Consider a system that contains two neurons and one cardiac SA node cell in a culture dish bathed in normal physiological saline. All three cells are healthy. Neuron A synapses onto Neuron B. Neuron B synapses onto the SA node cell. Neuron A has glycine in its synaptic vesicles.

Neuron B has acetylcholine (ACh) in its synaptic vesicles. The only ligand-gated channels in the plasma membrane of Neuron A are AMPA receptors. The only ligand-gated channels in the plasma membrane of Neuron B are glycine receptors. Both neurons have no metabotropic receptors in their plasma membranes.

Neuron A, Neuron B, and SA node cell each have a chloride equilibrium potential of -20 millivolts, a potassium equilibrium potential of -86 millivolts, and a sodium equilibrium potential of +60 millivolts. The SA node cell has a calcium equilibrium potential of +70 millivolts. The threshold for an action potential in all 3 cells is -55 millivolts. The SA node cell has its usual set of molecules. At 1:55AM, glutamate is added to the physiological saline.

At 2:00 AM, Neuron A’s action potential firing rate is 100 Hz, Neuron B’s action potential firing rate is 100 Hz, and the SA node cell’s action potential firing rate is 1.00 Hz. Which of the following will lead to a decrease in the SA node cell’s action potential firing rate?