For most excitable cells, the threshold stimulus is

Options

A. `+ 40` mV
B. ` - 55 ` to ` - 60` mV
C. ` + 60` mV
D. ` -70` mV

Answer

### Step-by-Step Solution:

1. **Understanding Excitable Cells**:
- Excitable cells, such as neurons, have the ability to respond to stimuli. They maintain a resting membrane potential which is crucial for their function.

2. **Resting Membrane Potential**:
- In a resting state, neurons have a membrane potential of approximately -70 millivolts (mV). This means the inside of the cell is negatively charged compared to the outside.

3. **Stimulus and Depolarization**:
- When a neuron is stimulated, sodium (Na+) channels open, allowing sodium ions to flow into the cell. This influx of positively charged sodium ions causes the inside of the cell to become less negative (depolarization).

4. **Threshold Stimulus**:
- The threshold stimulus is defined as the minimum change in membrane potential that must occur for an action potential to be generated. In this case, the influx of sodium ions changes the resting membrane potential by approximately +10 mV.

5. **Calculating the Threshold**:
- Starting from the resting membrane potential of -70 mV, if we add the change due to sodium influx (+10 mV), we get:
\[ -70 \, \text{mV} + 10 \, \text{mV} = -60 \, \text{mV} \]
- This value (-60 mV) represents the threshold potential that must be reached for the neuron to fire an action potential.

6. **Identifying the Closest Answer**:
- Given the options, we need to find the range that includes -60 mV. The closest range provided is -55 mV to -60 mV, which aligns with our calculated threshold.

### Final Answer:
The threshold stimulus for most excitable cells is approximately **-60 mV**, and the closest option is **-55 mV to -60 mV**.

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