Pain signals often have two components: an acute sensation of "first pain" which is felt almost immediately after an injury, and a sensation of "second pain" which is felt slightly later after the injury. This suggests that neurons that send the signals that we perceive as first pain ___, and neurons that send the signals that we perceive as second pain ___.

This suggests that neurons that send the signals that we perceive as first pain Myelinated Tpye A-delta___, and neurons that send the signals that we perceive as second pain Unmyelinated-Type C.___.

Explanation:

Physiologically, primary afferent neurons that innervated the the touch, pain, temperature, and pressure receptors in the effectors to input sensory signals to the spinal cord and the brain are categorized based on their diameter, sizes, and nature of myelination. Based on this;

A-alpha has the largest diameter, myelinated, and sensitive to sensation from the muscle cells

A-beta are next in diameter, myelinated and sensitive to sensation of touch.

A-delta followed in diameter, and size. They are mylinated, and sensitive to pain and temperature.

C-fibres are least in diameter, they are unmyelinated, but sensitive to pain, touch and temperature.

Therefore, if the speed of conduction, of action potential in a neuron depends on the diameter (thickness) and myelination of the axon (saltatory conduction):

''then, acute sensation / first pain should be transmitted by the A-delta fibre. While the the slower/second pain should be transmitted by the slower unmyelinated - C-fibres''.