Final answer:
The polarization and depolarization of nervous tissue are due to the influx and outflow of ions, primarily Na+ for depolarization and K+ for repolarization, across the neuron membrane.
Step-by-step explanation:
Nervous tissue polarizes and depolarizes primarily through the action of ion movement across the neuron's membrane. During depolarization, a nerve impulse opens the Na+ channels, allowing sodium ions (Na+) to enter the cell due to a concentration gradient. This influx of Na+ changes the electric charge inside the neuron, making it less negative relative to the outside and thus initiating the depolarization process.
Upon reaching a certain threshold, more Na+ channels open, further depolarizing the cell. Subsequent to this, repolarization occurs as K+ channels open, allowing potassium ions (K+) to move out of the neuron, which restores the negative charge inside the neuron relative to the outside. After repolarization, the Na+/K+ transporter helps to re-establish the original concentration gradients of these ions, preparing the neuron for the next action potential.