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The control of the heart by the autonomic nervous system acts through changes in heart rate and degree of contraction.

-Activation of sympathetic nervous sytem --> tachycardia and a positive ionotropic action.

-Release of noradrenaline from postgang neurons --> binds to B1 adrenreceptors which are found in the pacemaker cells of the SA node and on the atrial and ventricular myocytes.

Activation of parasympathetic nervous system --> brachycardia, with a minor negative ionotropic effect. Release of acetycholine from postgang neurons --> binds to M2 muscarinic receptors located on the postsyn membrane of the pacemaker cells of the SA and AV nodes, and myocytes.

B1 receptors

Agonists --> noradrenaline and adrenaline

Antagonists --> propanolol

On binding of NA to the receptor, activation of the G protein a subunit leads to the activation of adenylyl cyclase, and increase in cAMP and therefore activation of protein kinase A. This leads to an increase in the rate of depolarisation (the pacemaker cells reach threshold faster and can therefore fire more frequently producing tachycardia

M2 receptors

Agonists --> acetylcholine

Antagonists --> atropine -- blocks the receptor

Neostigmine - inhibits cholinesterase - prolongs the exposure to Ach (due to reduction in breakdown of ACh).

On binding of ACh to the M2 receptor there is release of the By G protein subunits from the a subunit, shifting the diastolic potential in a more negative direction, increasing the time taken for the potential to reach threshold, increasing action potential duration and decreasing heart rate.

In the ventricular myocytes the sequence of events involves activation of cGMP, inhibition of the calcium channels and a decrease in contractability

Each type of cardiac cell has the ability to fire APs, but the shape and basis for these differ between the pacemaker cells and the conducting cells of the purkinje fibres, atrial myocytes and ventricular myocytes. The major difference is the presence of absense of a plateau phase and representative traces for each type of AP.

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