NERVE IMPULSES

AIMS

• Describe the establishment of a resting potential in terms of the differential membrane permeability and the presence of cation pumps

• Explain the initiation of an action potential and its all-or-nothing nature, explained by changes in membrane permeability leading to depolarisation

• Describe the passage of an aciton potential along non-myelinated and myelinated axons resulting in nerve impulses

• Explain the nature and importance of the refractory period in producing discrete nerve impulses

RESTING POTENTIAL

How is information carried along neurones?

• An electrical impulse (not an electric current)

• A fleeting change in the electrical potential difference across the cell surface membrane of the neurone

What does it mean if a neurone is ‘resting’?

• When a neurone is not transmitting a signal

• However a resting neurone is very active

What is the potential difference (difference in charge) across the cell membrane of a resting neurone?

• About –70mV (milivolts) – resting potential (gives a neurone the potential to transmit a nerve impulse)

• The membrane is said to be polarised

How is a difference in concentration of ions across the neurone membrane maintained?

• Sodium potassium pump (carrier / cation pump) found in the cell surface membrane of axons

How does the sodium potassium pump create a negative electrical charge inside the axon compared with outside?

• It uses energy from ATP to move sodium ions (Na+) – cations- out of the cell and potassium ions (K+) in

• Three sodium ions are pumped out for every two potassium ions pumped in

• This results in a slightly higher positive charge outside than inside

How does the sodium potassium pump create a negative electrical charge inside the axon compared with outside?

• Potassium ions are able to diffuse freely back out of the cell – facilitated diffusion

• but the cell surface membrane is much less permeable to sodium ions, so they can diffuse (facilitated diffusion) back in only very slowly

• this results in the tissue fluid outside the cell containing more positive ions than the cytoplasm inside it

ACTION POTENTIALS

What are voltage-gated channels?

• Proteins which act as specific channels for sodium and potassium and are also sensitive to potential difference or voltage

• They can be opened or closed

What are gated channels like when the neurone is resting?

• They are closed

What meant by depolarisation?

• Something happens to reduce the difference in charge across the membrane

• Caused by a stimulus from the environment (received by a receptor) and passed down the length of the axon of a neuron

• The charge difference between the inside and outside becomes less than - 40mV

Depolarisation

What happens to the voltage-gated channels when the neurone becomes depolarised?

• The sodium ion channels suddenly open

• Sodium ions diffuse into the cytoplasm of the neurone

What happens to the potential difference in the axon as the sodium channels open?

• The positive charge of the sodium ions depolarises the membrane even more

• Even more sodium ion channels open

• The potential difference across the membrane changes form -40 mV inside to + 40 mV

What is an action potential?

• Travelling depolarisation of the cell membrane

• The resting potential disappears for a few milliseconds

• Starts at the cell body and travels to a synapse

Repolarisation

What happens as the potential difference becomes positive?

• Sodium ion channels begin to close

• Sodium ions stop moving in

• Voltage gated potassium ion channels open

• Potassium ions diffuse out and start to restore the resting potential

• Sodium ions are actively pumped out

• The potential difference across the membrane returns to about -75 mV inside

• Called repolarisation

What is meant by the refractory period?

• For a short period after repolarisation the membrane cannot be depolarised and therefore no impulse can pass

• This is because the gated channels for sodium and potassium ions remain closed

Why is the refractory period important?

• Each action potential is kept discrete – not overlapping of potentials

• It ensures the action potentials pass in only one direction

What is meant by the threshold level?

• The minimum level at which a stimulus can set up an action potential

What is the all-or-nothing principle?

• *** the size of the action potential does not depend on the size of the stimulus

• the action potential remains exactly the same size as it travels along the neurone (no big or small action potentials)

If all action potentials are the same size, how is information about the strength of the stimulus transferred?

• The frequency of nerve impulses carries information

• A strong stimulus produces a high frequency of nerve impulses

Myelinated vs. non-myelinated

How does the transmission of nerve impulses differ between non-myelinated axons and myelinated axons? Which types of organsism have which?

NON-MYELINATED AXONS

• Invertebrates

• Nerve impulses travel along the length of the axon

How does the transmission of nerve impulses differ between non-myelinated axons and myelinated axons? Which types of organsism have which?

MYELINATED AXONS

• Vertebrates

• The axon membrane is exposed at the nodes of Ranvier

• Nerve impulses ‘jump’ from one node to the next, making the speed of travel of the nerve impulse faster than in non-myelinated axons (salatory conduction)

Propagation of the action potential

Channel gating during an action potential