1979
The Journal of Experimental Biology 202, 1979–1989 (1999)
Printed in Great Britain © The Company of Biologists Limited 1999
JEB2168
REVIEW
FENESTRATION NODES AND THE WIDE SUBMYELINIC SPACE FORM THE BASIS
FOR THE UNUSUALLY FAST IMPULSE CONDUCTION OF SHRIMP MYELINATED
AXONS
KE XU (KE HSU)1,* AND SUSUMU TERAKAWA2
1Shanghai Institute of Physiology, Chinese Academy of Sciences, Shanghai, China and 2Photon Medical Research
Center, Hamamatsu University School of Medicine, Hamamatsu, Japan
*Present address: Department of Molecular Neuropharmacology, Shanghai Institute of Physiology, Chinese Academy of Sciences,
320 Yue-Yang Road, Shanghai 200031, China (e-mail: [email protected])
Accepted 27 April; published on WWW 7 July 1999
Summary
Saltatory impulse conduction in invertebrates is rare and
has only been found in a few giant nerve fibres, such as the
pairs of medial giant fibres with a compact multilayered
myelin sheath found in shrimps (Penaeus chinensis and
Penaeus japonicus) and the median giant fibre with a loose
multilayered myelin sheath found in the earthworm
Lumbricus terrestris. Small regions of these nerve fibres are
not covered by a myelin sheath and serve as functional
nodes for saltatory conduction. Remarkably, shrimp giant
nerve fibres have conduction speeds of more than 200 m s−1,
making them among the fastest-conducting fibres
recorded, even when compared with vertebrate myelinated
fibres. A common nodal structure for saltatory conduction
has recently been found in the myelinated nerve fibres of
the nervous systems of at least six species of Penaeus
shrimp, including P. chinensis and P. japonicus. This novel
node consists of fenestrated openings that are regularly
spaced in the myelin sheath and are designated as
fenestration nodes. The myelinated nerve fibres of the
Penaeus shrimp also speed impulse conduction by
broadening the gap between the axon and the myelin
sheath rather than by enlarging the axon diameter as in
other invertebrates. In this review, we document and
discuss some of the structural and functional
characteristics of the myelinated nerve fibres of Penaeus
shrimp: (1) the fenestration node, which enables saltatory
conduction, (2) a new type of compact multilayered myelin
sheath, (3) the unique microtubular sheath that tightly
surrounds the axon, (4) the extraordinarily wide space
present between the microtubular sheath and the myelin
sheath and (5) the main factors contributing to the fastest
impulse conduction velocity so far recorded in the Animal
Kingdom.
Key words: invertebrate, Penaeus, shrimp, myelin sheath, nodal
structure, saltatory conduction, nodal current, microtubular sheath,
submyelinic gap space.
Introduction
The great majority of nerve axons in various species are
surrounded by a sheath of glial cells. The complexity of this
glial sheath varies from a single layer to loose folds, and
culminates in the formation of a compact multilayered sleeve
of glial membranes known as the myelin sheath. Nerve axons
surrounded by such a sheath are usually referred to as
myelinated fibres. These have been found in all vertebrates
with the exception of lamprey and hagfish (Bullock et al.,
1984) and are considered to be exclusive to vertebrates,
whereas all the other varieties of ensheathment occur in both
vertebrates and invertebrates.
The myelin sheath covering the axon in vertebrates acts as
an electrical insulator of high resistance and low capacitance.
It is interrupted regularly by ‘nodes of Ranvier’, which are
unmyelinated gaps 1–1.5 µm wide in the peripheral nervous
system (PNS) and 1–10 µm or wider in the central nervous
system (CNS) (see Roots, 1995). Voltage-gated Na+ and K+
channels are distributed unevenly on the axolemma: the nodal
membrane is rich in Na+ channels but poor in K+ channels,
whereas the internodal membrane is rich in K+ channels and
has relatively few Na+ channels. Thus, the myelin sheath and
node of Ranvier form the structural basis for the saltatory
conduction of a nerve impulse from node to node. Myelination
in vertebrates is considered as one of two strategies developed
during evolution to increase impulse conduction velocity, the
other being the enlargement of axon diameter (see Bullock et
al., 1977; Schmidt-Nielsen, 1990; Kandel et al., 1995).
A sheath of multilayered membranes of glial cells devoid of
cytoplasm has been reported in two species of earthworm,
Eisenia foetida (Hama, 1959) and Lumbricus terrestris
1980
K. XU AND S. TERAKAWA
(Günther, 1973), in the tubificid worm Branchiura sowerbyi
(Zoran et al., 1988), in two species of prawn, Macrobrachium
niponensis (Yeh and Huang, 1962) and Palaemonetes vulgaris
(Heuser and Doggenweiler, 1966), in the crab Cancer irroratus
(McAlear et al., 1958) and in two species of shrimp, Penaeus
orientalis (Huang et al., 1963) and P. japonicus (Kusano, 1966;
Hama, 1966). However, among these species of invertebrate,
saltatory conduction has only been demonstrated in pairs of
medial giant fibres in P. orientalis (Hsu et al., 1964, 1975) and
P. japonicus (Kusano and LaVail, 1971; Terakawa and Hsu,
1991) and in the median giant fibre of L. terrestris (Günther,
1976). Moreover, in the shrimp, both an unmyelinated branch
of the giant fibres in each thoracic and abdominal ganglion and
synapses between the medial giant fibre and every motor giant
fibre serve as the functional nodes for saltatory conduction
(Hsu et al., 1964, 1975; Kusano and LaVail, 1971; Terakawa
and Hsu, 1991). In the earthworm, a few spot openings in the
myelin sheath of the single median giant fibre serve as
functional nodes in addition to several unmyelinated branches
(Günther, 1976). To date, no common mode of saltatory
conduction and no common structure similar to the node of
Ranvier have been demonstrated in invertebrate nervous
systems.
Recently, a novel common node for saltatory conduction,
termed a ‘fenestration node’, has been found in myelinated
fibres in the nervous systems of at least six species of shrimps
from the Family Penaeus (Xu and Terakawa, 1993; Hsu and
Terakawa, 1996). In addition, the myelinated nerve fibres of
all six species of shrimp speed impulse conduction by
broadening the gap between the axon and the myelin sheath
rather than by enlarging the axon diameter as in other
invertebrates. In this review, the fenestration node and some
other functional and structural characteristics of the myelinated
fibres of Penaeus shrimps are described and discussed with
reference to their vertebrate counterparts.
Anatomy of the ventral nerve cord of Penaeus shrimp
The novel fenestration node has been demonstrated in the
following six species of the Family Penaeus (Hsu and
Terakawa, 1996): (1) genus Penaeus, P. orientalis Kishinouye
(P. chinensis Osbeck, more specifically), P. japonicus Bate, P.
monodon Fabricius, P. semisulcatus De Haan; (2) genus
Trachypenaeus, T. curvirostris; (3) genus Metapenaeopsis, M.
barbata.
The term Penaeus is used in this review to indicate these six
species. Nerve fibres in several other species of the genus
Penaeus, such as P. aztecus (brown shrimp), P. duorarum
(pink shrimp) and P. setiferus (white shrimp), which are found
in the Gulf of Mexico, generally share the same myelin
characteristics and have a submyelinic space like that in the
species mentioned above (Kusano and LaVail, 1971). It is
likely that these shrimps also have fenestrated nodes in their
myelinated fibres. Thus, these three species can be included in
the group of Penaeus shrimps and are so considered in this
review. Moreover, we expect most, if not all, decapod
Head ganglion
Suboesophageal
ganglion
Thoracic
ganglia
First abdominal
ganglion
First root
Second root
Third root
Fenestration
nodes
Medial giant
fibres
Lateral giant
fibres
Unmyelinated
branch
Motor giant
fibre
Telson
ganglia
Synapse
Fig. 1. Anatomical diagram of the ventral nerve cord of Penaeus
chinensis (taken from Fan et al., 1961) with three enlarged drawings
to show the arrangement of the fenestration nodes (open circles), the
giant synapses (filled circles) and the unmyelinated intraganglionic
branches of the medial and lateral giant fibres.
crustaceans of the Family Penaeus to share common
characteristics in their myelinated nerve fibres, including the
presence of fenestration nodes.
The CNS of the Penaeus shrimp is located close to the
ventral surface of the body along the median line and is termed
the ventral nerve cord. The anatomical structure of the ventral
nerve cord of P. orientalis is shown schematically in Fig. 1. It
consists of a head ganglion, a suboesophageal ganglion, five
thoracic ganglia, five abdominal ganglia and a telson ganglion.
A pair of circumoesophageal connectives links the head and
the suboesophageal ganglia, and connectives lie between other
adjacent ganglia. Two pairs of nerve branches (the first and
second roots) originate directly from each thoracic and
abdominal ganglion, and a pair of nerve branches (the third
root) leaves the ventral nerve cord from the anterior part of
every abdominal connective.
Neurons are located on the abdominal side of the thoracic
and abdominal ganglia, and two pairs of giant nerve fibres pass
through their dorsal region. The connectives contain nerve
fibres and a large blood vessel. A cross section of the ventral
nerve cord of P. chinensis at the level of an abdominal
connective between the second and third roots is shown in
Fig. 2A. Among numerous small- and middle-sized nerve
fibres, there are five giant fibres: two pairs of giant fibres
designated as medial and lateral giant fibres towards the dorsal
side of the connectives, and a motor giant fibre running along
the ventral nerve cord dorsal to the mid-line.
Conduction velocities greater than 200 m s−1 have been
Fenestration nodes in shrimp myelinated fibres
1981
V
MF
M
LF
GMF
A
B
Fig. 2. Shrimp nerve fibres observed by conventional light microscopy. (A) A cross section of the ventral nerve cord at a connective in Penaeus
chinensis (osmium-stained preparation, taken from Huang et al., 1963). V, blood vessel; MF, medial giant fibre; LF, lateral giant fibre; M, a
bundle of middle-sized motor fibres; GMF, motor giant fibre. An arrow indicates the axon of the medial giant fibre. (B) Two unfixed medial
giant fibres isolated from the ventral nerve cord of Penaeus chinensis. Part of the myelin sheath was cut away from the upper fibre to expose its
axon (arrow). Scale bar, 100 µm (A,B).
measured in the medial giant fibres of P. chinensis and P.
japonicus (Fan et al., 1961; Kusano, 1966). Giant fibre systems
occur in the ventral nerve cord of most crustaceans that can
swim by flapping their abdomen. They are considered to
mediate rapid escape responses (Bullock, 1984).
The myelin sheath of Penaeus nerve fibres
In vertebrates, myelin sheaths surrounding peripheral nerve
axons are formed by Schwann cells and those around central
nerve axons are formed by oligodendrocytes, although in both
cases their compacted laminae are arranged in a similar way.
For invertebrates, the term CNS is used to describe
aggregations of nervous tissue such as ganglia, i.e. the term
refers to the gross neuronal organization (Hildebrand et al.,
1993). Oligodendrocytes, which form the myelin sheath of
nerve fibres in the CNS of vertebrates, have not been found in
invertebrates (see Bunge, 1968), and the distinction between
glial cell types in the CNS and the PNS, typical of vertebrate
nervous systems, may not apply to invertebrates (Hildebrand
et al., 1993). In this review, all the glial cells myelinating and
ensheathing axons in the connectives are referred to as
Schwann cells.
As in vertebrates, the myelin sheath of living nerve fibres of
P. chinensis shows distinct birefringence when observed with
a polarized light microscope. This property was used to make
measurements of the relative thickness of the myelin sheath in
110 freshly dissected nerve fibres ranging in diameter from 10
to 230 µm. The ratio of internal diameter to external diameter
of the myelin sheaths was constant at 0.69±0.10 (mean ±
S.E.M.) (Hao and Hsu, 1965). A ratio of 0.7 is considered to be
optimal for impulse conduction in vertebrate myelinated nerve
fibres (see Hodgkin, 1964).
At the turn of the century, reports of light microscopy studies
suggested that many nerve fibres in decapods such as shrimps,
prawns and crabs possessed a sheath that resembled the myelin
sheath of vertebrates (see Holmes, 1942). Electron microscopy
later showed that the myelin sheath of the vertebrate nerve
fibres has multilayered laminae that repeat concentrically and
regularly with a period of approximately 11 nm. Electron
micrographs of the fibres of two prawn species,
Macrobrachium niponensis (Yeh and Huang, 1962) and
Palaemonetes vulgaris (Heuser and Doggenweiler, 1966),
showed that their sheaths differ from those of vertebrates. In
particular, the period of lamination in the sheath of the prawns
is more than 20 nm, which is much wider than that in
vertebrates. However, Huang et al. (1963) found using electron
microscopy that most of the larger nerve fibres greater than
5 µm in diameter in the ventral nerve cord of P. chinensis
possess a compact multilayered myelin sheath. Later, similar
results were also obtained from P. japonicus (Kusano, 1966).
The period of lamination in these sheaths was 8 nm for P.
chinensis and 9 nm for P. japonicus.
The segment of myelin sheath between two neighbouring
nodes in vertebrates is formed by closely apposed sheets of
Schwann cell plasma membrane which form a spiral wrapping
around the axon. The spiral is reported to be formed by a
progressive advance of the inner lip of the Schwann cell over
the axon surface rather than by an advance of the outer lip of
the spiralling Schwann cell cytoplasm over its outer surface
(Bunge et al., 1989). The spiral of laminated membranes starts
from the internal mesaxon, where the outer faces of the plasma
membranes come together to form an interperiod line, and ends
outside the sheath at the outer mesaxon. The major dense line
in the lamination is formed by apposition of the cytoplasmic
faces of the membrane. It has been established that both the
1982
K. XU AND S. TERAKAWA
Fig. 3. Electron micrographs showing the fine structure of
myelinated fibres of Penaeus chinensis in cross section. (A) A
middle-sized axon (a) surrounded in turn by the microtubular sheath
(ms), the submyelinic space (s) and the myelin sheath (m). In this
sheath, there are two regions where the enlarged tips of the laminae
form a seam (arrows). An attachment zone (arrowhead) appears as a
dense line. *, nucleus of a myelinating Schwann cell; **, nucleus of
a Schwann cell forming the microtubular sheath. (B) A developing
nerve fibre from an immature shrimp (body length 5 mm). In this
fibre, the submyelinic space and microtubular sheath have yet to be
formed. A well-developed myelin sheath with a single seam line
adjacent to an attachment zone can be seen (arrow). (C) A higherpower view of part of the microtubular sheath in C. Scale bars,
10 µm (A), 1 µm (B) and 0.1 µm (C).
major dense line and the interperiod line in the myelin sheath
of P. chinensis (Huang et al., 1963) and P. japonicus (Kusano,
1966) are also formed by apposition of the cytoplasmic and
outer faces of the Schwann cell membrane. However, there is
a clear difference in the fine structure of the myelin sheath
between shrimps and vertebrates: the compact laminae of the
shrimp sheath are not formed by a spiral wrapping of glial cell
membrane. Instead, a number of laminae extend from a single
Schwann cell to enwrap the axon from both sides, leaving a
wide gap between the laminated sheath and the axon. Each
lamina completely or partially encircles the axon just once by
extending its ends from both sides and by connecting them
each with other or with those of laminae from other Schwann
cells to form a seam (Xu and Sung, 1980; Xu et al., 1994). In
the seam region, the slightly swollen tips of the laminae form
terminal loops, which contain a few microtubules. The
junctions between the two tips of each lamina are arranged
more or less regularly along a line radial to the axon
(Fig. 3A,B, arrows). These junctions are frequently associated
with an attachment zone (see Fig. 3A, arrowhead), which
closely links the adjacent laminae and thus reinforces the
sheath. Usually, only one seam line can be seen in a cross
section of small-sized myelinated fibres, while two to three
seam lines can be observed in cross sections of larger fibres,
indicating that each stack of laminae originates from two or
three Schwann cells. The seam line can also be observed in
longitudinal sections of the fibres, showing that the laminae
from adjacent Schwann cells are connected by the terminal
loop structure. This tip-to-tip connection of the terminal loops
to form a seam may provide some flexibility to the compact
multilayered myelin sheath.
Another difference between the myelin sheath of vertebrates
and that of the Penaeus shrimp is that in vertebrates the
Schwann cell nucleus is located on the outer edge of a segment
of the sheath, while in the shrimp the nucleus is randomly
located between the sheath laminae (Fig. 3A). The differences
in fine structure between the myelin sheaths of vertebrates and
of the shrimp are shown schematically in Fig. 4.
From these observations in Penaeus shrimp, it is evident that
a compact multilayered myelin sheath has evolved in the
nervous system of at least some invertebrates, although the
sheath is formed in a different manner from that of vertebrates.
The fine structure of the Penaeus shrimp myelin sheath is,
however, also quite distinct from that of the myelin enveloping
the giant axon of earthworms, where there is a mixture of
uncompacted, compacted and spirally arranged myelin
lamellae (Hama, 1959; Günther, 1976; Zoran et al., 1988).
Two structures unique to the myelinated fibres of the
Penaeus shrimp
A distinctive feature of Penaeus shrimp myelinated fibres is
that the axon diameter is much smaller than the fibre diameter.
This unusual relationship between the axon and the myelin
sheath is depicted in Fig. 2B. In a pair of medial giant fibres
dissected from the ventral nerve cord of P. chinensis, one fibre
(the lower one) was kept intact, whereas the other (the upper
one) was partly desheathed to expose the axon. A thin straight
axon is visible in both giant fibres (Fig. 2B). The same
structural characteristic is also evident in the middle- and
small-sized myelinated fibres of the shrimp (Fig. 2A). In most
cases, the axon is as little as one-eighth to one-tenth of the
diameter of the host fibre. Although some motor fibres have
thicker axons, the axon:fibre diameter ratio is less than 1:4
(Fig. 2A; GMF and M). Two structures unique to the Penaeus
shrimp occupy the space between the axon and the myelin
sheath: a microtubular sheath and a wide gap filled with a gel.
The microtubular sheath
The axolemma of the myelinated fibres of P. chinensis (Yeh
et al., 1963; Hsu et al., 1980) and P. japonicus (Hama, 1966)
Fenestration nodes in shrimp myelinated fibres
1983
Fig. 4. Schematic drawings to show the A
B
structure of the myelin sheath in vertebrates
Penaeus shrimp
Vertebrates
Nucleus of
(A) and in Penaeus shrimp (B). (A) The
Schwann cell
myelin sheath of vertebrates is tightly wrapped
Major dense line
around the axon and forms a continuous spiral
of membrane, with the nucleus of the Schwann
Attachment
zone
cell on the outer edge of the sheath. (B) The
myelin sheath of the shrimp is separated from
Interperiod
the axon by the submyelinic space. Seams are
line
formed in the sheath in the regions where the
Terminal
loop
tips of the laminae (terminal loops) meet. The
nucleus of the Schwann cell is located within
Axon
the sheath. The dotted line shows the
Submyelinic
axoplasmic face of the Schwann cell
space
membrane and the thick solid line shows the
Microtubular
major dense line, which is considered to be
sheath
formed by the close apposition of two
axoplasmic faces. The thin solid line shows both the outer face of the Schwann cell membrane and the interperiod line, which is considered to
be formed by apposition of two adjacent outer faces of the Schwann cell membrane.
is closely surrounded by the microtubule-rich processes of a
distinct type of Schwann cell. The innermost process is
separated from the axon by a gap of just 20 nm. The
microtubules in the processes of this Schwann cell are
assembled in bundles, which lie more or less parallel to the
longitudinal axis of the axon. Each process, flat in the radial
direction and elongated longitudinally, is stacked up to other
similar processes. This stack of cell processes forms an
irregular spiral layer that encloses the axon and forms a unique
wall around the axon designated the microtubular sheath
(Fig. 3A,C). The nucleus of this Schwann cell has a random
location along the sheath (Fig. 3A). The microtubular sheath
is usually very thin but, in general, the larger the fibre size the
thicker is the sheath. Thus, myelinated nerve fibres of the
Penaeus shrimp have three subtypes of Schwann cell, one
forming myelin, another ensheathing without myelinating and
a third forming the microtubular sheath. The electrical
resistance of the microtubular sheath should be low, since there
is little difference in the amplitudes of action potentials
recorded from the axon and from the submyelinic space
(Fig. 5). The most likely function for the microtubular sheath
is as a mechanical support for the axon.
and microtubular sheath is located asymmetrically in the
submyelinic space and tends not to contact the myelin sheath.
The electrolyte composition of the submyelinic space is
similar to that of sea water (K. Xu, unpublished results), and
no difference in direct current potential is recorded when
electrodes are placed in the external medium and in the
submyelinic space in the giant fibres of P. chinensis and P.
japonicus. The resting membrane potential of the axon
measured by microelectrode insertion is −64.3 ±7.4 mV (mean
± S.E.M., N=7) (Xu and Terakawa, 1991). As shown in Fig. 5,
it is interesting to note that a monophasic positive action
potential is recorded intracellularly from the axon
(approximately 75 mV in amplitude) and also extra-axonally
from the submyelinic space (approximately 65 mV in
amplitude) (Xu and Terakawa, 1993).
This fact indicates that no excitation takes place in the
internodal axolemma during impulse conduction and that the
submyelinic space is sufficiently well insulated to serve as a
conductor that is equipotent to the axolemma so far as impulse
B
A
Axon
Submyelinic space
The axon and its associated microtubular sheath are
surrounded by a wide gap and then by the myelin sheath. This
gap between the microtubular sheath and the myelin sheath is
unique to the myelinated fibre of the Penaeus shrimp and is
designated the submyelinic space (Fig. 3A). The submyelinic
space is filled with an amorphous gel, into which a few cells
extend numerous processes shaped like thin flaps. Some
processes are attached to the outer layer of the microtubular
sheath as well as to the inner layer of the myelin sheath. The
processes tend to run parallel to the longitudinal axis of the
fibre. In small fibres, the axon with its microtubular sheath
usually attaches to the inner layer of the myelin sheath,
whereas in middle-sized and giant fibres, the complex of axon
FN
Myelin
1 ms
20 mV
Fig. 5. Monophasic positive action potentials recorded with a
microelectrode inserted (A) into the submyelinic space (extra-axonal
recording) and (B) into the axon (intracellular recording) of the
motor giant fibre of Penaeus japonicus (modified from Xu and
Terakawa, 1993). The broken line marks 0 V.
1984
K. XU AND S. TERAKAWA
A
B
C
Fig. 6. Pathways of local current flow during impulse conduction in a
non-myelinated nerve fibre (A), in a vertebrate myelinated nerve
fibre (B) and in a shrimp myelinated nerve fibre (C).
conduction is concerned. Indeed, the internodal axolemma has
proved to be experimentally inexcitable in the motor giant fibre
of P. japonicus (Xu and Terakawa, 1991). The specific
resistance of the gel within the submyelinic space of a fibre
120 µm diameter in P. japonicus has been estimated to be as
low as 23 Ω cm (Kusano, 1966). The large submyelinic space
is, in fact, a low-resistance pathway for the majority of the
longitudinal internal current of shrimp nerve fibres. The pattern
of local current flow during impulse conduction is shown
schematically in Fig. 6.
It is worthwhile noting that, during ontogeny, the axon of
the shrimp nerve fibre is first surrounded by the myelin sheath
directly, and then the microtubular sheath and submyelinic
space gradually develop between the axon and myelin sheath.
A developing nerve fibre in the ventral nerve cord of an
immature shrimp 5 mm in body length is shown in Fig. 3B. In
this fibre, the microtubular sheath and submyelinic space have
yet to appear.
Functional nodes
Both medial and lateral giant fibres run the entire length of
the ventral nerve cord of the Penaeus shrimp without
noticeable interruption. Morphological studies have shown
that, at each thoracic and abdominal ganglion, the four giant
fibres give off unmyelinated branches to nerve cell bodies in
the opposite side of the ganglion. Motor giant fibres originating
from the last thoracic ganglion and each abdominal ganglion
run posteriorly along the connective and divide into two
branches. Each branch of the motor giant fibre forms giant
synapses with both medial and lateral giant fibres on the
ipsilateral side before leaving the connective. Then, through
the third root, these branches innervate the abdominal muscles
on the ipsilateral side. The anatomical locations for the giant
fibres with their unmyelinated intraganglionic branches and
giant synapses in the ventral nerve cord are shown
schematically in the middle and lower enlarged drawings in
Fig. 1.
The action current could only be recorded from the
ganglionic and synaptic regions of the giant nerve fibres, and
the unmyelinated intraganglionic branches and synaptic
membranes were further proved to be the functional nodes for
saltatory conduction (Hsu et al., 1964, 1975; Kusano and
LaVail, 1971; Terakawa and Hsu, 1991). Moreover, voltageclamp studies showed that the nodal current of the synaptic
membrane consisted of a large Na+ current and a small K+
current, which is similar to the situation in the amphibian node
of Ranvier. It is interesting to note that both activation and
inactivation of the Na+ current of the functional nodal
membrane in the shrimp are the fastest that have been recorded
from any animal to date. The time from onset to maximum Na+
current in the presynaptic membrane was as short as 100 µs at
20 °C in the fastest case, whereas values measured at the nodes
of Ranvier were approximately 150 µs in the frog and 180 µs
in the rabbit at the same temperature. The fast rate constant of
Na+ inactivation in the shrimp was 200 µs at 21 °C, compared
with 462 µs in the frog and 513 µs at 20 °C in the rabbit. The
rapid kinetics of the shrimp giant fibres is advantageous for fast
impulse conduction over long internodal distances (see
Terakawa and Hsu, 1991). Indeed, the internodal distances
found in the medial giant fibres of adult P. chinensis and P.
japonicus range from 3 mm to more than 10 mm.
The record high conduction speed of the shrimp giant fibres
may not mean that the conduction efficiency is as high as
that of warm-blooded vertebrates. The velocity/diameter
relationship for myelinated fibres (Hursh, 1939) predicts that a
mammalian myelinated fibre would have a conduction velocity
of 600 m s−1 at 37 °C, if the diameter of the fibre was 100 µm
(see Terakawa and Hsu, 1991).
Fenestration node
The medial and lateral giant fibres in the circumoesophageal
connective and in the caudal half of the last abdominal
connective do not have synapses or branches for a length that
may exceed 20 mm in adult P. japonicus and P. chinensis. If
the unmyelinated intraganglionic branches and synaptic
membranes were the only source of local currents for saltatory
Fenestration nodes in shrimp myelinated fibres
Fig. 7. Differential interference contrast image of
the fenestration node in the circumoesophageal
connective of a medial giant fibre from Penaeus
japonicus (top view). Both images (A,B) were taken
from the same preparation, but at slightly different
focal planes. M, myelin sheath; G, submyelinic gap
space; a, axon. Arrowheads indicate the fenestration
in the myelin sheath; arrows indicate vacuoles in the
axon. Scale bar, 20 µm (modified from Xu and
Terakawa, 1996).
a
conduction, the internodal distance for fibres in these
connectives would appear to be too long for impulses to
propagate. Moreover, neither synaptic structures nor
unmyelinated branches were found in many middle- and smallsized myelinated fibres in the connectives of the ventral nerve
cord of the shrimp. This led us to suspect the presence of an
unknown type of node in shrimp myelinated fibres. No nodelike structure was apparent in fixed or living preparations
examined by conventional microscope. However, by scanning
a single living nerve fibre preparation from P. japonicus using
a differential interference contrast (DIC) microscope, we found
a novel type of nodal structure. Using voltage-clamp
procedures, we proved that the structure functions like the node
of Ranvier in vertebrate myelinated fibres (Xu and Terakawa,
1993; Hsu and Terakawa, 1996).
With DIC microscopy, the myelin sheath of the nerve fibres
of Penaeus shrimp can be clearly seen (Fig. 7). The distance
between the axon and the myelin sheath, the submyelinic
space, is quite uniform, the same separation being maintained
over long distances. Along a whole single fibre preparation, a
few regularly spaced spots are present where the smooth
continuity of the myelin sheath is interrupted by many circular
or ellipsoidal areas arranged concentrically. In the DIC
microscope, these concentric circles appear to form a craterlike structure, shown in top view in Fig. 7A,B.
When viewed from the side, this structure is characterized
by a lack of myelin, forming a round ‘window’ onto the axon.
In this region, the thickness of the myelin sheath tapers,
suggesting that the layers of myelin at the edge of the window
gradually decrease in number so that the window resembles a
crater on the axolemma. Hereafter, we refer to this structure as
the fenestration node. In the fenestration node region, the axon
is swollen and attached to the myelin sheath. Once the site
of the fenestration node has been noted under the DIC
1985
a
microscope, it can be detected with a conventional bright-field
microscope.
The fenestration node was first found in the
circumoesophageal and telson segments of the medial and
lateral giant fibres of P. japonicus. The number of fenestration
nodes is variable, ranging from one to three in the
circumoesophageal segment and from five to six in the telson
segment (Hsu and Terakawa, 1996). The arrangement of the
fenestration nodes as well as other excitable membranes that
function as nodes (unmyelinated intraganglionic branches and
synaptic membranes) is shown schematically in the enlarged
drawings of Fig. 1. The fenestration node was later also found
as a regularly spaced structure in numerous smaller myelinated
fibres in the ventral nerve cord of the shrimp. The fenestration
node has now been identified in the myelinated fibres of five
other species of Penaeus shrimp, as described above.
The internodal distance is roughly proportional to the
diameter of the fibre. For example, the internodal distance was
3 mm in nerve fibres with a diameter of approximately 40 µm,
but was as long as 12 mm in the circumoesophageal segment
of a medial giant fibre 170 µm in diameter. The diameter of the
fenestration also depends on the diameter of the nerve fibre.
For example, the largest diameter of the outermost ring of the
myelin layer around the fenestration was 50 µm in a medial
giant fibre 150 µm in diameter and approximately 5 µm in
fibres 30–40 µm in diameter. In the region of the fenestration
node, the axon is swollen and contains many vacuoles of
different shapes and sizes (Figs 7, 8). This does not appear to
be a sign of morphological damage, since the vacuoles were
observed in fresh nerve fibre preparations showing normal
impulse conduction (Hsu and Terakawa, 1996).
Preservation of the axolemma at the fenestration node was
actually very difficult using conventional fixation, probably
because of the thick myelin sheath and the many large vacuoles
1986
K. XU AND S. TERAKAWA
Fig. 8. Electron micrographs of a fenestration node in the myelinated fibres of Penaeus chinensis. (A) A cross-sectional view of the fibre with a
fenestration node marked by an arrow. (B) Enlarged view of the fenestration node region showing the edge of the myelin sheath bridged by
perineurial cells and microtubular sheath processes. The arrow indicates the location of the nodal membrane. (C) Enlarged view of the
infoldings formed by the axolemma with processes of the microtubular sheath. These structures were found in the regions indicated by an
asterisk in B. a, axon; m, myelin sheath; ms, microtubular sheath; smG, submyelinic (gap) space; p, perineurium; v, vacuole. Scale bars, 10 µm
(A), 5 µm (B), 1 µm (C) (modified from Xu and Terakawa, 1996).
in the axon. Of the various methods tried, glutaraldehyde used
together with paraformaldehyde or with intermittent
microwave irradiation resulted in relatively good fixation.
Electron microscopic studies revealed that both the myelin
sheath and the microtubular sheath are absent in the
fenestration region, and instead a few perineurial cells loosely
cover the axolemma with digitated infoldings of various
lengths (Fig. 8B). The opening in the myelin sheath is formed
just over the swollen part of the axon (Fig. 7), and the myelin
layers are seamed and fused together at the edge around the
fenestration node (Fig. 8A,B). At this edge, the myelin sheath
is tightly attached to the axolemma, presumably forming a
close junction between them. The edge of the microtubular
sheath is also tightly attached to the myelin sheath so that the
submyelinic space is electrically insulated from the external
space. The excitable nodal membrane (axolemma attached to
the window) is located slightly above the narrowest part of the
fenestration in the myelin. Many infoldings are formed by the
axolemma at the nodal region, partly by perineurial cell
processes but mainly by multiple processes of the microtubular
sheath (Fig. 8B,C). These infoldings contribute to the total area
of the excitable membrane in the node, which would help to
increase the density of the local loop current for saltatory
conduction.
To enable comparison of the fine structure of the fenestration
node with that of the spot opening in the myelin sheath of the
earthworm L. terrestris, two electron micrographs modified
from Günther (1973, 1976) are shown in Fig. 9. The
differences in fine structure between the fenestration node and
the spot opening are evident. In the earthworm spot opening,
the nodal membrane is extended towards the upper level of the
myelin sheath and is connected directly with the collagenous
capsule of the nerve cord. Neither infoldings of axolemma nor
vacuoles within the axoplasm are present in the earthworm spot
opening.
The nodal currents were recorded and analyzed using the
sucrose-gap voltage-clamp method. The results showed that
the action current largely arises from the activity of Na+
channels located in the nodal membrane. The current/voltage
(I/V) curve of the nodal membrane is shown in Fig. 10.
Although there is a small outward K+ current at the late phase
of the clamping pulse, even in the voltage range 50–120 mV,
this current would not be enough to terminate the excitation
during the action potential. In fact, the fast kinetics of the Na+
channel per se is responsible for both the rapid termination and
the rapid onset of the action potential (Hsu and Terakawa,
1996).
Roots (1995) suggested that a myelin sheath evolved
independently in chordates, arthropods and annelids. The
widespread distribution of fenestration nodes in the nerve
Fenestration nodes in shrimp myelinated fibres
1987
Fig. 9. Electron micrographs of a spot opening in the myelin sheath of the median giant fibre of Lumbricus terrestris. (A) Transverse section
through the fibre (modified from Günther, 1973). The arrow indicates the nodal opening. Gl, glial tissue; Co, collagenous capsule of the nerve
cord; M, muscles of the cord envelope. (B) Enlarged view of the spot opening (modified from Günther, 1976). The arrow indicates the location
of the nodal membrane. Co, the collagenous capsule of the nerve cord; mi, mitochondria; ssc, subsurface cisternae; des, desmosomal
attachments in the myelin sheath; M, muscles of the cord envelope. Scale bars, 40 µm (A), 5 µm (B).
fibres of various species of the Penaeus shrimp makes the
position of their myelin sheath in the evolutionary tree
significant. The differences in fine structure of the myelin
sheath between vertebrates and invertebrates suggest that the
20
10
Current (nA)
0
40
80
Voltage (mV)
120
-10
-20
-30
-40
-50
Fig. 10. Current/voltage relationship of the nodal membrane of
Penaeus japonicus measured using the sucrose-gap voltage-clamp
technique. Depolarizing pulses of 2 ms duration were applied at
various amplitudes. The peak value of the initial inward current is
indicated by open circles and the late steady value of the outward
current by filled circles (taken from Hsu and Terakawa, 1996, with
permission).
ability to generate the myelin sheath and the different types of
node were developed after an evolutionary separation between
these two branches of animal (Hsu and Terakawa, 1996).
Conclusions
For most animals, rapid propagation of nerve impulses is of
functional importance, and two distinct mechanisms have been
developed to achieve this during evolution. One adaptive
strategy is to increase the diameter of the axon core and the
other is through myelination of axons (Kandel et al., 1995). A
myelin sheath interrupted by nodes of Ranvier forms the
structural basis for the rapid conduction of impulses from node
to node in a saltatory manner. It is usually considered that the
former method is largely available to invertebrates, and the
latter only to vertebrates, since giant axons are most commonly
found in invertebrates, whereas the compact multilayered
myelin sheath together with the node of Ranvier have been
exclusively observed in vertebrates (Bullock et al., 1977;
Schmidt-Nielsen, 1990). In fact, however, a compact
multilayered myelin sheath (Huang et al., 1963; Hama, 1966)
with regular interruptions by a common node for saltatory
conduction (Xu and Terakawa, 1993; Hsu and Terakawa,
1996) has also evolved in the nervous system of the Penaeus
shrimp.
The myelinated fibres of the Penaeus shrimp have the
following structural and functional characteristics. (1) The
period of lamination in the myelin sheath is 8 nm (Huang et al.,
1963) or 9 nm (Kusano, 1966), but each lamina is interrupted
1–3 times by a seam formed by the terminal loops, which
contain a few microtubules meeting tip-to-tip. The sheath is
formed by Schwann cells, which extend numerous laminae to
enwrap the axon in a manner similar to a hug with two arms.
The seams are arranged in a radial line and are typically
accompanied by an attachment zone. The seam structure also
1988
K. XU AND S. TERAKAWA
serves as a connection between the myelin layers of adjacent
Schwann cells along the nerve fibre (Xu et al., 1994). The tipto-tip connected terminal loop structure is believed to provide
some flexibility to the myelin sheath. (2) The compact myelin
sheath is regularly spotted with fenestration nodes, which are
characterized by a round window lacking myelin. Nodes of this
type have been found in the nervous systems of six species of
shrimp. (3) Two unique structures, the microtubular sheath and
the submyelinic (gap) space, are located between the axon and
the myelin sheath. The microtubular sheath may provide
mechanical support for the axon. The wide submyelinic space,
which is filled with gel, is tightly sealed at the nodal region
and is electrically equivalent to an increase in axon diameter
such as occurs in the giant axons of other invertebrates. The
longitudinal axoplasmic resistance of shrimp nerve fibres is
greatly lowered by the parallel conductance of the gel within
the submyelinic space. This gel could provide the nerve fibres
with some flexibility. A submyelinic space of this type has
been found in nerve fibres of nine species of the Penaeus
shrimp and is the third strategy adopted by nature for
increasing the conduction velocity in nerve fibres. (4) The
conduction velocities of the medial giant fibres were
80–200 m s−1 in P. chinensis (Fan et al., 1961) and
90–210 m s−1 in P. japonicus (Kusano, 1966). The values vary
because of the wide range of sizes of shrimps used and
because, once nerve fibres have been myelinated, the number
of nodes becomes stabilized. The factors contributing to the
extreme rapidity of impulse conduction are: (i) myelination
with long internodal distances, (ii) a wide submyelinic space
with low electrical resistance, and (iii) the rapid kinetics of
activation and inactivation of the Na+ current. The efficiency
of impulse conduction in the shrimp giant nerve fibres,
however, may not be as high as in warm-blooded vertebrates.
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