cardiac muscle Flashcards
(28 cards)
1
Q
properties of cardiac muscle
A
- Small cells (same as smooth)
- Striated (same as skeletal)
- Branched cells connected by intercalated discs
- Involuntary muscle
2
Q
does cardiac muscle have troponin
A
- yes
- Troponin is the calcium sensing protein
- In cardiac muscle, there are different isoforms of troponin (can distinguish between troponin in skeletal and cardiac muscle)
3
Q
does cardiac muscle have sarcomeres
A
yes
4
Q
are cardiac cells electrically coupled
A
yes
5
Q
3 components of intercalated discs
A
- interdigtating folds- in folds at the end of cells that increase the surface area of cell-cell connection
- mechanical junction-2 types: fascia adherens and desmosomes
- electrical junctions- electrically coupled by gap junctions. Enables each cell to stimulate the next
6
Q
gap junctions
A
- A hexamer composed of connexins provided by each cell
- Connexin = protein subunit that forms gap junctions
- Gap junctions hexamers (6 subunits) provided by adjacent cells form a gap junction with 12 subunits which connects the cells
- Present in intercalated discs
- low-resistance pathway between cells
- cell-cell conductance by electrical coupling
- Gap junction plaques - 5 to 500 gap junctions which offer low resistance pathways and cell to cell conduction
7
Q
description of blood supply in cardiac muscle
A
- Requires continuous supply of oxygen for mitochondria to be able to rapidly oxidise substrates and generate ATP
- Rich capillary supply - approx 1 capillary per fibre
- Short diffusion distances for oxygen and waste products and waste
8
Q
2 types of electrical activity
A
- pace maker potential
2. prolonged cardiac action potential
9
Q
3 ion conducting mechanisms
A
- Funny channel
- T-type calcium channels
- L-type calcium channels
10
Q
what is contraction in cardiac muscle cells in response to
A
- membrane action potential that propagates through the T-tubules
11
Q
L type Ca 2+ channels
A
- (long lasting current)
- Modified DHP receptors that act as the voltage sensor in skeletal muscle excitation-contraction coupling
- Enable depolarization of the cell due to influx of extracellular Ca2+
- VGCs
12
Q
Where does Ca2+ for contraction come from
A
- When L-type channels are open, there is an influx of calcium
- Calcium levels in cytoplasm increase
- Calcium regulates the activity of the ryanodine receptor
- 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
- Causes ryanodine receptor to open and release more calcium into the cytoplasm from the sarcoplasmic reticulum
- This binds to troponin
- Allows cross bridge cycling to occur
- Calcium is pumped back into the sarcoplasmic reticulum via a protein called serca
13
Q
Any differences between skeletal muscle filament activation, cross bridge cycling etc and in cardiac muscle
A
No
14
Q
when does contraction end
A
- when cytosolic Ca2+ concentration restores to its low value by Ca2+ ATPase pumps in the sarcoplasmic reticulum and sarcolemma (cell membrane of muscle cell)
15
Q
Sequence of excitation-contraction coupling
A
- Membrane depolarised by Na+ as an AP begins
- Depolarisation opens L-type channels in T tubules
- Trigger Ca2+ enters cytosol, contributing to cell depolarisation. Binds to and opens ryanodine receptor
- Ca2+ flows into cytosol, cross bridge cycling occurs
- Once Ca2+ has been restored to same level, membrane is repolarized when K+ exits the cell to end the action potential
16
Q
uses of cardiac troponins
A
- Cardiac troponins are released into the blood when heart muscle is damaged
- This is used to detect myocardial infarction
17
Q
Troponin I
A
inhibitory
18
Q
Troponin c
A
calcium binding
19
Q
Troponin T
A
tropomyosin binding
20
Q
Describe adhering junctions
A
- Couple the membrane to the contractile apparatus (actin cytoskeleton) of the cardiomyocytes
- Attached to the thin filaments
- Aid the transmission of contractile force from one myocyte to the next
21
Q
Describe adhering junctions
A
- Couple the membrane to the contractile apparatus (actin cytoskeleton) of the cardiomyocytes
- Attached to the thin filaments
- Aid the transmission of contractile force from one myocyte to the next
22
Q
What is pacemaker potential
A
- Spontaneous gradual depolarization
23
Q
Describe the sinoatrial node
A
- Has rhythmical pacemaker activity
- It does not have a steady resting potential but it slowly depolarizes
- Self generating depolarisation
24
Q
Describe the role of T-type calcium channels
A
- Voltage gated calcium channels
- Contribute to the final depolarizing boost of the pacemaker potential
- Opened when threshold for opening calcium channels is reached by the drifting baseline potential
- This allows the threshold for an action potential to be reached
25
Describe the role of L-type calcium channels
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
26
What is cardiac muscle resting potential
Resting potential is close to -90mV
27
Describe how an action potential is generated
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
28
Describe the steps in the contraction of the heart
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
