The Nerve Impulse (Edexcel International A Level Biology)

• Neurones transmit electrical impulses which travel along the neurone cell surface membrane from one end of a neurone 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
• The different states of membrane potential across a neurone cell surface membrane during transmission of a nerve impulse include
  • Resting potential
  • Action potential

Resting potential

• In a resting axon, i.e. one that is not transmitting impulses, the inside of the axon always has a negative electrical potential compared to outside the axon 
  • The difference in charge between the inside and outside of the neurone is due to different numbers of ions on each side of the neurone cell surface membrane
  • When there is a difference in charge across a membrane, we say that the membrane is polarised
• This potential difference, or difference in charge, across the membrane when there are no impulses is usually about -70 mV (millivolts) i.e. the inside of the axon has an electrical potential about 70 mV lower than the outside
  • -70 mV is the resting potential of the neurone
• Two factors contribute to establishing and maintaining resting potential
  • The active transport of sodium ions and potassium ions
  • A difference in membrane permeability to sodium and potassium ions

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 ions 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 membrane permeability to 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 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 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)

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, generates a negative resting potential across the membrane.

Action potential

• Once resting potential is reached the neurone membrane is said to be polarised
• To initiate a nerve impulse in a neurone the 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 +30 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

Generating an action potential

• Some of the ion channels in the membrane of a neurone are voltage gated, meaning that they open and close in response to changes in the electrical potential across the membrane
  • Voltage gated ion channels are closed when the membrane is at rest, but they are involved in the generation and transmission of action potentials
  • Note that not all of the channels in a neurone membrane are voltage gated e.g. some types of potassium ion channels are open when a neurone is at rest to enable potassium ions to diffuse out of the axon and generate a resting potential
• When a neurone is stimulated the following steps occur
  • A small number of sodium ion channels in the axon membrane open
  • Sodium ions begin to move into the axon down their concentration gradient
    • During resting potential there is a greater concentration of sodium ions outside the axon than inside due to the action of sodium-potassium pumps
  • This reduces the potential difference across the axon membrane as the inside of the axon becomes less negative
  • If the potential difference reaches around -55 mV, known as the threshold potential, more sodium ion channels open, leading to a further influx of sodium ions
    • This second set of sodium ion channels are voltage gated channels
    • Note that an action potential is only initiated if the threshold potential is reached
  • Once the charge has been reversed from -70 mV to around +30 mV the membrane is said to be depolarised and an action potential has been generated

Repolarisation

• About 1 millisecond after an action potential is generated all the voltage gated sodium channels in this section of membrane close
• Voltage gated potassium channels in this section of axon membrane now open, allowing the diffusion of potassium ions out of the axon down their concentration gradient
  • Remember that the sodium-potassium pumps have not stopped working during the action potential; hence the potassium ion gradient is still present
• This movement of potassium ions causes the inside of the axon to become negatively charged again, a process known as repolarisation
  • There is a short period during which the membrane potential is more negative than resting potential; this is known as hyperpolarisation
  • The period during which the membrane is hyperpolarised is known as the refractory period
    • The membrane is unresponsive to stimulation during the refractory period, so a new action potential cannot be generated at this time
    • This makes the action potentials discrete events and means the impulse can only travel in one direction
    • This is essential for the successful and efficient transmission of nerve impulses along neurones
• The voltage gated potassium channels then close, and the sodium-potassium pumps work to restore resting potential
  • Only once resting potential is restored can the membrane be stimulated again

The depolarisation and repolarisation of an action potential can be clearly seen in a graph of membrane potential against time

Transmission of an action potential

• Once an action potential has been generated it can be propagated, or transmitted, along the length of the axon
  • The depolarisation of the membrane at the site of the first action potential causes sodium ions to diffuse along the cytoplasm into the next section of the axon, depolarising the membrane in this new section, and causing voltage gated sodium channels to open
  • This triggers another action potential in this section of the axon membrane
  • This process then repeats along the length of the axon
    • Note that any sodium ions that diffuse backwards along the membrane are unable to initiate a new action potential due to the hyperpolarised nature of the membrane in the moments following an action potential
• The action potential is said to move along the axon in a wave of depolarisation
• In the body, this allows action potentials to begin at one end of an axon and then pass along the entire length of the axon membrane

Nerve impulses can be transmitted along axons by the diffusion of sodium ions

The all-or-nothing principle

• Action potentials are either generated or not generated depending on whether the threshold potential is reached; there is no such thing as a small or large action potential
  • If a stimulus is weak only a few sodium ion channels will open and the membrane won’t be sufficiently depolarised to reach the threshold potential; an action potential will not be generated
  • If a stimulus is strong enough to raise the membrane potential above the threshold potential, then an action potential will be generated
• This is the all-or-nothing principle
  • An impulse is only transmitted if the initial stimulus is sufficient to increase the membrane potential above a threshold potential
• Stimulus size can be detected by the brain because as the intensity of a stimulus increases, the frequency of action potentials transmitted along the neurone increases
  • This means that a small stimulus may only lead to one action potential, while a large stimulus may lead to several action potentials in a row

As the strength of a stimulus increases, the frequency at which action potentials are generated also increases

Preventing impulse transmission

• The transmission of nerve impulses is essential to survival as it allows the body to detect and respond to stimuli
• On occasion, however, it is useful to be able to prevent the transmission of impulses e.g. in painkillers and anaesthetics
• Our understanding of the way that action potentials are transmitted means that it is possible to design medications that prevent impulse transmission
  • Such drugs may bind to sodium ion channels, preventing them from opening and therefore preventing an influx of sodium ions when an axon is stimulated
  • Preventing sodium ion influx prevents membrane depolarisation and an action potential cannot be generated