Transmission of nerve impulses

axon

During a nerve impulse Na+ axon

During a nerve impulse K+ Na+ axon

More positive – outside Resting state More positive – outside axon Less positive - inside

During an action potential axon Less positive - inside

Inside becomes DEPOLARISED During an action potential Na+ axon Inside becomes DEPOLARISED

Inside becomes DEPOLARISED During an action potential Na+ axon Inside becomes DEPOLARISED Action potential is generated

Inside becomes negative again During an action potential K+ axon Inside becomes negative again

More positive – outside Resting state More positive – outside axon Less positive - inside

Transmission of nerve impulses Neurones transmit impulses as electrical signals These signals pass along the cell surface membrane of the axon as a nerve impulse

Transmission of nerve impulses It is NOT the same as an electric current passing down a wire (which is much faster) The mechanisms is the same throughout the animal kingdom

Transmission of nerve impulses Experiments have been carried out using squid axons which are big enough to have electrodes inserted in them. One electrode can be placed inside the axon and one on its surface.

Transmission of nerve impulses When at rest, the inside of the membrane has a negative electrical potential compared to the outside. This difference in potential is called the resting potential and is typically about between -70 mV to -80 mV In this resting state the axon is said to be polarised.

Transmission of nerve impulses This is maintained because the neurone has an internal composition which is different to the outside. Sodium ions and potassium ions are transported across the membrane against their concentration gradients by active transport

Transmission of nerve impulses Carrier proteins pick up Na+ ions and transport them to the outside. At the same time K+ ions are transported into the axon. This is known as the sodium-potassium pump and relies on ATP from respiration

Transmission of nerve impulses Inside the axon there are large numbers of negatively charged organic ions which can not move out of the axon.

Transmission of nerve impulses The Na+ ions are passed out faster than the K+ ions are bought in. Approx. three Na+ ions leave for every two K+ ions that enter. K+ ions can also diffuse back out quicker than Na+ ions can diffuse back in. Net result is that the outside of the membrane is positive compared to the inside.

Axon membrane

Axon membrane Inside axon

The action potential A nerve impulse can be initiated by mechanical, chemical, thermal or electrical stimulation When the axon is stimulated the resting potential changes. It changes from –70 mV inside the membrane to +40 mV For a very brief period the inside of the axon becomes positive and the outside negative

The action potential This change in potential is called the action potential and lasts about 3 milliseconds 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

depolarisation reploarisation

depolarisation reploarisation ‘overshoot’

direction of impulse depolarisation reploarisation ‘overshoot’

Depolarisation When the membrane depolarises changes occur in the membrane to the permeability of both Na+ ions and K+ ions

Depolarisation When the axon is stimulated, channels open on its cell surface which allow Na+ ions to pass through. Na+ ions flood in by diffusion The Na+ ions create a positive charge of +40 mV inside the membrane, reversing the resting potential and causing the action potential

Repolarisation Potassium channels open in the membrane and K+ ions diffuse out along their concentration gradient. This starts of repolarisation At the same time, sodium channels in the membrane close preventing any further influx of Na+ ions.

Repolarisation The resting potential is re-established as the outside of the membrane becomes positive again compared to the inside. So many K+ ions leave that the charge inside becomes more negative that it was originally. This shows up as an ‘overshoot’.

resting potential (no net ion movement)

Na+ start to move in resting potential (no net ion movement)

Na+ ions diffuse in rapidly Na+ start to move in resting potential (no net ion movement)

K+ ions diffuse out rapidly Na+ ions diffuse in rapidly Na+ start to move in resting potential (no net ion movement)

K+ ions diffuse out rapidly Na+ ions diffuse in rapidly Sodium ions pumped out potassium ions pumped in Na+ start to move in resting potential (no net ion movement)

Repolarisation The potassium channels close and the sodium-potassium pump starts again. Normal concentrations of sodium and potassium ions is re-established. The membrane is once again at its resting potential

direction of impulse + + + + + + + + - - - - - - - - - - - - - - - - + + + + + + + + a) In the resting axon, there is a high conc. of Na+ ions outside and a high conc. of K+ ions inside. But the net effect is that the outside is positive compared to the inside giving the resting potential

+ + + + + + + - - - - - - - - + - - - - - - - + + + + + + + + - Leading edge of impulse + + + + + + + - - - - - - - - + - - - - - - - + + + + + + + + - b) The axon is stimulated producing an action potential, setting up local circuits on the axon membrane

Na+ + + + + + + - - - - - - - - + + - - - - - - + + + + + + + + - - direction of impulse Na+ + + + + + + - - - - - - - - + + - - - - - - + + + + + + + + - - Na+ c) Sodium ions rush into the axon along a diffusion gradient depolarising the membrane causing an action potential

K+ + + + + + + - - - - - - - - + + - - - - - - + + + + + + + + - - K+ direction of impulse K+ + + + + + + - - - - - - - - + + - - - - - - + + + + + + + + - - K+ d) As the action potential passes along the axon potassium ions diffuse out along a concentration gradient, starting off the process of repolarisation

K+ Na+ K+ + + + + + - - + - - - - - + + - - - - - - + + - direction of impulse K+ Na+ K+ + + + + + - - + - - - - - + + - - - - - - + + - + + + + + - - + K+ Na+ K+ e) The sodium-potassium pump is re-established, fully repolarising the membrane