efferent autonomic regulation of the heart Flashcards
what is chronotropy?
heart rate
what is inotropy?
contractile force of heart
what is lusitropy?
rate of relaxation of the heart
what is dromotropy?
AV node sonduction velocity
where do sympathetic nerves innervate the heart?
- they richly innervate the ventricles and the atria and pacemaker
where do the parasympathetic post ganglionic fibres innervate?
- just the pacemaker cells - SA & AV node
where do the preganglionic sympathetic fibres originate from and compare the length of the pre and post ganglionic fibres..
- preganglionic sympathetic fibres originate from the T1-T6 segments of the spinal chord
- the pre ganglionic fibres are short
- the post ganglionic fibres are long
where do the preganglionic **parasympathetic **fibres originate from and compare the length of the pre and post ganglionic fibres..
- they originate from the vagal motor nuclei in the brain stem and the preganglionic fibres travel in the vagus nerve
- the preganglionic fibres are long
- the post ganglionic fibres are short
what is the resting heart rate dominated by?
- it is dominated by the parasympathetic system
- the vagus nerve exerts a tonic inhibitory action on the SA node to slow the intrinsic heart rate
what NT is involved with the sympathetic system and what effects does it have?
- noradrenaline is released by the post-ganglionic fibres and it binds onto the B1 receptors in the heart
- activation of the B1 receptors causes an increase in heart rate (chronotropic effect),
Describe the cellular mechanism for the activation of the B1 receptor with NA or A
- the B1 receptor is attached to a G stimultaory protein
- when NA or A binds to the receptor.. it causes the G protein to activate adenylate cyclase which catalyses the conversion of ATP to cAMP
- cAMP then has 4 main effects
what are the main effects of cAMP** when NA or A binds to the B1 receptor?
- cAMP interacts directly with the pacemaker channels to increase their open state probability - they increase the rate of delay of the pacemaker potential which causes the SA node to reach threshold more quickly
- cAMP activates the enzyme protein kinase A which has 3 main effects
after cAMP activates protein kinase A, what effects does this enzyme have?
- PKA phosphorlyates L type CA2+ channels which increases the contractile force (inotropic effect)
- PKA phosphorlyates the delayed rectifier K+ channels which increases the outward current of K+ (repolarisation) which shortens the ventricular AP and allows more excitations per minute
- PKA also phosphorlyates phospholamban which reduces its inhibitory effect on the Ca2+ ATPase pump
what are the **chronotropic and dromotropic effects **of sympathetic stimulation of the heart?
- the** rate of decay of the pacemaker potential is increased** which allows the threshold to be reached sooner and the heart rate increases (caused by increase in depolarising current LCA2+ and deactivation of rectifer K+)
* conduction through the AV node is increased by activated B1 receptors in the AV node - action potential of atrial and ventricular myocytes is shortened
what NT does the parasympathetic system release and what effects does it have?
- Ach is released by the post-ganglionic neurons
- Ach slows the rate of decay of the pacemaker potential and therefore decreases heart rate
- it also increases the membrane permeability to K+ through the KACH channels
Describe the cellular mechanism of acetylcholine with the M2 receptors
- The M2 receptor is linked to an **inhibitory G protein **
- when Ach binds to the M2 receptor, the** G protein reduces activity of adeylate cyclase** and reduces cAMP and protein kinase A which reduces the activation of L type Ca2+ channels etc
- The M2 receptor is also linked to a class of K+ channel called Kach
- the activation of the Kach channel increases the outward K+ current and hyperpolarises the membrane
