Transmission of impulses

• An impulse is a temporary reversal in the electrical potential difference (the voltage) across the neurone cell surface membrane
• The inside of a resting axon always has a negative electrical potential compared to outside the axon (this is due to a difference in the number of ions on each side) making the membrane polarised
• the resting potential of the neurone is -70mV
• factors involved in the establishment and maintenance of the resting potential;

  • the active transport of sodium (Na+) ions and potassium (K+) ions
  • a difference in membrane permeability to sodium and potassium ions
• Carrier proteins (Sodium-potassium pumps) are present in the cell surface membranes of neurones and use ATP to actively transport sodium ions out of the axon and potassium ions into the axon
• Sodium and potassium ions are pumped in and out of the cell surface membrane at uneven rates (more sodium is pumped out than potassium is pumped in) - this creates a concentration gradient across the membrane for both ions
• due to the concentration gradient created by the sodium-potassium pumps, both sodium and potassium ions will diffuse back across the membrane through channel proteins in facilitated diffusion
• the neurone membrane is less permeable to sodium ions than potassium ions so the potassium ions can diffuse out at a faster rate than sodium ions can diffuse back in - resulting in more positive ions on the outside of the neurone than inside (this generates a negative charge inside - the -70mV)
• to initiate a nerve impulse in a neurone, the membrane needs to be depolarised
• the depolarisation of the membrane occurs when an action potential is generated (when the resting potential is reversed from -70mV to +40mV)
• action potentials are produced when a neurone is stimulated. They involve the rapid movement of sodium ions and potassium ions across the membrane of the axon.
• voltage gated channels open and close in response to changes in the electrical potential across the membrane (they are closed when the membrane is at rest)
• process that follows the stimulation of a neurone;

  • a small number of sodium ion channels in the axon membrane open
  • sodium ions begin to move into the axon down their concentration gradient (lowering potential difference across the axon membrane - the inside becomes less negative)
• when a neurone is stimulated, the potential difference reaches -55mV (known as the threshold potential) causing more sodium ion channels to open (the voltage gated channels)
• an action potential is only initiated if the threshold potential is reached
• roughly 1 millisecond after an action potential is generated, all the voltage gated sodium channels in that section of the membrane close and all the voltage gated potassium channels open - allowing the diffusion of potassium ions out of the axon and down the concentration gradient and causing the axon to become negatively charged again, known as repolarisation.
• there is a short period during which the membrane potential is more negative than resting potential - known as hyperpolarisation
• the period during which the membrane is hyperpolarised is known as the refractory period - when the membrane is unresponsive to stimulation so a new action potential cannot be generated at this time (this allows the successful and efficient transmission of nerve impulses along neurones)
• once an action potential is generated, it is propagated along the length of the axon:

  i.e. 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 the next section which continues along the length of the axon (think dominoes!)
• 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 (large stimuli will lead to several action potentials in a row)
• Myelination speeds up action potential transmission by preventing sodium and potassium ion diffusion in myelinated sections, forcing depolarisation to occur only at the nodes of Ranvier.
• Saltatory Conduction

  local currents cause depolarisation at nodes of Ranvier, making the action potential appear to 'jump' between nodes
• Local currents/circuits

  Sodium ions diffuse along the axon within Schwann cells - depolarisation occurs at the nodes of Ranvier when the sodium ions arrive
• Saltatory conduction allows the nerve impulse to travel much faster than in an unmyelinated axon
• Medications that prevent impulse transmission bind to sodium ion channels preventing them from opening - prevents an influx of sodium ions when an axon is stimulated which prevents membrane depolarisation so no action potential is generated.