Biology Nervous Coordination ETEA 2023 Retake
PMDC Verified Question 47 of 86
What brings membrane potential to threshold?
A
Outflow of odium ions across the post synaptic membrane
B
Influx of sodium ions across the post synaptic membrane
C
Outflow of calcium ions across the post synaptic membrane
D
Influx of potassium ions the post synaptic membrane
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option B: Influx of sodium ions across the post synaptic membrane
Concept:

Excitation of a neuron requires shifting its resting negative voltage towards a more positive critical threshold (-55 mV) to fire an action potential.

Solution:

  • When excitatory neurotransmitters bind to receptors on the post-synaptic membrane, they trigger ligand-gated sodium channels to open. The subsequent influx of positively charged sodium (\(\text{Na}^+\)) ions reduces the negative charge inside the cell, driving the membrane potential up to the threshold.


Why other options are incorrect:

  • Outflow of sodium / Influx of potassium: These ion movements would generally hyperpolarize the cell (make it more negative), moving it away from the threshold.


  • Calcium: Primarily involved in neurotransmitter exocytosis at the terminal, not the initial postsynaptic depolarization in typical neurons.

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