10.3 The nerve impulse

The nerve impulse is described as a self propagating wave of electrical disturbance whichtravels along the surface of the axon membrane.

It is a temporary reversal of the potential difference across the membrane, switching fromthe resting potential to the action potential.

The resting potential of the cell is maintained due to the different concentrations of sodiumand potassium ions. Both of these ions are positive.

• The phospholipid bilayer is impermeable to both ions, and prevents either one fromdiffusing naturally through it

• The membrane is dotted with intrinsic proteins that travel all the way from theoutside in. These proteins act as channels and can allow the sodium and potassium.  However, some of the gates are 'gated' and will only only certain ions to move in certain directions across chemical and electrical gradients.

• Some proteins actively transport ions through the membrane, against their electrochemical gradients. This requires ATP from respiration. These gates are calledsodium potassium pumps

The operation of these mechanisms work to maintain a constant, negative potential of -70millivolts, relative to the outside of the cell. This is the Resting Potential of the axon. The axon is said to be polarised in this state.

the resting potential of the cell is maintained by the following mechanism:

• Sodium ions are actively transported out of the axon

• Potassium ion are actively transported into the axon

• The active transport of each is not equal. for every three sodium ions transported out of the axon, only three potassium ions are transported into the axon

• The outward movement of sodium and the inward movement of potassium creates chemical gradients for both of the ions, which they naturally try to follow; sodiumtries to diffuse into the cell and potassium tries to diffuse out of the cell

• However! The phospholipid bilayer prevents this movement, leaving only the intrinsic 'gated' proteinswhich the two ions can move through.

• These gates have different permeabilities to different ions. The sodium gates aremainly closed, while the potassium gates are mainly open.

• This disparity means that potassium ions freely move out of the cell, while sodium ions cannot move in. The result of this is that the inside of the axon is more positive to the outside, which has the larger concentration of positive charges.

• After a while though, the electrical gradient of the potassium becomes the more dominant factor in its net movement.  Potassium ions stop moving across their chemical gradient and an equilibrium is established

• This equilibrium is the resting potential of the axon: -70 mV

In their resting state, all nerve cells maintain a potential difference of -70 mV across their membranes. This is the resting potential of the cell

But what happens during a nerve impulse?

A nerve impulse is simply a wave of depolarisation along the membrane of the axon in a nerve cell. Energy from a stimulus hits a receptor. This energy  causes part of the receptor membrane to depolarise. A description of this process is described below:

1. At resting potential, the potassium gates are open, allowing potassium out of the cell. However, the sodium gates are closed, and sodium ions cannot flow down their chemical gradient

2. The energy from a stimulus somehow causes the sodium gates to open. In the Pacinian Corpuscle, stretch mediated sodium channels open when mechanical pressure is applied

3. Sodium ions rush into the axon along their chemical and electrical gradient. As sodium ions are positive, this causes a reversal in the potential difference of the membrane. The influx of sodium causes more sodium gates to open, increasing the flow of sodium into the cell

4. At around +40 mV, the sodium gates close, preventing further influx of sodium ions. At this point, the potassium gates that were closed now open.

5. The electrical gradient that prevented potassium from leaving the cell during the resting state has now been reversed. The inside of the cell, where most potassium ions are, is now positive. Potassium ions will now want to flow out of the cell through the newly opened potassium gates. this starts to re-polarize the cell

6.The outward diffusion of potassium causes a slight over shoot of the electrical gradient, called hyper-polarisation, causing the inside of the axon to be slightly more negative than usual. The potassium gates close at this point, and the activities of the sodium potassium pumps quickly restore a resting potential of -70 mV