A postsynaptic neuron has three presynaptic inputs - from neurons X, Y, and Z. Stimulation of neuron X causes the postsynaptic neuron to depolarize by 0.5 mV. When X and Y are stimulated simultaneously, the postsynaptic neuron depolarizes by 1 mV. When X and Z are stimulated simultaneously, however, there is no change in the membrane potential of the postsynaptic neuron. What is most likely true about presynaptic neurons Y and Z?

Y is probably excitatory and Z is probably inhibitory

Explanation:

Two neurons communicate with each other generally by releasing neurotransmitters. The neuron which releases the neurotransmitter is termed as presynaptic neuron and the neuron to which the neurotransmitter binds to exert its effects is called as postsynaptic neuron. Based on the nature of the neurotransmitter released, the presynaptic neurons can either be inhibitory or excitatory in nature. The change in the membrane potential of the postsynaptic neurons depends on the total sum of inputs coming from different pre synaptic neurons at a given time. From the given situation, the stimulation by X alone is causing deploarization of 0.5 mV which shows that it is causing excitation, and since depolarization is increasing when X and Y are stimulaneously stimulated Y is also excitatory in nature. However the depolarisation is not occurring when X and Z are stimualted together and hence, Z is probably inhibitory in nature.