LECTURE 9 Flashcards
structure of cardiac muscle
Mononucleated cells. 100µm in length
Striated: crossbands of the muscle contraction apparatus are visible (z-bands)
Intercalated discs between: made up of gap junctions, which allow the transmission of ions and the action potential
This arrangement means that the muscle works as a functional syncytium: it works as one unit.
3 specialist features for conduction in the cardiac muscle
desmosomes, intercalated discs and gap junction
specialist features for conduction in the cardiac muscle: desmosomes
hold the muscle cells together tightly
specialist features for conduction in the cardiac muscle: intercalated discs
link muscle cells together and contain desmosomes and gap junctions
specialist features for conduction in the cardiac muscle: gap junctions
Allow passage of action potentials from one cell to the next, very quickly – allows the cardiac muscle to function together as a syncytium
Difference in action potential in cardiac vs skeletal muscle
- Cardiac resting potential : -90mV (not -70mV)
- action potentials are slower
- extended refractory, depolarisation and repolarisation
What is hypoxia
- resting potential is higher
- affects Na and K ion channels
- can lead to arrythmias
Why do cardiac action potentials have a long refractory period
The long refractory period is essential to allow relaxation (and filling) of ventricles between contractions – if further action potentials were triggered too quickly, the heart would not function effectively as a pump.
What external influences can interrupt rhythm
- caffeine
- nicotine
What regulatory system co-ordinates heart contraction
autonomic nervous system
Neural control of Heart Rate
Sensory information from sensors (eg chemo and baro etc) is processed in the medulla oblongata
Triggers an ANS response
- Sympathetic NS (cardiac accelerator nerves) increase HR and contractility. Physical and emotional stress will increase HR
- Parasympathetic NS (Vagus nerves) decrease HR
factors that increase SA node firing
Muscarinic receptor antagonist
β Adrenergic receptor agonists
Circulating catecholamines
Hypokalemia
Hyperthyroidism
Hyperthermia
factors that decrease SA node firing
Muscarinic receptor agonist
β blockers
Ischaemia/hypoxia
Hyperkalemia
Sodium and calcium channel blockers
Hypothermia
which sex has a higher heart rate
females
what is tachycardia
increased heart rate (>100bpm) through stress, drugs, heart disease
If persistent, leads to death
what is fibrillation
rapid, regular and unco-ordinated contraction
what is brachycardia
Brachycardia- <60bpm, low temperature, drugs, endurance training. If not an athlete, leads to poor circulation. Indicative of head trauma
what is an ecg
ECG is an electrical trace of the action potentials in all the heart muscle fibres
