Nervous control of the heart

We have seen that the heart beat originates as repetitive activity in the cells of the pacemaker regions of the heart. This activity can be modified by the action of nerve fibres innervating the heart via the autonomic nervous system. The neurotransmitters, acetylcholine or noradrenaline, activate cellular cascades that have complex effects on cell function (Fig. 8.9).

Activity in sympathetic accelerator nerve fibres increases the force and rate of the contraction. Noradrenaline is the neurotransmitter, acting on ^-adrenergic receptors. When the noradrenaline binds to its receptor, this activates a G protein (Gs) so that the Ga subunit binds GTP and is released from the receptor and the ffy subunit. The Ga subunit then activates the enzyme adenylyl cyclase, so producing cyclic AMP. The cyclic AMP has two effects. Firstly, it combines with open If channels to keep them open: this increases the pacemaker current and so increases the heart rate. Secondly, it activates protein kinase A, which then phosphorylates L-type voltage-gated calcium channels in the heart muscle cell membrane. Phosphorylation increases the open probability of the calcium channels, so more calcium ions enter the cell when it is next depolarized, and so the contraction force is increased.

Activity in parasympathetic inhibitor nerve fibres, on the other hand, slows the heart rate and decreases the force of the contraction. Here acetylcholine is the neurotransmitter, acting on muscarinic receptors. When the acetylcholine binds to its receptor, this activates a G protein (Gi2) so the Ga subunit binds GTP and splits off from the receptor and the fly subunit. The Gfy subunit binds to a particular type of potassium channel (called GIRK1) and opens it, so the membrane potential is held near to EK. There is also some inhibition of cyclic AMP production in the pacemaker cells (DiFrancesco, 1993).

Both these systems show considerable amplification. Activation of one ff-adrenergic receptor activates many G proteins, each of these will activate an enzyme molecule which will produce many cyclic AMP molecules, and each activated protein kinase A molecule will phosphorylate several calcium channels. Activation of one muscarinic receptor produces many Gfy subunits and so opens many GIRK1 channels.

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