2 Learning Objectives:How does an action potential pass along an unmyelinated axon?How does an action potential pass along a myelinated axon?What factors affect the speed of conductance of an action potential?What is the refractory period?What is meant by the “all or nothing” principle?
3 Unmyelinated Neurones Localised electrical currents are set up and the action potential is propagated along the neurone.
4 Task: Complete the cut and stick activity to show transmission in an unmyelinatedneurone.
5 Myelinated NeuronesThe axons of many neurones are encased in a fatty myelin sheath (Schwann cells).Where the sheath of one Schwann cell meets the next, the axon is unprotected.The voltage-gated sodium channels of myelinated neurons are confined to these spots (called nodes of Ranvier).Na+Sodium channelNodes of Ranvier
6 Myelinated NeuronesThe in rush of sodium ions at one node creates just enough depolarisation to reach the threshold of the next.In this way, the action potential jumps from one node to the next (1-3mm) – called saltatory propagationResults in much faster propagation of the nerve impulse than is possible in unmyelinated neurons.Na+Sodium channelNodes of Ranvier
8 Factors Affecting the Speed of an AP 1. Myelin sheath – electrical insulator – the AP jumps from one Node of Ranvier to another = SALTATORY CONDUCTION.Myelinated = 90ms-1Unmyelinated = 30ms-1
9 Factors Affecting the Speed of an AP 2. Diameter of the axon – greater diameter = faster conductance (due to less leakage).
10 Factors Affecting the Speed of an AP 3. Temperature – higher temp = faster nerve impulse (rate of diffusion is faster, enzyme activity is faster e.g. ATPase.
11 The Refractory PeriodThere is a time after depolarisation where no new AP can start – called the refractory period.Time is needed to restore the proteins of voltage sensitive ion channels to their original resting conditions.Na+ channels cannot be opened, as it can’t be depolarised again.WHY?AP travel in one direction only.Produces discrete impulses.Limits the frequency of impulses.AP can only depolarise the membrane ‘in front’ as the membrane ‘behind’ is in its refractory period and cannot be depolarised again
12 AP – All or nothingAP only happens if the stimulus reaches a threshold value.Stimulus is strong enough to cause an APIt is an ‘all or nothing event’ because once it starts, it travels to the synapse.AP is always the same sizeAn AP is the same size all the way along the axon.The transmission of the AP along the axon is the nerve impulse.
13 How do we detect the size of a stimulus? The number of impulses in a given time – the larger the stimulus, the more impulses generated.By having neurones with different threshold values – the brain interprets the number and type of neurones and therby determines its size.