excitable cells

• excitable cell

  rapidly adjust membrane potential
• membrane potential

  difference in electron charge inside and outside cell
• resting membrane potential

  around -70mv
• potassium leak channel

  potassium down gradient and out of cell
• depolarisation

  Na enters cell
• repolarisation

  k leaves cell
• what happens when voltage gated K channels open

  k leaves cell and membrane repolarises
• what happens when voltage gates Na channels open

  Na leaves cell and membrane depolarises
• how can membrane potential change

  graded potentials or action potentials
• graded potential

  small deviation from resting membrane potential
• hyperpolarising graded potential

  cell more negative
• depolarising graded potential

  cell more positive
• acetylcholine

  na moves out, depolarisation
• glycine

  cl in, hyperpolarisation
• summation

  stimuli add together to produce a bigger change in membrane potential
• positively charged ions into cell

  depolarise
• negatively charged ions into cell

  hyperpolarise
• action potential

  rapid and brief reversal of membrane potential
• where are action potentials initiated

  axon initial segment
• voltage gated sodium channels

  na into cell, in response to depolarisation
• voltage gated potassium channels

  k out of cell, in response to depolarisation
• action potential threshold

  point that voltage gated sodium channels open
• absolute refractory period

  sodium channels cannot be activated by further depolarisation
• relative refractory period

  another action potential can be activated in response to a larger membrane depolarisation
• action potentials are initiated at axon initial segment and propagate down to axon terminals
• unmyelinated neuron

  continuous conduction, reinitated at all points
• unmyelinated neuron speed

  0.5 - 2 m/s
• myelinated neuron

  saltatory conduction, reinitiated at Nodes of Ranvier
• myelinated neuron speed

  30 - 40 m/s
• membrane is more permeable to K than Na so resting potential is closer to the equillibrium potential of k
• Na are in high concentration outside of the cell normally
• positive inside

  depolarise
• negative inside


  hyperpolarise
• axon initial segment contains a high concentration of voltage-gated sodium channles
• after hyperpolarisation

  cell more negative than start, very brief