Neurones

• the resting membrane potential: - in a neurone's resting state (when it is not being stimulated) the outside of the membrane is positively charged compared to the inside

  • the is bc there are more positive ions outside the cell than inside
  • so the membrane is polarised - there is a difference in charge (potential difference/ voltage) across it
  • the voltage across the membrane when it is at rest is called the resting potential - about 70mV
• Movement of sodium and potassium ions:

  • the resting potential is created and maintained by sodium-potassium pumps and potassium ion channels in a neurone's membrane
  • sodium-potassium pumps use active transport to move three sodium ions (Na+) out of the neurone for every 2 potassium ions (K+) moved in - active transport is needed for this
  • potassium ion channels allow facilitated diffusion of potassium ions (K+) out of the neurone, down their concentration gradient
• 1. the sodium-potassium pumps move sodium ions out of the neurone, but the membrane is not permeable to sodium ions so they cannot diffuse back in
  2. this creates a sodium electrochemical gradient (concentration gradient of ions) bc there are more positive sodium ions outside the cell than inside
  3. the sodium-potassium pumps also move potassium ions in to the neurone
  4. when the cell is at rest, most potassium ion channels are open
  5. this means that the membrane is permeable to potassium ions so some diffuse back out through potassium ion channels
• Even though positive ions are moving in and out of the cell, in total more positive ions move out than enter. this makes the outside of the cell positively charged compared to the indside
• Action potentials:

  • when a neurone is stimulated, other ion channels in the cell membrane - sodium ion channels - open
  • if the stimulus is big enough - it will trigger a rapid change in potential difference
  • this causes the cell membrane to become depolarised (no longer polarised)
• 1. stimulus - this excites the neurone cell membrane, causin sodium ion channels to open. the membrane becomes more permeable to sodium, so sodium ions diffuse into the nuerone down the sodium ion electrochemical gradient. this makes the inside of the neurone less negative
• 2. depolarisation - if the potential difference reaches the threshold (around 55mV), more sodium ion channels open. more sodium ions diffuse into the membrane
• 3. repolarisation - at a potential difference of around +30mV - the sodium ion channels close and potassium ion channels open

  the membrane is more permeable to potassium so potassium ions diffuse out of the neurone down the potassium ion concentration gradient

  this starts to get the membrane back to its resting potential
• hyperpolarisaton: - potassium ion channels are slow to close so there is a slight 'overshoot' where too many potassium ions diffuse out of the nuerone

  the potential difference becomes more negative than the resting potential (i.e. less than -70mmV)
• 5. resting potential:

  • the ion channels are reset
  • the sodium-potassium pump returns the membrane to its resting potential by pumping sodium ions out and potassium ions in
  • maintains the resting potential until the membrane's excited by another stimulus
• The refractory period:

  • after an action potential, the neurone cell membrane can't be excited again straight away
  • this is bc the ion channels are recovering and they can't be made to open
  • sodium on channels are closed during repolarisation
  • potassium ion channels are closed during hyperpolarisation
  • this period of recovery = the refractory period
• • the refractory period acts as a time delay between one action potential and the next
  • this makes sure that action potentials don't overlap but pass along as discrete (separate) impulses
  • the refractory period also means that there is a limit to the frequency at which the nerve impulses can be transmitted
  • and that action potentials are unidirectional (only travel in one direction)
• Waves of depolarisation:

  • when an action potential happens, some of the sodium ions that enter the neurone diffuse sideways
  • this causes the sodium ion channels in the next region of the neurone to open and sodium ions diffuse into that part
  • this causes a wave of depolarisation to travel along the neurone
  • the wave moves away from the parts of the membrane in the refractory period bc these parts can't fire an action potential
• the electrical impulse can be said to propagate along the neurone
• the all-or-nothing principle stops the brain from getting overstimulated by not responding to very small stimuli
• All-or-nothing principle:

  • once the threshold is reached
  • an action potential will always fire with the same change in voltage, not matter how big the stimulus is
  • if the threshold is not reached - an action potential won't fire
  • this is the all-or-nothing nature of action potentials
  • a bigger stimulus won't cause a bigger action potential but it will cause them to fire more frequently
• speed of conduction:

  • three factor affect the speed of conduction f action potentials:
  • myelination (saltatory conduction)
  • axon diamteter
  • temperature
• Myelination:

  • some neurones including motor neurones are myelinated - they have a myelin sheath
  • the myelin sheath is an electrical insulator
  • in the PNS - the sheath is made of a type of cell called a Schwan cell
  • between the Schwan cells are tiny patches of bare membrane called the nodes of Ranvier
• dendrites are extensions of the cell body that connect with other neurones
• Saltatory conduction:

  • in a myelinated neurone, depolarisation only happens at the nodes of Ranvier (where sodium ions can get through the membrane)
  • the neurone's cytoplasm conducts enough electrical charge to depolarise the next node, so the impulse jumps from node to node
  • this is called saltatory conduction - very fast
• in a non-myelinated neurone:

  • the impulse travels as a wave along the whole length of the axon membrane - get depolarisation along the whole length of the membrane
  • this is slower than saltatory conduction - still quite quick
• Axon diameter:

  • action potentials are conducted quicker along axons with bigger diameters bc there is less resistance to the flow of ions than in the cytoplasm of a smaller axon
  • with less resistance, depolarisation reaches other parts of the neurone cell membrane quicker
• temperature:

  • the speed of conduction increases as the temperature increases too, because ions diffuse faster
  • the speed only increases up to around 40 degrees
  • after that the proteins begin to denature and the speed decreases