Cardiac and Smooth Muscle

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  • Created by: LBCW0502
  • Created on: 19-01-19 14:38
What are some of the differences between cardiac and smooth muscle?
Molecular mechanism for switching contraction on and off, organisation of myofilaments
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What are some of the similarities between cardiac and smooth muscle?
Electrical and mechanical coupling between fibres, innervation by ANS
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What can be identified on the cell structure of cardiac cells using microscopy and staining?
Intercalated discs, nuclei - cell size of 200 micrometres
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What can be identified on the cell structure of smooth muscle cells using microscopy and staining?
Adventitia, smooth muscle layer, nuclei - cell size of 150 micrometres
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Describe features of cardiac muscle structure (1)
Fibres are small with a single central nucleus. Striated (have the same molecular mechanism for force production as in skeletal muscle). Linked to each other both mechanically by adherens junctions, electrically by gap junctions at ICDs
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Describe features of cardiac muscle structure (2)
Short branching fibres to allow spherical contraction, blood vessels (coronary supply – oxygen), less circular muscle fibres due to branching, striated
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Describe the function of cardiac muscle
Wave of electrical activity triggers contraction (has to sweep around the heart from fibre to fibre). When fibres contract they must pull on each other in order to reduce the size of blood-filled chambers, eject blood into major output vessels
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State features of a myofibril in a striated cardiac muscle
I-band, A-band, sarcomere, Z-discs (highly organised)
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The cardiac muscle uses which type of mechanism to enable Ca to operate the on-off switch at molecular level?
Steric blocking mechanism - involves Ca, troponin complex, tropomyosin, myosin and actin along with binding sites (actin, ATP)
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Describe how intracellular Ca 2+i ons are managed in cardiac muscle (1)
Some Ca 2+ needed for contraction comes from SR and is released by a signal from T-tubules but Ca 2+ also enters cell from extracellular space (via voltage gated Ca channels during AP)
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Describe how intracellular Ca 2+ ions are managed in cardiac muscle (2)
Much of Ca release from SR triggered by Ca 2+ ions entering from EC space (CICR) and exact control of intracellular Ca 2+ concentrations is used by other mechanisms (e.g. sarcolemmal Ca-Na exchangers) to control strength of contraction
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What is CICR? (1)
Calcium-induced calcium release. Much of Ca release from cardiac muscle SR is triggered by Ca 2+ ions entering from EC space. Ca ions enter through voltage gated Ca channels during plateau phase of the cardiac APRyR of cardiac SR open in response t
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What is CICR? (2)
RyR of cardiac SR open in response to local rise in Ca concentration, releasing more Ca ions
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What is the difference between the T-tubules in skeletal and cardiac muscle?
Skeletal muscle has triads but cardiac muscle has dyads
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Cardiac muscle cannot make what type of contractions?
Tetanic contractions. Heart is a pump which must contract then relax at least 60 x/min
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What is the key difference in relative length of AP in cardiac and skeletal muscle? (1)
Skeletal muscle - AP duration is ~ 2 m/s, twitch contraction lasts ~ 200 m/s. Cardiac muscle - AP and contraction both last ~ 300 m/s. Results in cardiac muscle with twitch finished by the time a new AP can be generated
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What is the key difference in relative length of AP in cardiac and skeletal muscle? (2)
Over of contractile responses (as occurs in tetanus) does not occur (see graphs for skeletal/cardiac APs)
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What are the ion currents in skeletal muscle AP?
Na+ and K+
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What are the ion currents in cardiac muscle AP?
Na+, Ca 2+ and K+
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Describe features of the length-tension curve in cardiac muscle (1)
Similar to skeletal muscle, active tension depends on effective overlap of thick/thin filaments in sarcomere. Overlap depends on how stretched sarcomere is
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Describe features of the length-tension curve in cardiac muscle (2)
Skeletal muscles work at optimum length but cardiac muscle works on ascending portion of curve (force of contraction can be increased/decreased by stretching cardiac muscle cells more or less).
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Describe features of the length-tension curve in cardiac muscle (3)
Ascending portion is steeper/more sensitive to changes in length
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What are the factors affecting the force of contraction of cardiac muscle? (1)
Muscle length (length-tension relationship, as ventricle fills more, fibres are stretched and next contraction is stronger). Treppe effect (as HR increases, fall in [Ca] at end of each beat is not complete so intracellular [Ca] rises slightly
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What are the factors affecting the force of contraction of cardiac muscle? (2)
(Contractions are stronger). Ionotropic agents (these increase intracellular [Ca] by various mechanisms, increasing strength of contraction
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Describe the structure of smooth muscle fibres (1)
Have actin and myosin filaments but are not striated, they are coupled to each other. Mechanically/paired by MDAs, electrically by gap junctions. No T-tubules or troponin and little SR. Intermediate fibres/dense bodies join myofibrils. Nuclei bulged
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Describe how Ca 2+ triggers contraction in smooth muscle
Ca 2+ binds to calmodulin, binds to myosin light chain kinase (uses ATP), phosphorylated the myosin light chains, so myosin head can bind to actin and start the contraction (which stops when another enzyme dephosphorylates them)
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Describe how intracellular Ca 2+ ions are managed in the smooth muscle (1)
Intracellular Ca conc used to regulate strength of contraction. Some Ca 2+ needed for contraction comes from SR but Ca release mechanism from SR is different (no T-tubules). Most Ca needed enters cells from EC space via several routes.
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Describe how intracellular Ca 2+ ions are managed in the smooth muscle (2)
Extrusion of Ca 2+ from fibre via pumps and exchangers is more important for relaxation than reuptake into SR
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How is [Ca 2+] raised for contraction?
Ca 2+ enters smooth muscle cell via voltage gated Ca channel (move down conc gradient/pharmaco-mechanical coupling) or receptor gated ion channel (electrochemical coupling). IP3 pathway leads to release of Ca from SR (little)
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How is [Ca 2+] lowered for relaxation?
Ca 2+ pump to remove Ca form cell. Ca returns to SR. Na-Ca pump (1 Na in, 3 Ca out)
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What are the two main routes to increase [Ca 2+] and contraction?
Pharmaco-mechanical coupling (chemical route using intracellular messengers, no APs/depolarisation needed). Electro-mechanical coupling (electrical route, depolarisation required)
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Describe features of pharmaco-mechanical coupling
Transmitter binds to receptor. Receptor activates GCRP. Generates intracellular messenger (e.g. cGMP). Cause change in [Ca 2+]. Regulates actin-myosin interactions (outcome of contraction/relaxation depends on type of transmitter/receptor/messenger)
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Describe features of electro-mechanical coupling
Depolarisation opens ion channels. Ca 2+ enters cell (down conc gradient). Raises intracellular [Ca 2+]. Enables actin/myosin interaction (outcome - force increases as [Ca 2+] increases
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What is depolarisation caused by?
Electrical transmission (gap junctions). Closing of K+ channels. Inhibition of Na pump
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Describe the phasic contraction in smooth muscle
Rise and fall in [Ca 2+] phosphorylation of myosin and force are tightly coupled to each other (e.g. removing food down GIT, co-ordination)
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Describe the tonic contraction in smooth muscle (1)
Lasts much longer, force is higher than expected from Ca 2+ concentration. Very economic contraction for long periods
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Describe the tonic contraction in smooth muscle (2)
E.g. blood vessels (dilation/constriction), sphincter muscles (constantly closed), Ca increases then decreases to lower level (latch – myosin interacts with actin at a slower rate/slower contraction), prolonged contraction
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Describe features of single units
Few synaptic contacts and many gap junctions so acts as a single unit e.g. visceral (control portion of the muscle, spread signal/contraction)
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Describe features of multi units
Synaptic contacts on every cell but fewer gap junctions so cells can act independently e.g. some vascular (don’t need as much co-ordination, occur straight away)
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What are the similarities between skeletal, cardiac and smooth muscle? (1)
All have fibres with an electrically excitable sarcolemma. All use ATP as energy source for contraction. All have actin and myosin as main contractile proteins, arranged into thick/thin filaments
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What are the similarities between skeletal, cardiac and smooth muscle? (2)
All use interaction of actin/myosin proteins to generate force and shortening. All use calcium to initiate contraction at molecular level
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What are the differences between skeletal, cardiac and smooth muscle?
Cell shape, striated/non-striated, on-off switch, source of calcium ions, how force is regulated, electrical activity (AP shape and duration) etc.
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What are some of the similarities between cardiac and smooth muscle?

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Card 3

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What can be identified on the cell structure of cardiac cells using microscopy and staining?

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Card 4

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What can be identified on the cell structure of smooth muscle cells using microscopy and staining?

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Card 5

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