Download Evolutionary Patterns of Axial Muscle Systems in Some

AMER. ZOOL., 29:5-18 (1989)
Evolutionary Patterns of Axial Muscle Systems in
Some Invertebrates and Fish1
Q. BONE
The Marine Laboratory, Citadel Hill, Plymouth, U.K.
SYNOPSIS. Axial muscles used for oscillatory swimming are found not only in fish and
other vertebrates but also in some protochordates and invertebrates. Chaetognaths have
unsegmented locomotor musculature with some unusual features, but larvacean tunicates
and the tadpole larvae of ascidians show the simplest variant of the chordate segmented
axial muscle arrangement for flexing a notochordal column, where all muscle cells along
one side are electrically coupled. With amphioxus, the basic fish myotomal layout is
established, with two main fibre types probably used for different patterns of swimming
(as in fish). There are, however, several unique features, including the flattened fibre
shape and the paramyosin system of the notochord. Agnatha have two fibre types in the
myotomes, a third type perhaps being a developmental stage in the ontogeny of fast fibres.
In lampreys, the central fibres of the characteristic fibre sandwiches in the myotomes are
flattened (though less so than in amphioxus); they have a dual innervation of unknown
function seen also in the fast fibre system of many Gnathostome fish groups. Hagfish fast
fibres are not flattened nor do they have a dual innervation. Gnathostome fish axial muscles
are strikingly uniform in design with two possible exceptions: (1) higher teleost fast fibres
which, unlike those of other groups, are multiply-innervated and (2) tonic fibres in a few
fish, which seem not to be involved in locomotion.
INTRODUCTION
As well as fish and some higher vertebrates, several deuterostome groups generate swimming thrust by using axial muscles to produce alternating lateral flexions
of the body. In the tunicates which swim
in this way, the axial musculature is segmented and operates to flex a notochord
or vertebral column, just as in fish, so that
the ascidian tadpole larva and the adult
larvacean perhaps provide glimpses of an
early stage in the evolution of the vertebrate axial musculature system. However,
the rather uniform dual fibre pattern of
the axial musculature in the myotomes of
all fish groups, despite their distant taxonomic relationships, is first seen in
amphioxus (Branchiostoma). There are good
reasons to suppose that the fundamental
duality of fibre types in the axial musculature arose in response to the stringent
demands that the density and viscosity of
an aquatic medium impose on fish-like
forms swimming through it (Bone, 1978),
so that it is less surprising than it might
1
From the Symposium on Axial Movement Systems:
Biomechanics and Neural Control presented at the
Annual Meeting of the American Society of Zoologists, 27-30 December 1986, at Nashville, Tennessee.
seem at first sight that all fish groups have
this arrangement.
This review of axial muscle fibre types
and their innervation deals briefly with the
non-segmented chaetognath system, and
the segmental electrically-coupled axial
muscle cells of tunicates, before considering the myotomal axial system of amphioxus and fish in more detail. In these sections, it is argued that the functional
division of the axial musculature into two
differently specialised fibre types, each with
a different innervation pattern, is an ancient
one and homologous in all fish groups. The
significance of other fibre types is considered and, paradoxically, it seems that true
tonic fibres are found in some fish; however, they may play no part in locomotion.
The innervation patterns of fish fast muscle fibres are considered, and it is concluded that the fast fibres of higher teleosts
have been uniquely modified from an
ancestral pattern seen today in all other
fish and in urodele amphibia.
Table 1 summarises the organisation of
the axial muscle systems in the forms considered in this review. The group on the
left of Table 1, the Chaetognatha, is very
different from all the others; here there is
an alternative solution to the design of an
axial muscle system for providing thrust by
TABLE 1.
Comparisons between axial muscle fibres in different groups.
Gnathostome fish
Tunica la
Chaelognathu
Ascidian tadpole
Segmented
Non-segmented
Many muscle fibres
Gap junctions but
no coupling
Cholinergic
Doubtful chordate
relationships
A different solution
using axial
muscle for oscillatory swimming
Many muscle
cells in tiers
Agnatha
Acrania
Larvacean
Few muscle
cells
All coupled
Lampreys
Hagfish
Fibres flat
and very thin
Dual fibre types
and intermediate
Flattened
Dual fibre types
fast fibres and intermediate
Coupled
Fast coupled
Cholinergic
Single
Dual
innervation
innervation
No myotomes
Cholinergic
Unusual central
endplates
Notochordal
paramyosin
First chordate-like arrangement,
but only one muscle fibre type
Some special
features, but
basic fish plan
None coupled
Elasmobranchs
Dipnoi
Chondroslei
Lower teleosts
Higher teleosts
Dual fibre types and
some apparently
tonic fibres in a few
species
No coupling in adults
Cholinergic
Cholinergic
Dual fast
Single, fast
innervation
innervation
In most, dual
Multiple
innervation of innervation of
fast
fibres
fast fibres
Onwards and
The "typical"
upwards to
vertebrate
the Amphibia
pattern (since
higher teleosts are the
most abundant
vertebrates)
Lampreys closer to
basic fish plan?
AXIAL MUSCLE: FISH AND INVERTEBRATES
06
FIG. 1. Scheme of arrangement of axial musculature in the chaetognath Sagitta. The motor innervation of
the two muscle fibre types (B fibres dark stipple) lies on the outer side of the basement membrane (BM) under
the multi-layered outer epithelium (OE). Dark blocks indicate numerous gap junctions between muscle fibres.
At lower right, an extracellular record of spontaneous swimming activity (from a suction electrode enclosing
the tail tip) shows that Sagitta can vary its movements. Time marker: seconds.
found, one much less abundant than the
other.
The A fibres make up some 85% of the
total, and are larger than the B fibres that
CHAETOGNATHA
lie in small clumps between them. The two
Chaetognaths or arrow worms are a small fibre types differ in myofibrillar array, and
marine group, almost all planktonic, whose the B fibres contain relatively more SR and
50 or so species are of rather uniform mitochondria (Dress and Duvert, 1983).
design so that although only a couple of The A fibres are some 300-400 pm long,
species in the genus Sagitta have been 1-2 nm wide, and around 8 fim deep, and
examined in detail, it is probably safe to unlike the smaller B fibres, span the whole
suppose that what follows is typical of all. depth of the muscle layer. Unfortunately,
The axial musculature is non-seg- nothing is known of possible physiological
mented, and forms a tube around the fluid- differences between these two fibre types,
filled body cavity. External to the muscle but it seems clear that the much more
layer is a thick basement membrane con- abundant A fibres are fast fibres, used for
taining helically wound collagen layers, the rapid darts the animal makes to catch
which forms the outer limit of the hydro- its prey and for the brief swimming bursts
static skeleton. Figure 1 illustrates the sys- made every 20 sec or so which drive it
tem. Sagitta swims by oscillating its body in upwards in the water column (Feigenbaum
the dorso-ventral plane (like a cetacean), and Reeve, 1977). Recent physiological
and does so in rapid darts; the animals are studies (Duvert and Savineau, 1986) have
formidable predators on copepods, which shown that acetylcholine is probably the
they seize with the spines on the side of the neuromuscular transmitter. Curiously,
head. Ultrastructural studies by Duvert and although the muscle fibres are coupled by
his colleagues (Duvert and Salat, 1979; numerous gap junctions, they do not appear
Duvert et al., 1980) have shown that two to be electrically coupled, and in line with
types of cross-striated muscle fibre are this, each fibre is probably separately
body oscillations that is interesting because
it presents some rather mysterious features.
8
Q. BONE
innervated (Bone et ai, 1987), by nerve
terminals lying outside the basement membrane, and hence not in direct contact with
the muscle fibre membrane (Duvert and
Barets, 1983).
Here then is a very unusual axial muscle
system that evidently has little to do with
the chordate arrangement. Chaetognath
affinities are entirely obscure, and even
though some zoologists have allied them
with chordates, others have felt that molluscs, coelenterates, or annelids are their
closest allies. Thus their possible chordate
affinities cannot be taken too seriously!
TUNICATES
lower tier is also innervated (Fig. 2a). All
muscle cells are coupled dorso-ventrally and
antero-posteriorly by gap junctions, and in
this way, all the muscle cells along one side
of the body are electrically coupled (Fig.
2b, c).
Electrophysiological investigations have
been limited to Dendrodoa (Mackie and
Bone, 1976 and unpublished observations)
and to Ciona, and in Dendrodoa at least,
three types of electrical event can be
recorded from the muscle cells of intact
larvae (Fig. 2d). The largest, rapid nonovershooting spikes are seen as the tail rapidly flexes unilaterally. Longer bursts of
smaller spikes are seen during the short
bursts of symmetrical swimming movements; and finally, much smaller events can
be recorded which do not correspond to
obvious tail movements.
At present, these results are rather hard
to interpret, since it is difficult to see (for
example) what the function of the dorsal
and ventral innervation of the most anterior tiers of muscle cells may be, or indeed
why each dorsal muscle cell should be
innervated if the whole system is closely
coupled. Perhaps the largest potentials
accompanying unilateral flexions result
from simultaneous activation of the dorsal
and ventral innervation of the anterior
cells, whilst symmetrical swimming only
involves the dorsal innervation along the
tail; but these are only speculations. What
does seem clear, though, is that the result
of this rather complicated arrangement is
that different kinds of movement can be
produced from a single type of axial muscle
cell.
Since Kowalewsky's demonstration
(Kowalewsky, 1867, 1871) that the ascidian
tadpole larva was chordate-like, it has been
clear that the tunicates are related in some
way to vertebrates, although their exact
status has been disputed. In any event, ascidian tadpole larvae, adult larvacean tunicates, and some doliolid larvae all have a
finned tail supported by a notochord, and
flexed by segmented axial muscle cells. The
myogenic tail movements of some doliolid
larvae (others only have vestigial tails [Braconnot, 1970]) are desultory and can hardly
be of much significance in distributing this
short-lived larva of a planktonic adult, but
those of the ascidian tadpole and the larvacean adult are quite different. Adult ascidians are sessile, and larval locomotion plays
an essential role in distribution and site
selection, whilst in larvaceans, tail movements are used not only for swimming, but
also for producing the feeding current
within the filtering house, and indeed, for
expanding the house itself.
Larvaceans
In some respects, larvaceans are simpler
Ascidian tadpole larvae
than ascidian tadpole larvae, for there are
The segmented caudal muscle cells have only ten caudal muscle cells, and they are
been examined in several species, and are arranged in a single row rather than in tiers
rather similar in all, apart from differences in each segment. Each is coupled electriin the sarcotubular systems probably cally to those adjacent (Fig. 3a) but in conresulting from size differences between trast to the ascidian tadpole, where there
species (Burighel et ai, 1977). Three or are apparently no motoneuron cell bodies
sometimes four tiers of muscle cells are along the dorsal nerve cord in the tail, in
found in each segment; only the dorsal row larvaceans the muscle cells are innervated
is innervated from axons in the dorsal nerve by motoneurons whose cell bodies are segcord, except at the anterior end, where the mentally arranged in the tail (Fig. 3c). Each
AXIAL MUSCLE: FISH AND INVERTEBRATES
FIG. 2. Dendrodoa. (a) Schematic diagram of tadpole larva. There are three tiers of muscle cells in each
segment; the upper (U) is innervated by a few motor axons in the dorsal nerve cord above the notochord
(N). A second motor innervation (VM) supplies the most anterior cells of the ventral row (L). The middle
tier (M) and the caudal lower tier are not innervated, but are coupled to neighbouring cells. All motoneuron
somata lie in the brain (B). (b) Current steps injected into a muscle cell of the upper tier (above) are seen in
a cell of the ventral tier (below), (c) Simultaneous intracellular records from ipsilateral muscle cells (anterior
below) during a burst of symmetrical swimming, showing close coupling, (d) Intracellular record from caudal
muscle cell showing two series of unilateral flexions (u) followed by a burst of symmetrical swimming (s). Low
amplitude electrical activity following the first series of unilateral flexions is not correlated with obvious tail
movements. Vertical scale as (c).
muscle cell receives two kinds of motor
innervation. One type forms large terminal rosettes, the other elongate finer beaded
terminals; under the former alone is acetylcholinesterase found (Flood, 1973). Here
then is the same paradox as in the ascidian
tadpole, viz. individual innervation of electrically coupled axial muscle cells, though
in larvaceans the situation is complicated
by the dual innervation of all muscle cells.
Once again it has to be admitted that the
operation of the system is not well understood, but as in ascidian tadpoles, it permits
different types of movement from a single
type of muscle cell. In the case of larvaceans, however, some clues to the variety
of contractile responses from this single
fibre type are provided by recordings of
escape swimming responses (Bone, 1984).
In oikopleurid larvaceans the outer epithelium is excitable, and mechanical or
electrical stimulation of the epithelial cells
evokes rapid escape swimming bursts.
Intracellular records from the muscle cells
during normal and escape swimming show
that not only does the frequency of swim-
10
Q. BONE
Fie. 3. Oikopleura. (a) Current steps injected into one caudal muscle cell (upper) are recorded in the next
but one cell along the tail, (b) Intracellular record of muscle potentials during normal swimming (vertical and
horizontal scales as in (d)). (c) Schematic diagram showing dual innervation of caudal muscle cells from
motoneurons in cord driven by caudal ganglion (CG), and connexion of the outer excitable epithelia (stippled)
with the caudal ganglion via the paired receptors whose bristles touch the walls of the house (H). The anterior
epithelium (not stippled) is not excitable, (d) Intracellular record of muscle potentials during escape swimming
burst (to same scales as (b)).
ming movements increase during escape
swimming (see also Bone and Mackie, 1975)
but the amplitude of the spike-like potentials much increases (Fig. 3b, d). It is possible therefore that the second motor
innervation of the caudal muscle cells modulates the responses evoked by the cholinergic endings. The transmitter employed
at the second motor ending is unknown
(unfortunately neuropeptide immunocytochemistry has yet to be applied successfully to larvaceans).
Evidently, ascidian tadpole larvae and
oikopleurid larvaceans (perhaps indirectly
derived from ascidian tadpoles by neoteny)
show two rather different variations on an
axial muscle theme where only a single fibre
type is found. These are almost all very
small animals in which the tail is under 5
mm long (there are a few giant larvaceans
whose tails may be 30 mm or longer, but
these have not been examined in detail),
and it is perhaps the small scale of their
design that has prevented the "invention"
of the added complication of two muscle
fibre types specialised for different types of
swimming such as are found in all fish. The
obverse of this coin, however, is that to
enable them to obtain a variety of patterns
of tail movement from the single type of
muscle fibre, they are obliged to use a more
complex system of control than is found in
fish, involving electrical coupling between
muscle cells as well as dual innervation.
ACRANIA
Although there are several unique features, the axial musculature of amphioxus
AXIAL MUSCLE: FISH AND INVERTEBRATES
11
FIG. 4. The amphioxus axial system (partly after Bone 1984). The notochord (N) contains paramyosin fibres,
innervated by motor fibres (NM) in the ventral part of the cord. Two types of muscle fibre (SUP: superficial)
and (D: deep) send tails to different regions of the central motor end-plate, where (inset above) their motor
terminals contain vesicles of different sizes and where the muscle cell tails seem to be linked by gap junctions.
Intermediate fibres (INT) send their tails with the deep fibres. (Re-drawn from Flood, 1968, 1969.)
is constructed essentially on the fish plan,
with myosepta dividing myotomes that
contain two histologically distinct muscle
fibre types (Flood, 1968), which probably
(though not certainly) are used for different modes of swimming, as are the two
main fibre types in the fish myotome. In
amphioxus, the two types (superficial and
deep) differ in mitochondrial and glycogen
content, volume of sarcotubular system,
and in position just as in fish. As in most
fish, the mitochondria-rich fibres lie superficially in the myotome (Fig. 4), and form
a much smaller part of the myotome than
the deeper mitochondria-poor fibres, making up only around 1% of the total. An
intermediate fibre type also occurs, but the
significance of this fibre type is unclear;
perhaps it represents a developmental stage
in the formation of the deep mitochondriapoor fibres.
Amphioxus muscle fibres are very different from those offish in their shape and
mode of innervation (Fig. 4), for they are
flattened, very thin plates with curious thin
tails which approach the sheath of the central nervous system to reach the neuromuscular junction just outside the sheath
(Flood, 1966). Here, the tails of the mitochondria-rich superficial fibres form a separate bundle above the much larger bundle
formed by the tails of the deeper fibres.
The nerve terminals associated with each
type of muscle tail contain vesicles of different diameter: those in nerve terminals
associated with the tails of the superficial
fibres are electron-lucent and around 70
nm diameter, whereas those associated with
the deeper fibres are also electron-lucent
but around 117 nm in diameter.
Several points remain to be clarified
about this remarkable arrangement. First,
Flood (1966) observed apparent gap junctions not only between the tails of the muscle fibres (including between tails of deep
and superficial fibres), but also between the
tails and sheath cell processes, and suggested that the apparent gap junctions
might be artefactual. However, Hagiwara
and Kidokoro (1971) have demonstrated
electrical coupling between amphioxus
muscle cells, so that it is unclear whether
the two types of muscle fibre can be activated independently. Secondly, Flood
(1974) has shown acetylcholinesterase at
both superficial and deep muscle fibre cen-
12
Q.BONE
tral endings, despite their different vesicle
morphology, so that both fibre types seem
to have a cholinergic innervation.
The other striking feature of the axial
system, peculiar to amphioxus, is that the
notochordal lamellae are contractile and
contain a thick and thin fibre array very
like those of the slow adductor or "catch"
muscles of lamellibranch molluscs. The
thick filaments are up to 1,500 A in diameter and show a regular 145 A periodicity;
they contain paramyosin (tropomyosin A)
(Flood, 1967), as do the molluscan muscles.
Contraction of the notochordal cells via
cholinergic synapses at the base of the spinal
cord modifies the stiffness of the notochord (Flood, 1969; Flood et al., 1969;
Guthrie and Banks, 1970). Guthrie and
Banks have shown that electrical events in
the notochordal cells follow spikes in the
axons of the giant Rhode cell system of the
spinal cord which is also involved in myotomal muscle contraction, and suggest that
the notochordal system is chiefly important during fast swimming. During bursts
of rapid swimming, amphioxus contracts
its myotomes at frequencies up to 20 Hz
(Guthrie and Banks, 1970), and at these
frequencies the notochordal muscles
respond tonically. In fast swimming the
angles of flexion of the body are small, so
Guthrie and Banks suggest that the tonic
notochordal response effectively doubles
the stiffness of the notochord and hence
the elastic recoil of the system. During slow
swimming at contraction frequencies of 12 Hz, notochordal responses are phasic and
do not impede initial body flexures. Webb
(1973) suggests in addition, that control of
notochordal stiffness along the body permits amphioxus to swim and burrow in the
sand equally well in either direction (as
indicated by its common name).
From the evolutionary point of view it is
remarkable that this complex notochordal
system is unique to amphioxus, and considering that it is alone amongst the chordates in being able to burrow very rapidly
head or tail first into the sediment, it seems
most likely that Webb's (1973) opinion is
correct, and that the notochordal system
arose in conjunction with the burrowing
habit. It would be worth examining the
notochord of amphioxus larvae, in particular, those of the apparently neotenous
pelagic "amphioxides" larvae (Wickstead,
1964).
AGNATHA
In lampreys and hagfish, two muscle fibre
types are arrayed in characteristic sandwiches in the myotomes; in hagfish both
types are circular, but in lampreys the central fast fibres in each sandwich are flattened plates (Fig. 5). In both groups the
slow parietal fibres are multiply-innervated, whilst the fast fibres are focallyinnervated2; the two fibre types differ also
in mitochondrial content, myofilament
pattern and so forth, as they do in other
fish. As in amphioxus, there are some central fibres in the muscle sandwiches of both
lampreys and hagfish, which lie next to the
parietal fibres and are in some respects
intermediate between the two main fibre
types (Flood and Storm-Mathisen, 1962;
Dahl and Nicolaysen, 1971 for hagfish; Lie,
1974 for lampreys). In lampreys these
intermediate fibres are electrically-coupled to the other central fast fibres (Teravainen, 1971) so it does not seem that they
may be separately activated. This problem
has been pointed out previously, in considering tunicates and amphioxus (pp. 8 and
11), and it may perhaps be resolved by supposing that the degree of gap junction permeability and hence electrotonic coupling
can be regulated in vivo, to permit separate
activation of coupled muscle fibre types.
Whilst there is no direct evidence for this
suggestion, gap junction permeability can
be altered experimentally, e.g., by changes
in intracellular pH (Turin and Warner,
1980). In hagfish, where the fibres are not
electrically coupled, Andersen,?* al. (1963)
have not observed any physiological dif-
2
The patterns of innervation of axial muscle fibers
distinguished are: Focal: the fiber is innervated at a
single site only. Multiple: the fiber is innervated at
several sites along its length; if by several different
axons, this is polyneuronal multiple innervation. Terminal: the innervation site is at one or both ends of
the fibre. Dual: two different axons contribute to
closely adjacent innervation sites on a fibre with terminal innervation.
13
AXIAL MUSCLE: FISH AND INVERTEBRATES
Lampetra
Myxine
FIG. 5. Myotomes of lamprey and hagfish seen from inner aspect. Note lack of innervation of some central
and intermediate fibres in lamprey. There is no coupling between muscle fibres in hagfish. 2 axons (1 & 2)
supply lamprey central fibres. (Modified from Bone, 1978.)
ference between intermediate and fast
fibres. Perhaps, as in amphioxus, these
intermediate fibres may be developmental
stages in the formation of the central fast
fibres (they are lacking in ammocoete larvae of lampreys).
An interesting and puzzling feature of
the innervation of lamprey fast central
fibres is that their motor end-plates are
supplied by two apparently separate axons,
whose terminals contain vesicles of different diameters (Kashapova and Sakharov,
1976). In hagfish, only a single axon supplies each fast muscle fibre (Bone, 1963).
The dual innervation of lamprey fast fibres
is found also in elasmobranchs and some
other fish groups (see below) and deserves
further investigation. In this difference in
innervation pattern, as in muscle fibre
shape and in many other features, hagfish
differ from lampreys, and lampreys are
closer to the cephalochordate arrangement.
GNATHOSTOME FISH
In all gnathostome fish, from elasmobranchs to dipnoi, there are two main axial
muscle fibre types, very similar in all; where
investigations have been made, each type
performs a different function (references
in Bone, 1978). Slow red fibres usually lie
superficially, forming up to 15% of the total
myotomal cross-sectional area, and beneath
them is the main mass of white fast fibres.
It is this zoning of the fibre types within
the fish myotome that makes the study of
the structure, biochemistry and function
of the different fish fibre types much simpler than in higher vertebrates. Histological, ultrastructural, histochemical, electrophysiological, and biochemical studies
all support the view that the slowfibresare
designed for sustained aerobic operation
during cruise swimming, whilst the fast
fibres are specialised for anaerobic glycolysis during short bursts of rapid swimming.
Technical difficulties have so far prevented
in amphioxus the same kind of direct evidence for this generalisation provided by
electromyographic studies in gnathostome
fish, but there seems no reason to doubt
that the duality of function in gnathostome
axial muscle arose early in chordate phylogeny.
No biologist will be surprised that there
are complications which have to be introduced into this simple picture of the
gnathostome fish axial musculature; probably the most interesting are the questions
raised by other fibre types, and by the patterns of innervation found in different
groups.
Whilst recognising the fundamental
division of myotomal fibres into slow red
14
Q. BONE
W2
different manner, and the slow fibres do
not propagate action potentials. The distinction between them is one of kind, rather
than degree, as it is in mammals. It is therefore inappropriate to consider fish fast and
slow fibres as part of a continuum (as in
mammals), although within fast and slow
categories it seems sensible to adopt this
approach rather than distinguishing a multiplicity of sub-types.
FIG. 6. Outer part of Scyliorkinus myolome in trans- Slow fibres
verse section, showing superficial (tonic?) fibres (S),
The slow fibres are invariably multiplyand the inner and outer red (Rl & R2), and white
(Wl & W2) fibres. A coiled indirect proprioceptive innervated by two or more axons, bearing
ending is seen between skin and outer myotomal sur- many cholinergic end-formations along
face.
their length, and despite Stanfield's finding
that, in dogfish, some possessed sufficiently
large inward Na+ currents in voltage clamp
fibres and fast white fibres, several workers experiments as to suggest that they could
have sub-divided these categories, or dis- propagate action potentials (Stanfield,
tinguished other fibre types that do not fit 1972), action potentials have never been
into them. For example, dogfish slow and observed from slow fibres in any fish
fast fibres may each be sub-divided on the species. Slow red fibres specialised for susbasis of enzymatic activity and position tained low speed swimming are rather uniwithin the myotome (Bone and Chubb, form in design in all fish, but in a few fish
1978). In dogfish and some higher teleosts, small numbers of a different type of slow
a small category of myotomal fibres are fibre have been found.
unlike either slow or fast fibres and appear to be true tonic fibres (Kilarski and Tonic fibres
Kozlowska, 1983; Bone etal., 1986). Again,
In several freshwater teleosts such as
in many higher teleosts, there are distinct Gasterosteus (Kilarski and Kozlowska, 1983)
pink or intermediate fibres lying between "tonic" fibres have been described, which
the outer slow red fibres and the deeper differ markedly from typical slow fibres.
white fibres, which are not only interme- They stain weakly for succinic dehydrodiate in mitochondrial and enzyme content genase, have relatively sparse mitochonand so on, but also appear to be recruited dria and at the ultrastructural level show
at sustained swimming speeds intermediate long sarcomere lengths, thick Z-lines, and
between slow cruise and maximum burst lack M-lines. In these respects they resemspeed (Johnston et ai, 1977).
ble amphibian tonic fibres, but so far the
In mammals, where all fibre types are mechanical properties of this kind of fibre
focally-innervated and propagate action have only been examined in dogfish (Bone
potentials, their inherent plasticity is man- et al., 1986). In each myotome of Scylioifested by the way in which their ultrastruc- rhinus an interrupted layer of 80-90 superture, biochemistry and mechanical prop- ficial fibres lies outside the slow fibre zone,
erties can be altered experimentally by just under the connective tissue sheets of
appropriate chronic electrical stimulation, the sub-dermis (Fig. 6). They are entirely
so that different fibre types represent stages distinct from the underlying slow red fibres
within a dynamic equilibrium, as Pette for they are of much larger diameter, con(1985) remarks. Although there is no rea- tain2+few mitochondria, and stain weakly for
son to suppose that fish muscle fibres are CA -activated ATPase (Table 2). They
any less plastic, they differ importantly from make up less than 0.5% of total myotomal
mammalian fibres, because the slow and fibre number (0.6% of total cross-sectional
fast fibre types are innervated in a quite area at the post-anal level, compared with
15
AXIAL MUSCLE: FISH AND INVERTEBRATES
TABLE 2.
Comparison of some structural and mechanical properties of muscle fibre types in the myotomes o/Scylio-
rhinus.*
Fibre type
Innervation
% mitochondria!
volume
Maximum Ca2+-activated force developed (kN m~2)
Maximum Ca2+-activated force developed corrected for myofibril density (kN m"2)
Unloaded contraction velocity V,bcl(L0 sec"1)
V ^ U sec-)
Superficial
Multiple
2.39 ± 0.49
49 ± 4
Slow red
Multiple
21.55 + 3.39
70 ± 8
Fast white
Focal terminal
0.99 ± 0.16
180 ± 5
65
113
231
0.47 ± 0.03
0.58
1.44 ± 0.01
1.53
4.4 ± 0.3
4.5
* From Bone ti al, 1986.
24.4% for slow fibres and 75% for fast importance) has not yet been carefully
fibres). Single skinned superficial fibres examined and, so far as I am aware, elecproduce around half the maximum force tromyographic and mechanical studies of
per unit cross-sectional area produced by the axial musculature in terrestrial
slow fibres and around a quarter of that amphibia have yet to be performed to see
produced by fast fibres, and their contrac- if some of the fibre types there can be
tion velocity is correspondingly lower than regarded as tonic and equivalent, for
that of the other fibre types (Table 2).
example, to the tonic type 5 fibres of XenAs can be seen from Table 2, the mul- opus limb muscles (Lannergren, 1978).
tiply-innervated Scyliorhinus superficial
fibres fulfil many tonic fibre criteria, dif- Fast fibres
fering from those of amphibia mainly in
In all fish groups apart from the higher
their large diameter. It is difficult to know teleosts, the fast fibres are focally-innerwhat role they play, for they have only been vated and almost always terminally-innerfound in Scyliorhinus canicula and S. stellaris, vated (Bone, 1970; Bone and Ono, 1982).
and they are absent from all other sharks In contrast to the slow fibres, they propaexamined as well as from batoid axial mus- gate muscle action potentials. In sharks,
cles. In view of their number and the low fast fibres are innervated only at one end,
forces they can exert, it seems very improb- but in some teleosts, such as catfish (Barets,
able that they can play any part in loco- 1961) they are innervated at each end. An
motion.
interesting feature of the terminal innerIn some cases at least, the large end-for- vation in several groups (including urodele
mations along the superficial fibres are sup- amphibia) is that the terminal endplate is
plied by axons which branch also to inner- derived from the endings of two separate
vate the outermost slow fibres, but it is axons, which may contain different types
unclear whether this means that the super- of synaptic vesicles (Bone, 1972). Ono
ficial fibres are activated during slow swim- (1983) has surveyed the occurrence of such
ming driven by the slow fibres. It seems dual innervation of fast fibres, adding one
most probable that they are not and, par- clupeid and holocephali to the forms preadoxical as it may seem, the superficial viously reported, which include sturgeons
fibres are not involved in locomotion; per- and axolotls, as well as elasmobranchs (Best
haps they have a postural role in keeping and Bone, 1973; Sakharovand Kashapova,
head and tail slightly raised when the fish 1979). There seems good reason to suprests on the bottom. If this is correct, the pose that dual innervation is the primitive
superficial fibres would be equivalent in innervation pattern of vertebrate fast axial
function (as well as in their structure and muscle fibres, but there is as yet no evimechanical properties) to the tonic fibres dence for (a) different transmitters in the
two axons, nor (b) any difference in their
of amphibian limb muscles.
function if both are cholinergic. It is an
The origin of tonic fibres in terrestrial obvious possibility that one axon might
forms subject to gravity from the ancestral release a "modulator" neuropeptide to
aquatic condition (where gravity is of little
16
Q. BONE
alter the contractile response resulting
from cholinergic release by the other, but
it is not clear why the fish should need to
grade the fast muscle system in this way if
it is only employed during burst swimming.
In a very few teleost fish, fast muscle
fibres appear to have both terminal and
some degree of multiple innervation. One
of the groups of the deep sea Stomiiformes,
the Photoichthya (Weitzman, 1974) has a
few axons passing through the fast zone of
the myotome, and innervating fibres in a
distributed manner, but also has fibres
which are innervated terminally (Bone and
Ono, 1982). It would be interesting to
examine larval stomiiforms to see if the
beginnings of multiple-innervation, as seem
to be shown by the adults, appears only at
metamorphosis.
In higher teleosts, fast fibres are multiply-innervated and may have relatively
more mitochondria and a more extensive
vascular bed than the apparently entirely
anaerobic fast fibres in those groups where
they are focally innervated. Electromyographic investigations (e.g., Bone et al.,
1978) of fish with focal and multiple fast
fibre innervation swimming in a water
treadmill showed that the herring with focal
innervation only used their fast fibres at
high speeds and were unable to drive the
fast system for other than short bursts,
whereas in carp, where the fast fibres are
multiply-innervated, there was low-amplitude electrical activity from the fast fibres
at intermediate swimming speeds; only at
higher swimming speeds were muscle
action potentials recorded from the fast
fibres.
The multiple innervation of fast muscle
fibres in higher teleosts is polyneuronal,
ranging from only a few separate axons
innervating each muscle fibre in zebra fish
(Westerfield, et al, 1986), up to 22 in the
marine teleost Cottus (Hudson, 1969). In
their interesting study of the zebra fish,
Westerfield et al. found that motoneuron
EPPS generate action potentials in the
majority of muscle fibres, but it is not yet
clear whether this is always the case for
those species where the degree of polyneuronal innervation is much greater, and
where fast fibres are known from electromyography to be active in sustained locomotion.
CONCLUDING REMARKS
This brief survey of axial muscle patterns in protochordates and fish suggests
that originally, axial muscle fibres were of
a single type, and rather than being organised into serial myotomes, electrically "separate" from each other, formed a series of
electrically-coupled muscle cells along the
length of the body. With size increase, perhaps related to a more efficient technique
of filter-feeding, serial activation of the
axial muscle fibres along the body was
required (and myosepta may have been
needed for mechanical reasons) so that the
system became transformed into separate
myoseptal units, in which two major fibre
types were specialised in different directions. The universal distinction in fish
between slow red aerobic fibres employed
during slow sustained cruising, and the fast
white fibres for burst swimming shows not
only how successful this solution has been
to the need for a wide range of power output from the axial musculature over even
modest speed ranges, but also that it must
have early appeared in chordate evolution.
It is clear too, from the distribution of terminal innervation in all fish (and in urodele
amphibia) that this mode of innervation of
the fast fibres must have evolved early, and
that probably (as Ono, 1983 suggested) dual
innervation of fast fibres by two separate
axons is to be regarded as the primitive
vertebrate condition.
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