Question

what opens first in response to a threshold stimulus?
voltage - gated na+ channels
ligand - gated cl- channels
ligand - gated cation channels
voltage - gated k+ channels

Explanation:

When a threshold stimulus is reached in a neuron, voltage - gated ion channels play a crucial role. Voltage - gated Na⁺ channels open first. This allows Na⁺ ions to rush into the cell, depolarizing the membrane. Ligand - gated channels (for Cl⁻ or cations) open in response to a ligand (a chemical signal) binding, not directly to a threshold electrical stimulus. Voltage - gated K⁺ channels open after voltage - gated Na⁺ channels, during the repolarization phase.

Answer:

Voltage - gated Na⁺ channels