Answer
Internal: Nodal tissue is a unique kind of muscle that allows for the contraction of the atria and ventricles. There are two nodes that internally control the heart; the sinoatrial node (SA node) and the atrioventricular node (AT node). The SA node starts the heartbeat by automatically sending an excitation signal every 0.85 seconds. The excitation signal causes the atria to contract. After the signal reaches the AV node there is a delay, at which time the atria finish their contraction before the ventricles begin to contract.
External: There is a cardiac control center in the medulla oblongata, which is located in the brain. Parasympathetic and sympathetic portions of the nervous system send signals to the cardiac control center, which has the ability to control heart rate.
Work Step by Step
Internal: Nodal tissue is a unique kind of muscle that allows for the contraction of the atria and ventricles. There are two nodes that internally control the heart; the sinoatrial node (SA node) and the atrioventricular node (AT node). The SA node starts the heartbeat by automatically sending an excitation signal every 0.85 seconds. The excitation signal causes the atria to contract. After the signal reaches the AV node there is a delay, at which time the atria finish their contraction before the ventricles begin to contract.
External: There is a cardiac control center in the medulla oblongata, which is located in the brain. Parasympathetic and sympathetic portions of the nervous system send signals to the cardiac control center, which has the ability to control heart rate.
