cardiac muscle

Question 1

properties of cardiac muscle

Answer 1

1. Small cells (same as smooth)
2. Striated (same as skeletal)
3. Branched cells connected by intercalated discs
4. Involuntary muscle

Question 2

does cardiac muscle have troponin

Answer 2

1. yes
2. Troponin is the calcium sensing protein
3. In cardiac muscle, there are different isoforms of troponin (can distinguish between troponin in skeletal and cardiac muscle)

Question 3

does cardiac muscle have sarcomeres

Answer 3

yes

Question 4

are cardiac cells electrically coupled

Answer 4

yes

Question 5

3 components of intercalated discs

Answer 5

1. interdigtating folds- in folds at the end of cells that increase the surface area of cell-cell connection
2. mechanical junction-2 types: fascia adherens and desmosomes
3. electrical junctions- electrically coupled by gap junctions. Enables each cell to stimulate the next

Question 6

gap junctions

Answer 6

1. A hexamer composed of connexins provided by each cell
2. Connexin = protein subunit that forms gap junctions
3. Gap junctions hexamers (6 subunits) provided by adjacent cells form a gap junction with 12 subunits which connects the cells
4. Present in intercalated discs
5. low-resistance pathway between cells
6. cell-cell conductance by electrical coupling
7. • Gap junction plaques - 5 to 500 gap junctions which offer low resistance pathways and cell to cell conduction

Question 7

description of blood supply in cardiac muscle

Answer 7

1. Requires continuous supply of oxygen for mitochondria to be able to rapidly oxidise substrates and generate ATP
2. Rich capillary supply - approx 1 capillary per fibre
3. Short diffusion distances for oxygen and waste products and waste

Question 8

2 types of electrical activity

Answer 8

1. pace maker potential

2. prolonged cardiac action potential

Question 9

3 ion conducting mechanisms

Answer 9

1. Funny channel
2. T-type calcium channels
3. L-type calcium channels

Question 10

what is contraction in cardiac muscle cells in response to

Answer 10

1. membrane action potential that propagates through the T-tubules

Question 11

L type Ca 2+ channels

Answer 11

1. (long lasting current)
2. Modified DHP receptors that act as the voltage sensor in skeletal muscle excitation-contraction coupling
3. Enable depolarization of the cell due to influx of extracellular Ca2+
4. VGCs

Question 12

Where does Ca2+ for contraction come from

Answer 12

1. When L-type channels are open, there is an influx of calcium
2. Calcium levels in cytoplasm increase
3. Calcium regulates the activity of the ryanodine receptor
4. ryanodine receptors on external surface of sarcoplasmic reticulum are Ca2+ channels, and not opened by voltage as in skeletal muscle, instead by binding of trigger Ca2+ in the cytosol
5. Causes ryanodine receptor to open and release more calcium into the cytoplasm from the sarcoplasmic reticulum
6. This binds to troponin
7. Allows cross bridge cycling to occur
8. Calcium is pumped back into the sarcoplasmic reticulum via a protein called serca

Question 13

Any differences between skeletal muscle filament activation, cross bridge cycling etc and in cardiac muscle

Answer 13

No

Question 14

when does contraction end

Answer 14

1. when cytosolic Ca2+ concentration restores to its low value by Ca2+ ATPase pumps in the sarcoplasmic reticulum and sarcolemma (cell membrane of muscle cell)

Question 15

Sequence of excitation-contraction coupling

Answer 15

1. Membrane depolarised by Na+ as an AP begins
2. Depolarisation opens L-type channels in T tubules
3. Trigger Ca2+ enters cytosol, contributing to cell depolarisation. Binds to and opens ryanodine receptor
4. Ca2+ flows into cytosol, cross bridge cycling occurs
5. Once Ca2+ has been restored to same level, membrane is repolarized when K+ exits the cell to end the action potential

Question 16

uses of cardiac troponins

Answer 16

1. Cardiac troponins are released into the blood when heart muscle is damaged
2. This is used to detect myocardial infarction

Question 17

Troponin I

Answer 17

inhibitory

Question 18

Troponin c

Answer 18

calcium binding

Question 19

Troponin T

Answer 19

tropomyosin binding

Question 20

Describe adhering junctions

Answer 20

1. Couple the membrane to the contractile apparatus (actin cytoskeleton) of the cardiomyocytes
2. Attached to the thin filaments
3. Aid the transmission of contractile force from one myocyte to the next

Question 21

Describe adhering junctions

Answer 21

1. Couple the membrane to the contractile apparatus (actin cytoskeleton) of the cardiomyocytes
2. Attached to the thin filaments
3. Aid the transmission of contractile force from one myocyte to the next

Question 22

What is pacemaker potential

Answer 22

1. Spontaneous gradual depolarization

Question 23

Describe the sinoatrial node

Answer 23

1. Has rhythmical pacemaker activity
2. It does not have a steady resting potential but it slowly depolarizes
3. Self generating depolarisation

Question 24

Describe the role of T-type calcium channels

Answer 24

1. Voltage gated calcium channels
2. Contribute to the final depolarizing boost of the pacemaker potential
3. Opened when threshold for opening calcium channels is reached by the drifting baseline potential
4. This allows the threshold for an action potential to be reached

Question 25

Describe the role of L-type calcium channels

Answer 25

1. open to cause the action potential after T-type
2. Voltage gated K+ ion channels
3. Open during repolarization phase
4. Start to close at negative potentials
5. Progressive reduction in potassium permeability
6. Restores resting membrane potential

Question 26

What is cardiac muscle resting potential

Answer 26

Resting potential is close to -90mV

Question 27

Describe how an action potential is generated

Answer 27

1. Activated in cardiomyocyte
2. Fast depolarizing phase due to voltage gated sodium channels opening
3. Voltage gated sodium channels rapidly inactivate
4. The membrane depolarisation activates L type voltage gated calcium channels
5. L-type calcium channels are long lasting (L = long lasting)
6. Sustained depolarization is observed - plateau phase
7. Repolarization occurs as the L-type calcium channels close and voltage potassium channels open after a delay

Question 28

Describe the steps in the contraction of the heart

Answer 28

1. Depolarisation is transmitted to atrium
2. Contraction of atrium
3. Passes to ventriatricular node
4. There is a slight delay
5. Depolarisation spreads to bundle of hiss which separates into right and left bundles
6. Passes to purkinje fibres
7. Spreads to ventricles
8. Contraction of ventricles