1. Nervous system Junior Roberts December 8, 2011

2. Objectives
List and explain the four features of action potentials in nerve fibres.
Explain the intensity of the action potential in myelinated and non-myelinated nerve fibre; mention the importance of the nodes of Ranvier. Labeled diagram of a neuron will be an asset.

3. What are action potentials
The experimental stimulation of an axon by an electrical impulse which results in a change in the potential across the axon from negative to positive is known as the action potential. It experiences a voltage change of -70mV to +40mV and appears as a spike on the CRO cathode ray tube.

4. Action potentials cont’d
This is generated by a change in the sodium ion channel. This channel and some of the potassium ion channels, are known as gated channels i.e they can be open or closed with a polypeptide chain called a gate. They are four major features of action potential .

5. Action potentials cont’d
These four are:
Propagation (conduction )
Speed of conduction
All or nothing law and coding for stimulus intensity
Effect of temperature on speed of conduction

6. A neuron

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8. Propagation
A nerve impulse is a wave of depolarization that moves along the surface of a nerve cell. Action potentials are self generated along the axon by the effect of sodium ions entering the axon. This leads to a flow of current to be set up in a local circuit between the active region and the negatively charged region ahead.

9. Propagation
This current flow reduces the membrane potential in the resting region and this depolarization causes and increase in sodium permeability which leads to an action potential being developed. Repeated depolarizations of immediately adjacent regions of the membrane results in propagated along the axon.

10. Propagation cont’d
It is theorize that action potentials posses the ability to be transmitted over long distances without losing strength. This is due to the fact that the production of action potentials at each point in the axon is a self generated event.

11. Speed of conduction
In vertebrates (organism possessing a backbone) the majority of neurons are structured with a layer of myelin sheath formed from Schwann cells. Myelin is very resistant to electricity (i.e does not conduct charge) hence it is used as an insulator by neurons.

12. Speed of conduction cont’d
The combined resistance of the axon membrane and myelin is very high, but where there are gaps in the myelin sheath as at the nodes of Ranvier conduction does occur. The type of conduction which occurs is termed saltatory since the action potential ‘jumps’ from node to node.

13. All or nothing law and coding for stimulus intensity
This states that the size of the action potential is dependent on the size of the stimulus. This therefore means that an action potential is either created or not and its size is constant.

14. All or nothing law and coding for stimulus intensity cont’d
Furthermore it would be safe to say that there is a threshold stimulus intensity above which the action potential will be triggered irrespective of the strength of the stimulus. Also the speed at which the impulse is transmitted is not affected by the strength of the impulse.

15. All or nothing law and coding for stimulus intensity cont’d
The strength of the stimulus is determined by the frequency of the action potential (the number in a given time ). The stronger the stimulus, within limits, the greater the frequency of the action potentials set up. Therefore the frequency of the action potential is proportional to intensity.

16. Effect of temperature on speed of conduction
Temperature significantly affects the rate of conduction of an action potential. This is so since when temperature rises to 4o degrees Celsius then rate of conduction of action potentials is increased.

17. Action potentials in myelinated and non-myelinated nerve fibres
As previously mentioned action potentials move by a saltatory action in which the impulse ‘jumps’ from one node to the next. Saltatory conduction greatly increases the conduction speed because the nodes of Ranvier makes it unnecessary for action potentials to travel along the cell membrane.

18. Action potentials in myelinated and non-myelinated nerve fibres
Action potential conducted in myelinated nerve fibre is like a grasshopper jumping, whereas in un-,myelinated axon it is like a grasshopper walking. Note that lightly myelinated axons conduct action potentials at the rate of about 3- 5m/s, whereas heavily myelinated nerve fibres conduct action potentials at the rate of m/s.

19. Action potentials in myelinated and non-myelinated nerve fibres
It was also founded that, several hundred times fewer ions cross the cell membrane during conduction in myelinated fibres than in un- myelinated axons. This leads to less energy required for the sodium-potassium exchange pump to maintain the ion distribution.

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