Question

ACTION POTENTIAL (AP) ON A NEURON

1. For each of the following channels, indicate what specifically opens it, what specifically closes it, and what goes through (& in what direction) when it is open.

•Voltage-gated Na+ channel

•Voltage-gated K+ channel

2. What are leak channels? What purpose do they serve in neurons?

Answer 1

Ans :

1. Voltage gated channels :

Na channel :  

Voltage Gated Sodium channels (Na⁺ VGC) are found within the plasma membrane of nerve cell axons. Their main role is to induce an action potential (AP). The Na⁺ VGC can occupy 3 states: Open, inactive and closed. There are 2 gates in Na⁺ VGC (unlike K⁺ VGC which only has 1). There is the activation gate and inactivation gate. The activation gate is voltage dependent and the inactivation gate is time dependent.

Open - When the plasma membrane begins to depolarize, it triggers the opening of a few Na⁺ VGC which increases the amount of Na⁺ entering the cell and therefore continues to depolarise the membrane. This in turn triggers the opening of more Na⁺ VGC. At this point the cell is permiable to Na⁺. The open phase is said to be voltage dependent, as they open when the plasma membrane is depolarised.

Inactive - This stage is said to be time dependent as the inactivation gates close a set time after the activation gates open in the open stage. The cell now becomes impermiable to Na₊.

Closed - When the membrane repolarizes, the Na⁺ VGC takes on it's closed state. The activation gate closes a set time after the inactivation gate opens so it also time dependent.

K channel :

Voltage-gated potassium channels are activated by depolarization, and the outward movement of potassium ions through them repolarizes the membrane potential to end action potentials, hyperpolarizes the membrane potential immediately following action potentials, and plays a key role in setting the resting membrane potential

2. Leak channels:

Leakage channels are the simplest type of ion channel, in that their permeability is more or less constant.

The types of leakage channels with the greatest significance in neurons are potassium and chloride channels.

The cell possesses potassium and sodium leakage channels that allow the two cations to diffuse down their concentration gradient.

However, the neurons have far more potassium leakage channels than sodium leakage channels.

Purpose :  The neuron cell membrane is super permeable to potassium ions, and so lots of potassium leaks out of the neuron through potassium leakage channels (holes in the cell wall). The neuron cell membrane is partially permeable to sodium ions, so sodium atoms slowly leak into the neuron through sodium leakage channels.

The leak channels allow Na+ and K+ to move across the cell membrane down their gradients (from a high concentration toward a lower concentration). With the combined ion pumping and leakage of ions, the cell can maintain a stable resting membrane potential.