Download Structure and Function in Vertebrate Skeletal Muscle1

AMER. ZOOL., 29:221-234 (1989)
Structure and Function in Vertebrate Skeletal Muscle1
SUSAN E. PETERS
Department of Biology, University of North Carolina at Charlotte,
Charlotte, North Carolina 28223
SYNOPSIS. The functional diversity of vertebrate skeletal muscle largely depends upon
its structure. An important aspect of this is its hierarchical design. At the cellular level,
muscle fibers form three categories whose functional properties grade into each other:
slow-oxidative fibers with high endurance to fatigue, fast-oxidative/glycolytic fibers also
endurant but with greater metabolic diversity, and fast-glycolytic fibers with limited endurance but quick response. This partitioning of functional properties found among single
muscle fibers is conserved at a second level of the structural hierarchy, since the group
of myofibers innervated by a single motor neuron (together called a motor unit) is composed of the same fiber type. Different motor units may be recruited in an orderly pattern
depending upon the functional demands of a particular behavior. Finally, groups of motor
units innervated by axons travelling together in the primary nerve branches may form
discrete neuromuscular compartments at a third level of structural hierarchy. Different
motor units may be found in regional arrays in these compartments so that slow or fast
units tend to be clumped together and may be recruited together as larger functional
units. This hierarchical organization of skeletal muscle may be a fundamental vertebrate
plan that allows the diversity of functions so evident in vertebrate behavior.
INTRODUCTION
Diversity of muscle function is evident
in all kinds of movements, from the shear
digging-power of a giant armadillo, to the
lithe beauty of a leaping gazelle; the endurance of a hunting dog on the chase, to the
controlled precision of a neurosurgeon's
hands. To understand this diversity, we
must be able to describe the behaviors produced by the skeletal muscles. Milton Hildebrand has provided us with an effective
quantitative method to describe locomotor
behavior, and has suggested a unifying
model for the evolution of tetrapod gaits
(cf. Hildebrand, 1976). Hildebrand often
emphasizes the precision and diversity of
vertebrate locomotor behavior, admonishing us that locomotion is much more varied
than can be expressed within defined gaits.
In the transitions between gaits, in the exuberant and seemingly random locomotion
of kittens at play, we glimpse some of the
complexity and variety of movement of
which animals are capable.
What are the factors that can account
for all of these different types of move-
ment? One must, of course, consider the
neural control of movement; however, the
present paper deals with properties of muscle, and my theme is that a large amount
of the variation in quality and force of muscle activity is due to inherent properties of
the muscles themselves.
To provide a focus for this paper, I would
like to examine recent work describing the
functional units of vertebrate skeletal muscles. From the cellular to the whole-muscle
level, muscles are organized in a hierarchical manner (Fig. 1): individual muscle
fibers are at the primary level of contraction, embodying the basic functional properties of force, shortening velocity and
endurance. But individual fibers are normally arranged in groups forming a muscle
unit that is innervated by a single motor
neuron. These motor units are then grouped
within some muscles into compartments
which may act as autonomous neuromuscular entities. Several compartments may
be joined into larger structures that we
commonly recognize as individual muscles.
What is the importance of each level of this
hierarchy in the emerging picture of functional diversity within muscles?
1
From the Symposium on Vertebrate FunctionalMorpholoFf: A Tribute to Milton Hildebrand presented at the
Annual Meeting of the American Society of Zoologists, 27-30 December 1986, at Nashville, Tennessee,
.
,
MUSCLE FIBERS
,
.
.
.
During development and throughout
life, the large multinucleate muscle fibers
221
222
SUSAN E. PETERS
Muscle nerve
Primary nerve branch
, Motor axon
Neuromuscular
compartments
Myofiber
Myofibril
FIG. 1. The hierarchy of muscle structure can be seen in this representation of a typical vertebrate skeletal
muscle. Individual muscle fibers with their contractile proteins organized into myofibrils (with A-bands and
I-bands, and sarcomeres [S] defined by the Z-lines) are innervated in a group (muscle unit) by a single motor
axon forming a motor unit. Groups of motor axons run together within the primary nerve branches to
innervate a neuromuscular compartment.
synthesize huge quantities of the fibrous
contractile proteins, actin and myosin, as
well as the attendant proteins troponin,
tropomyosin and numerous others. It is not
the purpose of this paper to examine the
excitation-contraction mechanism of muscle fibers (see Pollack, 1983, and references therein). Once excited, all skeletal
muscle fibers contract by the same mechanism. What, then, are the bases for functional differences in muscle contraction?
How can we discover and describe these
differences?
Three primary functional properties that
we can measure for myofibers are force of
contraction, velocity of shortening, and
endurance. All three of these properties
may vary according to molecular and cellular differences between myofibers.
Because of their small size, however, it is
very difficult to measure each property in
individual fibers. Common methods used
to estimate muscle fiber characteristics
involve histochemical and molecular techniques.
Histochemical techniques
Cross-sections of muscle tissue can be
stained in a variety of ways to help us evaluate their functional properties. For example, the amount of force produced by a
muscle fiber will depend upon the amount
of contractile proteins within the fiber. This
can be estimated by measuring the crosssectional area of the myofibers seen in histological sections. A number of non-specific chemicals can be used to stain the fibers
in order to measure their areas; however,
a much more powerful histological technique involves using stains which are specific for particular molecules. This method
has been used to show that muscle fibers
have some significant differences in a variety of important molecules which may help
to explain their functional diversity.
The speed with which a muscle fiber can
shorten will depend upon the speed of the
molecular interactions between actin and
myosin, and this may depend upon the
ATPase found in the myosin molecules.
VERTEBRATE SKELETAL MUSCLE
223
FIG. 2. In Figure 2A, a cross-section of the medial gastrocnemius of the opossum has been stained for myosin
ATPase after alkaline preincubation (pH = 10.1). The ATPase of the slow fibers (type I) has been denatured
and they do not stain. The fast fibers' (type II) ATPases are alkaline-stable and stain darkly under these
conditions. With acid preincubation (pH = 4.35) (2B), the type I (slow) ATPase is stable and stains darkly.
The type II fibers separate into two forms; one is less acid-stable and stains lightly (type IIA), and the other
is more acid-stable and stains intermediately (type IIB). Modified from Peters et al. (1984).
Padykula and Herman (1955) developed a
method for specifically staining myosin
ATPase, and found that there are indeed
different types of ATPase in slow and fast
muscle fibers. With alkaline pretreatment,
the myosin ATPase of slow fibers denatured so that they did not stain, but the
fast fibers stained darkly for myosin
ATPase (Fig. 2A). Brooke and Kaiser
(1970) found that at a low pH (<5.0) the
myosin ATPase of fast fibers denatured,
while the slow fibers' ATPase was acid-stable and stained darkly. Interestingly, at a
moderately low pH (4.6), some of the fast
fibers retained significant staining, while
the rest were only lightly stained (Fig. 2B).
Brooke and Kaiser (1970) used this analysis to classify myosin ATPases into the two
primary types, type I (slow) and type II
(fast), with type II having a less acid-stable
form (IIA), and a more acid-stable form
(IIB) (Fig. 2B). These results undoubtedly
reflect a fundamental difference in expression for those genes controlling the structure of myosin ATPases. However, at this
point it was uncertain as to what molecular
differences in the myosin ATPases were
correlated with these staining differences.
It remained for molecular studies (see
below) to clarify the bases for these differences.
Besides force production and contraction speed, a third functional property of
muscle fibers, endurance, also lends itself
to analysis by histochemical means. The
fatigability of a myofiber will depend upon
its ability to produce quantities of ATP
rapidly enough to supply the contractile
demand. ATP can be produced slowly and
continuously by oxidative means, or rapidly for short periods by glycolysis. Serial
tissue sections can be stained for oxidative
or glycolytic enzymes, and, depending on
how much of each is present, one can estimate the endurance or metabolic capacity
of a muscle fiber.
In 1972, Peter et al. combined these
techniques by staining serial sections alternately for myosin ATPase and the metabolic enzymes. They were able to describe
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SUSAN E. PETERS
TABLE 1.
Fiber types
Myosin ATPaseJ
Metabolic enzymes'
Relative oxidative capacity
Relative glycolytic capacity
Motor unit types'
Relative twitch contraction speed
Relative tetanic force
produced
Relative endurance
I
SO
Muscle fiber and motor unit types.
IC and I I C
very high
IIA
FOG
IIAB"
IIB
FG d
low
moderate to high
moderate to high
moderate to low
high
S
FR
FI r
FF
slow
fast
fast
fast
small
very high
medium
high
intermed.
intermed.
large
low
a
Brooke and Kaiser, 1970. For equivalent classification schemes, see Stuart et al. (1984).
b
See discussion of intermediate fiber types (defined in Staron and Pette, 1986).
c
Peter et al., 1972.
d
See cautionary discussion.
'Burke et al, 1971.
f
McDonagh et al., 1980.
a classification of muscle fibers into three
types (Table 1). Slow-twitch fibers (myosin
type I) had high oxidative capacity and were
called slow-oxidative (SO) fibers. Some fasttwitch fibers (myosin type IIA) stained
darkly for both oxidative and glycolytic
enzymes and were named fast-oxidative/
glycolytic (FOG). The other fast-twitch
fibers (myosin type IIB) had low oxidative
but high glycolytic capacity and were
dubbed fast-glycolytic (FG) fibers. These
three muscle fiber types have been
described in many different species of
mammals (Burke et al., 1971; Peter et al.,
1972; Ariano et al, 1973; Armstrong,
1980), and a similar diversity has been
found in lower vertebrates (Putnam et al.,
1980; Spurway, 1982).
Molecular techniques
With increasingly sophisticated biochemical and molecular techniques, it has
been possible to analyze differences in
activity and structure of reactive molecules
like the enzymes just discussed. Using these
methods, it has been known for at least 20
years that variations exist in myosin ATPase
activity, and that this correlates with variations in the speed of shortening of different muscles (Barany, 1967). The structural
basis for these differences is only now being
worked out.
Each filamentous myosin molecule ends
in two heads, which bind to active sites on
the actin filaments when free Ca++ and ATP
are present. The heads are composed of a
combination of heavy chain and light chain
segments, with each of the heads having
ATPase activity (see Swynghedauw [1986]
for a review), which will determine the rate
of breakdown of ATP, thus determining
the speed of sarcomere shortening.
We know that myofibers defined as slow
or fast by histochemical means have fundamentally different myosin heavy chains
(MHC) (cf. Gauthier and Lowey, 1979; Billeter et al., 1981). Molecular techniques
have revealed a single slow MHC, and two
MHC isomers of fast myosin have been
identified, corresponding to the IIA and
IIB forms (Swynghedauw, 1986). Within
the myosin heads there is also heterogeneity in the myosin light chains (MLC);
three isomers have been discovered in fast
myosin (Lowey et al., 1979), and two different MLCs have been demonstrated for
slow myosin (Pinter et al., 1981). Recent
evidence shows that the histochemical fiber
types I (slow) and IIA and IIB (fast) are
defined by their MHC composition, and
that no correlation exists between the
myosin ATPase staining and the forms of
MLCs found in the muscle fibers (Reiser et
al., 1985; Staron and Pette, 1986).
These biochemical methods corroborate
the general results of histochemistry and
show the same basic subdivision of fast and
slow myosin ATPases. These analyses also
provide quantitative data with which we
VERTEBRATE SKELETAL MUSCLE
may correlate structural differences with
the differences in enzyme activity, and ultimately sort out causal factors determining
functional diversity of muscle fibers.
225
in myofibers is preserved at the level of the
motor unit. The tripartite classification
scheme for muscle fibers can, thus, be
extended to motor units (Table 1): those
composed of SO fibers are known as slow
(S) motor units, those having FOGs are fastMOTOR UNITS
fatigue resistant (FR) motor units, and those
What is the functional significance of the with FG fibers are fast-fatigable (FF). In
tripartite diversification of muscle fibers addition to the increasing gradient of speed
into SO, FOG and FGs? Obviously, the from S -• FR -» FF, most results also show
muscle contractions necessary to produce an increase in force, with S units producing
the myriad types of movements that are the smallest forces, FR units larger, and FF
seen can be finely controlled by preferen- units the largest forces within muscles
tial recruitment of different types of mus- (Burke, 1981).
Results from many different studies have
cle fibers. However, muscle fibers work
within the context of the motor unit. When supported an orderly pattern of motor unit
a single motor neuron fires, it conducts an recruitment in which motor units producaction potential to all of the fibers of its ing the smallest amounts of force that have
muscle unit, and each is stimulated to con- the slowest contraction speeds (S units) are
tract with an all or none response. In order recruited first for posture and slow ambling.
to preserve the functional diversity of the The FRs become active in trotting or other
different muscle fiber types, all of the fibers fast sustained activity, and FFs are used
composing a muscle unit should be of the only in fastest gaits, darting, accelerating
or jumping (Desmedt and Godaux, 1977;
same type.
Direct evidence for uniformity of muscle Stephens and Usherwood, 1977; Taylor,
fiber types within individual muscle units 1978).
Patterning of recruitment may result
came with the studies of Edstrom and
Kugelberg (1968). They isolated individ- from differences in excitability of the motor
ual motor units in rat muscles and stimu- neurons. Henneman (cf. 1979) postulated
lated them repeatedly to deplete their fibers that the low threshold (smallest) neurons
of glycogen. When the muscles were innervate the small, slow units, while the
removed and sectioned, the first of a series neurons with the highest thresholds (largof sections was stained for glycogen to est) innervate the large, fast units. Other
identify the fibers in the single experimen- evidence suggests that there may actually
tal motor unit. When subsequent sections be a recruitment order that is specifically
were stained for the oxidative and glyco- tied to motor unit type (see Burke, 1981;
lytic enzymes, they found that the histo- Fleshman et al., 1981). The details remain
chemical characteristics of the fibers were to be worked out, but it appears evident
that recruitment follows an orderly and
uniform within single muscle units.
predictable
pattern depending upon the
Burke and Tsairis (1974) used the same
requirements
of a particular behavior.
technique to verify the uniformity of fibers
in motor units of the cat, and other histochemical studies of select species have
COMPLEXITY AND CAUTIONS
reported similar results. In a quantitative
histochemical study, Nemeth el al. (1981) Problems with muscle fiber categories
found that the levels of activity of two
A great deal of work over the last 40 yr
enzymes (malate dehydrogenase and fruc- has gone into developing the satisfying
tose- 1,6-diphosphatase) were virtually scenario of myofiber and motor unit orgaidentical between myofibers of the same nization that I have just presented. As long
motor unit, while random fiber sampling as it is used with caution, this scheme can
showed a wide range of variation in enzyme be a very powerful model to describe musactivities throughout the muscle.
cle function. However, as one reviews the
As a result of these studies, we know that great volume of literature in this field, the
the diversity of functional properties found authors continually caution against too
226
SUSAN E. PETERS
rigid an interpretation of these categories.
There is much variation in speed of contraction, force and endurance within each
category, and their ranges have only begun
to be examined.
Several studies have shown that the functional characteristics of motor units defined
by histochemistry to be of the same type,
can vary widely both between different animals and within the same individual. For
instance, the S units of cat soleus have a
slower range of twitch contraction times
than do the S units of the lateral gastrocnemius (Burke, 1981). Other studies (cf.
Reinking et al., 1975; McDonagh et al.,
1980) indicate that there is a broad overlap
in the twitch contraction times and force
production of motor units so that it is very
difficult to separate S and FR units solely
on functional grounds. The FFs can usually
be classified by fatigability, but their contraction times may widely overlap those of
the FRs. It is apparent from these studies
of motor unit properties that the functional categories of motor units are by no
means uniform and exclusive, but broadly
grade into each other.
Results from a recent study of opossum
motor units (Peters et al., 1984) can serve
to raise several useful cautions. In their
ankle extensor muscles, the medial and lateral heads of the gastrocnemius (MG & LG),
and the soleus (Sol), we found an apparent
lack of correlation in the opossum between
the myosin ATPase fiber types and the
functional fiber types defined by metabolism. In Figure 2B, staining for ATPase
using acid-preincubation showed that
darkly staining slow fibers (type I) are present, as are more acid-stable fast fibers (type
IIB) and highly acid-labile fibers (type IIA)
(Brooke and Kaiser, 1970). One would
expect that both FG and FOG fibers should
be found using metabolic stains; however,
when we stained for the oxidative enzyme
NADH-diaphorase, we found no clear distinction in staining intensity. Many of the
fast fibers stained just as darkly as did the
slow ones, and all had significant staining
(Fig. 3). Moreover, of the 73 motor units
tested in both muscles, over 90% did not
fatigue at all upon repeated stimulation,
and we observed only moderate fatigue in
the others.
This lack of correspondence of the
ATPase-based categories with the metabolic categories of muscle fibers has been
observed using optical scanning and singlefiber biochemical assays to generate quantitative comparisons of enzyme amounts
and activities (Nemeth et al., 1979; Nemeth
and Pette, 1981; Reichman and Pette,
1982; Gollnick et al, 1983; Pette and Tyler,
1983). In summary, these investigators
found that though in many animals type I
myosin ATPase is correlated with SO fibers
and type IIA is usually found in FOG fibers,
fibers with type IIB ATPase are highly
variable in oxidative and glycolytic
enzymes. Many IIB fibers have oxidative
activity in the same range as the IIA fibers,
and, as we found in the opossum, are metabolically FOGs (Nemeth and Pette, 1981).
Hence, the myosin types do not unambiguously define the metabolic types, and standard staining for ATPase cannot be used
alone to infer myofiber functions. In fact,
in mice the IIA fibers normally have a
higher content of glycolytic enzymes and
lower oxidative content than the IIB fibers
(Reichman and Pette, 1984). Until a much
wider variety of vertebrates have been
studied comprehensively, we must be wary
of over-generalizing.
Plasticity
Variability in muscle fibers and motor
units has been apparent for a long time.
Brooke and Kaiser (1970) described a small
percentage of fast fibers that are intermediate in acid-lability between the type I
and IIA fibers. These they called the IIC
fibers. In motor unit studies, Edstrom and
Kugelberg (1968) and Burke (1975, 1981)
found a small number of motor units that
could not be classified as S, FR, or FF. They
were fast contracting, but produced force
and fatigued at levels in between the average FR and FF motor units. McDonagh et
al.(\ 980) described these as a fourth motor
unit type, the fast-intermediate (FI) motor
units. This recognition of fibers and motor
units that were intermediate in their characters, and were typically found in small
VERTEBRATE SKELETAL MUSCLE
227
FIG. 3. Combined staining for alkaline-stable ATPase (3A) and for the oxidative enzyme, nicotine adenine
dinucleotide diaphorase (NADH-d) (3B), shows a lack of correlation between ATPase types and the metabolic
capacity of muscle fibers. All had significant staining for NADH-d, and variation in staining intensity does
not clearly differentiate the fibers. Modified from Peters et al. (1984).
numbers, led to speculation that they may
represent transitional forms.
For years, studies have suggested that
muscle fibers are plastic; not just that variation is widespread from fiber to fiber, but
that individual fibers can change their phenotypic expression. Buller et al. (I960) cut
the nerves to a fast and slow muscle and
sutured the slow nerve so it would reinnervate the fast muscle, and the fast nerve
so it would grow back into the slow muscle.
They found a dramatic change in the contraction times of the muscle fibers, with
previously slow fibers speeding up, and the
reinnervated fast fibers slowing under the
influence of their new nerves. Many studies
have now shown conversions of twitch contraction times and forces in cross-reinnervated muscle (Romanul and Van Der Meulen, 1967; Burke et al., 1979; Dum et al.,
1979;Bagust^a/., 1981; Chan et al., 1982),
or in experiments where superimposed
electrical stimulation changes the pattern
of neural input (Pette et al., 1973, 1975).
This plasticity of function in muscle fibers
may be based on their ability to change
biochemical phenotypes. In immunohistochemical experiments, Pierobon-Bormioli et al. (1981) found that there was
unique immunoreactivity of slow fibers
(type I), and the two fast fiber types (IIA
and IIB), but that some fibers reacted to
antibodies against both type I and type IIA
fibers (probably the fibers that Brooke and
Kaiser [1970] classified as type IIC); others
reacted to antibodies against both types IIA
and IIB (probably the fibers present in FI
motor units), which they classified as type
IIAB fibers. They observed no cross-reactivity of fibers with both anti-I and antiIIB, suggesting a gradient of conversion
from type I «-> IIA «-> IIB.
Staron and Pette (1986) have shown that
the phenotypic differences between fibers
lie in the types of MHCs present in the
myosin heads. Fiber types I, IIA and IIB
have unique MHCs, while intermediate
fibers (they recognize types IC, IIC and
IIAB—see Table 1) have combinations of
the I: IIA and IIA: IIB MHCs. These
results support the idea of a continuum
among muscle fibers, in which three unique
fiber types exist, but where, under appropriate conditions, intermediate forms may
coexist; however, most studies point out
that intermediate fiber types are low in
number relative to the basic types, and conversion of fiber types in normal adult animals without extreme experimental intervention is probably rare (see Pette, 1980).
This suggests a strong phenotypic bias to
remain unchanged under normal circum-
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SUSAN E. PETERS
occupy circumscribed regions known as
compartments.
Some confusion exists about what is
meant by a "compartment." Gonyea and
Ericson (1977) used the term as it applied
to the non-homogeneous and regional distribution of different muscle fiber types
within cat flexor carpi radialis. In fact, this
regionalization of muscle fibers, with a high
percentage of SO fibers in the deep and
distal part of the muscle, and FOGs and
FGs concentrated more in the periphery
(see, also, Armstrong et al., 1982), was an
early clue that muscles were partitioned in
some way at a level above the motor unit.
Over the last ten years, however, a compartment has come to be recognized as a
subvolume of muscle which is innervated
FIG. 4. Lateral and posterior views of the ankle
extensor muscle, lateral gastrocnemius (LG) of the by its own primary nerve branch (Letbetcat are shown relative to its synergists, medial gas- ter, 1974; English and Letbetter, 1982a,
trocnemius (MG), soleus (S) and plantaris (P). LG is b). Compartmentalization has thus come to
comprised of three unipinnate heads. The medial head refer to the neuromuscular compartments of
(LGm) lies dorsal to plantaris and lateral to MG, orig- a muscle (English and Letbetter, 1982a;
inating from the posterior aspect of the lateral femsee Stuart et al., 1988, for review).
oral condyle. Its fibers course laterally to insert on an
aponeurosis where the intermediate head (LGi) origThe most complete description of cominates. Fibers of the LGi are also unipinnate, running
partmentalization,
to date, is that of English
laterally to insert on the medial aspect of a second
aponeurosis. From the lateral side of this tendon the and Letbetter (1982a) in the ankle extensors of the cat. A description of the comfibers of the lateral head of LG (LGI) insert after
originating on a large lateral tendon which can be partmentalization of the LG will serve as
seen in the side view. Modified from English and Let- our example. They found that LG is a combetter (1982a).
plex muscle, consisting of three unipinnate
heads joined by tendons of origin and
insertion (Fig. 4). There are five primary
stances. The fact that in cross-reinnerva- nerve branches of the LG/S nerve, one of
tion studies a 90-100% conversion in con- which goes into Sol and the other four enter
traction speed is accompanied by only a into LG (Fig. 5). By individually stimulat15-20% conversion of fiber types (Chan et ing each primary nerve branch to deplete
al., 1982) also suggests that muscle fibers, the fibers it innervates of glycogen, they
within type, may be diverse enough in met- were able to map the compartmentalizaabolic and functional properties to respond tion of the muscle.
to normal behavioral demands with little
As seen in Figure 5, the three heads of
need to change their fundamental gene LG are partitioned into four neuromusexpression.
cular compartments supplied by these four
primary nerve branches. Some of the borCOMPARTMENTS
ders between compartments were "fuzzy,"
We have seen that muscle fiber diversity indicating some overlap in muscle fibers,
is preserved and expressed through the but this zone of overlap was narrow, and
motor units. How is motor unit diversity the borders between compartments were
organized within whole muscles? Though relatively sharp. From these results alone
the fibers of a muscle unit are not closely one cannot conclude that the compartclumped, they are not randomly scattered ments define anatomical regions of motor
either. Muscle units have been found to unit populations. If the axons branch above
VERTEBRATE SKELETAL MUSCLE
229
FIG. 5. The primary nerve branches and the neuromuscular compartments of a cat's left LG are shown
relative to the three muscle heads seen from a posterior view. One branch goes to soleus, while the other
four branch into LG. The muscle cross-sections are seen from distal perspective with the superficial surface
of the muscle to the right. The medial head (LGm) forms a single compartment (diagonal lines). The most
distal nerve branch (light shading) innervates a superficial and proximal compartment, whose fibers were
mainly from LGi with a small number of proximal LGl fibers. Another nerve branch (dark shading) innervates
a lateral compartment, made up almost entirely of LGl fibers with a small group of bordering LGi fibers.
The fourth compartment (blank) is deep and distal within LGi. Modified from English and Letbetter (1982a).
the primary nerve branches, terminals to nization (Botterman et al., 1983a; Weeks
other muscle fibers of a motor unit may and English, 1985). Though there appear
have been cut when the primary nerve to be some muscles that are not compartbranches were severed prior to stimula- mentalized in this way (Botterman et al.,
tion, and they have merely shown that the 19836), a large enough sample has now
axons do not rebranch once they enter the been examined to suggest that neuromusbody of the muscle. In order to show that cular compartmentalization is a common
motor units are indeed compartmental- phenomenon in mammals.
ized, English and Weeks (1984) isolated
Is compartmentalization a new phenomindividual motor neurons at the spinal cord enon that evolved in mammals or is it a
and stimulated them to deplete their mus- primary organizational plan of the vertecle units of glycogen. With subsequent his- brates? What kind of muscles are comtochemical staining to identify the muscle partmentalized? Much more work needs to
unit's fibers, it was clear that the distri- be done to address these questions; howbution of muscle fibers lay within the ever, a recent study from my lab suggests
defined compartments.
that compartmentalization is an old feaSeveral studies now support the notion ture of mammals, and may be typical of
that a neuromuscular compartment is com- reptiles as well (Peters, 1984; Peters et al.,
posed of a group of motor units whose 1984).
motor neurons are isolated in the primary
The North American opossum is probnerve branch to that compartment (English ably the best living model for estimating
and Letbetter, 1982a; English and Weeks, the neuromuscular structure and function
1984, 1987; Iliya and Dum, 1984); the sen- of early mammals and their reptilian presory neurons from spindles and tendon decessors. It is very primitive in skeletal
organs in the compartment may also be morphology and, by inference, in locofound in the primary nerve branch (Cam- motor behavior (Jenkins, 1971). In reperon etal., 1981). These sense organs have tiles the ankle extensor homologous to the
synaptic connections onto motor neurons, gastrocnemius/soleus complex of the
most of which go back to their own com- opossum is a single large muscle. Histopartment, and the motor nuclei in the spinal chemical studies of it in the lizard, Dipsocord reflect the same compartmental orga- saurus dorsalis, show a deep, distal concen-
230
SUSAN E. PETERS
possum), the LG and Sol are also unified.
In the more specialized kangaroos and
bandicoots, convergent in skeletal adaptations with cursorial eutherian mammals,
the LG and Sol have separated. These
observations suggest that Sol arose as a
neuromuscular compartment within a
larger muscle mass, and became increasingly more independent with locomotor
specialization.
This hypothesis has several important
implications: first, it suggests a mechanism
by which muscle numbers can increase,
through splitting of compartments away
from parent muscles; and second, that
compartmental organization may allow for
specialization of muscle function by diversifying the fiber architecture, sites of origin
FIG. 6. The ankle extensor complex of the opossum and insertion, and regionalization of musis shown from lateral and posterior aspects. Note that cle fiber types in different compartments.
MG is smaller relative to LG than in the cat, and its
Preliminary dissections in my lab showed
tendon of insertion joins that of LG/S more distally
that
the LG/S muscle of the opossum is a
(Fig. 4). Though its fiber architecture is different than
in the cat, the LG portion is similar in that it composes complexly pinnate muscle, consisting of
three distinct heads. The medial (LGm) and inter- four heads (Peters, 1984) (Fig. 6). As in
mediate (LGi) heads originate from the posterior and other mammals, the Sol region is in a deep,
lateral aspects of the lateral femoral condyle. Both
heads have internal tendons to which fibers insert lateral position. Its fibers angle posteriorly
bipinnately before joining in the common Achilles' to insert on the underside of the LG. The
tendon. The lateral head (LGI) originates from a less lateral gastrocnemius comprises three
extensive lateral tendon than in the cat. Its fibers are heads: the medial head (LGm) is in contact
unipinnate, inserting on the lateral aspect of an apo- with MG medially, overlies plantaris and
neurosis with LGi where some of the fibers of soleus
(Sol) attach medially. Soleus originates from the pos- covers the medial surface of Sol. The interterolateral surface of the tibia and from the head of mediate head (LGi) lies lateral to the LGm
the fibula. The fibers are closely apposed to those of and covers the main mass of Sol. The latthe deep LGi and just lateral to the LGm. It cannot eral head (LGI) lies along the lateral aspect
be separated along the deep aspect of LGi and it is of Sol and LGi. This arrangement of heads
no more clearly a separate muscle than are the other
heads comprising LG. Note that though the LG/S is the same as that described in the cat (Fig.
complex is absolutely smaller than that of the cat, it 4) (English and Letbetter, 1982a); howis fleshier, with much less tendon.
ever, the architecture of the muscle heads
differs from the cat in that only the lateral
head of LG is unipinnate. The LGi and
tration of highly oxidative fibers (both tonic LGm are both primarily bipinnate, with
and red twitch) (Putnam et ai, 1980). The central tendons which join to form the
anatomy is suggestive that this may be a medial portions of the tendon of insertion
true compartment, occupying the same of LG/S.
position, and possibly serving a similar
Four primary branches of the nerve servfunction as the soleus of mammals.
ing LG/S were isolated, and, using the same
Though soleus is easily separable from glycogen depletion techniques of English
the overlying LG in the cat, we found that and Letbetter (1982a), I was able to find
LG and Sol are unified in the opossum. the compartments serviced by each branch
Lewis (1962) observed that in the most (Fig. 7). The most distal nerve branch
primitive of the Australian marsupials, innervates the Sol region. The most proxTrichosurus vulpecula (brush-tailed possum) imal nerve branch into LG innervates the
and Pseudochirus laniginosus (ring-tailed LGm as a whole. The other two branches
231
VERTEBRATE SKELETAL MUSCLE
LGm
LGi
FIG. 7. Glycogen-depletion studies of the primary nerve branches of LG/S in the opossum demonstrate
neuromuscular compartments. A posterior view of the left LG with muscle cross-sections is shown oriented
as in Figure 5. As in the cat, soleus is serviced by its own discrete branch (iv). Also the medial head (LGm)
has its own primary nerve branch (i) and forms a self-contained compartment (see Fig. 5) (diagonal lines).
Another primary nerve branch (ii) enters into LGi and forms a compartment in the proximal and superficial
region (light shading). The last primary nerve branch (iii) services a deep, distal compartment with fibers
from the distal LGi and all of LGI (dark shading).
split shortly after entering into the belly of
LGi; the proximal branch innervates a
proximal and superficial compartment in
LGi, and the more distal branch services a
compartment comprising the deep, distal
LGi and all of LGI (Fig. 7).
These experiments do not necessarily
define neuromuscular compartments. As
with the English and Letbetter study
(1982a), this should be followed by experiments to map the distribution of individual muscle units (English and Weeks, 1984),
to ensure that axons do not branch proximal to the splitting of the primary nerve
branches. Until such studies are done I am
assuming that in the opossum, as in the cat,
the primary nerve branches define the
realms of neuromuscular compartments.
If these results are borne out, they suggest that Sol originated as a compartment
of the LG/S, and that the LG portion in
the opossum consists of other neuromuscular compartments which are found in
similar relative positions as in the cat. Thus,
compartmentalization is probably primitive for the higher tetrapods, and may be
a conservative vertebrate structural design.
Compartmentalization may have arisen
as a simple way by which to organize the
development and growth of muscles and
their nerves. It may simply reflect compartmentalization of the spinal motor
nuclei as a basic organizational plan with-
out necessarily strong adaptive significance.
A functional reason for compartmentalization may be to allow for diverse mechanical activities. Results from English and
Weeks (1987) suggest that compartments
can be independently recruited to take
advantage of differences in origins and
insertions. Further evidence for this can
be seen in Herring et al. (1979), in which
EMG recordings from diverse regions of
the complex masseter muscle of pigs
showed different periods of activity
depending upon the gape. Those regions
whose fibers were at greatest mechanical
advantage when the jaws were open fired
first during closure, and those regions
which had the most advantageous force
vectors at occlusion fired last.
As noted above, many studies have now
shown that muscle fiber types may be distributed in vastly different ratios between
compartments (Gonyea and Ericson, 1977;
Armstrong et al., 1982; English and Letbetter, 19826). This is compelling evidence
for adaptive significance of compartmentalization. Regional innervation from compartmental motor neuron pools suggests a
pattern of recruitment that is organized
for energetic efficiency and mechanical
advantage. SO fibers may be concentrated
in compartments deep to the bones (having
the shortest leverage) because they are
232
SUSAN E. PETERS
recruited for posture, which may require
the least torque at the joint. The peripheral location of the faster fibers, and larger
motor units may supply a better mechanical advantage for these compartments if
greater torque is required when they are
recruited in the fast gaits (Taylor, 1978).
The study of compartmentalization is in
its infancy compared with other areas of
muscle structure and function. However,
even at this early stage, it is clear that compartments may act as functional units in
the hierarchy of muscle structure above
the level of the motor unit, but more discrete than the whole muscle.
The hierarchical design of vertebrate
skeletal muscle from the cellular to the
gross anatomical level is fundamental, and
may have both evolutionary and functional
implications. We have come a long way in
the last half century toward understanding
muscle structure and function. However,
we must recall that most descriptions of the
diversity of muscle structure and function
are generalizations from studies of relatively few species. It is now the job of comparative functional morphologists and
physiologists to extend these studies to a
greater range of vertebrates and a wider
variety of muscles. Through these efforts
we can come closer to separating the unique
from the general, and to understanding
the causal relationships between the diverse
functions of muscles and their structural
design.
ACKNOWLEDGMENTS
Experimental work was supported in part
by a grant from the United Medical
Research Foundation, and by the University of North Carolina and the State of
North Carolina. I am indebted to M. B.
Thomas and G. E. Goslow, Jr. for their
comments on the manuscript, and to two
anonymous reviewers. My thanks go to
Sandra F. Zane who produced thefinalfigures. Heartfelt thanks go to the other Symposium editors, Dennis Bramble, Ken Gordon and Ted Goslow for making this an
enjoyable experience.
This work is dedicated to Milton Hildebrand, scientist, educator and mentor.
His excellence in teaching and his holistic
approach to understanding vertebrate
structural adaptations has been an inspiration to me and functional morphologists
throughout the world.
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