The human nervous system uses specific types of nerve fibers to transmit different sensory modalities such as touch and pain. Understanding these fiber types helps explain clinical phenomena such as why external pressure on a nerve may cause loss of touch but preserve pain, whereas local anesthetics may have the opposite effect.

Nerve Fiber Types and Their Sensory Roles

• Type A fibers are myelinated and subdivided into four classes: Aα, Aβ, Aγ, and Aδ.

  • Aβ fibers (Type II) are large diameter fibers primarily responsible for carrying light touch and pressure sensations.

  • Aδ fibers (Type III) are smaller myelinated fibers that carry sharp, well-localized pain and cold temperature sensations.

  • Aα and Aγ fibers are mainly involved in proprioception and motor functions.

• Type C fibers are unmyelinated, small-diameter fibers that conduct impulses slowly. They are responsible for carrying dull, aching, or burning pain sensations and some temperature sensations.

Explanation of the Observations in the Question

• External pressure on a mixed nerve causes loss of touch sensation while pain sensation remains intact.
  Large myelinated fibers (Type Aβ), which carry touch impulses, are more susceptible to mechanical compression and thus lost first, resulting in loss of touch. Smaller fibers like Aδ and unmyelinated C fibers, which carry pain, tend to be more resistant to pressure and continue conducting pain signals.

• Local anesthetics cause loss of pain sensation while sparing touch sensation to some extent.
  Local anesthetics preferentially block small diameter fibers (like Aδ and C fibers) responsible for pain transmission while sparing the large diameter fibers responsible for touch, which are less sensitive to the anesthetic at low concentrations.

Evaluation of the Statements

• A. External pressure causes loss of conduction of impulses in small diameter sensory nerve fibres.
  This is incorrect. It is the large diameter myelinated fibers carrying touch that are more affected by pressure, not small diameter fibers.

• B. Local anesthetics depress the conduction of impulses in large diameter sensory nerve fibres.
  This is incorrect. Local anesthetics preferentially block small fibers (pain) before large fibers (touch).

• C. Touch-induced impulses are carried by fibre Type A.
  This is correct. Touch impulses primarily travel via large, myelinated Type Aβ fibers.

• D. Fibre type C is responsible for pain sensation.
  This is correct. Small unmyelinated C fibers transmit dull, aching pain.

Which Statements Are INCORRECT?

• Statements A and B are incorrect.

Final Answer:

The INCORRECT statements are:

(1) A and B