Transmission of Nerve Impulses GHB 2004

Information is carried along a neurone as an electrical impulse

The signal in a neurone is not an electrical current The signal is a fleeting change in the potential difference across the cell surface membrane of the neurone. This change in potential difference sweeps along the neurone from one end to the other.

The Resting Potential When a neurone is not transmitting a signal it is said to be resting A ‘resting’ neurone is NOT resting! The cell surface membranes contain sodium- potassium pumps… …these use ATP to move sodium ions out of the cell and potassium ions in. Three sodium ions are moved for every two potassium ions.

Na + K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ 3Na + 2K + Na – K Pump ATPADP + P i Outside Axon Inside Axon

Potassium ions are able to diffuse back out of the cell, down their concentration gradient, however… The membrane is less permeable to sodium ions so… …they diffuse in more slowly. This means that the tissue fluid outside the cell contains more positive ions than the cytoplasm, so… …the inside is negatively charged compared to the outside The potential difference across the membrane is about –70mV (millivolts), inside compared with outside.

Na – K Pump Potential Difference (70mv) Outside Axon Inside Axon Na + K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K K+K+ K+K+ K+K+

Depolarisation If something happens to reduce the difference in charge across the membrane, the neurone is said to be depolarised

Action Potential The cell surface membrane contains sodium and potassium channels which are sensitive to potential difference… …they are voltage gated channels When the neurone is resting, and the potential difference across the membrane is –70mV, the gated channels are closed.

The Sodium Channels 1.When the membrane is depolarised such that the potential difference across the membrane is less than –40mV, the sodium channels open 2.Sodium ions diffuse in down the electro-chemical gradient. 3.The positive charge of the sodium ions further depolarises the membrane, opening more channels. 4.Within less than a millisecond, the potential difference across the membrane changes from –40mV inside to +40mV inside 5.The sodium channels then begin to close. The sodium ions stop moving

The Potassium Channels 1.As the sodium channels close, the potassium channels open 2.Potassium ions flow rapidly outwards down their electro- chemical gradient. 3.As the potassium ions take positive charge back out of the neurone, the potential difference across the membrane switches back to a value of about –75mV inside… 4.… overshooting the resting value 5.The Potassium ion channels close, and the resting potential is restored once more. THE WHOLE PROCESS LASTS ABOUT 4 ms!

Na + K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ -70mV Outside Axon Inside Axon Na + K+K+ K+K+ Potassium channel +40mV mV Sodium channel

The Refractory Period When an action potential has just taken place at a particular point in a neurone, that point is unable to produce a second action potential immediately… …the sodium ion channels have closed and cannot immediately reopen The membrane must wait until a good proportion of sodium channels are ready to reopen. This waiting time is called the refractory period

How the Impulse Travels When positively charged sodium ions have rushed into a depolarised region of the neurone, they are attracted sideways… … as the regions on either side of the depolarised region have more negative charge A localised current or circuit is set up. The sodium ions depolarise the adjacent regions… …and so the action potential travels along the neurone.

AXON Direction of Impulse The net effect is that the outside is positively charged compared to the inside, giving the… …resting potential In the resting axon, there is a high concentration of sodium ions ouside the axon… Na + …and a high concentration of potassium ions inside K+K+ K+K+ K+K+ K+K+ K+K+

AXON Direction of Impulse Leading edge of impulse … action potential 1. Axon is stimulated producing an… 2. The action potential sets up local circuits in the axon membrane

AXON Direction of Impulse Na + 3. Sodium ions rush in… …depolarising the membrane… …and causing an action potential

AXON Direction of Impulse As the action potential passes along the axon… K+K+ K+K+ … potassium ions diffuse out along a concentration gradient… … starting the process of repolarisation.

AXON Direction of Impulse K+K+ K+K Na + K+K+ K+K+ 5. The sodium-potassium pump is re-established… … fully repolarising the membrane

AXON Direction of Impulse Leading edge of impulse Na K+K+ K+K K+K+ K+K K+K+ K+K K+K+ K+K+ K+K+ K+K K+K+ K+K+ K+K+ K+K K+K+ K+K

Details of the electrical activity of neurones were discovered by studying the giant axons of squid… … these axons have a diameter of 1mm! giant axon bathing saline solution The giant axon is placed in a bathing solution (saline) The potential difference across the axon’s membrane is detected by two electrodes: earth electrode - the earth electrode in the bathing solution, and; microelectrode - a microelectrode inserted into the axon.

microelectrode giant axon bathing saline solution earth electrode oscilloscope The earth electrode and the microelectrode are connected to a dual beam oscilloscope … the distance between the beams indicates the potential difference between the two electrodes. This value is called the resting potential When the tip of the microelectrode is inserted into the axon, the beams of the oscilloscope separate…

microelectrode giant axon bathing saline solution oscilloscope earth electrode stimulator A stimulator produces a current… … which generates an action potential in the axon This action potential is detected by the microelectrode… … and can be displayed on the oscilloscope.

Membrane Potential (mV) time (ms) resting potential depolarisation repolarisation ‘overshoot’ resting potential restored

How do neurones transmit information? Action potentials in a neurone are always the same size… …a depolarisation is either large enough to produce a full- sized action potential or not. Action potentials are BINARY – they are either ON or OFF! Information about the strength of stimulus is contained in the frequency of action potentials in a neurone. The brain recognises the type of stimulus (e.g. light or sound) by the position of the neurone.