Cardiac myocytes Flashcards
What does the cardiac myocyte do? (2)
The interactions transform the CHEMICAL ENERGY derived from ATP into the MECHANICAL WORK that moves blood under pressure from the great veins in the pulmonary artery and from the pulmonary veins into the aorta.
What does the cardiac myocyte do? (4)
- Excitation-contraction coupling begins when the action potential depolarises the cell and ends when ionised calcium (in the cytosol) binds to the Ca receptor of the contractile apparatus.
- The contractile proteins are activated by a signalling process called excitation-contraction coupling.
What does the cardiac myocyte do? (6)
- The heart relaxes when ion exchanges and pumps transported ca2+ uphill out of the cytosol
- Movement of Ca2+ into the cytosol is a passive downhill process mediated by ca2+ channels.
What is cardiac muscle?
Cardiac muscle (heart muscle) is involuntary striated muscle
Describe the structure of myoctes
- regular array of thick and thin filaments or myofibrils
- A-band, thick filaments
- I-band thin filaments (contains tropomyosin and troponins)
- Z lines bisect each I-band
Describe the contractile proteins of the heart
- Myosin
- Actin
- Tropomysoin
- Troponin C
- Troponin I
- Troponin T
- Myosin - thick, hydrolyses ATP interacts with actin
- Actin - thin, activates myosin ATP interacts with myosin
- Tropomysoin - thin, modulates actin-myosin interaction
- Troponin C - thin, binds Ca2+
- Troponin I - thin, inhibits actin-myosin interation
- Troponin T - thin, binds troponin complex to thin filament
Explain the cardiac cycle in terms of excitation beginging in the SAN node
1) SA node depolarises generating an action potential
2) This spreads through the myocardium passing along gap junctions
3) Passes to AV node - brief delay (PRM delay)
4) This allows the contraction on the atria, blood passes to ventricles and valves snap shut
5) Action potential passes to the bundle of his from the AV node and onto the purkjinke fibres
6) This means that the ventricles contract from the apex up forcing blood into the aorta and pulmonary artery
What determines heart beat?
Sa node discharge rate (pacemaker cells)
What is the velocity of conduction in atrial and ventricular muscle fibres and in purkinje fibres
Atrial and ventricular = 0.3-0.5 m/s
Purkinje = 4 m/s
What is the structure of myosin?
Myosin
2 heavy chains, also responsible for the dual heads
4 light chains
The heads are perpendicular on the thick filament at rest, and bent towards the centre of the sarcomere during contraction
Alpha and beta myosin
What is the structure of Actin?
Actin
Globular protein
Double-stranded macromolecular helix
Both form the F actin
What is the structure of Tropomyosin ?
Tropomyosin
Elongated molecule, made of two helical peptide chains
It occupies each of the longitudinal grooves between the two actin strands
Regulates the interaction between other components
What is Cushing’s phenomenon?
-The Cushing reflex consists of an increase in sympathetic outflow to the heart as an attempt to increase arterial blood pressure and total peripheral resistance, accompanied by bradycardia.
- The ischemia activates the sympathetic nervous system
- This causes an increase in the heart’s output by increasing heart rate and contractility along with peripheral constriction of the blood vessels.
- This accounts for the rise in blood pressure, ensuring blood delivery to the brain.
- The increased blood pressure also stimulates the baroreceptors (pressure sensitive receptors) in the carotids, leading to an activation of the parasympathetic nervous system, which slows down the heart rate, causing the bradycardia.
The Cushing reflex is usually seen in the terminal stages of acute head injury.
What is the absolute refractory period?
The period during and after an action potential when an excitable membrane cannot be re-excited . This is due to the inactivation of the Na+ channels.
Contraction 20-100msecs
In skeletal muscle - absolute refraction time 1-2msec
therefore a second action potential can occur while the first action is still occurring.
In cardiac muscle - the absolute refractory period of cardiac muscle is almost as long as the contraction, and so the muscle cannot be re-excited multiple times during an ongoing contraction (this allows the ventricles to fill while they are relaxed)
What is the relative refractory period in the heart?
-Relative refractory period : is a period that follows the absolute period , and the excitability begins to recover gradually until it reaches its normal value. Relative refractory period coincides with the period of rapid polarization following the plateau.
