What is Tachycardia and Bradycardia?
Tachycardia = sustained increase in heart rate above normal Bradycardia = the opposite
What effect do the parasympathetic and sympathetic nervous systems have on heart rate?
Parasympathetic keeps heart rate at a normal level via vagal tone , sympathetic increases heart rate
How does an action potential occur?
- Stimulus to innervate a nerve impulse received by neuron 2. Na+ gated channels open and Na+ = enters the cells (depolarisation) 3. Action potential reaches threshold value and propagates 4. Na+ channels close K+ channels open 5. K= enters (repolarisation)
What ion is responsible for contractility?
Ca2+
What is the sodium-calcium exchanger?
After hyperpolarisation and K+ decreased permeability Na+ and Ca2+ enter via leak channels until the threshold value is met
What is the normal Order of Conduction within the heart?
- SA Node via sympathetic innervation 2. Atria 3. AV node 4. Purkinje system 6. Ventricular muscle
What is the role of the vagal nerve in the heart?
Innervates both the SA and AV nodes as well as a small amount of atria for controlling heart rate
What is the role of the sympathetic nerves in the heart?
Innervate SA and AV plus atria and ventricles, stimulates release of noradrenaline for increased heart rate during exercise
In terms of Vagal and Sympatehtic tone how do we increase heart rate?
Decrease Vagal Tone, Increase Sympatehtic Tone
What is a hormonal way of increasing heart rate?
Increased adrenaline/epinephrine
What is the pathway of noradrenaline?
- Noradrenaline binds to Beta 1 adreonoreceptor (G couple receptor) 2. Activates adenyl cyclase (conversion of ATP into cAMP) 3. Activation of PKA which phosphorylates Fast Na+ channels, Slow Ca2+ channels (leak channels), Ryanodine receptor (Ca2+ Sarcoplasmic reticulum release channel) and SER Ca2+ alpha pump.
Releases intracellular calcium from sarcoplasmic reticulum causing contraction
Also causes faster repolarisation via K+ channel activation
What does more [Ca2+] result in?
Stronger more forceful contraction
What is the role of acetylcholine from vagal tone in the heart?
Acts on muscarinic receptors in AV and SA nodes.
Muscarinic receptor is a G-Coupled protein receptor which is linked to K+ channels (negative chronotrope)
Also linked via an inhibitory G protein to adenylate cyclase to inhibit formation of cAMP (weak negative inotrope)
Presynaptic muscarinic receptors can inhibit norepinpherin release from sympathetic nerve terminals
Define Arrhythmia
Abnormalities of cardiac rhythm affect filling of the heart as well as heart rate therefore compromising cardiac output
In terms of cardiac output what effect does a low or very high heart rate cause?
Low heart rate = Decreased cardiac output
Very high heart rate = inadequate filling of the heart chambers = decreased stroke volume = decreased cardiac output
