Velocity of impulse transmission is least in the nerve fiber:
C fiber is unmyelinated and it has least diameter among all the fiber types. Its conduction velocity is the slowest. For unmyelinated nerves: Conduction velocity is propoional to the square root of the diameter. Hence, faster conduction velocity is primarily based on the diameter alone. A greater diameter means less axoplasmic resistance for the flow of ions. In addition, myelin decreases the effective resistance of the membrane.
Aa
Ab
B
C
Nerve