The nerve impulseLearning objectives:• How a resting potential is achieved• How changes in permeability create an action potential (and the all-or-nothing idea)• How passage of action potentials along a neurone leads to a nerve impulse being conducted• The nature and importance of the refractory period in producing discrete nerve impulses
A resting neurone• In a resting neurone there are more positive ions on the outside of the axon and fewer positive ions inside the axon• Resting potential – the potential difference (p.d.) across a membrane when a neurone is not conducting an impulse• The p.d. across the membrane is –70mV and is polarised
Q: How is a resting potential achieved? A: The sodium-potassium pump!
Sodium – potassium pumpa) Uses energy from ATP to pump 3 x Na ions OUT in exchange for 2 x K ionsb) Allows both Na and K ions to diffuse but is MORE permeable to K + ions than to Na2+ ions
Gated channels• The channels are closed in a resting membrane• Only open when the p.d. across the membrane reaches a specific value
Gated channels• Na gated channels open at the point of a stimulus• NA2+ ions move in• If enough move in, more channels open, causing MORE to come in.• If not enough, nothing happens (all-or-nothing nature)• Reduction in p.d. = depolarisation of the membrane
Gated channelsSodium ions diffuse along:• Concentration gradient• Electrical gradientSo many have moved in that inside becomes more positive than outside. The p.d. is now +40mV
Repolarisation• Sodium gated channels now close and potassium gated channels open• K+ ions now flow along a concentration and electrical gradient• Membrane is repolarised to its original resting potential of -70mV
Recovery• There is a brief time of hyperpolarisation when the p.d. is more negative due to potassium channels staying open a millisecond longer• The is also a brief refractory period – when the sodium channels have closed before the neurone can respond to the next stimulus• The original balance of ions is restored by the sodium- The whole process is called potassium pump an action potential
Changes in permeability of the neurone during anaction potential
How does an impulse travel along a neurone? Passage of action potentials!
Different sized neurones• Nerve impulses move at constant speed• Wide neurones - conduct impulses more quickly than narrow• Myelinated neurones - conduct impulses more quickly unmyelinated