3. the nervous impulse

• nervous impulses are the electrical charges transmitted along a neuron and are created by the movement of sodium and potassium ions across a neuron cell membrane
• in neurons resting state, the outside of the membrane is positively charged because there are more positive ions outside the cell than in, so the membrane is polarised ( theres a difference in charge )
• voltage across the membrane when at rest is called the resting potential ( about -70mv )
• Sodium-potassium pumps use active transport to move 3 sodium ions out the neuron for every 2 potassium moved in and ATP is required to do this
• potassium ion channels allow for facilitated diffusion of potassium ions out the neuron down their concentration gradient
• 1. Sodium-potassium pumps move sodium ions out of the neuron, but the membrane isn't permeable to sodium ions and can't diffuse back in. This creates a sodium ion electrochemical gradient because there is more positive sodium ions outside the cell than inside
  2. sodium-potassium pumps also move potassium ions into the neuron
  3. When cells at rest, most potassium ion channels are open so membrane is permeable to potassium ions so some diffuse back out through the potassium ion channels
• when a neurone is stimulated sodium ion channels in the cell membrane open and if big enough will trigger a rapid change in potential difference which causes the cell membrane to become depolarised. The sequence of events are called an action potential
• 1. Sodium channels open
  2. voltage gated sodium channels open
  3. voltage gated sodium channels close
  4. Potassium channels open
  5. Potassium channels close
• voltage gated channels are channels that only open at a certain voltage threshold
• 1. stimulus - excites the neuron cell membrane causing sodium ion channels to open. membrane becomes more permeable to sodium so it diffuses into the neuron down the sodium ion electrochemical gradient making the inside of the neuron less negative
• 2. depolarisation - if potential difference reaches threshold level voltage gated sodium ion channels open and more sodium ions diffuse into the neuron