Biology · Neural Control and Coordination · NEET
Polarised means the two sides of the membrane carry opposite charges. In a resting neuron the outer surface is positive and the inner surface is negative. This charge difference across the membrane is called the resting potential. The membrane is not damaged or active in this state, it is simply electrically ready to fire an impulse.
Three things make the inside negative: (1) the membrane keeps more Na+ ions outside, (2) it keeps more K+ ions inside, and (3) large negatively charged proteins are trapped inside the cell and cannot leave. Even though K+ is high inside, the trapped negative proteins plus the outward leak of K+ leave the inner side net negative.
Ion channels are protein gates in the membrane that let specific ions pass. At rest the membrane is much more permeable (leaky) to K+ than to Na+ because more K+ channels are open. So K+ can move but Na+ mostly cannot. This unequal permeability is a key reason the resting membrane stays polarised.
The sodium-potassium pump (Na-K pump) actively pushes 3 Na+ out and pulls 2 K+ in, using ATP energy. Because it moves 3 positive ions out for every 2 brought in, it also helps make the outside more positive. The pump plus selective channels together set up and maintain the concentration gradients.
Yes, for NEET they are linked. The polarised membrane is the physical state (charges separated), and the resting potential is the measured voltage of that state, about -70 mV. When you read 'membrane is polarised at rest', it means the neuron is sitting at its resting potential, ready but not conducting an impulse.
A stimulus opens many Na+ channels, so Na+ rushes in and the inside briefly becomes positive. This loss of the resting charge difference is called depolarisation. So 'polarised' is the resting condition and 'depolarised' is the active condition during an action potential.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. Even at rest the sodium-potassium pump keeps working and using ATP to hold Na+ outside and K+ inside. This active work maintains the polarised state so the neuron is always ready to fire.
Potassium (K+). At rest the membrane has more open K+ channels, so it is far more permeable to K+ than to Na+. This is a common NEET one-liner.
About -70 mV (inside negative relative to outside). NEET usually just expects you to know the inside is negative and the membrane is polarised at rest.
No. The large negatively charged proteins and organic anions are trapped inside because the membrane is not permeable to them. Their fixed negative charge helps keep the inside negative.
