What happens during an action potential?

1. What happens during an action potential?

2. 5.1.3 Neuronal Communication Myelinated Neurones

3. How does the axon change?

4. Local Currents Opening the sodium ion channels during the action potential obviously changes the resting potential on the membrane. The sodium ions entering the cytoplasm causes sodium ion channels further along the membrane to open and depolarise (positive feedback). This is called a local current. http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter14/animation__the_nerve_impulse.html Use figure 1 page 52 to describe how local currents are formed

5. Learning Objectives Success Criteria • To understand what affects the transmissions of action potentials • Describe the structure of a myelinated neurone (Grade E - D) • Describe and explain how an action potential is transmitted in a myelinated neurone, with reference to the roles of voltage gated sodium ion and potassium ion channels (Grade C –B) • Explain factors that might affect transmission speed of APs (Grade B – A)

6. The all or nothing principle A stimulus has to be above a certain threshold value if it is to cause an action potential. Below the threshold value no action potential occurs and therefore no impulse is generated. ‘ALL’ – It doesn’t matter how much above the threshold value the stimulus is it will still only generate one action potential – Each one produces a depolarisation of -40mV. ‘NOTHING’ – Any stimulus of any strength below the threshold value will not generate an action potential. . 40 0 failed initiations potential difference (mV) thresholdvalue –70 0 1 2 3 4 5 time (ms) Once the threshold value is reached, the action potential generated is always the same size regardless of the strength of the stimulus. It is an ‘all or nothing’ response.

7. Different strengths of stimuli

8. Frequency of transmission All action potentials are the same intensity (all or nothing), but we can still detect stimuli of different intensities, such as loud or quiet sounds. Our brain determines this by the frequency of action potentials. A higher frequency of action potentials mean a more intense stimulus. When a stimulus is at a higher intensity more sodium channels are opened in the sensory receptor, producing more generator potentials. More frequent action potentials enter the CNS.

9. The axon after an action potential

10. The Refractory period • The transmission of the action potential along the axon of a neurone is the nerve impulse. • The Refractory period is the period in which the membrane of the axon of a neurone can’t be depolarised and no action potentials can be initiated (no inward movement of Na ions, the channel is closed) What are the reasons for the refractory period? • 1) Action potential will only pass in one direction – passing from an active region to a resting region. • 2)Discrete impulses – Action potentials can’t be formed immediately after the first one, s they are spread out • 3) Limits the number of action potentials

11. What affects the speed of an impulse? An action potential moves faster along myelinated neurones than those without a myelin sheath. Ion channels present at the nodes of Ranvier allow the movement of sodium and potassium ions across the membrane. It is at these points that an action potential can be generated in a myelinated neurone. The action potential moves from node to node.This is referred to as saltatory conduction and is much quicker than the step-by-step conduction that occurs in a non-myelinated neurone.

12. Myelinated axon There is a lipid-rich insulin layer surrounding the axon called a myelin sheath. There are small gaps of up to 1.5mm between the Schwann cells called nodes of Ranvier. Depolarisation only happens at nodes of Ranvier, impulse jumps from node to node. This is called saltatory conduction. Describe what saltatory conduction is and the advantages of it.

13. Speed of a nerve impulse 1)Looking at the table what affects the speed of transmission of an axon potential, and how? 2) Are there any other factors that might affect transmission speed? 3)What is the name of the cells whose membranes make up the myelin sheath? 4)Which has the greatest effect on speed conductance, myelin or axon diameter, use the table to explain. 5)Squid is an ectothermic animal, how would this affect the speed of action potentials?

14. Answers – Peer mark • 1) Myelin sheath acts as an electrical insulator , APs can only form where there is no myelin, at nodes of Ranvier, so it jumps from one node to the next (saltatory conduction) making conductance faster. Greater diameter of axon the faster conductance because there is less leakage of ions from the axon, making it easier to maintain • 2) Temperature – Higher temperature the faster the impulse, as there is an increase in rate of diffusion of ions. The energy for active transport comes form respiration, like the sodium-potassium pump is controlled by enzymes. • 3)Schwann cells • 4) Myelin has a greater effect as a myelinated human sensory axon conducts twice as fast as the squid giant axon despite only being a fifth of its diameter • 5) The concordance of AP s in the squid changes as the environment al temperature changes, so it will react slowly at low temperatures.

16. Plenary • Draw a table to summarise the differences between myelinated and non myelinated neurones

17. Figure 5.2 Figure 5.1

18. Learning Objectives Success Criteria • To understand what affects the transmissions of action potentials • Describe the structure of a myelinated neurone (Grade E - D) • Describe and explain how an action potential is transmitted in a myelinated neurone, with reference to the roles of voltage gated sodium ion and potassium ion channels (Grade C –B) • Explain factors that might affect transmission speed of APs (Grade B – A)