Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 4Excitable Tissue: Nerve 89

with their diameters, electrical characteristics, and functions.
By comparing the neurologic deficits produced by careful
dorsal root section and other nerve-cutting experiments with
the histologic changes in the nerves, the functions and histo-
logic characteristics of each of the families of axons responsi-
ble for the various peaks of the compound action potential
have been established. In general, the greater the diameter of a
given nerve fiber, the greater its speed of conduction. The
large axons are concerned primarily with proprioceptive sen-
sation, somatic motor function, conscious touch, and pres-
sure, while the smaller axons subserve pain and temperature
sensations and autonomic function. The dorsal root C fibers
conduct some impulses generated by touch and other cutane-
ous receptors in addition to impulses generated by pain and
temperature receptors.
Further research has shown that not all the classically
described lettered components are homogeneous, and a numer-
ical system (Ia, Ib, II, III, IV) has been used by some physiolo-
gists to classify sensory fibers. Unfortunately, this has led to
confusion. A comparison of the number system and the letter
system is shown in Table 4–2.
In addition to variations in speed of conduction and fiber
diameter, the various classes of fibers in peripheral nerves dif-
fer in their sensitivity to hypoxia and anesthetics (Table 4–3).
This fact has clinical as well as physiologic significance. Local
anesthetics depress transmission in the group C fibers before
they affect group A touch fibers. Conversely, pressure on a
nerve can cause loss of conduction in large-diameter motor,
touch, and pressure fibers while pain sensation remains rela-
tively intact. Patterns of this type are sometimes seen in indi-
viduals who sleep with their arms under their heads for long
periods, causing compression of the nerves in the arms.
Because of the association of deep sleep with alcoholic intoxi-
cation, the syndrome is most common on weekends and has

acquired the interesting name Saturday night or Sunday

morning paralysis.

NEUROTROPHINS

TROPHIC SUPPORT OF NEURONS

A number of proteins necessary for survival and growth of neu-

rons have been isolated and studied. Some of these neurotro-

phins are products of the muscles or other structures that the

neurons innervate, but others are produced by astrocytes. These

proteins bind to receptors at the endings of a neuron. They are

internalized and then transported by retrograde transport to the

neuronal cell body, where they foster the production of proteins

associated with neuronal development, growth, and survival.

Other neurotrophins are produced in neurons and transported

in an anterograde fashion to the nerve ending, where they main-

tain the integrity of the postsynaptic neuron.

TABLE 4–1 Nerve fiber types in mammalian nerve.a

Fiber Type Function

Fiber

Diameter (μm)

Conduction

Velocity (m/s)

Spike

Duration (ms)

Absolute Refractory

Period (ms)

A

α Proprioception; somatic motor 12–20 70–120

β Touch, pressure 5–12 30–70 0.4–0.5 0.4–1

γ Motor to muscle spindles 3–6 15–30

δ Pain, cold, touch 2–5 12–30

B Preganglionic autonomic <3 3–15 1.2 1.2

C

Dorsal root Pain, temperature, some mechano-reception 0.4–1.2 0.5–2 2 2

Sympathetic Postganglionic sympathetic 0.3–1.3 0.7–2.3 2 2

aA and B fibers are myelinated; C fibers are unmyelinated.

TABLE 4–2 Numerical classification

sometimes used for sensory neurons.

Number Origin

Fiber

Type

Ia Muscle spindle, annulo-spiral ending A α

Ib Golgi tendon organ A α

II Muscle spindle, flower-spray ending; touch,

pressure

A β

III Pain and cold receptors; some touch receptors A δ

IV Pain, temperature, and other receptors Dorsal

root C