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Proprioception in extraocular muscles of the rat
Brain Research, 278 (1983) 291-294
291
Elsevier
Proprioception in extraocular muscles of the rat W. J. D A U N I C H T
Dept. of Biocybernetics, University of Dasseldorf, D-4000 Diisseldorf (F. R. G.) (Accepted July 5th, 1983)
Key words: extraocular muscles - - semilunar ganglion - - stretch receptor - - first-order n e u r o n s - - single unit horseradish peroxidase - - rat
Stretch receptor afferents from extraocular muscles were found in the rat. Their first-order somata, responding specifically to eye muscle displacements, were restricted to the ipsilateral semilunar ganglion. Over a broad range of stimulus frequencies the sensitivity of receptors increased by a factor of 2.2 per decade with a m e a n sensitivity of 125 imp. s-1 m m -l at 1 Hz.
Proprioception from extraocular muscles (EOM) has been reported for a wide variety of mammalian speciesll; in rat, however, morphological evidence 6 was negative. Since rats are commonly used laboratory animals and may be of benefit in oculomotor research 3, it seems worthwhile to re-examine the presence of EOM receptors on both anatomic and physiologic bases in this animal. Further, the localization of first-order cell bodies has also been a debatable issue, viz. whereas the semilunar ganglion is expected to contain first-order somata 7, the implication of the mesencephalic nucleus of the trigeminal nerve (mes V) remains unclear. Also, response characteristics of the stretch receptors in E O M have not yet been described. The present experiments were designed to identify the presence of stretch receptors in the EOM of rat, the localization of their cell bodies and the properties of their signal processing. Albino rats of 200-300 g weight were anesthetized with Nembutal and placed in a stereotaxic frame. The eyelids and conjunctiva were retracted after incision, and threads were tied around the tendons of extraocular muscles. The threads were tied together to pull all muscles simultaneously and secured as close as possible by a small clamp which was fixed to the lever of a galvanometer (MFE, Salem). This device was supplied with a position sensor, and feedback loops for position and velocity allowed for a bandwidth of 200 Hz. In 22 rats eye muscles were displaced passively by sinusoidal length changes with
amplitudes between 140 and 355 ktm; assuming a diameter of 6 mm for the globe, these correspond to eye rotations of 2.7-6.8 °. Coated tungsten microelectrodes were advanced into the semilunar ganglion and nucleus mes V, and single units were recorded extracellularly. The impulse intervals were converted into an instantaneous impulse rate by an electronic circuit. The localization of the electrode was marked by an electrolytic, lesion (20ktA, 20 s) and histologically confirmed. In 7 rats E O M were injected with 0.5-1/~1 horseradish peroxidase (HRP) (20%) through a glass micropipette. The rats were treated according to the TMB method8 to identify first-order afferent neurons. Precision of the injection was checked by the specific labeling of brainstem motor nuclei. During passive stretching of EOM, 8 units were found whose impulse rates were very regular and dosely related to muscle displacement (Fig. 1, top left). The specificity of this stimulus to modify the discharge of these receptors was confirmed by the lack of response to stretch of jaw muscles, movement of vibrissae and touch to eyeball and surrounding tissue. All units were recorded from the ophthalmic (anteromedial) subdivision of the ipsilateral ganglion semilunare. The ganglion could be identified by the abundance of neurons responding to touch of skin and vibrissae. In most cases a part of the cortex was also removed by aspiration to expose the ganglion.
292
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0.01
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Fig. 1. Top left: response of EOM proprioceptor to passive sinusoidal displacement; single unit recording (upper trace), impulse rate (middle trace), mechanical displacement (lower trace). Top right: schematic of the semilunar ganglion; projection of positions of 8 labelled neurons into a horizontal plane, after injection of HRP into m. rectus superior and m. rectus lateralis. Bottom: bode plots of EOM afferents (n = 8); mean (geom.) of sensitivities (upper curve), mean (arithm.) of phase angles between response and displacement (lower curve), vertical bars denote standard deviation.
293 Electrolytic lesions identified histologically were always located close to large neuronal somata. As a second possible site of E O M afferents the nucleus mes V was examined; it could easily be identified as many neurons responded to jaw muscle stretch. However, in no instance (14 experiments) could any activity be recognized that would indicate a specific relation to passive eye movements. In another series of experiments, 1-3 E O M were injected with HRP. The retrograde transport of the enzyme resulted in labeling of 1-8 afferent neuronal somata in the ophthalmic subdivision of the ipsilateral ganglion semilunare (Fig. 1, top right); no labeled neurons were observed in either of the mes V nuclei, confirming the findings obtained using electrophysiological techniques. When a unit responding specifically to stretch of E O M was isolated, the length of the muscle was adjusted isometrically so as to maintain a steady impulse rate. For sinusoidal displacement a fundamental sinusoid was fitted to the impulse rate to obtain average values for amplitude and phase. Rising and falling phase were evaluated separately, and if they resulted in different values, the more distorted part was omitted. We found that even small displacements evoke appreciable impulse rate changes: calculating the sensitivity from the ratio of impulse rate amplitude to stimulus amplitude, e.g. at 1 Hz, the mean sensitivity is 125 imp. s-1 mm -1 (Fig. 1, bottom), with individual sensitivities ranging from 28 to 275 imp. s-I mm-l; the mean value corresponds to 6.5 imp. s-1 deg.-l of eye rotation. The static sensitivity extends up to 130 imp. s-t mm -x. In the range between 0.1 and 10 Hz the sensitivity increases with, but less than proportional to, frequency. The constant slope in double logarithmic scales may I~e described by a power law relationship to frequency with an exponent of 0.34; exponents of individual frequency characteristics range from 0.21 to 0.54. In other words, an increase of frequency (and thus velocity) by a factor of 10 leads to an increase of response amplitude by a factor of only 2.2. Assuming a minimum phase system such a
relationship predicts a constant phase lead of 31 °. This value agrees well with the constant phase lead of about 34 ° shown in Fig. 1 (bottom). At frequencies below 0.1 Hz the sensitivity plots flatten out and the phase lead tends to decrease. In some of the individual frequency plots (not shown) this behavior is even more pronounced. Thus the behavior of the receptors deviates from a pure power law relationship to frequency and tends to approach proportionality to position. The behavior of the stretch receptors in the EOM of the rat may be summarized as an intermediate behavior between position- and velocity-dependence. The results of the present paper show that stretch receptors are present in E O M of rats. That these animals are supplied with E O M proprioceptors is remarkable in view of the notion that they do not have foveae. Presumably the function of E O M receptors is not confined to support the precision of directing foveal gaze; instead the E O M receptors seem to play a more basic role in the coordination of body, head and eye movements. Recordings from single units and retrograde labeling of afferent somata indicate that first-order somata are localized solely in the ophthalmic subdivision of the ipsilateral semilunar ganglion in this species. This finding is in line with studies of localization in lamb 2, cat 10 and monkey 9, but not with reports of a proprioceptive representation in the nucleus mes V in cat 1 and goat 4. The analysis of responses to sinusoidal displacements reveals dynamic properties that are similar to those of hind limb muscle spindles in the catS. It should be pointed out that the regularity and the high sensitivity with its enhancement for higher velocities, allow the monitoring of eye movements in the physiological range with high precision. It is a great pleasure to thank Prof. R. Eckmiller for advice and encouragement, Ms. E. Jaworski for expert histological service, W. MiJller for building the impulse rate meter, and Ms. A. Thelen for graphic assistance. Thanks are due to Dr. R. Boyle for his critical reading of the manuscript.
294 l Alvarado-Mallart, M. R., Batini, C., Buisseret-Delmas, C. and Corvisier, J., Trigeminalrepresentation of the masticatory and extraocular proprioceptors as revealed by horseradish peroxidase retrograde transport, Exp. Brain Res., 23 (1975) 167-179. 2 Bortolami, R., Manni, E., Lucchi. M. L., Callegari, E., De Pasquale, V. and Lalatta Costerbosa, G.. Labelled trigeminal ganglion cells after injection of horseradish peroxidase in the extraocular muscles and IIIrd nerve of the lamb, Boll. Soc. hal. Biol. Sper., LV (1979) I206-1209. 3 Cazin, L.. Precht, W. and Lannou, J.. Pathways mediating optokinetic responses of vestibular nucleus in the rat, Pfliigers Arch., 384 (1980) 19-29. 4 Cooper, S., Daniel, P. M. and Whitteridge, D., Nerve impulses in the brainstem of the goat. Short latency responses obtained by stretching the extrinsic eye muscles and the jaw muscles, J. Physiol. ~Lond.). 120 (1953) 471-490. 5 Hunt, C. C. and Wilkinson, R. S., An analysis of receptor potential and tension of isolated cat muscle spindles in response to sinusoidal stretch, J. Physiol. (Lond.), 302 (1980) 241-262. 6 Maier, A.. DeSantis, M. and Eldred, E., The occurrence of
7
8
9
10
11
muscle spindles in extraocular muscles of various vertebrates, J. Morph., i43 (1974) 397-408. Manni, E., Bortolami, R. and Desole, C., Eye muscle proprioception and the semilunar ganglion, Exp, NeuroL. 16 (1966) 226-236. Mesulam, M. M,, Tetramethyl benzidine for horseradish peroxidase neurohistochemistry: a non-carcinogenic blue reaction product with superior sensitivity for visualizing neural afferents and efferents, J, Histochem. Cytochem., 26 (1978) 106-117. Porter, J. D,, Guthrie, B. L. and Sparks. D. L., Localization of neurons providing afferent and efferent innervation of monkey extraocular muscles. Soc. Neurosci. Abstr., 8 (1982) 155. Porter, J. D. and Spencer, R. F., Localization and morphology of cat extraocular muscle afferent neurones identified by retrograde transport of horseradish peroxidase, J. comp. Neurol.. 204 (1982) 56-64. Schaaf, P., Muskelspindeln in den ~iusseren Augenmuskeln der Vertebraten. In H. Drischel and W. Kirmse (Eds.), Das okulomotorische System, physiologische und klinische A~spekte. VEB Georg Thieme, Leipzig, 1979, pp. 173-180.
291
Elsevier
Proprioception in extraocular muscles of the rat W. J. D A U N I C H T
Dept. of Biocybernetics, University of Dasseldorf, D-4000 Diisseldorf (F. R. G.) (Accepted July 5th, 1983)
Key words: extraocular muscles - - semilunar ganglion - - stretch receptor - - first-order n e u r o n s - - single unit horseradish peroxidase - - rat
Stretch receptor afferents from extraocular muscles were found in the rat. Their first-order somata, responding specifically to eye muscle displacements, were restricted to the ipsilateral semilunar ganglion. Over a broad range of stimulus frequencies the sensitivity of receptors increased by a factor of 2.2 per decade with a m e a n sensitivity of 125 imp. s-1 m m -l at 1 Hz.
Proprioception from extraocular muscles (EOM) has been reported for a wide variety of mammalian speciesll; in rat, however, morphological evidence 6 was negative. Since rats are commonly used laboratory animals and may be of benefit in oculomotor research 3, it seems worthwhile to re-examine the presence of EOM receptors on both anatomic and physiologic bases in this animal. Further, the localization of first-order cell bodies has also been a debatable issue, viz. whereas the semilunar ganglion is expected to contain first-order somata 7, the implication of the mesencephalic nucleus of the trigeminal nerve (mes V) remains unclear. Also, response characteristics of the stretch receptors in E O M have not yet been described. The present experiments were designed to identify the presence of stretch receptors in the EOM of rat, the localization of their cell bodies and the properties of their signal processing. Albino rats of 200-300 g weight were anesthetized with Nembutal and placed in a stereotaxic frame. The eyelids and conjunctiva were retracted after incision, and threads were tied around the tendons of extraocular muscles. The threads were tied together to pull all muscles simultaneously and secured as close as possible by a small clamp which was fixed to the lever of a galvanometer (MFE, Salem). This device was supplied with a position sensor, and feedback loops for position and velocity allowed for a bandwidth of 200 Hz. In 22 rats eye muscles were displaced passively by sinusoidal length changes with
amplitudes between 140 and 355 ktm; assuming a diameter of 6 mm for the globe, these correspond to eye rotations of 2.7-6.8 °. Coated tungsten microelectrodes were advanced into the semilunar ganglion and nucleus mes V, and single units were recorded extracellularly. The impulse intervals were converted into an instantaneous impulse rate by an electronic circuit. The localization of the electrode was marked by an electrolytic, lesion (20ktA, 20 s) and histologically confirmed. In 7 rats E O M were injected with 0.5-1/~1 horseradish peroxidase (HRP) (20%) through a glass micropipette. The rats were treated according to the TMB method8 to identify first-order afferent neurons. Precision of the injection was checked by the specific labeling of brainstem motor nuclei. During passive stretching of EOM, 8 units were found whose impulse rates were very regular and dosely related to muscle displacement (Fig. 1, top left). The specificity of this stimulus to modify the discharge of these receptors was confirmed by the lack of response to stretch of jaw muscles, movement of vibrissae and touch to eyeball and surrounding tissue. All units were recorded from the ophthalmic (anteromedial) subdivision of the ipsilateral ganglion semilunare. The ganglion could be identified by the abundance of neurons responding to touch of skin and vibrissae. In most cases a part of the cortex was also removed by aspiration to expose the ganglion.
292
[mm]
Eimp/s]
5
10
0
L
o
,~,
~,,
.~.~'i
.
.,.,;';
ophthalmic
;-.~.,
~k,..,.-,--,~
'~-,..-.--
+°3E
o
maxillary
5
+
mandibular
- 0.3
[mm] I
J
1 [-S']
0
1000
-
'E E E I '-I
d
J
Q.
i
100
r
J
T,
J
.m
c ®
10
T ~t,J
T
T
0 I
0.01
I
i
I
0.1
1.0
10
frequency
[Hz]
Fig. 1. Top left: response of EOM proprioceptor to passive sinusoidal displacement; single unit recording (upper trace), impulse rate (middle trace), mechanical displacement (lower trace). Top right: schematic of the semilunar ganglion; projection of positions of 8 labelled neurons into a horizontal plane, after injection of HRP into m. rectus superior and m. rectus lateralis. Bottom: bode plots of EOM afferents (n = 8); mean (geom.) of sensitivities (upper curve), mean (arithm.) of phase angles between response and displacement (lower curve), vertical bars denote standard deviation.
293 Electrolytic lesions identified histologically were always located close to large neuronal somata. As a second possible site of E O M afferents the nucleus mes V was examined; it could easily be identified as many neurons responded to jaw muscle stretch. However, in no instance (14 experiments) could any activity be recognized that would indicate a specific relation to passive eye movements. In another series of experiments, 1-3 E O M were injected with HRP. The retrograde transport of the enzyme resulted in labeling of 1-8 afferent neuronal somata in the ophthalmic subdivision of the ipsilateral ganglion semilunare (Fig. 1, top right); no labeled neurons were observed in either of the mes V nuclei, confirming the findings obtained using electrophysiological techniques. When a unit responding specifically to stretch of E O M was isolated, the length of the muscle was adjusted isometrically so as to maintain a steady impulse rate. For sinusoidal displacement a fundamental sinusoid was fitted to the impulse rate to obtain average values for amplitude and phase. Rising and falling phase were evaluated separately, and if they resulted in different values, the more distorted part was omitted. We found that even small displacements evoke appreciable impulse rate changes: calculating the sensitivity from the ratio of impulse rate amplitude to stimulus amplitude, e.g. at 1 Hz, the mean sensitivity is 125 imp. s-1 mm -1 (Fig. 1, bottom), with individual sensitivities ranging from 28 to 275 imp. s-I mm-l; the mean value corresponds to 6.5 imp. s-1 deg.-l of eye rotation. The static sensitivity extends up to 130 imp. s-t mm -x. In the range between 0.1 and 10 Hz the sensitivity increases with, but less than proportional to, frequency. The constant slope in double logarithmic scales may I~e described by a power law relationship to frequency with an exponent of 0.34; exponents of individual frequency characteristics range from 0.21 to 0.54. In other words, an increase of frequency (and thus velocity) by a factor of 10 leads to an increase of response amplitude by a factor of only 2.2. Assuming a minimum phase system such a
relationship predicts a constant phase lead of 31 °. This value agrees well with the constant phase lead of about 34 ° shown in Fig. 1 (bottom). At frequencies below 0.1 Hz the sensitivity plots flatten out and the phase lead tends to decrease. In some of the individual frequency plots (not shown) this behavior is even more pronounced. Thus the behavior of the receptors deviates from a pure power law relationship to frequency and tends to approach proportionality to position. The behavior of the stretch receptors in the EOM of the rat may be summarized as an intermediate behavior between position- and velocity-dependence. The results of the present paper show that stretch receptors are present in E O M of rats. That these animals are supplied with E O M proprioceptors is remarkable in view of the notion that they do not have foveae. Presumably the function of E O M receptors is not confined to support the precision of directing foveal gaze; instead the E O M receptors seem to play a more basic role in the coordination of body, head and eye movements. Recordings from single units and retrograde labeling of afferent somata indicate that first-order somata are localized solely in the ophthalmic subdivision of the ipsilateral semilunar ganglion in this species. This finding is in line with studies of localization in lamb 2, cat 10 and monkey 9, but not with reports of a proprioceptive representation in the nucleus mes V in cat 1 and goat 4. The analysis of responses to sinusoidal displacements reveals dynamic properties that are similar to those of hind limb muscle spindles in the catS. It should be pointed out that the regularity and the high sensitivity with its enhancement for higher velocities, allow the monitoring of eye movements in the physiological range with high precision. It is a great pleasure to thank Prof. R. Eckmiller for advice and encouragement, Ms. E. Jaworski for expert histological service, W. MiJller for building the impulse rate meter, and Ms. A. Thelen for graphic assistance. Thanks are due to Dr. R. Boyle for his critical reading of the manuscript.
294 l Alvarado-Mallart, M. R., Batini, C., Buisseret-Delmas, C. and Corvisier, J., Trigeminalrepresentation of the masticatory and extraocular proprioceptors as revealed by horseradish peroxidase retrograde transport, Exp. Brain Res., 23 (1975) 167-179. 2 Bortolami, R., Manni, E., Lucchi. M. L., Callegari, E., De Pasquale, V. and Lalatta Costerbosa, G.. Labelled trigeminal ganglion cells after injection of horseradish peroxidase in the extraocular muscles and IIIrd nerve of the lamb, Boll. Soc. hal. Biol. Sper., LV (1979) I206-1209. 3 Cazin, L.. Precht, W. and Lannou, J.. Pathways mediating optokinetic responses of vestibular nucleus in the rat, Pfliigers Arch., 384 (1980) 19-29. 4 Cooper, S., Daniel, P. M. and Whitteridge, D., Nerve impulses in the brainstem of the goat. Short latency responses obtained by stretching the extrinsic eye muscles and the jaw muscles, J. Physiol. ~Lond.). 120 (1953) 471-490. 5 Hunt, C. C. and Wilkinson, R. S., An analysis of receptor potential and tension of isolated cat muscle spindles in response to sinusoidal stretch, J. Physiol. (Lond.), 302 (1980) 241-262. 6 Maier, A.. DeSantis, M. and Eldred, E., The occurrence of
7
8
9
10
11
muscle spindles in extraocular muscles of various vertebrates, J. Morph., i43 (1974) 397-408. Manni, E., Bortolami, R. and Desole, C., Eye muscle proprioception and the semilunar ganglion, Exp, NeuroL. 16 (1966) 226-236. Mesulam, M. M,, Tetramethyl benzidine for horseradish peroxidase neurohistochemistry: a non-carcinogenic blue reaction product with superior sensitivity for visualizing neural afferents and efferents, J, Histochem. Cytochem., 26 (1978) 106-117. Porter, J. D,, Guthrie, B. L. and Sparks. D. L., Localization of neurons providing afferent and efferent innervation of monkey extraocular muscles. Soc. Neurosci. Abstr., 8 (1982) 155. Porter, J. D. and Spencer, R. F., Localization and morphology of cat extraocular muscle afferent neurones identified by retrograde transport of horseradish peroxidase, J. comp. Neurol.. 204 (1982) 56-64. Schaaf, P., Muskelspindeln in den ~iusseren Augenmuskeln der Vertebraten. In H. Drischel and W. Kirmse (Eds.), Das okulomotorische System, physiologische und klinische A~spekte. VEB Georg Thieme, Leipzig, 1979, pp. 173-180.