Question

Which of the following factors do not increase the impulses generated by SAN?
A High thyroxine level in blood
B An increase in body temperature by 1°C
C Discharge from vagus nerve
D Increase in nicotine level due to smoking

Answer 1

Hint The sinoatrial node (SA node) is a specialised cardiac structure situated in the atrial wall at the intersection of the superior caval vein and the right atrium that initiates electrical impulses to induce contraction.

Complete step by step answer:
When the SA node sends an electrical impulse, the following sequence of events occurs: The electrical signal flows from your SA node to your right and left atria muscle cells. The signal activates the muscle cells in your atria, causing them to contract. Blood is pumped into your left and right ventricles as the atria contract.
In most Aplysia neurons specialised for this type of activity, temperature increases cause a regular and reproducible increase in the frequency of generation of pacemaker potentials, which can only be explained as a direct stimulating effect of temperature on the ionic mechanisms responsible for pacemaker potentials.
The SA node is mostly innervated by efferent branches of the right vagus nerves, while some left vagus innervation is also seen. If the resting heart rate is less than 100 beats/min, experimental denervation of the right vagus to the heart causes a sudden increase in SA nodal firing rate.
Nicotine damages blood vessel walls and lowers the quantity of oxygen delivered to the heart, causing the heart to beat quicker and the damaged blood vessels to work harder. This little amount of time allows your body to begin the process of self-repair.

Only increases in thyroxine does not affect the SA node, so option A is the correct answer.

Note: The rate at which the sinoatrial node produces action potentials determines heart rate. Heart rate ranges between 60 and 100 beats per minute when at rest. This is due to the activity of two sets of neurons, one of which acts to slow down the creation of action potentials.