Biological Membranes - A. N. Ogurtsov 2012

Electrogenesis of Biomembranes
Mechanisms of Action Potential Generation
Detection of Stimulus Intensity

A single Action Potential impulse alone contains only information about whether or not a nerve receptor is excited, but it does not reflect the intensity of the external stimulus acting on the receptor. For example, let us consider a pain receptor in a human finger (Figure 137). The Cell body of this sensory neuron is located near the Spinal Cord, its distal axon terminal is in the finger, and its proximal axon terminal transmits the pain signal to an associative (Relay) pain neuron via a synapse located in the spinal cord.

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Figure 137 - Transmission of stimulus intensity from a receptor

As long as the stimulus intensity remains below the sensitivity threshold, the depolarization of the receptor's distal axon terminal does not reach the threshold value, and no action potential is triggered. Once the stimulus intensity exceeds the sensitivity threshold and the depolarization of the distal terminal surpasses the threshold value, an avalanche-like generation of a Nerve Impulse occurs According to the "all-or-none" law; the action potential propagates along the axon and reaches the proximal terminal in the spinal cord, where it is transmitted to the pain nerve and subsequently reaches the Brain.

If the external stimulus persists, the axon membrane depolarizes once again, and the first impulse is followed by another, and so on. If the stimulus intensity is now increased, repeated membrane depolarization will occur more rapidly, meaning that subsequent action potential impulses will be triggered more frequently.

Thus, the information regarding the stimulus intensity is encoded in the frequency of nerve impulses—the higher the frequency, the stronger the external stimulus acting on the receptor.

METABOLISM/35.html">Selection/41.html">Review Questions and Exercises

1. What is an action potential that is generated and propagated in Neurons?

2. What four functionally distinct parts can be distinguished in a neuron?

3. In which part of the neuron is the action potential generated, which then propagates along the neuron's axon?

4. Which voltage-Gated Ion Channels are opened in the neuron's axon terminal by the action potential arriving from the axon?

5. What membrane process is triggered by the rise in calcium ion concentration within the presynaptic terminal?

6. What are the molecules called that enter the synaptic cleft As a result of synaptic vesicles fusing with the axon membrane?

7. What is the function of the neuron's dendrites?

8. Which membrane channels are open in the resting state?

9. Which membrane channels open upon membrane depolarization?

10. Which membrane channels open during membrane repolarization?

11. Describe the operational cycle of the voltage-gated sodium channel.

12. What are the Similarities and differences in the functioning of voltage-gated sodium and potassium channels?

13. What serves as the voltage sensor in ion channels?

14. How does an action potential propagate?

15. What are the local currents generated during the propagation of an action potential?

16. Why does a nerve impulse propagate along the axon in only one direction?

17. Describe the molecular Structure OF THE voltage-gated sodium channel.

18. Describe the molecular structure of the voltage-gated potassium channel.

19. Describe the molecular structure of the voltage-gated calcium channel.

20. What is the function of the S5 and S6 a-helices in ion channels?

21. What is the function of P-segments in ion channels?

22. What is the function of blocking segments in ion channels?

23. What is the purpose of myelin sheaths in mammalian neurons?

24. By how many times does myelination increase the propagation velocity of impulses along an axon?

25. What is the structural Nature of the neuronal myelin sheath?

26. What do Schwann glial Cells synthesize?

27. WHAT IS A Node of Ranvier?

28. What are the KEY FEATURES OF saltatory action potential propagation?

29. How does the axonal membrane at the Nodes of Ranvier differ from the axonal membrane beneath the myelin sheath?

30. How is information regarding receptor excitation intensity transmitted along axons?