Nerve Impulses (SL IB Biology)

• Neurones transmit information in the form of impulses, which travel extremely quickly along the neurone from one end to the other
  • Note that an impulse is not an electrical current that flows along neurones as if they were wires
  • Instead, an impulse is a momentary reversal in the electrical potential difference across the neurone cell surface membrane
    • The electrical potential difference across a membrane can also be described as the voltage across a membrane, the difference in charge across a membrane, or the membrane potential
• In an axon that is not transmitting an impulse the inside of the axon always has a negative electrical potential, or charge, compared to outside the axon, which has a positive electrical potential
  • This membrane potential in a resting neurone is known as resting potential
• The resting potential is usually about -70 millivolts (mV)
  • This means that the inside of the resting axon has a more negative electrical charge than the outside by about 70 mV
• Two main processes contribute to establishing and maintaining resting potential:
  • The active transport of sodium ions and potassium ions
  • A difference in rates of diffusion of sodium ions and potassium ions
• In addition to these two main processes, negatively charged proteins inside the axon also contribute to the negative resting potential

The active transport of sodium ions and potassium ions

• Carrier proteins called sodium-potassium pumps are present in the cell surface membranes of neurones
• These pumps use ATP to actively transport sodium ions (Na⁺) out of the axon and potassium ions (K⁺) into the axon
• The two types of ion are pumped at an unequal rate; for every 3 sodium ions that are pumped out of the axon, only 2 potassium ions are pumped in
• This creates a concentration gradient across the membrane for both sodium ions and potassium ions

Difference in rates of diffusion of sodium ions and potassium ions

• • Because of the concentration gradient generated by the sodium-potassium pumps, both sodium and potassium ions will diffuse back across the membrane
    • The neurone cell surface membrane has sodium ion channels and potassium ion channels that allow sodium and potassium ions to move across the membrane by facilitated diffusion
  • The neurone membrane is much less permeable to sodium ions than potassium ions, so potassium ions inside the neurone can diffuse out at a faster rate than sodium ions can diffuse back in
  • This results in far more positive ions on the outside of the neurone than on the inside, generating a negative charge inside the neurone in relation to the outside
  • The result of this is that the neurone has a resting membrane potential of around -70 millivolts (mV)

Resting Potential Diagram

Sodium-potassium pumps in the membrane of a resting neurone generate a concentration gradient for both sodium ions and potassium ions. This process, together with the facilitated diffusion of potassium ions back out of the cell at a faster rate than sodium ions diffuse back into the cell, generates a negative resting potential across the membrane. 

• Once resting potential is reached, the neurone membrane is said to be polarised
• To initiate a nerve impulse in a neurone, the neurone membrane needs to be depolarised
  • Depolarisation is the reversal of the electrical potential difference across the membrane
• The depolarisation of the membrane occurs when an action potential is generated
  • Action potentials lead to the reversal of resting potential from around -70 mV to around +40 mV 
• Action potentials involve the rapid movement of sodium ions and potassium ions across the membrane of the axon
• An action potential is the potential electrical difference produced across the axon membrane when a neurone is stimulated e.g. when an environmental stimulus is detected by a receptor cell