Nerve fiber-Types and Function
In an organism there are different types of nerve fibers.The difference can be in function, diameter, conduction velocity etc..It was Erlanger and Gasser who studied mammalian nerve fibers and classified it into various types.They observed and compared the neurologic deficits produced by careful section of nerve fibers and other nerve-cutting experiment.Based on their experiments, they classified nerve fibers into A, B, and C groups.The A group is again subdivided into A alpha, A beta, A gamma and A delta fibers.C group also has got subdivisions called dorsal root fiber and sympathetic fiber.
A, B, and C are specific type of fibers in mammals.But in general we can make conclusions based on some features.The speed of conduction is directly related to the diameter of the nerve fiber.Larger the diameter, greater is the speed of conduction.Also large axons are primarily concerned with the proprioceptive sensation, somatic motor function, conscious touch and pressure, while the smaller axons are concerned with pain and temperature sensations and autonomic functions.Now we can see each nerve fiber type in detail.
The A and B fibers are myelinated axons.A alpha fibers has a diameter of 12-20 micro meter with a conduction velocity of 70-120 m/s.The main function is proprioception and somatic motor.A beta carry out the function of sensation of touch, pressure and also the motor function.It has diameter of 5-12 micro meter and conduction velocity of 30-70 m/s.A gamma on other hand has 3-6 micro meter fiber diameter with conduction velocity 15-30 m/s.The main function of A gamma is the motor impulse to muscle spindles.The last subdivision of A fiber is A delta which carry out the function of sensation of pain, cold and touch.It has 2-5 micro meter diameter and 12-30 m/s conduction velocity.Also all the A type fibers has a spike duration of 0.4-0.5 ms and absolute refractory period of 0.4-1 ms.
The second type is B type and it has no subdivisions.They are mainly preganglionic autonomic fibers with a fiber diameter less than 3 micro meter.The conduction velocity is about 3-15 m/s.The spike duration and absolute refractory period is 1.2 ms.The third and the last type is C fibers.They are unmyelinated axons.They are subdivided into dorsal root and sympathetic fibers.The dorsal root carry many important sensations like pain, temperature,some mechanoreception and reflex responses.It has a fiber diameter of 0.4-1.2 micro meter and conduction velocity of 0.5-2 m/s.The sympathetic fibers carry post-ganglionic sympathetics and it has a fiber diameter of 0.3-1.3 micro meter and conduction velocity of 0.7-2.3 m/s.The spike duration and absolute refractory period of all the C type fibers are 2 ms.So this is how the basic type of nerve fibers differ.
In addition to the basic features written above, there are some other differences between the nerve fibers.For example, sensitivity to hypoxia and anesthetics.When local anesthetics are given, it depress the transmission in group C fibers before they affect the touch fibers in the group A.Also in another condition, when there is pressure on the nerve, it can cause loss of conduction in large diameter motor, pressure and touch fibers, while pain sensation remain relatively intact.To study this we can use various agents to produce conduction block and find out the relative susceptibility of nerve fibers A, B, and C.It is found that B group is most susceptible to hypoxia and C group is least susceptible.An intermediate effect is for A group.In case of local anesthetics the most susceptible is C group followed by B and then A group.In case of pressure the order of susceptibility from highest to lowest is ABC.One syndrome caused by pressure in nerve can be seen in individual who sleeps with arm under the head for a long time because the nerve is compressed.Since such a position is common in deep sleep after taking too much alcohol,the syndrome is common on weekends and thus it is called as Saturday night or Sunday morning paralysis.
So this is how the nerve fibers are basically classified studied.But further research has come forward with a new type of classification based on numerical system.The fibers are classified as Ia, Ib, II, III and IV.This system also is used by some physiologist and it is mainly used for sensory neurons.The Ia has origin of sensation from muscle spindle and annulo-spiral endings.The Ib has origin from Golgi tendon organ.Both Ia and Ib is compared to A alpha fiber type.The number II has origin from muscle spindle, flower-spray ending, touch and pressure receptor.It is similar to fiber type A beta.The number III is similar to A delta which has the origin of sensation from cold and pain receptors and some touch receptors.Also the number IV has origin from pain, temperature and other receptors and the fibers are similar to dorsal root of C group.Thus using both the classification different type of nerve fibers can be studied in detail.