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
