- A mechanical pressure was exerted on a specific location of a peripheral nerve of a mammal. The touch or pain receptors were stimulated from the skin surface innervated by the same nerve. The action potential generated by touch receptor stimulation was blocked beyond the point of mechanical pressure. But the action potential generated by pain receptor stimulation passes through the point of mechanical pressure.
Following explanations were offered for these observations:
A. The large diameter ‘A’ fibres were affected by mechanical pressure
B. The small diameter ‘C’ fibres were not affected by mechanical pressure
C. The intermediate diameter ‘B’ fibres were affected by mechanical pressure
D. The large diameter ‘A’ fibres were not affected by mechanical pressure
Which of the above combinations is correct?
(1) A and B (2) B and C (3) C and D (4) A and D
When mechanical pressure is applied to a peripheral nerve, it can differentially affect the conduction of action potentials depending on the fiber type. This phenomenon explains why touch sensations may be blocked while pain sensations persist despite the same mechanical challenge.
Nerve Fiber Types and Their Sensitivity to Mechanical Pressure
Peripheral nerves contain fibers of varying diameters and myelination, grouped broadly as:
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Large diameter, heavily myelinated A fibers: These fibers include Aβ and Aα types responsible for conducting touch, pressure, and proprioception signals rapidly.
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Intermediate diameter B fibers: These are smaller myelinated fibers mainly associated with autonomic preganglionic functions.
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Small diameter, unmyelinated C fibers: Slow-conducting fibers responsible for transmitting pain and temperature signals.
Effect of Mechanical Pressure on Different Fiber Types
The sensitivity of nerve fibers to mechanical pressure correlates with their diameter and myelination:
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Large diameter A fibers are more susceptible to blockage by mechanical pressure. Their thick myelin sheaths and fast conduction velocities make them vulnerable to mechanical deformation, which disrupts their conduction.
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Small diameter C fibers are more resistant to mechanical pressure. Their lack of myelin and smaller size allow them to maintain conduction even under pressure.
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Intermediate diameter B fibers can be variably affected but generally are less implicated in sensory conduction compared to A and C fibers.
Explanation of Observations
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The action potential generated by touch receptors (carried by large diameter A fibers) is blocked beyond the mechanical pressure point due to their susceptibility.
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The action potentials generated by pain receptors (carried mainly by small diameter C fibers) pass through the pressure site relatively unhindered.
Thus:
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Statement A (“The large diameter ‘A’ fibres were affected by mechanical pressure”) is correct.
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Statement B (“The small diameter ‘C’ fibres were not affected by mechanical pressure”) is correct.
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Statement C (“The intermediate diameter ‘B’ fibres were affected by mechanical pressure”) is not typically relevant to this sensory scenario.
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Statement D (“The large diameter ‘A’ fibres were not affected by mechanical pressure”) is incorrect.
Correct Combination
The correct combination based on the above explanation is:
(1) A and B
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