Final answer:
Sympathetic stimulation of the heart increases the release of the neurotransmitter norepinephrine (NE), which raises heart rate and force of contraction.
Step-by-step explanation:
The student asked which neurotransmitter is released during sympathetic stimulation of the heart. During sympathetic stimulation, the neurotransmitter norepinephrine (NE) is released at the neuromuscular junction of the cardiac nerves. NE is a catecholamine like epinephrine, and they both play a role in the body's fight-or-flight response. This response includes increasing the heart rate and the force of contraction (positive inotropy), which is achieved by opening chemical- or ligand-gated sodium and calcium ion channels, thus speeding the rate of depolarization and contraction. In contrast, parasympathetic stimulation releases acetylcholine (ACh) and has the opposite effect on the heart.