**Core Concept**
The neuronal action potential is a complex process that involves changes in membrane potential, ion channel activation, and synaptic transmission. The threshold level for neuronal action potential refers to the minimum membrane potential required to generate an action potential. This is a critical concept in understanding how neurons communicate with each other.

**Why the Correct Answer is Right**
The correct answer is -55 mv because this is the threshold potential for action potential generation in neurons. When the membrane potential reaches -55 mv, voltage-gated sodium channels open, allowing a rapid influx of sodium ions, which depolarizes the membrane and triggers an action potential. This is a result of the gradual depolarization of the membrane due to the opening of voltage-gated sodium channels, which is facilitated by the slight outward movement of potassium ions through leak channels. The threshold potential is critical because it determines when a neuron will fire an action potential in response to a stimulus.

**Why Each Wrong Option is Incorrect**
**Option A:** -70 mv is the resting membrane potential of neurons, not the threshold potential for action potential generation.
**Option B:** +70 mv is not a realistic value for neuronal membrane potential, as it is far beyond the normal range of membrane potentials.
**Option D:** +55 mv is actually the threshold potential for action potential generation in some specialized neurons, but not in the general case.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the threshold potential for action potential generation can vary depending on the type of neuron and the presence of certain ions or modulators. This is why some neurons may require a more depolarized or hyperpolarized membrane potential to generate an action potential.

✓ Correct Answer: C. -55 mv