Group Title: BMC Physiology
Title: The Transduction properties of intercostal muscle mechanoreceptors
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Title: The Transduction properties of intercostal muscle mechanoreceptors
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Language: English
Creator: Holt, Gregory
Johnson, Richard
Davenport, Paul
Publisher: BMC Physiology
Publication Date: 2002
 Notes
Abstract: BACKGROUND:Intercostal muscles are richly innervated by mechanoreceptors. In vivo studies of cat intercostal muscle have shown that there are 3 populations of intercostal muscle mechanoreceptors: primary muscle spindles (1°), secondary muscle spindles (2°) and Golgi tendon organs (GTO). The purpose of this study was to determine the mechanical transduction properties of intercostal muscle mechanoreceptors in response to controlled length and velocity displacements of the intercostal space. Mechanoreceptors, recorded from dorsal root fibers, were localized within an isolated intercostal muscle space (ICS). Changes in ICS displacement and the velocity of ICS displacement were independently controlled with an electromagnetic motor. ICS velocity (0.5 – 100 µm/msec to a displacement of 2,000 µm) and displacement (50–2,000 µm at a constant velocity of 10 µm/msec) parameters encompassed the full range of rib motion.RESULTS:Both 1° and 2° muscle spindles were found evenly distributed within the ICS. GTOs were localized along the rib borders. The 1° spindles had the greatest discharge frequency in response to displacement amplitude followed by the 2° afferents and GTOs. The 1° muscle spindles also possessed the greatest discharge frequency in response to graded velocity changes, 3.0 spikes·sec-1/µm·msec-1. GTOs had a velocity response of 2.4 spikes·sec-1/µm·msec-1 followed by 2° muscle spindles at 0.6 spikes·sec-1/µm·msec-1.CONCLUSION:The results of this study provide a systematic description of the mechanosenitivity of the 3 types of intercostal muscle mechanoreceptors. These mechanoreceptors have discharge properties that transduce the magnitude and velocity of intercostal muscle length.
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Research article

The transduction properties of intercostal muscle
mechanoreceptors
Gregory A Holt1,2, Richard D Johnson1 and Paul W Davenport*1


Address: IDepartment of Physiological Sciences College of Veterinary Medicine University of Florida Gainesville, Florida, USA 32610 and 2Current
Address: School of Allied Health Sciences Florida Agricultural and Mechanical University Benjamin Banneker Building C Tallahassee, FL 32307
E-mail: Gregory A Holt tsdc2002@aol.com; Richard D Johnson johnson@ufbi.ufl.edu;
Paul W Davenport* davenportp@mail.vetmed.ufl.edu
* Corresponding author


Published: 22 October 2002
BMC Physiology 2002, 2:16


Received: 8 August 2002
Accepted: 22 October 2002


This article is available from: http://www.biomedcentral.com/1472-6793/2/16
2002 Holt et al; licensee BioMed Central Ltd. This article is published in Open Access: verbatim copying and redistribution of this article are permitted
in all media for any purpose, provided this notice is preserved along with the article's original URL.





Abstract
Background: Intercostal muscles are richly innervated by mechanoreceptors. In vivo studies of cat
intercostal muscle have shown that there are 3 populations of intercostal muscle
mechanoreceptors: primary muscle spindles (1), secondary muscle spindles (2) and Golgi tendon
organs (GTO). The purpose of this study was to determine the mechanical transduction properties
of intercostal muscle mechanoreceptors in response to controlled length and velocity
displacements of the intercostal space. Mechanoreceptors, recorded from dorsal root fibers, were
localized within an isolated intercostal muscle space (ICS). Changes in ICS displacement and the
velocity of ICS displacement were independently controlled with an electromagnetic motor. ICS
velocity (0.5 100 pIm/msec to a displacement of 2,000 pIm) and displacement (50-2,000 pIm at a
constant velocity of 10 lim/msec) parameters encompassed the full range of rib motion.
Results: Both I 0 and 20 muscle spindles were found evenly distributed within the ICS. GTOs were
localized along the rib borders. The 10 spindles had the greatest discharge frequency in response
to displacement amplitude followed by the 20 afferents and GTOs. The 1 muscle spindles also
possessed the greatest discharge frequency in response to graded velocity changes, 3.0 spikes-sec-
I/ljm-msec-1. GTOs had a velocity response of 2.4 spikes-sec-'/ltm-msec-1 followed by 20 muscle
spindles at 0.6 spikes-sec- I/lim-msec-1.
Conclusion: The results of this study provide a systematic description of the mechanosenitivity
of the 3 types of intercostal muscle mechanoreceptors. These mechanoreceptors have discharge
properties that transduce the magnitude and velocity of intercostal muscle length.


Background
The location and numbers of intercostal muscle mech-
anoreceptors suggest that they mediate the transduction
of chest wall contractile mechanics [1-3]. Intercostal mus-
cle spindles have been identified histologically [3]. These
afferents increase their discharge frequency during inspi-


ration and decrease their activity during expiration [3]. In-
tercostal muscle mechanoreceptors enter the spinal cord
through the thoracic dorsal roots [3]. These chest wall
mechanoreceptors are responsible for segmental and in-
tersegmental proprioceptive feedback from the intercostal
muscles. Intercostal afferents have been shown to alter in-


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Primary Muscle Spindles, n=l 1
Secondary Muscle Spindles, n=10
Golgi Tendon Organs, n= 10
Non-Characterized, n=25




Electromagnetic Linear
Displacement Motor


Rigid Clamp
for Fixed Rib


Figure I
A schematic representation of the experimental preparation. The cranial rib of the isolated rib space is fixed to a rigid
clamp. The caudal rib is connected to the electromagnetic displacement motor. The receptive field locations of the intercostal
muscle mechanoreceptors are shown. The I 0 muscle spindles are represented by the open circles. The 20 muscle spindles by
the closed circles, the Golgi tendon organs by the open squares, and the uncharacterized mechanoreceptors by the closed
squares. A total of 56 mechanoreceptors in 35 cats were found.


spiratory muscle motor activity [4-7]. The activity of res-
piratory muscle afferents provides essential muscle
mechanical information during ventilation [3]. This is
supported by previous reports that have shown these re-
ceptors to be sensitive to changes in muscle length and
stretch velocity and are active during spontaneous breath-
ing [1-3,8].

There have been relatively few studies investigating the
mechanical transduction properties of intercostal muscle
mechanoreceptors, with mechanical stimulation confined
to the tidal breathing range [1,2]. Hindlimb muscle prep-
arations [9-11] and similar studies have shown that there
are two basic types of muscle spindles receptors, 10 and 20
endings. The 10 muscle spindles were shown to be more
responsive to the velocity of stretch and the 20 afferents
were sensitive to static length changes. A third type of
mechanoreceptor, the Golgi tendon organ (GTO), was
also responsive to muscle movement (contraction). Von
Euler & Peretti [2] found that all three types of mech-
anoreceptors (GTOs, 10 and 20 muscle spindles) increase
their discharge frequency in response to changes in mus-


cle length and stretch velocity in the tidal breathing range.
This is supported by intra-axonal recording of 1 muscle
spindles demonstrating increased discharge with inspira-
tion during tidal breathing in the cat [3]. While it is
known that intercostal mechanoreceptors are sensitive to
muscle motion, the specific afferent response to inde-
pendently controlled changes in muscle displacement and
velocity over the tidal volume range of rib motion is un-
known. It was hypothesized that intercostal muscle 1
muscle spindles, 20 muscle spindles and GTO's transduce
muscle length changes with increased length associated
with increased discharge frequency. It was further hypoth-
esized that 10 intercostal muscle spindles and GTO's also
code muscle velocity changes with increased velocity asso-
ciated with an increase in discharge frequency.

Results
Fifty-six intercostal muscle mechanoreceptors were local-
ized. Muscle spindles accounted for 21 of these mech-
anoreceptors, 10 were identified as Golgi tendon organs,
and 25 mechanoreceptors were not characterized. The re-
ceptive field locations of these afferents were distributed


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A. Raw afferent frequency discharge


h 1t l l J1LM i lt llhfliiliufIIUhJiili llt iIlIllllli JIJ iIlll iillll Jl i J I I 1 1 ii


* .Ad".. JL 1 A d A l.. IU h.- .~ I I I I r IL


Rest phase \Ramp stretch


SI 2.0V


I Static displacement hold phase


500 msec

Figure 2
Intercostal muscle spindle response to controlled intercostal space displacement. (a) Afferent spike activity, (b)
The intercostal space displacement was to a plateau of 2,000 [tm at 10 Im/msec ramp stretch. The calibration for the length
change in this trace is: 2.0 V = 1000 pm displacement.


throughout the intercostal muscle (Fig. 1). The receptive
fields of the isolated mechanoreceptors were discretely lo-
calized within a 3 6 mm circumferential area. The mus-
cle spindles were distributed evenly along the dorsal-
ventral extent of the 7th intercostal space. The Golgi ten-
don organs were located primarily along the caudal edge
of the fixed cranial rib with a few isolated along the cranial
edge of the caudal rib.

Eleven receptors were identified as 10 muscle spindles and
10 were characterized as 20 muscle spindles. The 10 affer-
ents possessed higher conduction velocities (CV = 68.5
8.7 m/sec) than 20 endings (CV = 43.8 6.2 m/sec), al-
though there was an overlap between the two (Fig. 3). The
10 mechanoreceptors classified as Golgi tendon organs
had a mean CV of 41.9 8.8 m/sec.

Muscle spindles were studied with and without intact ven-
tral roots. There was no significant difference in the Lini
static discharge frequencies after the ventral roots were
severed indicating there was no appreciable activity in
gamma motor axons. Increasing ICS displacement result-
ed in an increased discharge frequency in all 1 muscle
spindles (Fig. 4). The Linit static discharge frequency for all
10 muscle spindles was 16 2 spikes/sec. The slope of stat-
ic displacement discharge frequency was found to be 8.8
spikes/sec/mm (Fig. 4). The range of static displacement
discharge frequency for 10 afferents was found to be 18.8
+ 5 spikes/sec at 50 |im displacement to 34.5 5 spikes/


sec at 2000 |im displacement. The peak instantaneous fre-
quency of 10 muscle spindles occurred during the ramp
phase of displacement. The peak instantaneous frequency
also increased with increasing velocity of stretch. 10 mus-
cle spindles had a characteristic adaptation of discharge
frequency at the end of the ramp phase of stretch. The Linit
static discharge and static hold phase discharge frequen-
cies were unaffected by ventral root section.

The 20 muscle spindles were also found to increase their
discharge frequency with increasing amplitudes of inter-
costal space displacement. The Linit static discharge fre-
quency of the 20 muscle spindles was not affected by
ventral root section. The response of 20 muscle spindles to
increasing displacement amplitudes had a slope of 3.7
spikes/sec/mm. The mean Linit static discharge of these af-
ferents was 17 2 spikes/sec (Fig. 4). The range for dis-
placement frequencies for 20 muscle spindles was 19 2
at 50 [tm to 26 3 spikes/sec at 2000 [tm.

Golgi tendon organs increased their discharge frequency
with increasing displacement amplitudes. Most of the
GTO's had only small increases in frequency with static
changes in intercostal muscle length. The slope of the
curve was 2.7 spikes/sec/mm. The mean Lini static dis-
charge frequency for GTO's was 15 4 spikes/sec (Fig. 4).
The range of displacement discharge frequencies was 24 +
5 to 33 7 spikes/sec.



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* i i . m l-- -Ve







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Muscle Afferent Conduction Velocity


<20 21- 50 51- 80 > 80
CONDUCTION VELOCITY (M/sec)


Figure 3
Distribution of conduction velocities for muscle
mechanoreceptors. The mean conduction velocity of 10
and 20 afferents was 68.5 + 8.7 and 43.8 + 6.2 m/sec, respec-
tively. The mean conduction velocity for Golgi tendon organs
was 41.9 + 8.8 m/sec.


The group mean Linit static discharge frequency for 10 af-
ferents was 16 2 spikes/sec. The range of peak instanta-
neous frequencies for 1 afferents was 41 3 spikes/sec at
0.5 pm/msec velocity to 221 29 spikes/sec at 100 pim/
msec velocity (Fig. 5). The curve was sinusoidal so the
slope of the middle, linear range of the sinusoidal curve
(between stretch velocities of 5 to 50 pim/msec) was 3.0
spikes sec-1/im msec-1. The static discharge hold phase
frequency for 10 muscle spindles was 35 3 spikes/sec.

2 muscle spindles increased peak instantaneous frequen-
cies with increasing intercostal muscle stretch velocity
(Fig. 5). The range of discharge frequency was 25 3
spikes/sec at 0.5 pim/msec to 85 21 spikes/sec at 100
lpm/msec intercostals muscle stretch velocity. The slope of
the frequency response for 20 afferents was 0.6 spikes-sec-
1/tm msec-1 and was taken from stretch velocities be-
tween 5 and 100 pim/msec. The static hold phase frequen-
cy was 24 3 spikes/sec and the Linit static discharge
frequency was 17 3 spikes/sec. 20 muscle spindles were
found to have the lowest velocity sensitivity of the three
types of mechanoreceptors studied.

Golgi tendon organs mean Linit static discharge frequen-
cies was 16 2 spikes/sec (Fig. 5). The peak instantaneous
frequency range for GTO's was from 31 6 to 278 52
spikes/sec as velocities were increased from 0.5 100 pim/
msec. GTO's were found to possess a sensitivity of 2.4


spikes sec-1/im msec-1 between stretch velocities of 5
and 100 im/msec. The GTOs had a mean static hold
phase frequency of 26 4 spikes/sec.

Discussion
The results of this study provide a systematic description
of the mechanosenitivity of the three types of intercostal
muscle mechanoreceptors, 1 muscle spindles, 2 muscle
spindles, and Golgi tendon organs. These mechanorecep-
tors have discharge properties that transduce the magni-
tude and velocity of intercostal muscle length changes
into an afferent code that is transmitted to the central
nervous system via their spinal dorsal roots. These affer-
ents were located throughout the intercostal muscles. All
the mechanoreceptors studied increased their frequency
of discharge with increased velocities of intercostal space
stretch. With the exception of three Golgi tendon organs,
all of the mechanoreceptors increased their frequency of
discharge with increased magnitudes of intercostal space
displacement. These afferents enter the spinal cord
through the thoracic dorsal spinal roots and provide es-
sential information to the central nervous system on inter-
costal muscle mechanics.

In the present study, intercostal muscle mechanoreceptors
were characterized as muscle spindles (primary or second-
ary) or Golgi tendon organs using criteria based primarily
on physiological response patterns (see Methods). It has
been shown by others that compared to muscles in the cat
hindlimb where spindle classification can be based al-
most entirely on conduction velocity, non-limb muscles
contain primary and secondary muscle spindle afferents
that are generally slower conducting and exhibit overlap-
ping bimodal or unimodal conduction velocity spectra
[12,13]. These studies have shown that only the very fast-
est afferents innervate primary endings (>75 m/s) and the
negatively adapting discharge frequency during static hold
and the abrupt decrease in firing at the end of static stretch
are the best criteria for characterization of primary spindle
afferents. In the present study, intercostal muscle spindles
exhibited a bimodal but overlapping conduction velocity
distribution necessitating the use of these physiological
criteria.

The maximal displacement amplitude used in this study
was 2,000 rim. This magnitude is approximately equal to
the change in intercostal muscle length recorded during a
tidal breath with artificial and spontaneous respiration
[ 2,8,14,15 ]. Mechanoreceptors were recorded from the in-
tercostal muscles of the 7th intercos tal space. These are
the lower parasternal intercostal muscles and increases in
their length can occur during spontaneous and mechani-
cal ventilation. Von Euler & Peretti [2] used a change in in-
tercostal muscle length of 2 mm at a velocity of
approximately 1-2 mm/sec to study intercostal mech-


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anoreceptors. The present study used a velocity range of
0.5 mm/sec 100 mm/sec to span the velocities observed
in spontaneously breathing and ventilated animals. The
responsiveness of intercostal muscle mechanoreceptors to
stretch velocities and displacement amplitudes used in the
present study encompassed the physiologic range of inter-
costal muscle motion.

The intercostal muscle spindles localized in this study
show that 1 and 20 afferents could be found with equal
frequency along the dorsal ventral extent of the intercos-
tal space. Golgi tendon organs were also evenly distribut-
ed along the intercostal space at the tendinous attachment
of the muscle to the rib. None of the receptors localized in
these experiments were found in the most ventral regions
of the rib cage. This is consistent with the results of Von
Euler & Peretti [2], who found intercostal muscle spindles
in the cat to be evenly distributed along the dorsal ven-
tral extent with a slight decrease in the numbers of affer-
ents towards the more ventral aspects. This decrease in the
number of afferents ventrally may be related to the fact
that all of the receptors studied were recorded from the
rostral three dorsal rootlets supplying the 7th intercostal
space. These rootlets were used primarily due to their
longer length compared to the more caudal rootlets. Inter-
costal muscle mechanoreceptors are distributed through-
out the intercostals muscle with afferent nerve fibers
distributed throughout all the dorsal rootlets. The activity
of the mechanoreceptors functioning within the more ros-
tral rootlets were randomly recorded in the present study
and should be representative for the discharge properties
of the afferents throughout the intercostal space.

The static displacement sensitivity for intercostal muscle
mechanoreceptors has not been studied previously. 1
muscle spindles were found to possess the highest firing
frequency range with displacements to 2000 ipm. Cooper
[9] recorded muscle spindle response to static displace-
ments in cat soleus muscle and found sensitivities of 10
spikes/sec/mm for 10 spindles and 5 spikes/sec/mm for 20
afferents when the muscle was stretched in 1 mm incre-
ments to 4 mm displacement. The results of the present
study are similar to the sensitivities reported for the cat so-
leus muscle but differ from muscles of the digits in cats.
These differences are probably due to the significant me-
chanical variations in the function of these different mus-
cle groups. The intercostal muscle has a unique
attachment between the ribs that results in an angle of 50
- 60 degrees between the muscle fibers and the rib. A lin-
ear displacement of the rib space will therefore not pro-
duce a linear displacement of muscle fibers. The change in
fiber length is probably less than the change in space
width, resulting in an underestimation of the displace-
ment sensitivity. This is particularly true for changes in
tension and the associated discharge of the GTO. The ex-


40
= 1 Muscle Spindle
2o Muscle Spindle
S- -= GTO
35

>- 30

25

20


0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200
DISPLACEMENT AMPLITUDE (pmm)

Figure 4
The peak instantaneous discharge frequencies during
changes in static displacement for intercostal muscle
mechanoreceptors. The discharge frequencies are for: a)
I muscle spindles (n = I I); b) 20 muscle spindles (n = 10);
and c) GTO (n = 10). The mean discharge frequencies, + S.E.
are plotted.


periments described above show that tendon organs are
responsive to static changes in intercostal muscle length,
but at a sensitivity lower than that of either the 10 or 20
muscle spindles. Increasing the width of the intercostal
space may change the length of the muscle fibers less than
space width, producing smaller changes in muscle ten-
sion. This would reduce the stimulation of GTOs and their
displacement sensitivity would be underestimated. While
the absolute sensitivity to fiber displacement may be un-
derestimated for the mechanoreceptors recorded in this
study, the above results do provide important quantifica-
tion of the sensitivity of intercostal mechanoreceptors to
changes in the functional displacement of the intercostal
space.

The dynamic sensitivities of the receptors were found to
correspond to the ranges of discharge frequencies of affer-
ents characterized by other investigators [2,9,111. Mat-
thews [101 found that de-efferented 10 muscle spindles in
the soleus muscle of the cat responded to varied velocity
changes (1.2 100 mm/sec) in the range of 95 400
spikes/sec. Discharge frequencies found for primary mus-
cle spindles in the present study are also similar to those
for 10 afferents in other muscles with intact y-innervation
[9,16,17] and without y-innervation [18-20]. In these
studies, the 10 muscle spindles were found to possess the
greatest sensitivity to changes in velocity. Our finding of a
decrease in the frequency response at the highest velocity
of stretch used (100 pm/msec) shows a limitation of the
primary muscle spindle in coding for rapid length chang-
es.


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* =1 Muscle Spindle
T = 2 Muscle Spindle
* -GTO


100 A-


1 10
STRETCH VELOCITY (pm/msec)


Figure 5
The peak instantaneous discharge frequencies during
changes in displacement velocity for intercostal mus-
cle mechanoreceptors. The discharge frequencies are for:
a) 1 muscle spindles (n = I 1); b) 20 muscle spindles (n = 10);
and c) GTO (n = 10). The mean discharge frequencies, + S.E.
are plotted. The axes are plotted on log-log scale.


Matthews [10] described a frequency range for two 20 af-
ferents to be approximately 45 185 spikes/sec with ve-
locity changes of 1.2 mm/sec to 100 mm/sec to a final
length change of 6 mm in the soleus muscle of the cat. The
instantaneous burst frequencies obtained in this study for
2 afferents were also similar to those found in other stud-
ies [9,11,18]. The results obtained by these investigators
were from hindlimb muscles of the cat at 2 3 different
velocities ranging from 0.5 52 mm/sec. Intercostal mus-
cle afferents studied by Critchlow & Von Euler [14] were
shown to discharge at rates of 29 74 spikes/sec during
the inspiratory phase, but were not characterized as being
either 10 or 20 muscle spindles. The velocity of intercostal
width change was not determined in their study. In the
present study, the 20 afferents were found to discharge
with a slope of 0.6 spikes sec-1 /tm msec-1 and were the
least sensitive of the three types of respiratory mechanore-
ceptors to dynamic changes in intercostal muscle fiber
length.

The Golgi tendon organs in this study were found to have
a much greater response to the dynamic phase of intercos-
tal muscle stretch compared to the static response. Von
Euler & Peretti [2] reported that a few GTOs in cat inter-
costal muscle could follow vibrational frequencies within
the range of 1 muscle spindles. This suggests that the Gol-
gi tendon organs supplying respiratory musculature may
be more sensitive to changes in muscle mechanics than
those functioning in other muscle groups. Houk & Hen-
neman [21] considered the variable response of GTO dis-
charge to tension changes to be due to differences in


motor unit recruitment. These differences, i.e., numbers
of muscle fibers activated either parallel to or in-series
with the mechanoreceptor, create changes in the afferent
receptive field and alter its discharge properties. Passive
length changes caused a linear increase in force acting on
these receptors. On the other hand, the forces acting on
the receptor with active contraction were dependent on
the motor units activated. When the motor units in-series
with the receptor were activated, there was a linear force
applied to the receptor, resulting in increased impulse dis-
charge. However, if the neighboring extrafusal fibers were
stimulated to contract, they unload the tendon organ,
thus bringing it to a subthreshold level. Houk & Henne-
man [211 also found the GTOs to exhibit a saturation ef-
fect where by the responses to one motor unit and that of
a second motor unit did not sum algebraically. The prima-
ry role of this type of intercostal mechanoreceptor is hy-
pothesized to be the same as in other muscle groups, i.e.
the transduction and coding of muscle tension.

The results of the present study provide a unique demon-
stration of intercostal muscle mechanoreceptor transduc-
tion properties throughout a wide range of intercostal
muscle displacement magnitudes and velocities. These af-
ferents transduce the intercostal muscle mechanical
events into a neural code that projects to the central nerv-
ous system [221 via thoracic spinal dorsal roots.

Conclusions
The results of this study provide a systematic description
of the mechanosenitivity of the 3 types of intercostal mus-
cle mechanoreceptors. These mechanoreceptors have dis-
charge properties that transduce the magnitude and
velocity of intercostal muscle length. These afferents trans-
duce the intercostal muscle mechanical events into a neu-
ral code that projects to the central nervous system.

Methods
These experiments were performed on 35 cats of either sex
weighing 3.1 0.8 kg. The animal was placed in an an-
esthesia chamber and anesthesia was induced with inha-
lation of halothane/oxygen gas. A catheter was placed in
the femoral artery to continuously measure blood pres-
sure (Konigsberg Instruments No. P36) and obtain arteri-
al blood for periodic blood gas analysis (Radiometer ABL
30). Arterial Pco2, P02 and pH were maintained at con-
trol levels throughout the experiment. A second catheter
was placed in the femoral vein for intravenous access.
Body temperature was maintained at 38 + IC by the use
of a heating pad (Neco Model 819) and monitored with a
rectal temperature probe (YSI Instruments). Following the
catheterization procedures, anesthesia was switched from
induction gas to a-chloralose (25 mg/kg) administered
i.v. A tracheal catheter was inserted and the animal artifi-
cially ventilated (Harvard Respiratory Pump). Sodium bi-


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carbonate was administered as necessary for maintenance
of arterial pH at 7.4 + 0.1.

The animal was placed in a stereotaxic apparatus and its
head fixed in a non-traumatic head-holder. The spinous
processes of T3 and T10 were clamped. A wide skin inci-
sion was made on the left flank and the cutaneous tissues
dissected away to expose the external intercostal muscles.
A laminectomy was performed from T4 to T8 to expose
the dorsal spinal cord. The spinal cord was flooded with
warm mineral oil and the dura cut longitudinally to ex-
pose the dorsal root filaments. A single dorsal root fila-
ment supplying the 7th intercostal space (ICS) was
severed at its attachment to the spinal cord and placed on
a platform lowered into a warm mineral oil pool. The fil-
ament was initially placed across bipolar platinum record-
ing electrodes. The signal was amplified (Grass P-5), band
pass filtered (100 Hz 1 kHz) and led to an oscilloscope
(Tektronix 5111), audio monitor (Grass AM 8) and FM
tape recorder (Vetter Model D) for subsequent analysis.
The 7th intercostal space was probed to assure that this in-
tercostal space contained the afferents in the dorsal root
filament.

Initial muscle length (Linit) was measured at functional re-
sidual capacity (FRC) with calipers. The intercostal mus-
cles of the rostral (6th) and caudal (8th) rib spaces were
severed, isolating the 7th intercostal space to be studied.
Lung inflation was maintained with a positive end expira-
tory pressure of 3 cmH20. The rostral rib of the isolated
space was fixed with a rigid clamp (Fig. 1). The caudal rib
was attached by two clamps to the armature of the dis-
placement motor (Ling V203). A regulated voltage was ap-
plied to the displacement motor which moved the
armature and delivered muscle stretch at controlled veloc-
ities and magnitudes. Except during mechanical stimula-
tion of the 7th intercostals space, the wound and exposed
intercostal muscles were covered with gauze soaked with
warm saline.

The isolated ICS was probed for muscle mechanoreceptor
activity. When activity was observed, the dorsal root fila-
ment was subdivided until the activity of a single intercos-
tal muscle mechanoreceptor was isolated as described
previously [23]. The receptive field was localized by prob-
ing with a fine tipped glass rod. The localized respiratory
muscle afferent (RMA) was characterized by the muscle
spindle "silent period" test [24,25]. Conduction velocity
(CV) was obtained by electrically stimulating the afferent
receptive field and measuring the distance between recep-
tive field and recording electrodes [10]. Further character-
ization was performed during analysis of RMA response to
stretch (see below). The rib space was set to Linit and base-
line RMA discharge frequency recorded. The displacement
protocol was then initiated. A square wave pulse of con-


trolled duration triggered a trapezoid generator (Frederick
Haer & Co. #2337) that delivered a controlled trapezoidal
waveform to the displacement motor. The motor, in turn,
produced stretch of the intercostal space with varying dis-
placements and velocities.

Displacement protocol
The response to intercostal muscle displacement was de-
termined by applying graded amplitudes of stretch to the
intercostal space. The intercostal space was set at Linit, the
width measured at FRC. The displacement amplitudes
were applied above Linit. The amplitudes were: 50, 100,
200, 400, 600, 800, 1000, 1200, 1400, 1600, 1800, and
2000 pm. These displacements were presented at a con-
stant velocity of 10 gim/msec. Each displacement test was
initiated by activating the displacement motor which
pulled the caudal rib at a constant velocity to the predeter-
mined amplitude. The stretch was then maintained for 3
seconds, after which the displacement motor returned the
rib to Linit. There was a 10 second rest period before the
next displacement test was initiated. The discharge of the
afferent was recorded throughout the stretch.

The displacement protocol began with at least 3 displace-
ment stretches of 2,000 gm to provide a constant stretch
history in the muscle. The ICS was displaced at each am-
plitude 3 times in an ascending order of amplitudes. The
response of the mechanoreceptors to graded displacement
of the ICS was determined by measuring the frequency of
discharge at two different points during the applied
stretch. The first measurement was taken at Linit. This fre-
quency was the baseline discharge of the respiratory mus-
cle afferent. The second discharge frequency was
determined 2 seconds after the end of the ramp phase
(Fig. 2). The discharge frequency at this point was termed
the static hold phase frequency.

Velocity protocol
The response of RMAs to dynamic changes in intercostal
muscle length was determined by varying the stretch ve-
locities of the intercostal space. This experimental trial fol-
lowed the displacement test. The intercostal space was set
at Linit and the velocities applied to displacements above
Linit The velocities tested were: 0.5, 1.0, 2.0, 5.0, 10, 20,
50, and 100 gim/msec. Each velocity test was initiated by
activating the displacement motor to deliver a 2000 gim
displacement at the predetermined velocity. The displace-
ment was maintained for 3 seconds after the completion
of the velocity ramp. The rib was then returned to Linit and
the muscle allowed to rest for 15 seconds. The next veloc-
ity test was then initiated. Each velocity was tested 3 times.
The discharge of the afferent was recorded throughout the
stretch. The velocity protocol again began with a series of
3 displacement stretches to 2,000 gm to provide a con-
stant stretch history.


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The response of the intercostal muscle afferents to graded
changes in displacement velocity were then determined.
The discharge frequencies measured were the peak instan-
taneous, static hold phase, and Linit static discharge fre-
quencies. The peak instantaneous frequency was obtained
during the ramp stretch of the intercostal muscle and was
the maximum frequency of discharge. The static Linit dis-
charge and static hold phase frequencies were taken from
positions as described in the displacement protocol.

Data analysis
Afferents were classified as Golgi tendon organs if electri-
cal stimulation of the adjacent extrafusal muscle fibers
caused the receptor to increase its discharge frequency.
Those afferents that exhibited a pause in discharge fre-
quency as the extrafusal fibers contracted ("silent period")
were classified as muscle spindles [24,25]. Primary muscle
spindles were differentiated from secondary muscle spin-
dles with the satisfaction of at least two out of three crite-
ria tested on every afferent: 1) a conduction velocity > 75
m/sec; 2) an increase in discharge frequency at 100 gm
displacement reflecting a low stretch threshold; 3) an
abrupt decrease in frequency discharge at the end of
stretch that returned to Linit static discharge levels. In ad-
dition to these criteria, the frequency discharge at the end
of the dynamic phase of stretch was used as a distinguish-
ing characteristic of 1 and 2 muscle spindles. Those
spindles that decreased discharge frequency gradually to
static hold phase levels could be identified as primary
endings. Secondary afferents reached static hold phase
discharge frequency at the end of ramp stretch and
showed little adaptation during the static phase. These dif-
ferentiation criteria have been used by others in studies of
non-limb muscles where conduction velocity of only the
very fastest spindle fibers can be associated with primary
spindle endings [12,13]. The data recorded on magnetic
tape was analyzed with the aid of the Spike 2 (Cambridge
Electronics Design, Ltd.) computer program. The analog
signal from the LVDT transducer connected to the dis-
placement motor provided a measure of length changes
and the output was sent from the magnetic tape recorder
to a digitizing signal processor (CED 1401). The afferent
spikes were passed through a slope/height window dis-
criminator (Frederick Haer & Co.) and then sent to the sig-
nal processor as transistor transistor logic (TTL) pulses.
The digitizing rate of the afferent signal was 3000 Hz. The
program was designed to provide the instantaneous fre-
quency of the afferent spikes at the corresponding ampli-
tudes of the displacement motor that produced the
muscle stretch.

The response frequencies used for analysis of the displace-
ment coding properties included the baseline or Linit static
discharge and the static hold phase frequency. The veloci-
ty protocol recorded these static frequencies in addition to


the peak frequency obtained during the dynamic phase of
stretch. The data were grouped by afferent population and
graphed as means + Standard Error (S.E.) with the de-
pendent variable being spike frequency and the independ-
ent variable being amplitude and/or velocity of muscle
stretch. An ANOVA and Scheffe's test for multiple compar-
isons were used to compare the coding properties of three
populations of mechanoreceptors. Statistical significance
was recorded at p < 0.05.

Authors' contributions
GAH conceived of the study, carried out the experimental
studies, participated in the data analysis and drafted the
manuscript. RDJ participated in the design of the study,
analysis of the data and manuscript preparation. PWD
also conceived of the study, and participated in its design,
participated in the data collection, participated in the data
analysis participated in study coordination and participat-
ed in manuscript preparation.

All authors read and approved the final manuscript.

Acknowledgements
This work was supported by a grant from NIH-NHLBI #HL-37596.

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