Biology questions (nervous controls)?!?

Question

1) A neuron stimulated in a laboratory will carry an impulse in either direction away from the point of stimulation. Why don't impulses travel in two directions along your neurons?

2) Can you suggest a reason why noradrenaline has no effect on skeleton muscle?

3) When does the sodium-potassium pump go into action? Why must it do so?

Thanks every one in advance!

Answer 1

That's pretty complex.  Here we go.

1) This all has to do with action potentials.  If you know about these then you know they are how impulses travel.  You would also know that in the late stage of the AP (action potential) there is a time when the potassium ions are still moving back into the cell after being let out during the AP and the sodium ions are moving back in.  When the ions are still being moved back to their original concentrations, another AP cannot occur, since there's not a big enough gradient in ion concentrations, and ion movement into and out of the cell is the basis of the AP.  This is called the latency period.  Consider an impulse travelling along an axon.  The section just behind the impulse will have just fired, and will be in the latency period.  This means it cannot fire again.  The section in front of the impulse has not fired, so it is ready for an AP.  This means the impulse can only travel forwards, and not in 2 directions.  There is also the reason that on an axon the point of stimulation is at the soma, or body of the nerve cell.  This is at one end of the axon.  So the impulse can only go from end to end.

2)  This is simply because there are no receptors for noradrenaline on skeletal muscle.  In terms of muscular effects, noradrenaline is only concerned with smooth muscle cells, which have a gradual contraction, such as muscle around blood vessels and things like that.  Skeletal muscle fibres contract in an 'all or nothing' event, not gradually.  Noradrenaline plays no part in this.

3) The sodium potassium pump kicks in in the late phase of the AP I was telling you about earlier.  The sodium ions that are now in the cell need to leave and potassium needs to enter in order to prime for another AP.  The concentration gradients will not allow this to happen by diffusion so a pump is required.