Final answer:
When ligand-gated sodium ion channels open in response to a stimulus, the neuron depolarizes as positive sodium ions enter the cell, making the inside less negative and initiating an action potential.
Step-by-step explanation:
If a stimulus opens ligand-gated sodium ion channels, the neuron will depolarize. This occurs because when the sodium channels open, positive sodium ions (Na+) enter the neuron, which reduces the difference in voltage between the inside and outside of the cell. This entry of Na+ causes the inside of the neuron to become less negative, moving the membrane potential toward a more positive value. This shift from the resting membrane potential toward a positive charge is referred to as depolarization, which is a critical step in the generation of an action potential. Depolarization makes the interior of the neuron less negative compared to the exterior, which is necessary for the propagation of an action potential along the neuron.
Once the neuron's membrane potential reaches around +40 mV, the Na+ channels close, and the neuron enters a refractory period. Following this, to return to the resting state, repolarization occurs as potassium (K+) channels open and K+ leaves the cell, restoring negative charge inside the neuron. Initially, the neuron may become even more negative than its resting potential, a process known as hyperpolarization, before returning to its normal resting membrane potential. However, the initial response to the opening of ligand-gated Na+ channels is the depolarization of the neuron.