Ligand gated sodium channel:

Here the channel protein is cylindrical which is normally closed. When a physiological ligand (molecule e.g. Acetylcholine) binds with the cylindrical protein at specific position, the channel is opened and sodium ion enters the cell. Here action potential causes the release of these ligands which bind with the channel to activate it.

Fig: Acetylcholine Gated Sodium Channel

Above is an example of ligand gated channel regulated by acetylcholine. The protein receptor is pentameric (has five subunits). It contains two α subunits, one β subunit, one γ subunit and one δ subunit. All of these five subunits cross the cell membrane and form walls of a cylinder. This cylinder has the diameter of 8 nm. Normally this cylinder is closed. But when two acetylcholine molecule bind with the α subunits (in the extracellular region of course) a conformational change occurs and the channel is opened for a brief moment. Then Na⁺ can enter the cell.

Acetylcholine is released from the presynaptic neuron or from neuron of neuro-muscular junction and affects the receptors present in postsynaptic neuron or muscle respectively.

The ligand gated sodium channel is to some extent permeable to K⁺.

Once again, my drawing is not very good, so I am sorry if the picture is not understandable.

Voltage gated sodium channel

Ion transport:

Transport of ions across cell membrane is important to maintain homeostasis (Normal healthy condition of the body) and normal bodily functions. Muscle contraction is an example of such functions. It is necessary for motion and also to pump blood throughout the body.

            Contraction of muscle involves the transport of several ions across the muscle cell (read fiber) membrane. The calcium ion is especially important. There are specific mechanisms for transporting ions across cell membrane and they are almost universally proteins. These transport mechanisms have been studied elaborately and many drugs act on these transport mechanisms to cause contraction or relaxation of the muscle.

            Some of the important transport mechanisms have been introduced below.

Voltage gated sodium channel:

This channel (protein) contains two gates – activation gate and the inactivation gate. It is activated when the membrane potential is raised from -70mV to say -55mV.

Fig: Deactivated state of voltage gated sodium channel

At the normal resting potential of the cell membrane, the channel is closed. As we can see that the activation gate is closed and the inactivation gate is opened. This is the deactivated state of the channel.

Fig: Activated state of voltage gated sodium channel

In response to an electrical current (like action potential which lowers the resting potential) the channel is opened and Na⁺ enters the cell. This is the activated state.

Fig: Inactivated state of voltage gated sodium channel

Finally when the potential reaches +35mV (peak of action potential) the channel is closed because the inactivation gate is closed. This is the inactivated state and entry of Na⁺ is prevented. Then the channel returns to its original deactivated state.

An important question is why this channel only allows the passage of sodium ions and not other ions. This will be explained later. For now lets notice the fact that in the above figures the channel protein contains negative charges. The whole process of entry through this channel is shown in following gif animation.

My drawing is not so good. So I am sorry for that.

There are several other transport mechanisms for sodium. They will be discussed one by one in the upcoming posts.

Reference:

Textbook Of Medical Physiology 11th Edition - Arthur C. Guyton & John E. Hall