2 Speed of the nerve impulse Aim: To recap work done last lesson!
3 Nerve impulses – key facts A nerve impulse is the movement of an action potentialNerve impulses are always a fixed size and speed in a given neuroneSpeed 0.5ms-1 – 120ms-1
4 Factors affecting speed of action potential Presence of myelin sheathDiameter:Temperature:
5 Presence of myelin sheath Impulse faster with in a myelinated axon than a unmyelinated axonWhat term is used to describe conduction in a myelinated axon?saltatory conduction!
6 Diameter Larger diameter, faster impulse Why? Larger diameter relatively less leakage of ions therefore membrane potentials easier to maintain.Narrower = greater resistanceIn thinner axons the localised circuits form in a much smaller area therefore conduction is slower.
7 TemperatureThe higher the temperature the faster the impulse … Up to a point!Why?Higher temperature gives a faster rate of diffusionEnergy for active transport from respiration, which is dependant on enzymes therefore higher temp faster rate of reaction.Too high a temp … enzymes and plasma membrane proteins denature … No more impulses!
8 Refractory periodAfter impulse sodium voltage gated channels remain closed for a period of time.Therefore its impossible to generate another action potential
9 Purpose of refractory period Only propagated in one direction. The action potential cannot be propagated in a region that is refractory so it can only move forwardsEnsures action potentials separated. This limits the number of action potentials in a given time
11 Extra info Actually there are 2 refractory periods! Absolute refractory period – Lasts 1ms and no impulses can passRelative refractory period – Lasts 5ms. New impulse only if the stimulus is more intense than the normal threshold
12 All or nothing principle Impulse only triggered at a certain level of stimulus, this is called the threshold valueThe impulse is always the same size and speed
13 How do organisms determine the size of the stimulus? Larger stimulus: More impulses in a given timeDifferent Neurones with different threshold values