Problem 1: Analysing a Nerve Impulse Experiment
Scenario: A researcher records from a neuron. At rest, the membrane potential is -70 mV. She applies stimulus A (subthreshold) and stimulus B (suprathreshold). For stimulus C, she applies stimulus B five times per second.
Q1: What would she record for stimulus A? Step-by-step solution:
- Subthreshold stimulus = below threshold (~-55 mV)
- Voltage-gated Na+ channels do NOT open (insufficient depolarization for positive feedback)
- Result: Small graded potential (EPSP) that decays without producing an AP
- Recorded: No action potential; resting potential of -70 mV maintained
Q2: What for stimulus B?
- Stimulus B exceeds threshold (~-55 mV)
- Voltage-gated Na+ channels open → Na+ rushes in → regenerative depolarization → AP fires
- Peak: +30 mV → then K+ channels open → K+ out → repolarization → brief hyperpolarization → return to -70 mV
- Recorded: Full action potential with all-or-none characteristics
Q3: If stimulus B were 10x stronger, how would the AP differ?
- Same amplitude — the all-or-none principle means the AP always reaches the same peak (+30 mV) regardless of stimulus strength above threshold
- The only difference: higher frequency of APs (more per second) would encode stronger stimuli
Q4: For stimulus C , what determines the maximum achievable firing rate?
- Maximum firing rate = 1 ÷ (absolute refractory period duration)
- Absolute refractory period ≈ 1-2 ms → maximum rate ≈ 500-1000 Hz
- At 5/second, the neuron is far below maximum — easily achievable