<Tr> <Td> Sodium </Td> <Td> Decreased Na </Td> </Tr> <Tr> <Td> Body temperature </Td> <Td> Increased body temperature </Td> </Tr> <Tr> <Td> Nicotine and caffeine </Td> <Td> Stimulants, increasing heart rate </Td> </Tr> <Tr> <Td> <Table> Table 2: Factors decreasing heart rate and force of contraction <Tr> <Th> Factor </Th> <Th> Effect </Th> </Tr> <Tr> <Td> Cardioinhibitor nerves (vagus) </Td> <Td> Release of acetylcholine </Td> </Tr> <Tr> <Td> Proprioreceptors </Td> <Td> Decreased rates of firing following exercise </Td> </Tr> <Tr> <Td> Chemoreceptors </Td> <Td> Increased levels of O; decreased levels of H and CO </Td> </Tr> <Tr> <Td> Baroreceptors </Td> <Td> Increased rates of firing, indicating higher blood volume / pressure </Td> </Tr> <Tr> <Td> Limbic system </Td> <Td> Anticipation of relaxation </Td> </Tr> <Tr> <Td> Catecholamines </Td> <Td> Decreased epinephrine and norepinephrine </Td> </Tr> <Tr> <Td> Thyroid hormones </Td> <Td> Decreased T3 and T4 </Td> </Tr> <Tr> <Td> Calcium </Td> <Td> Decreased Ca </Td> </Tr> <Tr> <Td> Potassium </Td> <Td> Increased K </Td> </Tr> <Tr> <Td> Sodium </Td> <Td> Decreased Na </Td> </Tr> <Tr> <Td> Body temperature </Td> <Td> Decrease in body temperature </Td> </Tr> </Table> </Td> </Tr>

What cellular changes occur in heart muscle when there is increased work of the heart