Neuroscience Letters, 125 (1991) 69-72
69
© 1991 Elsevier Scientific Publishers Ireland Ltd. 0304-3940/91/$ 03.50 ADONIS 030439409100156W NSL 07672
The efferent innervation of the suboccipital muscles in the guinea pig: a study with retrograde transport of horseradish peroxidase Maria-Sophie Hiller, Manfred Prihoda and T h o m a s Heller Institute of Anatomy, Department 2, University of Vienna, Vienna (Austria) (Received 10 October 1990; Revised version received 10 December 1990; Accepted 16 January 1991)
Key words: Neck muscle; Motoneuron; Ventromedial nucleus; Soma size; Horseradish peroxidase; Guinea pig The location and quantitative parameters of the motoneurons of three suboccipital muscles are studied by means of retrograde transport of horseradish peroxidase (HRP). Injections of HRP into the M. obliquus capitis superior and into the M. rectus capitis posterior maior result in labeling of somata within the first cervical spinal segment, Injections into the M. obliquus capitis inferior result in labeling within the second cervical spinal segment. All somata are situated within or close to the ventromedial nucleus and show average diameters of about 20-25/~m. Conclusions on the innervation of the suboceipital muscles are drawn.
It has long been recognized that neck muscle function cannot be understood as pure mechanical action. Especially deep neck muscles contain a high amount of muscle spindles and Golgi tendon organs as receptive units [12, 13]. The main projections from these suboccipital proprioceptors are found within precerebellar nuclei, i.e. the central cervical nucleus and the external cuneate nucleus [2, 6]. Neck proprioception thus constitutes an important element in the control of posture and eye/head coordination. Primary afferents from the suboccipital muscle group have been thoroughly examined in cat [2] and monkey [6]. In both studies, the efferent innervation of these muscles was described but not discussed in detail. There is only one recent paper that deals with the efferents of the suboccipital muscle group in the cat in particular [11]. The aim of the present study was to provide a description of the efferent innervation of the suboccipital muscle group that helps exercise postural neck reflexes, and to compare the results in the guinea pig concerning segmental origin, position, and size of the motoneurons with the differing results of above mentioned studies. Microdissection revealed that in the guinea pig each suboccipital muscle is not innervated by a single, well defined muscle nerve but by several nerve branches. The M. obliquus capitis inferior (OCI) receives at least two branches of the dorsal ramus of C2 close to the interver-
Correspondence: M.-S. Hiller, Institute of Anatomy 2, Waehringerstrasse 13, A-1090 Vienna, Austria
tebral foramen as well as a delicate branch of Ci. In order to label as many motoneurons as possible of the individual suboccipital muscles, we preferred intramuscular injection of horseradish peroxidase (HRP) to staining the muscle nerve stumps. Eighteen guinea pigs were anaesthetized with Valium (diazepam 1.4 mg/100 g b.wt.), Fentanyl (fentanyl 1.5 mg/100 g), and Sedalande (fluanisone 0.25 mg/100 g) in order to suppress any reflex activity and responses to pain. Following a median incision of skin and superficial neck muscles the suboccipital muscle group was exposed. A total amount of 3-5/A of a 40% aqueous solution of HRP (Sigma, grade I) was injected at multiple sites into the M. obliquus capitis superior (OCS; 5 cases), into the M. rectus capitis post. maior (RCM; 7 cases), or into the M. obliquus capitis inferior (OCI; 6 cases) with the help of a glass micropipette on a Hamilton syringe. To help avoid diffusion, the injection sites were covered with Parafilm immediately afterwards. Since diffusion of HRP into the surrounding muscles can never be totally excluded in these experiments, control experiments were performed in three animals. A piece of gelfoam soaked with HRP was installed upon the suboccipital muscle fascia [9]. All three experiments resulted in only few very slightly labeled motoneurons. As Haase and Hrycyshyn [9], we therefore assumed that intact fascial sheets to a certain degree act as a barrier to the diffusion of HRP. Spuriously labeled perikarya were not taken into consideration for the results. After a survival time of 72 h, the animals were reanaesthetized with Nembutal (pentobarbital 10 mg/100 g)
70
CELLN U ~ E R 6
2
OCS ;
f
: :
RCM
0
OCS
OCI
0
0
I I .
.
4.~
0
0
0
RCM 14
0
2
r
'
o
o
I
I
:i,:
J
:
oo
=
:
I
o
:
2
-oT,~ -1,4. -2,qO -3,36 -%$2 -5,2S -6,24 -7,20 -8,16 -9,12 -10,08 Cl
C~
DISTANCE FROM OBEX
Fig 1. Distribution of HRP-labeled motoneurons of the M. obliquus cap. superior (OCS), the M. rectus cap. post. maior (RCM), and the M. obliquus cap. inferior (OCI) within the spinal cord. The hatched lines indicate the borders of the first and second cervical spinal segment; the ordinate shows the average number cells per section (average of all animals).
and perfused transcardially with 0.9% NaC1 followed by 1.5% glutaraldehyde and 1.0% paraformaldehyde in 0.1 M phosphate buffer and by a 20% phosphate-buffered sucrose solution. The lower medulla and the upper spinal cord were removed together with the first 3 cervical dorsal root ganglia on both sides. The segmental borders were defined by measuring the breadth of the dorsal root entry zones and of the interroot distances under the microscope. Serial, transverse, 40 /zm frozen sections were processed histochemically according to Mesulam [10]. Selected sections were counterstained with Neutral red. Drawing of complete serial sections and mapping of labeled motoneurons were performed with the help of a camera lucida. Average diameters of the perikarya were measured by the method of Burke [4]. No allowance was made for shrinkage. The distribution of labeled cells along the longitudinal axis of the spinal cord after injecting each of the three muscles is shown in Fig. 1. Injections into the OCS resulted in labeled perikarya as far rostrally as the pyramidal decussation. The maximum density of labeled ventral horn cells was found in rostral and mid C1, and declined towards the caudal border of C]. Injections into the RCM resulted in a similar distribution of labeled motoneurons within Ch whereas retrograde projection from the OCI produced only few labeled cells in Ch a maximum quantity of labeled somata in rostral and mid C2, and a decreasing number in caudal C2. In the transverse plane, the majority of labeled perikarya was found within the ventromedial nucleus of lamina IX in all three experimental settings (Fig. 2).
[]
AREAOF MAINMOTORNUCLEUS
]
AREAOF ECTOPICMOTONEURONS
~
/
CC2
Fig. 2. Schematized, composite diagram showing sections through the right ventral horn at the level of the rostral, mid, and caudal third of Ct and at the level of the caudal CI and the rostral, mid, and caudal third of C2. The main motor nucleus of each muscle is always situated within the ventromedial nucleus of the ventral horn; areas of ectopic motoneurons are situated within the white matter and along the medial border of the ventral horn. Note the slightly different distribution of ectopic motoneurons of each muscle.
Ectopic motoneurons could be observed either within the ventral or lateral funiculus of the white matter, or along the border of the ventral horn within lamina VIII and near the area of the anterior cornucommissural nucleus. Motoneurons of all three muscles occupy the same area within the ventral horn, with their maximum density at different levels along the longitudinal axis of the spinal cord. Thus, it can be concluded that they form a common intermingled cell column in which the cells innervating OCS and RCM are located more rostrally and those innervating OCI in more caudal parts of Cl and in C2. Slight differences in the distribution of ectopic motoneurons (Figs. 2 and 3B,C) are not pronounced enough to conclude on a functional reason. Practically all labeled motoneurons were polygonal, with an average diameter of about 20-25/tm and 3-6 dendrites (Figs. 3A and 4). Cells with a diameter of more than 40/tm could not be detected in any case. Some perikarya especially at the rostral end of the cell column had a rather fusiform appearance. These cells were mostly situated near the border of the grey matter or even between the fibers of the pyramidal decussation. Since number of dendrites and average diameters correspond to those of the polygonal cells it can be assumed that they belong to one and the same cell population. There are contradictory statements concerning the segmental origin of motoneurons of the suboccipital muscle group as well as their position within the ventral horn. Generally, it is assumed that the suboccipital muscles are innervated by the dorsal ramus of Ch a fact that
71
!!!;i~ ~ ~i! ~i
l--~m
dB
,
/
o
Fig. 3. Photomicrographs showing the location and size of HRP-labeled motoneurons. A: group of cells labeled after injection of the RCM containing two of the largest motoneurons of this experiment (arrows). The soma sizes range from 17.5 to 31.25/am. B: section of the same animal as in A, about 0.6 mm further caudally. Note the ectopic cell within the anterior funiculus (arrow). C: motoneurons labeled after injection of an OCI with an ectopic cell near the medial border of the ventral horn (arrow).
is confirmed by Edney and Porter [6] in the monkey and of PAsaro et al. [11] in the cat. Bakker et al. [2] dissected and stained C2 dorsal rami innervating the OCI in the cat, and found labeled motoneurons both in C1 and C2 when examining the suboccipital muscles. This finding coincides well with our results; since the OCI motoneurons in the guinea pig lie within a sector of the spinal cord that belongs to the dorsal ramus of C2 [14], it can be concluded that the OCI is mainly innervated by C2. Besides, this innervation pattern has also been described early in this century in humans [7], a knowledge that seems to be lost in more recent anatomic textbooks. All authors report that the majority of labeled cell bodies is located within the ventromedial nucleus (VMN) of the ventral horn. P~saro et al. [11] in the cat describe labeled somata almost exclusively within the VMN, an observation that is contradicted by the study of Bakker et al. [2]. They found motoneurons innervating the suboccipital muscles distributed to the commissural nucleus and the lateral margin of the ventral horn RCM
OCS
OCI
N
N
30
50
3O
20
20
20
10
i0 i0
20
30
= 24,71 N = 108
40
. i UM 50
10
3.0 20
30
"X = 2 2 , 7 7 N = 112
•
.
qO
50
B JIM
•
10
20
50
40
J ~M
50
~ = 23,15 t~ = 1 2 8
Fig. 4. Diameters of labeled cells of M. obliquus cap. superior (OCS), M. rectus cap. post. maior (RCM), and M. obliquus cap. inferior (OCI). Cell diameters were sampled at 3 different levels within the cell columns of two animals of each experimental setting.
in addition to the VMN. Although we did not find labeled cells on the lateral margin of the ventral horn in the guinea pig, our results endorse those of Bakker et al. [2]. The exact shape of the suboccipital motor cell column will therefore have to be further examined in other animals. The cell bodies of all motoneurons labeled in the guinea pig were remarkably small. This coincides well with the results of P~saro et al. [11] who found cell diameters below 40 gm in the cat, while Bakker et al. [2] reported motoneuron sizes of 20-50 pm. The above studies as well as our own all do not comprise indications for a bimodal size distribution as it is documented for superficial neck muscles [1], limb muscles [3, 4], or infrahyoid muscles [8]. This bimodality is commonly associated with a division into larger at-motoneurons and smaller ?-motoneurons. According to that, all somata measured in the guinea pig would correspond to the group of 7-motoneurons. The high amount of muscle spindles within neck muscles is well known [13]. Nevertheless a total lack of ~-motoneurons seems highly improbable. It rather can be assumed that the ~-motoneurons contained in this cell column are small themselves. This could depend on the size of motor units, though the infrahyoid muscles for example do show a second peak of cell diameters over 35/xm. On the other hand, several studies [3, 5] show that the motoneuron size might scale with the muscle fiber types: slow-twitch fibers are innervated by smaller cells than fast-twitch fibers. Since the head of the guinea pig is rather large compared to its body, a high content of slow, 'tonic' muscle fibers could be one explanation for unimodal
72
motoneuron sizes of the suboccipital muscles. Further illumination will be obtained by histochemical muscle fiber analyses. The authors wish to thank Mrs. U. Fichtinger for excellent histochemical and photographic work and Mrs. R. Laske-Zundritsch for technical assistance. This study was supported by the Anton Dreher-Ged/ichtnisschenkung fiir medizinische Forschung. l Abrahams, V.C. and Keane, J., Contralateral, midline, and commissural motoneurons of neck muscles: a retrograde HRP-study in the cat, J. Comp. Neurol., 223 (1984) 448-456. 2 Bakker, D.A., Richmond, F.J.R. and Abrahams, V.C., Central projections from cat suboccipital muscles: a study using transganglionic transport of horseradish peroxidase, J. Comp. Neurol., 228 (1984) 409-42 I. 3 Burke, R.E., Dum, R.P., Fleshman, J.W., Glenn, L.L., Lev-Tov, A., O'Donovan, M.J. and Pinter, M.J., An HRP study of the relation between cell size and motor unit type in cat ankle extensor motoneurons, J. Comp. Neurol., 209 (1982) 17-28. 4 Burke, R.E., Strick, P.L., Kanda, K., Kim, C.C. and Walmsley, B., Anatomy of medial gastrocnemius and soleus motor nuclei in cat spinal cord, J. Neurophysiol., 40 (1977) 667~80. 5 Callister, R.J., Brichta, A.M. and Peterson, E.H., Quantitative analysis of cervical musculature in rats: histochemical composition and motor pool organisation. II. Deep dorsal muscles. J. Comp. Neurol., 225 (1987) 369-385.
6 Edney, D.P. and Porter, J.D., Neck muscle afferent projections to the brainstem of the monkey: implications for the neural control of gaze, J. Comp. Neurol., 250 (1986) 389 398. 7 Eisler, P., Die Muskeln des Stammes. In K.v. Bardeleben (Ed.), Handbuch der Anatomic des Menschen, II/2/1, 1912, pp. 433-441. 8 Gottschall, J., Neuhuber, W., M/intener, M. and Mysicka, A., The ansa cervicalis and the infrahyoid muscles of the rat. II. Motor and sensory neurons, Anat. Embryol., 159 (1980) 59 69. 9 Haase, P. and Hrycyshyn, A.W., On the diffusion of horseradish peroxidase into muscles and the 'spurious' labeling of motoneurons, Exp. Neurol., 91 (1986) 33~403. l0 Mesulam, M.M., Tracing Neural Connections With Horseradish Peroxidase, IBRO Handbook Series, Methods in Neuroscience, Wiley, New York, 1982. I l Pfisaro, R., Torres, P. and Delgado-Garcia, J.M., Location of short neck muscle motoneurons in the cat as revealed by horseradish peroxidase, Neurosci. Lett., 43 (1983) 131-135. 12 Peck, D., Buxton, D.F. and Nitz, A., A comparison of spindle concentrations in large and small muscles acting in parallel combinations, J. Morphol., 180 (1984) 243-252. 13 Richmond, F.J.R. and Bakker, D.A., Anatomical organization and sensory receptor content of soft tissues surrounding upper cervical vertebrae in the cat, J. Neurophysiol., 48 (1982) 49-61. 14 Smith, C.L. and Hollyday, M., The development and postnatal organization of motor nuclei in the rat thoracic spinal cord, J. Comp. Neurol., 220 (1983) 16-28.
69
© 1991 Elsevier Scientific Publishers Ireland Ltd. 0304-3940/91/$ 03.50 ADONIS 030439409100156W NSL 07672
The efferent innervation of the suboccipital muscles in the guinea pig: a study with retrograde transport of horseradish peroxidase Maria-Sophie Hiller, Manfred Prihoda and T h o m a s Heller Institute of Anatomy, Department 2, University of Vienna, Vienna (Austria) (Received 10 October 1990; Revised version received 10 December 1990; Accepted 16 January 1991)
Key words: Neck muscle; Motoneuron; Ventromedial nucleus; Soma size; Horseradish peroxidase; Guinea pig The location and quantitative parameters of the motoneurons of three suboccipital muscles are studied by means of retrograde transport of horseradish peroxidase (HRP). Injections of HRP into the M. obliquus capitis superior and into the M. rectus capitis posterior maior result in labeling of somata within the first cervical spinal segment, Injections into the M. obliquus capitis inferior result in labeling within the second cervical spinal segment. All somata are situated within or close to the ventromedial nucleus and show average diameters of about 20-25/~m. Conclusions on the innervation of the suboceipital muscles are drawn.
It has long been recognized that neck muscle function cannot be understood as pure mechanical action. Especially deep neck muscles contain a high amount of muscle spindles and Golgi tendon organs as receptive units [12, 13]. The main projections from these suboccipital proprioceptors are found within precerebellar nuclei, i.e. the central cervical nucleus and the external cuneate nucleus [2, 6]. Neck proprioception thus constitutes an important element in the control of posture and eye/head coordination. Primary afferents from the suboccipital muscle group have been thoroughly examined in cat [2] and monkey [6]. In both studies, the efferent innervation of these muscles was described but not discussed in detail. There is only one recent paper that deals with the efferents of the suboccipital muscle group in the cat in particular [11]. The aim of the present study was to provide a description of the efferent innervation of the suboccipital muscle group that helps exercise postural neck reflexes, and to compare the results in the guinea pig concerning segmental origin, position, and size of the motoneurons with the differing results of above mentioned studies. Microdissection revealed that in the guinea pig each suboccipital muscle is not innervated by a single, well defined muscle nerve but by several nerve branches. The M. obliquus capitis inferior (OCI) receives at least two branches of the dorsal ramus of C2 close to the interver-
Correspondence: M.-S. Hiller, Institute of Anatomy 2, Waehringerstrasse 13, A-1090 Vienna, Austria
tebral foramen as well as a delicate branch of Ci. In order to label as many motoneurons as possible of the individual suboccipital muscles, we preferred intramuscular injection of horseradish peroxidase (HRP) to staining the muscle nerve stumps. Eighteen guinea pigs were anaesthetized with Valium (diazepam 1.4 mg/100 g b.wt.), Fentanyl (fentanyl 1.5 mg/100 g), and Sedalande (fluanisone 0.25 mg/100 g) in order to suppress any reflex activity and responses to pain. Following a median incision of skin and superficial neck muscles the suboccipital muscle group was exposed. A total amount of 3-5/A of a 40% aqueous solution of HRP (Sigma, grade I) was injected at multiple sites into the M. obliquus capitis superior (OCS; 5 cases), into the M. rectus capitis post. maior (RCM; 7 cases), or into the M. obliquus capitis inferior (OCI; 6 cases) with the help of a glass micropipette on a Hamilton syringe. To help avoid diffusion, the injection sites were covered with Parafilm immediately afterwards. Since diffusion of HRP into the surrounding muscles can never be totally excluded in these experiments, control experiments were performed in three animals. A piece of gelfoam soaked with HRP was installed upon the suboccipital muscle fascia [9]. All three experiments resulted in only few very slightly labeled motoneurons. As Haase and Hrycyshyn [9], we therefore assumed that intact fascial sheets to a certain degree act as a barrier to the diffusion of HRP. Spuriously labeled perikarya were not taken into consideration for the results. After a survival time of 72 h, the animals were reanaesthetized with Nembutal (pentobarbital 10 mg/100 g)
70
CELLN U ~ E R 6
2
OCS ;
f
: :
RCM
0
OCS
OCI
0
0
I I .
.
4.~
0
0
0
RCM 14
0
2
r
'
o
o
I
I
:i,:
J
:
oo
=
:
I
o
:
2
-oT,~ -1,4. -2,qO -3,36 -%$2 -5,2S -6,24 -7,20 -8,16 -9,12 -10,08 Cl
C~
DISTANCE FROM OBEX
Fig 1. Distribution of HRP-labeled motoneurons of the M. obliquus cap. superior (OCS), the M. rectus cap. post. maior (RCM), and the M. obliquus cap. inferior (OCI) within the spinal cord. The hatched lines indicate the borders of the first and second cervical spinal segment; the ordinate shows the average number cells per section (average of all animals).
and perfused transcardially with 0.9% NaC1 followed by 1.5% glutaraldehyde and 1.0% paraformaldehyde in 0.1 M phosphate buffer and by a 20% phosphate-buffered sucrose solution. The lower medulla and the upper spinal cord were removed together with the first 3 cervical dorsal root ganglia on both sides. The segmental borders were defined by measuring the breadth of the dorsal root entry zones and of the interroot distances under the microscope. Serial, transverse, 40 /zm frozen sections were processed histochemically according to Mesulam [10]. Selected sections were counterstained with Neutral red. Drawing of complete serial sections and mapping of labeled motoneurons were performed with the help of a camera lucida. Average diameters of the perikarya were measured by the method of Burke [4]. No allowance was made for shrinkage. The distribution of labeled cells along the longitudinal axis of the spinal cord after injecting each of the three muscles is shown in Fig. 1. Injections into the OCS resulted in labeled perikarya as far rostrally as the pyramidal decussation. The maximum density of labeled ventral horn cells was found in rostral and mid C1, and declined towards the caudal border of C]. Injections into the RCM resulted in a similar distribution of labeled motoneurons within Ch whereas retrograde projection from the OCI produced only few labeled cells in Ch a maximum quantity of labeled somata in rostral and mid C2, and a decreasing number in caudal C2. In the transverse plane, the majority of labeled perikarya was found within the ventromedial nucleus of lamina IX in all three experimental settings (Fig. 2).
[]
AREAOF MAINMOTORNUCLEUS
]
AREAOF ECTOPICMOTONEURONS
~
/
CC2
Fig. 2. Schematized, composite diagram showing sections through the right ventral horn at the level of the rostral, mid, and caudal third of Ct and at the level of the caudal CI and the rostral, mid, and caudal third of C2. The main motor nucleus of each muscle is always situated within the ventromedial nucleus of the ventral horn; areas of ectopic motoneurons are situated within the white matter and along the medial border of the ventral horn. Note the slightly different distribution of ectopic motoneurons of each muscle.
Ectopic motoneurons could be observed either within the ventral or lateral funiculus of the white matter, or along the border of the ventral horn within lamina VIII and near the area of the anterior cornucommissural nucleus. Motoneurons of all three muscles occupy the same area within the ventral horn, with their maximum density at different levels along the longitudinal axis of the spinal cord. Thus, it can be concluded that they form a common intermingled cell column in which the cells innervating OCS and RCM are located more rostrally and those innervating OCI in more caudal parts of Cl and in C2. Slight differences in the distribution of ectopic motoneurons (Figs. 2 and 3B,C) are not pronounced enough to conclude on a functional reason. Practically all labeled motoneurons were polygonal, with an average diameter of about 20-25/tm and 3-6 dendrites (Figs. 3A and 4). Cells with a diameter of more than 40/tm could not be detected in any case. Some perikarya especially at the rostral end of the cell column had a rather fusiform appearance. These cells were mostly situated near the border of the grey matter or even between the fibers of the pyramidal decussation. Since number of dendrites and average diameters correspond to those of the polygonal cells it can be assumed that they belong to one and the same cell population. There are contradictory statements concerning the segmental origin of motoneurons of the suboccipital muscle group as well as their position within the ventral horn. Generally, it is assumed that the suboccipital muscles are innervated by the dorsal ramus of Ch a fact that
71
!!!;i~ ~ ~i! ~i
l--~m
dB
,
/
o
Fig. 3. Photomicrographs showing the location and size of HRP-labeled motoneurons. A: group of cells labeled after injection of the RCM containing two of the largest motoneurons of this experiment (arrows). The soma sizes range from 17.5 to 31.25/am. B: section of the same animal as in A, about 0.6 mm further caudally. Note the ectopic cell within the anterior funiculus (arrow). C: motoneurons labeled after injection of an OCI with an ectopic cell near the medial border of the ventral horn (arrow).
is confirmed by Edney and Porter [6] in the monkey and of PAsaro et al. [11] in the cat. Bakker et al. [2] dissected and stained C2 dorsal rami innervating the OCI in the cat, and found labeled motoneurons both in C1 and C2 when examining the suboccipital muscles. This finding coincides well with our results; since the OCI motoneurons in the guinea pig lie within a sector of the spinal cord that belongs to the dorsal ramus of C2 [14], it can be concluded that the OCI is mainly innervated by C2. Besides, this innervation pattern has also been described early in this century in humans [7], a knowledge that seems to be lost in more recent anatomic textbooks. All authors report that the majority of labeled cell bodies is located within the ventromedial nucleus (VMN) of the ventral horn. P~saro et al. [11] in the cat describe labeled somata almost exclusively within the VMN, an observation that is contradicted by the study of Bakker et al. [2]. They found motoneurons innervating the suboccipital muscles distributed to the commissural nucleus and the lateral margin of the ventral horn RCM
OCS
OCI
N
N
30
50
3O
20
20
20
10
i0 i0
20
30
= 24,71 N = 108
40
. i UM 50
10
3.0 20
30
"X = 2 2 , 7 7 N = 112
•
.
qO
50
B JIM
•
10
20
50
40
J ~M
50
~ = 23,15 t~ = 1 2 8
Fig. 4. Diameters of labeled cells of M. obliquus cap. superior (OCS), M. rectus cap. post. maior (RCM), and M. obliquus cap. inferior (OCI). Cell diameters were sampled at 3 different levels within the cell columns of two animals of each experimental setting.
in addition to the VMN. Although we did not find labeled cells on the lateral margin of the ventral horn in the guinea pig, our results endorse those of Bakker et al. [2]. The exact shape of the suboccipital motor cell column will therefore have to be further examined in other animals. The cell bodies of all motoneurons labeled in the guinea pig were remarkably small. This coincides well with the results of P~saro et al. [11] who found cell diameters below 40 gm in the cat, while Bakker et al. [2] reported motoneuron sizes of 20-50 pm. The above studies as well as our own all do not comprise indications for a bimodal size distribution as it is documented for superficial neck muscles [1], limb muscles [3, 4], or infrahyoid muscles [8]. This bimodality is commonly associated with a division into larger at-motoneurons and smaller ?-motoneurons. According to that, all somata measured in the guinea pig would correspond to the group of 7-motoneurons. The high amount of muscle spindles within neck muscles is well known [13]. Nevertheless a total lack of ~-motoneurons seems highly improbable. It rather can be assumed that the ~-motoneurons contained in this cell column are small themselves. This could depend on the size of motor units, though the infrahyoid muscles for example do show a second peak of cell diameters over 35/xm. On the other hand, several studies [3, 5] show that the motoneuron size might scale with the muscle fiber types: slow-twitch fibers are innervated by smaller cells than fast-twitch fibers. Since the head of the guinea pig is rather large compared to its body, a high content of slow, 'tonic' muscle fibers could be one explanation for unimodal
72
motoneuron sizes of the suboccipital muscles. Further illumination will be obtained by histochemical muscle fiber analyses. The authors wish to thank Mrs. U. Fichtinger for excellent histochemical and photographic work and Mrs. R. Laske-Zundritsch for technical assistance. This study was supported by the Anton Dreher-Ged/ichtnisschenkung fiir medizinische Forschung. l Abrahams, V.C. and Keane, J., Contralateral, midline, and commissural motoneurons of neck muscles: a retrograde HRP-study in the cat, J. Comp. Neurol., 223 (1984) 448-456. 2 Bakker, D.A., Richmond, F.J.R. and Abrahams, V.C., Central projections from cat suboccipital muscles: a study using transganglionic transport of horseradish peroxidase, J. Comp. Neurol., 228 (1984) 409-42 I. 3 Burke, R.E., Dum, R.P., Fleshman, J.W., Glenn, L.L., Lev-Tov, A., O'Donovan, M.J. and Pinter, M.J., An HRP study of the relation between cell size and motor unit type in cat ankle extensor motoneurons, J. Comp. Neurol., 209 (1982) 17-28. 4 Burke, R.E., Strick, P.L., Kanda, K., Kim, C.C. and Walmsley, B., Anatomy of medial gastrocnemius and soleus motor nuclei in cat spinal cord, J. Neurophysiol., 40 (1977) 667~80. 5 Callister, R.J., Brichta, A.M. and Peterson, E.H., Quantitative analysis of cervical musculature in rats: histochemical composition and motor pool organisation. II. Deep dorsal muscles. J. Comp. Neurol., 225 (1987) 369-385.
6 Edney, D.P. and Porter, J.D., Neck muscle afferent projections to the brainstem of the monkey: implications for the neural control of gaze, J. Comp. Neurol., 250 (1986) 389 398. 7 Eisler, P., Die Muskeln des Stammes. In K.v. Bardeleben (Ed.), Handbuch der Anatomic des Menschen, II/2/1, 1912, pp. 433-441. 8 Gottschall, J., Neuhuber, W., M/intener, M. and Mysicka, A., The ansa cervicalis and the infrahyoid muscles of the rat. II. Motor and sensory neurons, Anat. Embryol., 159 (1980) 59 69. 9 Haase, P. and Hrycyshyn, A.W., On the diffusion of horseradish peroxidase into muscles and the 'spurious' labeling of motoneurons, Exp. Neurol., 91 (1986) 33~403. l0 Mesulam, M.M., Tracing Neural Connections With Horseradish Peroxidase, IBRO Handbook Series, Methods in Neuroscience, Wiley, New York, 1982. I l Pfisaro, R., Torres, P. and Delgado-Garcia, J.M., Location of short neck muscle motoneurons in the cat as revealed by horseradish peroxidase, Neurosci. Lett., 43 (1983) 131-135. 12 Peck, D., Buxton, D.F. and Nitz, A., A comparison of spindle concentrations in large and small muscles acting in parallel combinations, J. Morphol., 180 (1984) 243-252. 13 Richmond, F.J.R. and Bakker, D.A., Anatomical organization and sensory receptor content of soft tissues surrounding upper cervical vertebrae in the cat, J. Neurophysiol., 48 (1982) 49-61. 14 Smith, C.L. and Hollyday, M., The development and postnatal organization of motor nuclei in the rat thoracic spinal cord, J. Comp. Neurol., 220 (1983) 16-28.
