Signals from the medulla to the heart via the vagus nerve primarily result in what effect?

The effect of signals from the medulla to the heart via the vagus nerve is primarily a decrease in heart rate. This is due to the role of the vagus nerve, which is part of the parasympathetic nervous system. When the brainstem's medulla oblongata sends signals through the vagus nerve, it releases neurotransmitters such as acetylcholine. This neurotransmitter acts on the sinoatrial (SA) node and the atrioventricular (AV) node of the heart, leading to a reduction in heart rate. The parasympathetic nervous system, therefore, counteracts the sympathetic nervous system's actions, which typically increase heart rate.

In this context, the other options do not align with the primary function of the vagus nerve signals. Increased heart rate is associated with sympathetic stimulation. Increased blood pressure typically occurs through mechanisms that involve vasoconstriction and increased heart contractility driven by sympathetic input. Decreased blood volume is related to fluid regulation by the kidneys and does not directly correlate with the vagus nerve's influence on heart rate.