Chapter 11 - Fundamentals of the Nervous System and Nervous Tissue - Review Questions - Page 426: 23

The classifying characteristic for nerve fibers is commonly fiber size or diameter, By this criterion there are three groups (classes) of fibers: Group A fibers : fibers of the largest diameters Group B fibers: fibers of intermediate diameter There are other auxiliary features that help to distinguish these types of fibers ; thee include myelination, function, and transmission speed. Absolute Refractory Period; The Absolute refractory period is the time interval immediately succeeding the firing of an axon ( generation of an action potential), when the axon cannot fire again in response to any stimulus--no matter how strong. Myelin sheath Gap.: Myelin is a whitish protein-lipoid substance that is laid down around many nerve fibers. The myelin coating is produced by oligodendrocytes in the CNS and by Schwann cells in the PNS. In either case the myelin sheath is not continuous but segmented. The gaps between the myelinated d segments are called myelin sheath gaps or Nodes of Ranvier

One usual classification of nerve fiber types is as follows: Group A Fibers Size: Largest diameter fibers Myelination: Thick myelin sheath Location/function--motor fibers; sensory fibers from skin muscles and joints Transmission speed--\approx 300 mph---saltatory conduction Group B Fibers Size: intermediate diameter Myelination--lightly myelination Transmission speed --\approx 30 mph--saltatory conduction Group C fibers Size : smallest diameter Myelination-- non-myelination--non-saltatory conduction Transmission speed--\approx 2mph Locations of Groups B and C Fibers These fibers are commonly found in the following locations: In ANS nerves ( motor fibers serving visceral organs, and as visceral sensory fibers). As smaller somatic sensory fibers that transmit impulses from the skin( pain and small touch fibers) Absolute refractory period: This is the period following the firing of a nerve fiber when no action potential can be generated. It starts immediately after the initiation of the AP and lasts until its peak The Na+ channels are inactivated during this period Saltatory Conduction Because myelin is such an efficient insulator of nerve fibers, impulses being propagated along a myelinated fiber cannot travel through the myelin internode; the impulse must jump from one node of Ranvier to the next in what is known as saltatory transmission/communication. This form of transmission is faster than the slow communication along non-myelinated fibers.