1. Refractory Periods

2. Why Can’t an Action Potential Travel the Wrong Way? When K+ ions flood out to repolarise the once depolarised action potential, the Na+ ions close. This closure of Na+ ions prevents an action potential happening in the wrong direction. This is called the REFRACTORY PERIOD Only when resting potential has been established in a once depolarised region can the Na+ potentially be opened again by a stimulus.

3. Refractory Period 2 parts Absolute Refractory Period – first ms after first action potential, no other action potentials can be made under any circumstances. Relative Refractory Period – time between 1 st ms and resting period being re-established. A second action potential can be made during this time but only if stimulus is much larger than usual.

5. Factors Affecting the Speed that an Action Potential Travels Speed can vary from 0.5 ms -1 to 120 ms -1 Myelin Sheath – Acts as an electrical insulator. Speed in unmyelinated neurones is a lot slower than in myelinated ones. Diameter of axon – Greater the diameter, faster the speed of conductance. Less leakage of ions. Temperature – Higher the temperature, faster the speed. (enzymes involved in respiration, ATP, NA+/K+ pumps) Concentration of Na+ ions – reduced conc, slows down diffusion as it reduces diffusion gradient between outside and inside.

6. Refractory Period Keeps impulse travelling in one direction Produces discrete impulses (they don’t merge into one) Limits the number of impulses (as action potentials are separated, it limits the number that can travel at one time).

7. All or Nothing Principle A certain energy level of stimulus has to be detected to cross the threshold value. If this is met, an action potential is created. If level of stimulus meets threshold level, no matter how strong the stimulus, the size of the action potential is the same.

8. How is the Size or Strength of a Stimulus is Determined by an Organism? Frequency of action potentials flowing through a neurone in a given time Having different neurones that possess different threshold levels Brain can determine number of impulses received and the type of neurone that delivered it, to determine size of stimulus.