406
UNIT 3
Regulation and Integration of the Body
11
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
30 mi/h).
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)
or less.
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-
nary incontinence.
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
Voltage-gated
ion channel
Stimulus
Myelin
sheath
Stimulus
Stimulus
Myelin
sheath gap
Myelin
sheath
(a) In bare plasma membranes, voltage decays.
Without voltage-gated channels, as on a dendrite,
voltage decays because current leaks across the
membrane.
(b) In nonmyelinated axons, conduction is slow
(continuous conduction).
Voltage-gated Na
+
and K
+
channels regenerate the action potential at each point
along the axon, so voltage does not decay. Conduction
is
slow
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
conduction).
Myelin keeps current in axons
(voltage doesn’t decay much). APs are generated
only
in the myelin sheath gaps and appear to jump
rapidly
from gap to gap.
1 mm
Figure 11.15
Action potential propagation in nonmyelinated and myelinated axons.
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