Cardiac Muscle

Question 1

Cardiac muscle structure

Answer 1

. Striated 
. Rigid sarcomere structure 
. extensive T-tubule and SR 
. Tons of mitochondria 
. Dense capillary supply 
. Gap junctions btw cells

Question 2

Characteristics of cardiac muscle

Answer 2

. SR and extracellular sources of activating Ca
. Troponin site of Ca regulation
. Slow speed of contraction
. Spontaneous production of APs in pacemaker cells
. No tone
. Can be excited or inhibited by nerve stimulation
. Controlled by hormones

Question 3

Intercalated disk

Answer 3

. Site of gap junctions
. Allow for rapid conduction of electrical activity from 1 cardiac m. Fiber to another
. Makes it so cardiac m. Acts as one unit (functional syncytium)
. Single depolarizing stimulus results in contraction of entire myocardium
. Cardiac m. Is fully recruited at all times

Question 4

T/F ANS modulates cardiac contractile performance

Answer 4

T

Question 5

SNS effect on cardiac cells

Answer 5

. Activate beta-1 receptors

. Inc. contractility

Question 6

PNS effect on cardiac cell

Answer 6

. Slow HR and conduction

. Effect on contractility is normally small

Question 7

Difference btw skeletal and cardiac mm. E-C coupling

Answer 7

. Ca in skeletal m. Comes from SR

. Ca in cardiac m. Comes from both SR and from outside

Question 8

Main regulatory site for cardiac function

Answer 8

L-type Ca channel

Question 9

Ca-induced Ca release in cardiac cells

Answer 9

. Extracellular Ca can activate contractile protein directly or trigger release from SR
. Ca release is positive feedback loop
. Ca channels are stimulated at low cytosolic levels and inhibited at high cytosolic levels

Question 10

Peak level of cytoplasmic Ca depends on ____

Answer 10

. Amount of Ca stored in SR
. HR
. Amount of Ca entering from extracellular space

Question 11

Resequesteration of Ca

Answer 11

. Primary mechanism to return Ca to resting levels
. ATP-dependent reuptake into SR via Ca-ATPase
. High levels Ca stimulate pump

Question 12

Extrusion of Ca in cardiac muscle

Answer 12

. Na-Ca exchange: removes 1 Ca from intracellular space for 3 Na moving into cell
. Ca-ATPase: not as important as Na-Ca exchanger

Question 13

Na-Ca exchanger

Answer 13

. 1 Ca out for 3 Na in
. Electro genie (net intracellular gain of 1 pos. Charge)
. Not directly ATP-requiring pumps indirectly dependent on Na gradient

Question 14

Major mechanisms for gradation of force in cardiac mm.

Answer 14

. Change in contractility

. Change in initial fiber length

Question 15

Recruitment of cardiac muscle cells

Answer 15

. Cardiac muscle is fully recruited
. Considered analogous to 1 giant motor unit
. Activity initiated by SA node

Question 16

Summation of cardiac muscle cells

Answer 16

. Inc. frequency of stimulation (inc. HR) does not result in summation and titanic contractions like it does in skeletal muscle

Question 17

Refractory period in cardiac cells

Answer 17

. Nearly as long as contractile process

. Cell can’t be re-excited in time to cause summation of contractile force

Question 18

Contractility

Answer 18

. Performance of heart at given period
. Ability of muscle tog enervate force from given resting fiber length
. Inc. in contractility (positive inotropic effect): inc. in tension developed at given fiber length
. Negative inotropic effect: dec. in tension development at given fiber length

Question 19

Role of Ca in changes in contractility

Answer 19

. Alteration of Ca flux is almost always predominant mechanism for alteration in contractility
. Ca flux regulated on beat-to-beat basis
. Ca flux from 1 AP is enough to activate 1/2 crossbridges
. Additional cytoplasmic Ca will result in additional force production

Question 20

Heart rate effect on Ca flux

Answer 20

. Inc. frequency of stimulation inc. contractility (Bowditch, Staircase, or Treppe effect)
. Intrinsic property of cardiac m.
. HR inc., number of APs per unit inc., inc. Ca current per unit time
. Less time btw beats to extrude excess Ca that entered
. Excess Ca is sequestered by SR and thus more Ca is available for release in subsequent beats
. If interbeat interval is too short, the basal level of intracellular Ca may inc.

Question 21

Length-tension relationship in cardiac muscle

Answer 21

. Operates below Lmax
. Significant role in modulation of force production bc large changes in initial cardiac m. Length (sarcomere length) can occur
. Normal operates on ascending part of relationship curve (normal initial resting lengths are below optimal length for producing force)

Question 22

What determines initial cardiac muscle length in the intact heart?

Answer 22

. Blood contained in ventricles just prior to beginning of contraction (end-diastolic volume) determines what degree ventricles are distended
. Greater EDV, greater force produced

Question 23

How does inc. In muscle fiber length result in increased force of contraction?

Answer 23

. Number of functional crossbridges inc.

. Ca-sensitivity to Tn-C is length-dependent (inc. in length results in inc. affinity of Tn-C for Ca)

Question 24

Starling’s law of the heart

Answer 24

. Force of contraction is dependent on initial fiber length

Question 25

Cardiac glycosides (digitalis)

Answer 25

. Drugs that treat HF
. Positive inotropic agents that inc. myocardial contractility
. Inhibits NA-ATPase causes accumulation of intracellular Na that is exchanged for Ca
. Inc. intracellular Ca inc. contractility
. Too high levels of drugs can cause arrhythmia

Question 26

Negative inotropic agents

Answer 26

. Ca channel blockers

. Limited use