiac Excitation-Contraction Coupling Summary - Answer--Cardiac muscle cells do
not have as much of a internal storage depot as skeletal muscles do (SR organelle is
not as extensive as skeletal muscle)
-Cardiac muscle cells need external Ca+ outside of the cell to come in to supplement
internal Ca+
-Action potential opens voltage-gated calcium ion channels: Slow Ca2+ channels,
causes calcium influx.
-RyR (ryanodine receptor) channels on SR membrane open due to calcium
-Calcium released from SR
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