Transmission of Nerve Impulses

Transmission of Nerve Impulses

Notes:

·         Neurones transmit impulses as a series of electrical signals

·         The electrical signals pass along the cell surface membrane surrounding the axon as a nerve impulse

Resting Potential 

In a resting axon the inside of the membrane has a negative electrical potential compared to the outside. This typically has a value of -70mV (this is the difference in charge between the Inside and outside of the axon membrane). The resting state of the axon is said to be polarised.

How is the resting potential produced and how is it maintained?

·         Neurons can maintain an internal composition which is different from that of the outside the neuron.

·         Na+ and K+ ions are transported across the membrane against their electrochemical gradient by active transport.

·         Carrier proteins in the membrane pick up the Na+ ions and transport them outside of the membrane

·         At the same time K+ ions are picked up and transported across the membrane into the axon.

·         This is known as the sodium potassium pump and it realise on ATP from respiration. The pump, pumps 3 sodium ions out of the axon while pumping only two potassium ions into the axon.

Remember

Outside = High Na+ concentration - Low K+ concentration

Inside = Low Na+ concentration High K+ concentration

 Why is the Outside Positive Compared to the inside?

·         Na+ ions are pumped out faster than K+ ions are brought back in.

·         K+ ions can diffuse out quicker than Na+ ions can be brought back in.

·         The net result is that the outside of the membrane is positive compared to the inside

·         The resting potential is established and the axon is said to be polarised

There are two gradients established across the axon membrane

1.      A concentration gradient

2.      A electrochemical gradient

3.      Action Potential

Notes

·         A nerve impulse can be initiated in a neurone by mechanical, chemical, thermal or electrical stimulations

·         When this happens the resting potential changes. It goes from around -70mV inside the membrane to +40

·         For a brief period of time, the inside of the axon becomes positive and the outside becomes negative.

·         This change on the potential is called the action potential and lasts for around 3ms

When an action potential occurs, the axon is said to be depolarised.

When the resting potential is re-established the axon membrane is said to be repolarised.

Depolarisation- How does it occur?

1.      The axon membrane changes its permeability to Na+ and K+ ions

2.      When the axon is stimulated, Na+ channels open in the membrane. Na+ ions move into the axon by diffusion down an electrochemical gradient.

3.      The influx of Na+ ions create a positive charge of +40mV inside the axon membrane

4.      K+ channels open and K+ ions diffuse out along their electrochemical gradients which starts of repolarisation

5.        At the same time sodium channels close so that Na+ ions can no longer enter

The resulting potential is re-established, the outside becomes positive and the inside becomes negative again

The membrane is said to have become repolarised.

In fact so many K+ ions leave that the inside becomes more negative than it was originally.The membrane is said to be hyperpolarised. 
K+ channels eventually close and the sodium potassium pump starts again . This restores the normal concentration of Na+ and K+ ions either side of the membrane, this re-establishes the resting potential