5.3.3(Nerve impulses: action potentials)

Describe what is meant by a resting neurone

• A neurone is called at rest when it is not transmitting an action potential

Describe how neurones at rest act

• The sodium potassium pump uses ATP to actively transport 3 Na+ ions out of a cell and 2 K+ ions into the cell.

• The gated sodium ion channels are kept closed

• Some potassium ion channels are open and therefore the plasma membrane is more permeable to potassium ions than sodium ions.

• Potassium ions tend to diffuse out of the cell

Describe the role of sodium and potassium channels around an axon

• These channels are voltage gates to regulate the diffusion of specific ions

• When the membrane is disrupted the channels open and sodium ions diffuse in

State the resting potential of nuerones

• -60mV

State where the concentration of sodium and potassium ions is the highest

• Sodium - outside the the cell

• Potassium - inside the cell

What volatge does the sodium gated channel open and close?

• Open: -50mV

• Close: +40mv

What voltage does the potassium gated channel open and close

• Open: +40mV

• Close: -50mV

Describe how an action potential is formed and how resting potential is later recharged

• If a receptor is stimulated it will release Na+ into the sensory neurone, this causes depolarisation. The resting potential becomes more positive. This is called the generator potential

• If there are enough Na+ entering the neurone, the threshold potential(-50mV) is met, then the voltage gated sodium channels open

• Na+ move down their concentration gradient into the neurone via facilitated diffusion from outside the neurone causing an action potential of +40mV. This is enough to close voltage gated sodium channels in that area

• Na+ diffuse along the neurone to cause V.G sodium channels to open further along the neurone

• at +40mV, voltage gated potassium channels open which allow K+ ions to move out of the neurone causing repolarisation.

• When -50mV is reached again, the V.G. potassium channels close slowly which allows more K+ ions to diffuse out of the neurone, this cause hyperpolarisation(-85mV) of the membrane

• After hyperpolarisation, the refractory period occurs: the non-gated potassium pump and K+/Na+ co transporter proteins restore the resting potential. During this phase, this area of the membrane is inactive