Final answer:
Norepinephrine is released onto the sino-atrial node by the cardiac accelerator nerve, resulting in an increased heart rate by shortening the repolarization period and speeding up depolarization and contraction.
Step-by-step explanation:
The neurotransmitter that is released onto the sino-atrial (SA) node by the cardiac accelerator nerve is norepinephrine (NE). Sympathetic stimulation through the cardiac accelerator nerves causes the release of norepinephrine at the neuromuscular junction of the cardiac nerves. The effect of norepinephrine on the SA node is an increase in heart rate (HR). Norepinephrine achieves this by shortening the repolarization period, speeding the rate of depolarization, and stimulating contraction. This is facilitated by its action of opening chemical- or ligand-gated sodium and calcium ion channels, which allows an influx of positively charged ions, thus increasing the heart's pace.