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Human muscle spindlesMicrofilaments in the group IA sensory nerve endings.

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Human Muscle Spindles: Microfilaments in the
Group IA Sensory Nerve Endings'
2 D e p a r t m e n t of
Neurology, University of Minncsota Medical School,
Minneapolis, Minnesota 55414 a n d Laboratory of Neuropathology a n d
Nezrroanatomical Sciences and Laboratory of Neurochemistry, National
I n s t i t u t e of Neurological Diseases a n d Stroke, Bethesda, Maryland 20014
The fine structure of the group IA sensory nerve endings from
normal human muscle spindles was studied in transverse and longitudinal sections. Two arrangements of microfilaments, approximately 75 A in diameter,
were found in each of ten spindles examined. The first was a central aggregate
of densely packed filaments. The aggregates were partly surrounded by mitochondria, and were oriented parallel to the longitudinal axis of the sensory ending as it encircled the intrafusal muscle fiber. Individual aggregate filaments
of glycerinated endings appeared to react with heavy meromyosin. The second
arrangement was a filamentous network in the periphery of the sensory ending
profiles. These microfilaments approached and appeared to merge with the surface membrane. They resembled the microfilaments that others have described
in growth cones of cultured neurons. Both types of microfilaments are thought
to be involved in changing the shape of the sensory endings during stretch and
relaxation of the spindle.
Electron microscopic studies have shown
that nerve cells contain two kinds of filaments. Neurofdaments are the major
filamentous constituent and have been
studied intensively for more than 20 years
(Palay and Palade, '55; Schmitt and Samson, '68; Wuerker and Palay, '69; Wuerker,
'70; Wuerker and Kirlipatrick, '72). When
transversely sectioned, neurofilaments are
about 100 A in diameter; they have clear
centers and their walls are formed by
globular subunits that probably are
arranged in a helix. Neurofilaments are
widely distributed in the cell soma and
processes and are much more numerous
in larger neurons. Presumably, they provide structural support for nerve cells and
may also participate in intracellular
More recently, focal collections of microfilaments, 50-70 A in diameter, have been
found in neuronal regions that move namely growth cones and their microspikes
(Tennyson, '70; Yamada et al., '70, '71;
Wessells et al., '71). In cultured neurons,
the movements of growth cones seems to
depend on the integrity of these microaaANAT. REC., 180: 521-532.
ments which can be reversibly disrupted
by cytochalasin B (Yamada et al., '70, '71;
Wessells et al., '71). Microfilaments are
arranged in a network or lattice and resemble actin in size (Spooner et al., '71;
Wessells et al., '71; Yamada et al., '71;
Bunge, '73; Ludena and Wessells, '73).
An actin-like protein has also been isolated
from similar cultures of actively growing
dissociated neurons (Fine and Bray, '71).
Microfilaments are also present in a
variety of other cells. Some are arranged
in a network while others are parallel to
the long axis of the cell and form a sheath
beneath the plasma membrane (Buckley
and Porter, '67; Pollard et al., '70; Goldman, '71; McNutt et al., '71; Spooner et al.,
'71; Ludena and Wessells, '73). Microfilaments may react with heavy meromyosin
(HMM) to form arrowhead complexes
(Ishikawa et al., '69; Pollard and Korn,
'71; Spooner et al., '73; Chang and Goldman, '73; Ludena and Wessells, '73). These
filaments are also thought to be important
Received Feb. 18, '74. Accepted May 14, '74.
1 Supported i n part by a grant from the ALS Foundation, St. Paul, Minnesota.
52 1
in cell movement. Because of the current
interest in this topic, it seemed worthwhile
to describe collections of microfilaments
75 A in diameter, that were found in
the group IA sensory endings (primary
sensory endings) of normal human muscle
Muscle spindles were obtained from the
Extensor Indicis muscle of human volunteers,J aged 21 to 47, who were judged
normal from the findings of neurological
examinations, nerve conduction tests, electromyography and muscle biopsy. Nerve
conduction studies were performed on
peroneal, ulnar, median and radial nerves
ipsilateral to the biopsied muscle. Needle
electrode testing was performed for at least
one muscle innervated by each of the
above nerves. Portions of each biopsy were
frozen, sectioned and stained with hematoxylin and eosin and with a modified
Gomori trichrome stain (Engel and Cunningham, '63); other frozen sections
were stained for adenosine triphosphatase
(Padykula and Herman, '55), alpha glyceryl phosphatase (Hess et al., '58), and
diphosphopyridine nucleotidase (Novikoff
et al., '59). In addition, intramuscular
nerves and motor end plates were stained
intravitally with methylene blue in order
to evaluate the terminal motor innervation
(Coers and Woolf, '59).
For this study, eight muscle spindles
were obtained by biopsy of spindle-rich
regions of Extensor Indicis of seven
normal subjects (Van Gorp and Kennedy,
'74). The biopsies were immersed in buffered glutaraldehyde in a stretched state
and the spindles isolated as previously described (Kennedy et al., '74). Then they
were post-fixed in osmium tetroxide, dehydrated, stained with uranyl acetate in absolute ethanol, and embedded in epon. Both
transverse and serially mounted longitudinal sections of the spindles' equatorial
regions that contained the primary sensory
endings were studied with the light and
electron microscopes.
Myosin and heavy meromyosin (HMM)
were prepared by methods that have been
reported (Jean et al., '73). Spindles from
an eighth subject were isolated in bicarbonate buffered Kreb's solution bubbled
with 95% 0, and 5% CO, at pH 7.35.
They were next placed into 50% glycerol
at room temperature for one hour, then
kept at 6°C overnight. The polar regions
and a portion of the capsule were removed
from the equatorial region which contained the primary sensory ending. Some
equatorial regions were reacted for 24
hours with HMM (3.5 mg/ml) in 25%
glycerol, others were incubated with HMM
plus 0.1 M adenosine triphosphate (ATP).
They were then washed in distilled water,
fixed in glutaraldehyde and prepared as
above for electron microscopy.
The general arrangement of the primary
sensory ending corresponded to that which
has already been described in man
(Kennedy, '70) and other species (Ruffini,
'98; Barker, '48; Boyd, '62; Corvaja et al.,
'69; Banker and Girvin, '71). Briefly, in
each of the spindles examined, the large
myelinated group IA sensory axon formed
a series of myelinated branches. Each
terminal branch became unmyelinated,
then lost its Schwann cell covering. As it
approached and joined a n intrafusal muscle fiber, the axon was covered only by
basement membrane (fig. 1). The primary
sensory endings partially encircled the nuclear region of the muscle fiber and occasionally also branched to supply an adjacent muscle fiber.
Each of the primary sensory endings
examined contained microfilaments about
75 A in diameter, along with neurofilaments, microtubules, mitochondria, agranular reticulum, vesicles, and glycogen
granules. Two arrangements of microfilaments were found. The first included long,
ovoid aggregates of parallel, densely
packed filaments. These filament aggregates were more commonly observed in
longitudinal sections of the spindles. In
this plane the intrafusal muscle fibers were
also sectioned longitudinally but the sensory endings which partially encircled each
muscle fiber were sectioned obliquely or
transversely (fig. 1 ) . In these sections the
filament aggregates were centrally located
in the ending and partially surrounded by
mitochondria (fig. 1) .
With the approval of the University of Minnesota
Committee on use of human volunteers in research
on January 21, 1972.
In single, random sections about onethird of the ending profiles along a nuclear
bag fiber contained aggregates. When the
spindle was examined in serial longitudinal
sections, microfilament aggregates were
found in endings at some level in all ten
spindles that were examined. Some aggregates bifurcated so that two to three were
occasionally seen within one ending profile
(fig. 1 ) ; they were less common in endings on nuclear chain fibers. Individual
microfilaments within aggregates followed
a slightly wavy course and occasionally
branched. The most peripheral filaments
of the aggregates were in close approximation to the mitochondria (figs. 2-4). Longitudinally or obliquely sectioned microfilaments appeared to either terminate or
change course at the aggregate-network
interface (fig. 3 ) . Dense bodies about 240
A in diameter were sometimes located in
this region.
The second microfilament arrangement
was a peripheral polygonal network which
occupied much of the outer circumference
of the ending immediately beneath the
plasmalemma (fig. 3 ) . Often the network
intervened between filament aggregates
and the plasma membrane and some filaments extended from the aggregates into
the network. Filaments in the network frequently approached and occasionally appeared to merge with the plasma membrane. The filament networks of endings
on nuclear bag and nuclear chain muscle
fibers were similar
The majority of microfilaments i n the
aggregates and in the networks measured
65-80 A (figs. 3 , 4). This was slightly
larger than actin filaments of the I band
of muscle within the same sections which
were 60-75 A (figs. 2, 5) and distinctly
smaller than neurofilaments of the axon
more proximally which averaged 100 A
(fig. 6 ) . Neurofilaments were infrequently
found within the sensory endings of
humans. Where present, they occurred in
small accumulations among the mitochondria. The interfilament spacing and cross
bridges of neurofilaments were similar to
those seen more proximally in the axon.
Glycerinated human and cat spindles
were reacted with HMM to determine
whether the microfilaments would form
arrowhead complexes similar to those
formed with actin and with actin-like
microfilaments from other cell types
(Ishikawa et al., '69; Pollard and Korn,
'71; Spooner et al., '73; Chang and Goldman, '73; Ludena and Wessells, '73).
Heavy meromyosin was bound to actin filaments of intrafusal muscle as evidenced
by obliteration of the I bands, but typical
arrowhead formations with actin were difficult to find. The aggregate microfilaments
in the sensory endings were more widely
spaced than microfilaments in glycerinated
endings treated with HMM plus ATP or in
the nonglycerinated endings. Thin side
projections, suggestive of arrowheads, were
seen on many filaments (fig. 7 ) . Arrowhead formations were not observed in
spindles from the same patient in the presence of Mg-ATP, which specifically dissociates the complex (fig. 8 ) . The peripheral
microfilamentous network was also distended in glycerinated spindles. This
allowed better visualization of the extension of aggregate filaments into the network, and of the merging of network filaments with the plasma membrane. The
network filaments of human and cat sensory endings were similar, but the cat endings did not contain aggregate filaments.
Two arrangements of microfilaments,
approximately 75 A in diameter, were
found i n the primary sensory endings of
human muscle spindles; a microfilamentous network under the plasma membrane
and centrally located aggregates of parallel
microfilaments. The microfilamentous network near the endings' plasma membrane
resembles the microfilamentous lattice in
growth cones and microspikes of cultured
neurons (Wessells et al., '71; Yamada
et al., '71; Spooner et al., '71; Bunge, '73).
There is now a substantial amount of evidence that these microfilaments are involved in the motion of microspikes and
ruffled membranes (Ludena and Wessells,
' 7 3 ) . When growing axons of cultured
neurons are exposed to cytochalasin B,
growth cones and their microspikes retract,
and their microfilamentous networks become disorganized. Removal of the agent
reverses this process (Yamada et al., '70,
'71 ). Membrane movement also ceases
when other types of cells are treated with
cytochalasin B (Schroeder, '70; Wessells
et al., '71; Spooner et al., '71; Cloney, '73).
Similar exposure of human muscle spindles to cytochalasin B during recording of
the sensory responses to spindle stretch,
and followed by examination of the fine
structure, would help clarify the role of the
microfilaments which we have observed.
However, techniques for recording the responses of human primary sensory endings have only recently become available
(Poppele and Kennedy, '74). Until the observations with cytochalasin B are made, i t
seems reasonable to suggest that this subplasmalemmal microfilamentous network
may be involved in adjusting the ending's surface membrane to the changes in
shape that accompany spindle stretch and
Analogues of the parallel microfilamentous aggregates of primary sensory endings are not found in cultured dorsal root
neurons, their axons or growth cones
(Ludena and Wessells, '73). However,
other cells, such as cultured glial cells,
fibroblasts and macrophages, contain
sheath filaments which share many features with filaments in the aggregates
(Buckley and Porter, '67; Ishikawa et al.,
'69; Spooner et al., '71; McNutt et al., '73;
Goldman, '71; Reavin and Axline, '73).
Sheath filaments are bundles of microfilaments which are parallel to the direction
of cell movement. They resemble actin in
size and they react with HMM to form
arrowhead complexes (Spooner et al., '73 ).
A thin layer of lattice microfilaments
separates the sheath microfilaments from
the plasma membrane. Some sheath filaments diverge from the bundle to enter
the lattice work in the region under the
microspikes on the cell body. Exposure to
cytochalasin B does not alter these filament
arrangements (Spooner et al., '71).
The aggregate microfilaments in human
spindles also combine with HMM, but the
arrowhead complexes were not as numerous on aggregate filaments or on the actin
filaments of intrafusal muscle as reported
for microfilaments in other cells. It is possible that this was due to poor penetration
of HMM through the several layers of
inner and outer capsule which could not
be completely removed by dissection, and
also to examination of embedded sectioned
material rather than negatively stained
suspensions. Like sheath filaments, the
aggregate filaments exist in large bundles
and have extensions into a microfilamentous network which separates it from the
plasma membrane. However, the aggregates are larger and longer than the sheath
bundles; the individual filaments are more
nearly parallel, are closer together, and
occasionally they branch. Aggregate filaments are also slightly larger than either
sheath filaments or actin found in intrafusal muscle, however they are similar
in diameter to actin in extrafusal muscle fibers (Huxley, '63; Hanson and
Lowry, '63).
The presence of sheath microfilaments
in migratory cells and their absence from
non-motile cells has suggested that they
possess contractile properties which help
the cell body move across a substrate behind the extensions of its ruffled membrane
(Ludena and Wessells, '73). Although
movement is not generally considered to be
a property of adult sensory nerve endings
the similarities between aggregate and
sheath microfilaments suggest that they
are involved in similar actions. Either a
contractile or structural support €unction
would conceivably be useful to adapt the
sensory ending to the rapid mechanical
forces to which it is subjected. During
muscle stretch these forces are accentuated for the primary sensory ending by the
ease of distortion of the underlying muscle
segment which has a low internal viscosity
because of the sparsity of myofibrils.
A difficulty in determining the functional significance of aggregate microfilaments is their absence from the well
studied primary endings of the cat spindle
(Corvaja et al., '69). This is surprising,
because linear analysis of the sensory
responses from cat and human primary
endings to small amplitude sinusoidal
stretch indicates identical sensory behavior (Poppele and Bowman, '70; Poppele and Kennedy, '74). It is possible
that other structures found in cat sensory
endings perform the same function as
aggregate filaments in human spindles.
Recent preliminary observations in our
laboratory have shown that cat spindles
fixed in a relaxed state contained many
microtubules i n the sensory endings and
large accumulations of actin size filaments arranged in peripheral networks and cenoutside the endings under the muscle tral aggregates which have similarities to
plasma membrane. I n endings of cat spin- the actin-like microfilamentous lattice in
dles fixed during stretch, there were fewer nerve growth cones and to the sheath filamicrotubules but many neuroflaments. ments in other cell types. The filaments i n
There was also a dramatic reduction in these sensory endings may participate in
size of the filament accumulations outside accommodation of the endings to stretch
of the endings (Kennedy and Poppele, un- and relaxation of the intrafusal muscle by
published observations).
changing the shape of the ending.
The careful selection of young subjects,
and the uniformity of the findings in all
studied spindles strongly suggest that miDr. Andrew Engel's helpful advice durcrofilaments are present in normal human ing the initial phase of this study was
spindles and are not associated with aging greatly appreciated.
or a neuropathologic process. None of the
subjects included i n the study gave a
P. Davies and S. DePetris 1971
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Nuclear bag intrafusal ( I F ) muscle fiber longitudinally sectioned at
the equatorial area. The myofibrils on either side of the bag nuclei
have a well defined H zone and M line. Two of the primary sensory
ending profiles contain prominent central aggregates of filaments cut
transversely (upper arrow) and obliquely (lower arrow). Clusters of
mitochondria partially surround and form a partition between adjacent aggregates. i: 6,800.
Longitudinal section of a sensory ending that is covered by basement
membrane of the IF muscle fiber. A large aggregate of longitudinally
sectioned parallel microfilaments is separated from the plasma membrane by a network of microfilaments. The actin filaments in the I
band are slightly smaller than the ending’s fine filaments. x 44,200.
W. Kennedy, H. Webster, K. Yoon s n d D. Jean
Same ending as figure 2 , shown at higher magnification. Aggregate
microfilaments are slightly wavy and intermingle with filaments of
the network. The double arrow indicates transversely sectioned (left)
and longitudinally oriented (right) network microfilaments. Occasionally network filaments appear to merge with the plasma membrane (top arrow). x 88,600.
Transverse section of a large aggregate in a sensory ending. The
microfilaments (circle) measure about 75 A in diameter. x 88,600.
Transverse section of intrafusal actin and myosin filaments at the
same magnification as figures 4 and 6. The thin (actin) filaments
measure about 65 A in diameter. x 88,600.
Transverse section of neurofilaments and microtubules in an adjacent
sensory axon at the same magnification as figures 4 and 5. The neurofilaments measure about 100 A in diameter and are much larger than
the microfilaments shown in figure 4. x 88,600.
W. Kennedy, H. Webster, K. Yoon and D. Jean
W. Kennedy, H. Webster, K. Yoon and D. Jean
Aggregate microfilaments in a sensory ending from a spindle incubated in HMM. Interfilament spacing is increased and thin side
projections suggestive of arrowhead complexes are on several microfilaments which are shown a t the same magnification as figures 4-6.
X 88,600.
Aggregate microfilaments from another spindle of the same biopsy
used in figure 7. The incubation with HMM was supplemented by the
addition of ATP. Side projections representative of arrowhead complexes were not present. x 88,600.
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muscle, group, ending, nerve, sensore, spindlesmicrofilaments, human
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