Regulation and Integration of the Body
Group A ﬁbers
are mostly somatic sensory and motor ﬁbers
serving the skin, skeletal muscles, and joints. Tey have the larg-
est diameter and thick myelin sheaths, and conduct impulses at
speeds ranging up to 150 m/s (over 300 miles per hour).
Group B ﬁbers
, lightly myelinated ﬁbers of intermediate di-
ameter, transmit impulses at an average rate of 15 m/s (about
Group C ﬁbers
have the smallest diameter and are nonmye-
linated. Consequently, they are incapable of saltatory conduc-
tion and conduct impulses at a leisurely pace—1 m/s (2 mi/h)
Te B and C ﬁber groups include autonomic nervous system mo-
tor ﬁbers serving the visceral organs; visceral sensory ﬁbers; and
may account for the remarkably variable cycles of remission
(symptom-free periods) and relapse (disability) typical of this
disease. Common symptoms are visual disturbances (including
blindness), problems controlling muscles (weakness, clumsi-
ness, and ultimately paralysis), speech disturbances, and uri-
Te advent of drugs that modify the immune system’s activ-
ity (such as interferons, glatiramer, and ﬁngolimod) will con-
tinue to improve the lives of people with MS. Tese drugs seem
to hold symptoms at bay, reducing complications and disability.
Recent studies show that high blood levels of vitamin D reduce
the risk of developing MS in the ﬁrst place.
Nerve ﬁbers may be classiﬁed according to diameter, degree
of myelination, and conduction speed.
Size of voltage
(a) In bare plasma membranes, voltage decays.
Without voltage-gated channels, as on a dendrite,
voltage decays because current leaks across the
(b) In nonmyelinated axons, conduction is slow
channels regenerate the action potential at each point
along the axon, so voltage does not decay. Conduction
because it takes time for ions and for gates of
channel proteins to move, and this must occur before
voltage can be regenerated.
(c) In myelinated axons, conduction is fast (saltatory
Myelin keeps current in axons
(voltage doesn’t decay much). APs are generated
in the myelin sheath gaps and appear to jump
from gap to gap.
Action potential propagation in nonmyelinated and myelinated axons.