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Answer.

The heart muscle can contract by itself, without the stimulation of a nerve. This is called myogenic muscle contraction. The region that initiates each contraction is found in the wall of the right atrium and is called the pacemaker. Every time the pacemaker sends out a signal, a heartbeat results.

The pacemaker is under the influence of nerves and adrenaline. One nerve carries messages from the medulla of the brain to the pacemaker and speeds up the beating of the heart. Another nerve carries messages from the medulla of the brain to the pacemaker and slows down the beating of the heart. Finally, adrenaline (epinephrine) is carried by the blood and once it reaches the pacemaker it signals it to increase the beating of the heart.

Summary:

(a) Heart muscle can contract by itself (myogenic muscle contraction).

(b) Pacemaker initiates contractions.

(c) One nerve carries messages from the brain to the pacemaker to speed up the beating of the heart.

(d) One nerve carries messages from the brain to the pacemaker to slow down the beating of the heart.

(e) Adrenaline signals the pacemaker to increase the beating of the heart.

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