biology - topic 6 - action potentials

resting neurone

neurone is not conducting a nerve impulse

inside of axon is slightly negatively charged (-70mV) compared to the outside

membrane = polarised

maintaining resting potential

sodium potassium pumps actively transport 3 sodium ions out of cell and 2 potassium ions into cell. K+ diffuses out of cell via open K+ leakage channels making membrane more permeable to K+ than Na+ (loss of positively charged ions means inside becomes slightly negatively charged)

depolarisation

stimulus reaches resting neurone, sodium voltage gated channels open, sodium ions diffuse into neurone down electrochemical gradient. if sufficient sodium ions diffuse in more gated sodium channels open. reduction in potential difference across membrane occurs causing depolarisation - inside of axon becomes positively charged (+40mV)

recovery/repolarisation

inside of axon has become positively charged, sodium voltage gated ion channels close and potassium gated channels open, potassium ions diffuse out of axon along electrochemical gradient - membrane is repolarised and returns to resting potential (-70mV)

Hyperpolarisation

potassium ion channels stay open for a few milliseconds after repolarisation meaning that membrane potential becomes more negative than resting potential. this causes a brief refractory period meaning that the membrane cannot conduct another AP. sodium potassium pump has to restore balance of ions required for resting potential

importance of refractory period

ensures APs only travel in one direction, ensures APs are separated from each other, limits number of APs

action potential

stimulation of an axon resulting in a change of potential across the membrane from a slight negative inside value of -70mV to a positive value of +40mV