An ion channel through a cell membrane specifically to allow the passage of sodium ions (Na⁺). Voltage-gated sodium channels are crucial in producing the rapid reversal in electrical polarity (i.e. the action potential) of excitable cells that occurs during passage of a nerve impulse. As an impulse arrives at any point, the sodium channels open in response to slight depolarization of the plasma membrane, allowing Na⁺ to flood into the cell and causing the action potential, which travels further along the neuron to transmit the nerve impulse. Within a millisecond, the sodium channels close, and potassium ion channels open to allow potassium ions (K⁺) to enter the cell, thereby restoring the negative polarity of the resting potential at that point in the cell. Ligand-gated sodium channels also occur. For example, in the postsynaptic membrane of an excitatory synapse or neuromuscular junction, such channels open in response to a neurotransmitter (e.g. acetylcholine in the peripheral nervous system) to cause depolarization. In the cone cells of the retina, ligand-gated sodium channels close in response to a chemical signal generated by light; the consequent hyperpolarization leads to release of neurotransmitter by the cone cell and generation of a nerve impulse in connecting neurons. See ligand-gated ion channel; voltage-gated ion channel. See also sodium-potassium pump.