Is a single synapse typically enough to reach threshold and trigger an action potential?

Is a single synapse typically enough to reach threshold and trigger an action potential?

• A. True
• B. Sometimes in some neurons
• C. Never
• D. False

Answer: (D)

Neurons integrate inputs from many synapses to decide whether to fire. An action potential is triggered when the depolarization at the axon hillock reaches the firing threshold, typically around a few tens of millivolts above the resting potential. A single excitatory synapse usually produces a small, graded change in voltage (an EPSP) that attenuates as it travels toward the hillock. Because of this, one EPSP on its own is rarely enough to push the membrane to threshold. To reach threshold, the neuron relies on summation: multiple EPSPs arriving close in time (temporal summation) or from multiple synapses at once (spatial summation) combine to produce a larger depolarization. Inhibitory inputs can counteract this depolarization, making it even harder for a single synapse to trigger an action potential. There are rare exceptions where a particularly large EPSP from a strong synapse or a neuron already near threshold could fire due to one input, but that’s not typical. Hence, the statement is false.

Neurons integrate inputs from many synapses to decide whether to fire. An action potential is triggered when the depolarization at the axon hillock reaches the firing threshold, typically around a few tens of millivolts above the resting potential. A single excitatory synapse usually produces a small, graded change in voltage (an EPSP) that attenuates as it travels toward the hillock. Because of this, one EPSP on its own is rarely enough to push the membrane to threshold.

To reach threshold, the neuron relies on summation: multiple EPSPs arriving close in time (temporal summation) or from multiple synapses at once (spatial summation) combine to produce a larger depolarization. Inhibitory inputs can counteract this depolarization, making it even harder for a single synapse to trigger an action potential. There are rare exceptions where a particularly large EPSP from a strong synapse or a neuron already near threshold could fire due to one input, but that’s not typical. Hence, the statement is false.