Setting up action potentials

• A change in the environment results in action potential being set up in a receptor
• Across the membrane there is a charge (voltage) called the potential difference (p.d). This charge can be measured using a voltmeter and a reading of around -65mV is measured. This value is telling you that the inside of the membrane is 65 times more negative compared to the outside of the membrane. The membrane is said to be polarised
• Potential difference when the cell is at rest is called resting potential
• changes in potential difference across a neurone during an action potential

  A) Na+ channels open

  B) voltage-gated Na+ channels open

  C) voltage-gated Na+ channels close

  D) K+ channels open

  E) K+ channels close

  F) refractory period

  G) threshold

  
• 1. Stimulus - this excites the neurone cell membrane, causing sodium ion channels to open. The membrane becomes more permeable to sodium, so sodium ions diffuse into the neurone 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 -55 mV), voltage-gated sodium ion channels open and more sodium ions diffuse into the neurone. This is positive feedback
• 3. Repolarisation - at a potential difference of around +30 mV the sodium ion channels close and voltage-gated 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. This is negative feedback
• 4. Hyperpolarisation - potassium ion channels are slow to close so there's a slight 'overshoot' where too many potassium ions diffuse out of the neurone. The potential difference becomes more negative than the resting potential (i.e. less than -70 mV).
• 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, and maintains the resting potential until the membrane's excited by another stimulus.