Summary

The heart’s contraction is controlled by pacemaker cells. In a normal heart, there will be cells in the sinus node (right atrium) that control the contraction frequencies. Pacemaker cells here, as elsewhere in the heart, have the property of depolarising spontaneously. Depolarisation occurs because the cell membrane of pacemaker cells contain leakage ion channels for sodium. Sodium enters the cell and the membrane potential slowly approaches the threshold value triggering the opening of calcium channels and depolarisation. An electrical impulse is generated that spreads through the heart’s electrical conduction system, and into the heart muscle. Depolarisation of muscle cells causes not only sodium channels to open in the cell membrane, but also slow calcium channels. The consequence of this is persistent diffusion of calcium ions (Ca²⁺) into the muscle cells. Here the calcium ions trigger a release of more calcium from the sarcoplasmic reticulum. The slow calcium transport into the muscle cells and the additional activation by released calcium from internal storage causes depolarisation of muscle cells. The large amounts of calcium accumulates within the cells that provides increased muscle strength. The increased strength is due to the function of calcium allowing bonding between myosin and actin filaments. The more bonding between myosin and actin, the more forceful the muscle cell contraction.

The autonomic nervous system regulates the heart’s contraction. Sympathetic nerves use noradrenaline as a neurotransmitter to stimulate the sinus node, as well as the heart muscle as a whole. Sympathetic stimulation results in increased contraction and increased mobilisation of calcium providing the myocardium with a higher force. The heart is emptied of more blood compared with an unstimulated heart. Parasympathetic nerve cells use acetylcholine as a neurotransmitter and stimulates the sinus node and AV-node. Acetylcholine inhibits spontaneous cardiac depolarisation, slowing down the heart. The spontaneous depolarisation frequency of the sinus node is about 75 to100 per minute. At rest your heart is being slowed by parasympathetic nerves leading to a resting heart rate of around 60 bpm.

The heart’s main function is to pump blood around to all tissues in the body. In order to do this the heart must create a pressure that pushes blood into the arteries with enough impact so that the transport of blood is appropriate to maintain homeostasis. The heart’s contraction creates this pressure. Each time the heart contracts it pushes around 70 ml of blood into the aorta, equating to approximately 5 to6 litres of blood per minute. This is a volume equal to the total blood volume of your body. Cardiac output can be regulated by the autonomic nervous system. Sympathetic stimulation. ESV will decrease and EDV increase, the latter due to increased venous return. Together this means that the heart’s stroke volume, SV, increases. In addition, sympathetic stimulation of the heart increases the heart rate, HR. When both SV and HR increases, cardiac output (CO) increases because CO = HR x SV.

If sympathetic stimulation of the heart decreases and / or parasympathetic stimulation increases, HR decreases. SV will fall due to a lower EDV and higher ESV. Leading to a lower cardiac output.