Brain Research, 123 (1977) 1-12
:~5 Elsevier/North-Holland Biomedical Press, Amsterdam - Printed in The Netherlands
Research Reports
M O R P H O L O G Y OF T H E F A C I A L N U C L E U S OF T H E RAT
MICHAEL R. M A R T I N and DAVID LODGE*
Department of Physio&gy, University of Bristol Medical School, Bristol BS8 1TD (Great Britain) (Accepted July 2nd, 1976)
SUMMARY
The facial nucleus of the rat can be divided into 5 morphological subdivisions. Using a method for the correlation of the observed subdivisions with antidromic field profiles, the origins of the major muscle branches of the facial nerve in the motor nucleus were determined. The posterior auricular branch is in the medial, the cervical in the ventromedial, the inferior and superior buccolabiales in the lateral, the zygomatic in the dorsal and the temporal and digastric in the intermediate subdivision. The results are consistent with an organization of the motor nucleus reflecting a corresponding topographic organization of the facial musculature.
INTRODUCTION
Several investigators have attempted to divide the facial nucleus into morphologically distinct subdivisions and delegate the origin of one or more branches to individual subdivisionsa,10,11,15,16,19. However there is some disagreement on the number and arrangement of the subdivisions within the nucleus in any one species (for reviews see refs. 1, 18). In the rat, from 3 to 6 subdivisions have been described5,14, 1,5. Only Papez 1~ has attempted to correlate the various morphological subdivisions of the rat with the origins of specific facial muscle branches. He found that the origin of the posterior auricular branch is in the medial, the anterior auricular (temporal) is in the intermediate, and the superior and inferior buccolabiales in the lateral and ventrolateral morphological subdivisions, respectively. The nuclear origins of 3 of these 4 branches, the posterior auricular and superior and inferior buccolabiales branches, are similar to those observed in other mammals. Attempts have been made in other * Animal Health Trust/Wellcome Trust Research Fellow. Present address: Department of Pharmacology, John Curtin School of Medical Research, A.N.U., Canberra, Australia.
mammals to ascertain the origin within the nucleus of the temporal (anterior auricular), zygomatic, cervical and deep facial (stapedius, stylohoid and digastric) muscle branchest, e,l,%ts, though there is considerable disagreement. This may be the result of the difficulty in the interpretation of cellular pathological changes due to the degeneration methods employed. Nonetheless efforts have been made ~o delineate an organization of the motor branches within the nucleus, reflecting the embryological development and topographic relations of the facial musclesV,~'~,~L This report describes the appearance, in the rat, of the facial nucleus and its subdivisions. These findings are correlated with the three-dimensional representation of the antidromic field potentials for 7 muscles branches (including the cervical, temporal, zygomatic and digastric) of the facial nerve within the nucleus. Not only does the method circumvent the difficulties in the analysis of cellular pathological changes due to degeneration, but it also permits the study of the intranuctear position of more than one branch of the nerve in a given animal. A further reason for using antidromic field profiles for determining the nuclear origins of motor branches as opposed to other anatomical methods, is that the ability to identify the nucleus both electrophysiologically and stereotaxically is essential in many physiological and pharmacological studies a',13. METHODS
Light microscopy Three male albino rats, 280-400g, were anaesthetized with pentobarbitone sodium (50 mg/kg) and fixed by intracardiac perfusion with 4°/i, neutral buffered formol saline. The brain stem was removed, embedded in paraffin wax and serially sectioned in the coronal, sagittal or horizontal planes. Sections were cut at 10 #m. Every third section was stained alternatively with cresyl violet and luxol fast blue or by Glees and Marsland's modification of Bielschowsky's silver technique.
Antidromic field profiles Experiments were performed on 7 male albino rats, 350-500 g. Anaesthesia was induced with pentobarbitone sodium (50 rag/kg) intraperitoneally and was maintained by intravenous injection, either periodically or by constant infusion (15 mg/kg/ h). During the period of recording the animals were paralysed with gatlamine triethiodide (10 mg/kg/h) and ventilated with oxygen. Body temperature was maintained with a thermistor-controlled heating blanket. The brain stem was approached ventrally as previously described3. The following 7 muscle branches of the facial nerve on one side were prepared for stimulation after removal of the parotid, submaxillary, and major sublingual glands and the sternomastoid muscle; digastric (DIG), posterior auricular (PA), cervical (CERV), inferior buccolabialis (IBL), superior buccolabialis (SBL), temporal (TZ I) and zygomafic (TZ II). The temporal and zygomatic muscle branches were tied together and stimulated as one root. The exposed tissues were immersed in a pool of liquid paraffin (37 ~C) made by
raising the skin flaps. Because of the restricted area of the pool, only 3 or 4 branches were prepared in any one experiment to minimize stimulus spread. The branches of the facial nerve were alternatively stimulated through bipolar silver wire electrodes, positioned to avoid stimulus spread, with rectangular pulses of 300/~A and 0.15 msec duration, from stimulus isolation units (Grass SIU 4678). Extracellular recordings were obtained with the 4 M NaCl filled centre barrel of 7-barrel micropipettes, the outer barrels of which contained substances used and described in a concurrent study 1~. Adequate retaining currents were applied to prevent diffusion of compounds which might affect the size of the antidromic field potential. One barrel in each electrode contained pontamine sky blue (2 ~ in 0.5 M Na acetate, pH 7.3). Antidromic field potentials were displayed on an oscilloscope and photographed for subsequent analysis. The three-dimensional negative-going field potential profiles evoked by antidromic stimulation of different muscle branches of the facial nerve were recorded from numerous tracks through the nuclear region. Although this was done as systematically as possible in 250 #m moves in both lateral and rostrocaudal directions the complex and dense nature of the vascular network imposed limitations to attaining a regular grid pattern of penetrations. The electrode was first advanced dorsally and the field potentials recorded during withdrawal of the electrode in 10-250/~m steps. The earbars of the head holder were used as a reference zero for comparisons between animals and the midline and other prominent surface landmarks on the brain stem acted as stereotaxic reference points in any one animal. The stereotaxic coordinates were read from a Narashige micromanipulator and the head holder conformed to the principles of K6nig and Klippel s. Pontamine sky blue was ejected at stereotaxically defined recording sites, and the resultant blue spots were found by cutting 25 or 50/~m thick frozen sections of the formalin-fixed brain stem 9. Fixation of central nervous system tissue for light microscopy causes differential shrinkage within any large nucleus. Because of this, direct translations of isopotential lines onto outlines of coronal sections from formalin-fixed, paraffin wax-embedded tissue are difficult to make. However by determining the position of the pontamine sky blue dye mark, both histologically and stereotaxically, correlations between the morphological subdivisions of the nucleus, the electrophysiological data and stereotaxic co-ordinates can be made. RESULTS
Anatomy of the facial nerve The peripheral course of the facial nerve in the rat is as described by Greene 4, except for the cervical branch. The cervical is contiguous with the inferior buccolabialis for a shorter distance than described by Greene and in contrast, generally two but sometimes as many as 6 branches are observed. The posterior auricular, superior buccolabialis and temporal-zygomatic branches are joined by numerous branches of the trigeminal nerve. But in each case the muscle branches of the facial nerve could be dissected free of all conjoining trigeminal branches. The inferior buccolabialis is
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Fig. 1. Sequential schematic outlines of the morphological subdivisions of the facial nucleus in the rat as they appear in formalin-fixed, paraffin wax-embedded, coronal sections. Positions of each coronal section in mm caudal to the external auditory meatus is given above each sections. Abbreviations: M, medial; I, intermediate; L, lateral: VM, ventromediat; and D, dorsal subdivision. The 1 mm calibration bar is for the tissue when prepared as described and is not to be confused with in vivo observations. rarely observed to have such associations with the trigeminal. The appearance of the anastomoses between peripheral facial branches in gross dissection closely resembles that of the dog described by Huber 6. Contrary to Huber however, such anastomosing bundles that exist, for example, between the temporal and zygomatic branches, are in every case found to be trigeminal and not facial in origin. Since stimulation of trigeminal fibres is likely to lead to synaptic potentials in the facial nucleus 12, these were carefully dissected from the facial branches prepared for stimulation.
Anatomy of the facial nucleus The facial nucleus of the rat was located near the ventrolateral surface of the medulla oblongata extending from 1.5 to 3.5 mm caudal to the interaural plane. The subdivisions of the facial nucleus in sequential coronal sections are illustratedin outline form in Fig. 1. The arrangement of subdivisions within the nucleus in the coronal plane presented here is consistent with the appearance of subdivisions in horizontal and sagittal sections. The position of the sections was determined by comparing frozen sections containing pontamine sky blue dye marks placed at stereotaxically defined points during the electrophysiological experiments (see below), with the formalin-fixed, paraffin wax-embedded sections. The nucleus is divided by fibre bundles running through it into rostrocaudal columnar subdivisions: the medial,
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Fig. 2. Comparison of the temporal-zygomatic (TZ) and digastric (DIG) branch antidromic field potentials. A : record of electrode track 2.4 m m caudal to the external auditory meatus and 1.8 mm lateral to the midline. Depth is in m m from the ventral surface. (a, TZ; b, DIG.) B: dorsoventral field profile for m a x i m u m negative amplitudes from same electrode track shown in (A). C: mediolateral field profile for potentials recorded in the coronal plane 2.5 m m caudal to the meatus and at a depth of 1.75 m m from the ventral surface. Ordinate is in m m from the midline. D : rostrocaudal field profile for the digastric branch, 1.7 1.9 m m lateral to the midline and 2.0 mm from the ventral surface. E: rostrocaudal field profile of the rostromedial maximum for the temporal-zygomatic branch (TZ I), 1.7 1.9 m m lateral to the midline and 1.25 m m from the ventral surface. F : rostrocaudal field profile o f the caudolateral maximum for the temporal-zygomatic branch (TZ ll), 2.1 2.4 mm lateral to the midline and 1.5 m m from the ventral surface.
ventromedial, intermediate, lateral and dorsal divisions. The medial division is distinct from other divisions throughout the extent of the nucleus and is slightly more rostral than the other divisions. In the more rostral part of the nucleus the intermediate and lateral groups are not clearly separated, but become more distinct in the median and caudal position. The lateral division extends more caudal than the other divisions. The dorsal division becomes distinct from the lateral division in the median position and remains distinct caudally. The small ventromedial division is separate from the medial division only in the median part of the nucleus. Throughout the text median refers to the mid-point of the axis being described and medial to the region closest to the midline of the brain stem.
A ntidromic field profiles Either 3 or 4 of the 6 (the temporal and zygomauc considered as one) major branches of the facial nerve were stimulated in 7 animals and the antidromic field potentials evoked in the nucleus were recorded. The combination of branches studied in any one animal was unique to that animal and any one branch was studied in at least two animals. This procedure allowed good comparisons to be made between animals in terms of both relative size and position of antidromically-evoked field potentials from the various branches. Quantitatively the amplitude of the antidromic field potential recorded for any one branch varied between experiments. However, the position within the nucleus of the maximum antidromic field potential evoked from each branch remained relatively constant. The characteristics of the antidromic field potential profile will be described for each branch of the facial nerve studied. More detail will be furnished for branches where the site of origin ~s more contentious. (1) Digastric (DIG) nerve. The digastric nerve was studied in 3 animals. An example of the antidromic field profile for a single electrode track is illustrated in Fig. 2Ab. The peak amplitude for each depth recording of this track was plotted in Fig. 2B. A small field was first observed at a depth of 2.0-2.5 m m from the ventral surface and reached a maximum between 1.5 and 1.75 ram, which rapidly diminished more superficially, and occasionally reversed in polarity. When the peak amplitudes were plotted in the mediolateral (Fig. 2C), and rostrocaudal (Fig. 2D), planes maxima were observed at approximately 1.8 m m lateral to the midline and 2.0 m m caudal to the external acoustic meatus. The field was recorded over an extremely narrow range in the most dorsal aspect of the nucleus. The nerve was represented by a column 800 # m long. The column was 250 ,urn wide and 500 # m in a dorsoventral direction at its largest transverse cross-sectional area. The location was confirmed by a pontamine sky blue mark placed at a depth of 1.67 m m from the ventral surface in the electrode track of Fig. 2A and subsequently found in the most dorsal aspect of the intermediate subdivision of the nucleus (t in Fig. 1). (2) Temporal-zygomatic (TZ) nerve. The temporal-zygomatic nerve was studied in two animals. In Fig. 2Aa the antidromic field is shown with that of the digastric previously described. In this coronal plane the temporal-zygomatic ~TZ) field is fairly diffuse extending from 0.5 to 2.0 mm from the ventral surface and 1.6-2.8 m m
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Fig. 3. C o m p a r i s o n of the temporal-zygomatic (TZ) and superior buccolabialis (SBL) branch antidromic field potentials. A : record of electrode track 2.1 m m caudal to the external auditory m e a t u s and 1.4 m m lateral to the midline. Depth is in m m f r o m the ventral surface. (a, SBL; b, TZ.) B: dorsoventral field profile for m a x i m u m negative amplitudes from same electrode track s h o w n in (A). C: mediolateral field profile for potentials recorded in the same coronal plane as (A) at a depth o f 1.0 m m from the ventral surface. Ordinate is in m m f r o m the midline. D : rostrocaudal field profile for the superior buccolabialis branch, 2.(~2.1 m m lateral to the midline a n d 1.0 m m from the ventral surface. E: rostrocaudal field profile of the rostromedial m a x i m u m for the temporal-zygomatic branch (TZ I), 1.4 m m lateral to the midline a n d 1.0 m m f r o m the ventral surface. F: rostrocaudal field profile of the caudolateral m a x i m u m for the temporal-zygomatic branch (TZ 11), 2.1-2.2 m m lateral to the midline a n d 1.0 m m f r o m the ventral surface. Ordinates in ( D - F ) are in m m caudal to the meatus.
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from the midline (Fig. 2B and C). However, in the rostrocaudal plane 2 very distinct maxima are observed (Fig. 2E and F). The first maximum (TZ 1) is 2.0 mm caudal. 1.7 m m lateral and 1.25 mm from the ventral surface (Fig. 2E). The second maximum (TZ lI) is 2.5 mm caudal. 2.2 mm lateral and 1.5 mm from the ventral surface (Fig. 2F). The postions of the 2 maxima were compared with the position of the pontamine sky blue mark placed at the maximum field for the digastric branch. The first maximum (TZ I) was rostral and ventral to this point but still within the intermediate subdivisions (| in Fig. 1). The second maximum (TZ II) was lateral, corresponding to the dorsal subdivisions (D in Fig. 1). In the second rat the temporat-zygomatlc antidromic field was compared wit h that of the superior buccolabialis (Fig. 3). At the more rostral coronal plane of 2.t mm the temporal-zygomatic field was shifted medially (Fig. 3C). The field was also more ventrally placed (Fig. 3B) with a maximum at about 1.0 m m (Fig. 3B). In the rostrocaudal direction the field again showed 2 maxima, one rostromedian and the other caudolateral (Fig. 3E and F). (3) Superior bueeolabialis (SBLj nerve. The amplitude of the superior buceolabialis field, studied in 4 animals, was usually one of the largest observed in the nucleus (Fig. 3Aa). At a depth of 1.0 m m from the ventral surtace and 2.1 mm caudal, the amplitude of the superior buccolabialis field was maximal at 2.3 mm from the midline and decreased abruptly further laterally, Fig. 3C. This result was characteristic, for the fields were always largest in the lateral aspect of the nucleus, tn a lateral rostrocaudal plane the field extended from 1.5 to 3.4 mm caudal to the earbars with a maximum at 2.1 m m (Fig. 3D). The superxor buccolabialis field was generally located ventral and somewhat lateral to the second, caudolateral, maximum of the temporal-zygomatic field (TZ ll). The field also extended more rostral as well as more caudal than the temporal-zygomatic field. (4) Inferior bueeolabialis (IBL) nerve. Like the superior buccotabialis field the inferior buccolabialis field (5 animals) was pronounced throughout a large part of the nucleus (Fig. 4Aa, B and D) but was maximal in the lateral aspect (Fig. 4C) from 1.95 to greater than 2.5 mm from the midline. The superior and inferior buccolabiales fields were compared in two animals which showed that the maximum from the inferior branch was medial and ventral as well as somewhat caudal (Figs. 3D and 4D) to that from the superior branch. Pontamine dye marks indicated that the superior buccolabialis is in the dorsal and the inferior buccolabialis is in the ventral portion of the lateral subdivision (L in Fig. 1). (5) Cervical (CERV) nerve. The cervical antidromic field was studied in 4 animals and was always found to be small, discrete and predominantly in the ventral two-thirds of the nucleus, Fig. 4E. In the electrode track illustrated (Fig. 4Ab and B) the field extended from 0.75 to 1.5 mm from the ventral surface. In the same coronal plane a ventral position for the field was generally confirmed and reached a peak at 2.15 m m from the midline (Fig. 4C) where it sharply decreased. In the rostrocaudal plane the maximum amplitude was found in the ventral portion of the median aspect of the nucleus with a peak at about 2.3 mm (Fig. 4E), diminishing caudally and no
4oo.
:~5 Elsevier/North-Holland Biomedical Press, Amsterdam - Printed in The Netherlands
Research Reports
M O R P H O L O G Y OF T H E F A C I A L N U C L E U S OF T H E RAT
MICHAEL R. M A R T I N and DAVID LODGE*
Department of Physio&gy, University of Bristol Medical School, Bristol BS8 1TD (Great Britain) (Accepted July 2nd, 1976)
SUMMARY
The facial nucleus of the rat can be divided into 5 morphological subdivisions. Using a method for the correlation of the observed subdivisions with antidromic field profiles, the origins of the major muscle branches of the facial nerve in the motor nucleus were determined. The posterior auricular branch is in the medial, the cervical in the ventromedial, the inferior and superior buccolabiales in the lateral, the zygomatic in the dorsal and the temporal and digastric in the intermediate subdivision. The results are consistent with an organization of the motor nucleus reflecting a corresponding topographic organization of the facial musculature.
INTRODUCTION
Several investigators have attempted to divide the facial nucleus into morphologically distinct subdivisions and delegate the origin of one or more branches to individual subdivisionsa,10,11,15,16,19. However there is some disagreement on the number and arrangement of the subdivisions within the nucleus in any one species (for reviews see refs. 1, 18). In the rat, from 3 to 6 subdivisions have been described5,14, 1,5. Only Papez 1~ has attempted to correlate the various morphological subdivisions of the rat with the origins of specific facial muscle branches. He found that the origin of the posterior auricular branch is in the medial, the anterior auricular (temporal) is in the intermediate, and the superior and inferior buccolabiales in the lateral and ventrolateral morphological subdivisions, respectively. The nuclear origins of 3 of these 4 branches, the posterior auricular and superior and inferior buccolabiales branches, are similar to those observed in other mammals. Attempts have been made in other * Animal Health Trust/Wellcome Trust Research Fellow. Present address: Department of Pharmacology, John Curtin School of Medical Research, A.N.U., Canberra, Australia.
mammals to ascertain the origin within the nucleus of the temporal (anterior auricular), zygomatic, cervical and deep facial (stapedius, stylohoid and digastric) muscle branchest, e,l,%ts, though there is considerable disagreement. This may be the result of the difficulty in the interpretation of cellular pathological changes due to the degeneration methods employed. Nonetheless efforts have been made ~o delineate an organization of the motor branches within the nucleus, reflecting the embryological development and topographic relations of the facial musclesV,~'~,~L This report describes the appearance, in the rat, of the facial nucleus and its subdivisions. These findings are correlated with the three-dimensional representation of the antidromic field potentials for 7 muscles branches (including the cervical, temporal, zygomatic and digastric) of the facial nerve within the nucleus. Not only does the method circumvent the difficulties in the analysis of cellular pathological changes due to degeneration, but it also permits the study of the intranuctear position of more than one branch of the nerve in a given animal. A further reason for using antidromic field profiles for determining the nuclear origins of motor branches as opposed to other anatomical methods, is that the ability to identify the nucleus both electrophysiologically and stereotaxically is essential in many physiological and pharmacological studies a',13. METHODS
Light microscopy Three male albino rats, 280-400g, were anaesthetized with pentobarbitone sodium (50 mg/kg) and fixed by intracardiac perfusion with 4°/i, neutral buffered formol saline. The brain stem was removed, embedded in paraffin wax and serially sectioned in the coronal, sagittal or horizontal planes. Sections were cut at 10 #m. Every third section was stained alternatively with cresyl violet and luxol fast blue or by Glees and Marsland's modification of Bielschowsky's silver technique.
Antidromic field profiles Experiments were performed on 7 male albino rats, 350-500 g. Anaesthesia was induced with pentobarbitone sodium (50 rag/kg) intraperitoneally and was maintained by intravenous injection, either periodically or by constant infusion (15 mg/kg/ h). During the period of recording the animals were paralysed with gatlamine triethiodide (10 mg/kg/h) and ventilated with oxygen. Body temperature was maintained with a thermistor-controlled heating blanket. The brain stem was approached ventrally as previously described3. The following 7 muscle branches of the facial nerve on one side were prepared for stimulation after removal of the parotid, submaxillary, and major sublingual glands and the sternomastoid muscle; digastric (DIG), posterior auricular (PA), cervical (CERV), inferior buccolabialis (IBL), superior buccolabialis (SBL), temporal (TZ I) and zygomafic (TZ II). The temporal and zygomatic muscle branches were tied together and stimulated as one root. The exposed tissues were immersed in a pool of liquid paraffin (37 ~C) made by
raising the skin flaps. Because of the restricted area of the pool, only 3 or 4 branches were prepared in any one experiment to minimize stimulus spread. The branches of the facial nerve were alternatively stimulated through bipolar silver wire electrodes, positioned to avoid stimulus spread, with rectangular pulses of 300/~A and 0.15 msec duration, from stimulus isolation units (Grass SIU 4678). Extracellular recordings were obtained with the 4 M NaCl filled centre barrel of 7-barrel micropipettes, the outer barrels of which contained substances used and described in a concurrent study 1~. Adequate retaining currents were applied to prevent diffusion of compounds which might affect the size of the antidromic field potential. One barrel in each electrode contained pontamine sky blue (2 ~ in 0.5 M Na acetate, pH 7.3). Antidromic field potentials were displayed on an oscilloscope and photographed for subsequent analysis. The three-dimensional negative-going field potential profiles evoked by antidromic stimulation of different muscle branches of the facial nerve were recorded from numerous tracks through the nuclear region. Although this was done as systematically as possible in 250 #m moves in both lateral and rostrocaudal directions the complex and dense nature of the vascular network imposed limitations to attaining a regular grid pattern of penetrations. The electrode was first advanced dorsally and the field potentials recorded during withdrawal of the electrode in 10-250/~m steps. The earbars of the head holder were used as a reference zero for comparisons between animals and the midline and other prominent surface landmarks on the brain stem acted as stereotaxic reference points in any one animal. The stereotaxic coordinates were read from a Narashige micromanipulator and the head holder conformed to the principles of K6nig and Klippel s. Pontamine sky blue was ejected at stereotaxically defined recording sites, and the resultant blue spots were found by cutting 25 or 50/~m thick frozen sections of the formalin-fixed brain stem 9. Fixation of central nervous system tissue for light microscopy causes differential shrinkage within any large nucleus. Because of this, direct translations of isopotential lines onto outlines of coronal sections from formalin-fixed, paraffin wax-embedded tissue are difficult to make. However by determining the position of the pontamine sky blue dye mark, both histologically and stereotaxically, correlations between the morphological subdivisions of the nucleus, the electrophysiological data and stereotaxic co-ordinates can be made. RESULTS
Anatomy of the facial nerve The peripheral course of the facial nerve in the rat is as described by Greene 4, except for the cervical branch. The cervical is contiguous with the inferior buccolabialis for a shorter distance than described by Greene and in contrast, generally two but sometimes as many as 6 branches are observed. The posterior auricular, superior buccolabialis and temporal-zygomatic branches are joined by numerous branches of the trigeminal nerve. But in each case the muscle branches of the facial nerve could be dissected free of all conjoining trigeminal branches. The inferior buccolabialis is
1.7 mm
2.0
2.2
2.4
2.7
2.9
3.1
3.4
1ram
Fig. 1. Sequential schematic outlines of the morphological subdivisions of the facial nucleus in the rat as they appear in formalin-fixed, paraffin wax-embedded, coronal sections. Positions of each coronal section in mm caudal to the external auditory meatus is given above each sections. Abbreviations: M, medial; I, intermediate; L, lateral: VM, ventromediat; and D, dorsal subdivision. The 1 mm calibration bar is for the tissue when prepared as described and is not to be confused with in vivo observations. rarely observed to have such associations with the trigeminal. The appearance of the anastomoses between peripheral facial branches in gross dissection closely resembles that of the dog described by Huber 6. Contrary to Huber however, such anastomosing bundles that exist, for example, between the temporal and zygomatic branches, are in every case found to be trigeminal and not facial in origin. Since stimulation of trigeminal fibres is likely to lead to synaptic potentials in the facial nucleus 12, these were carefully dissected from the facial branches prepared for stimulation.
Anatomy of the facial nucleus The facial nucleus of the rat was located near the ventrolateral surface of the medulla oblongata extending from 1.5 to 3.5 mm caudal to the interaural plane. The subdivisions of the facial nucleus in sequential coronal sections are illustratedin outline form in Fig. 1. The arrangement of subdivisions within the nucleus in the coronal plane presented here is consistent with the appearance of subdivisions in horizontal and sagittal sections. The position of the sections was determined by comparing frozen sections containing pontamine sky blue dye marks placed at stereotaxically defined points during the electrophysiological experiments (see below), with the formalin-fixed, paraffin wax-embedded sections. The nucleus is divided by fibre bundles running through it into rostrocaudal columnar subdivisions: the medial,
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Fig. 2. Comparison of the temporal-zygomatic (TZ) and digastric (DIG) branch antidromic field potentials. A : record of electrode track 2.4 m m caudal to the external auditory meatus and 1.8 mm lateral to the midline. Depth is in m m from the ventral surface. (a, TZ; b, DIG.) B: dorsoventral field profile for m a x i m u m negative amplitudes from same electrode track shown in (A). C: mediolateral field profile for potentials recorded in the coronal plane 2.5 m m caudal to the meatus and at a depth of 1.75 m m from the ventral surface. Ordinate is in m m from the midline. D : rostrocaudal field profile for the digastric branch, 1.7 1.9 m m lateral to the midline and 2.0 mm from the ventral surface. E: rostrocaudal field profile of the rostromedial maximum for the temporal-zygomatic branch (TZ I), 1.7 1.9 m m lateral to the midline and 1.25 m m from the ventral surface. F : rostrocaudal field profile o f the caudolateral maximum for the temporal-zygomatic branch (TZ ll), 2.1 2.4 mm lateral to the midline and 1.5 m m from the ventral surface.
ventromedial, intermediate, lateral and dorsal divisions. The medial division is distinct from other divisions throughout the extent of the nucleus and is slightly more rostral than the other divisions. In the more rostral part of the nucleus the intermediate and lateral groups are not clearly separated, but become more distinct in the median and caudal position. The lateral division extends more caudal than the other divisions. The dorsal division becomes distinct from the lateral division in the median position and remains distinct caudally. The small ventromedial division is separate from the medial division only in the median part of the nucleus. Throughout the text median refers to the mid-point of the axis being described and medial to the region closest to the midline of the brain stem.
A ntidromic field profiles Either 3 or 4 of the 6 (the temporal and zygomauc considered as one) major branches of the facial nerve were stimulated in 7 animals and the antidromic field potentials evoked in the nucleus were recorded. The combination of branches studied in any one animal was unique to that animal and any one branch was studied in at least two animals. This procedure allowed good comparisons to be made between animals in terms of both relative size and position of antidromically-evoked field potentials from the various branches. Quantitatively the amplitude of the antidromic field potential recorded for any one branch varied between experiments. However, the position within the nucleus of the maximum antidromic field potential evoked from each branch remained relatively constant. The characteristics of the antidromic field potential profile will be described for each branch of the facial nerve studied. More detail will be furnished for branches where the site of origin ~s more contentious. (1) Digastric (DIG) nerve. The digastric nerve was studied in 3 animals. An example of the antidromic field profile for a single electrode track is illustrated in Fig. 2Ab. The peak amplitude for each depth recording of this track was plotted in Fig. 2B. A small field was first observed at a depth of 2.0-2.5 m m from the ventral surface and reached a maximum between 1.5 and 1.75 ram, which rapidly diminished more superficially, and occasionally reversed in polarity. When the peak amplitudes were plotted in the mediolateral (Fig. 2C), and rostrocaudal (Fig. 2D), planes maxima were observed at approximately 1.8 m m lateral to the midline and 2.0 m m caudal to the external acoustic meatus. The field was recorded over an extremely narrow range in the most dorsal aspect of the nucleus. The nerve was represented by a column 800 # m long. The column was 250 ,urn wide and 500 # m in a dorsoventral direction at its largest transverse cross-sectional area. The location was confirmed by a pontamine sky blue mark placed at a depth of 1.67 m m from the ventral surface in the electrode track of Fig. 2A and subsequently found in the most dorsal aspect of the intermediate subdivision of the nucleus (t in Fig. 1). (2) Temporal-zygomatic (TZ) nerve. The temporal-zygomatic nerve was studied in two animals. In Fig. 2Aa the antidromic field is shown with that of the digastric previously described. In this coronal plane the temporal-zygomatic ~TZ) field is fairly diffuse extending from 0.5 to 2.0 mm from the ventral surface and 1.6-2.8 m m
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Fig. 3. C o m p a r i s o n of the temporal-zygomatic (TZ) and superior buccolabialis (SBL) branch antidromic field potentials. A : record of electrode track 2.1 m m caudal to the external auditory m e a t u s and 1.4 m m lateral to the midline. Depth is in m m f r o m the ventral surface. (a, SBL; b, TZ.) B: dorsoventral field profile for m a x i m u m negative amplitudes from same electrode track s h o w n in (A). C: mediolateral field profile for potentials recorded in the same coronal plane as (A) at a depth o f 1.0 m m from the ventral surface. Ordinate is in m m f r o m the midline. D : rostrocaudal field profile for the superior buccolabialis branch, 2.(~2.1 m m lateral to the midline a n d 1.0 m m from the ventral surface. E: rostrocaudal field profile of the rostromedial m a x i m u m for the temporal-zygomatic branch (TZ I), 1.4 m m lateral to the midline a n d 1.0 m m f r o m the ventral surface. F: rostrocaudal field profile of the caudolateral m a x i m u m for the temporal-zygomatic branch (TZ 11), 2.1-2.2 m m lateral to the midline a n d 1.0 m m f r o m the ventral surface. Ordinates in ( D - F ) are in m m caudal to the meatus.
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from the midline (Fig. 2B and C). However, in the rostrocaudal plane 2 very distinct maxima are observed (Fig. 2E and F). The first maximum (TZ 1) is 2.0 mm caudal. 1.7 m m lateral and 1.25 mm from the ventral surface (Fig. 2E). The second maximum (TZ lI) is 2.5 mm caudal. 2.2 mm lateral and 1.5 mm from the ventral surface (Fig. 2F). The postions of the 2 maxima were compared with the position of the pontamine sky blue mark placed at the maximum field for the digastric branch. The first maximum (TZ I) was rostral and ventral to this point but still within the intermediate subdivisions (| in Fig. 1). The second maximum (TZ II) was lateral, corresponding to the dorsal subdivisions (D in Fig. 1). In the second rat the temporat-zygomatlc antidromic field was compared wit h that of the superior buccolabialis (Fig. 3). At the more rostral coronal plane of 2.t mm the temporal-zygomatic field was shifted medially (Fig. 3C). The field was also more ventrally placed (Fig. 3B) with a maximum at about 1.0 m m (Fig. 3B). In the rostrocaudal direction the field again showed 2 maxima, one rostromedian and the other caudolateral (Fig. 3E and F). (3) Superior bueeolabialis (SBLj nerve. The amplitude of the superior buceolabialis field, studied in 4 animals, was usually one of the largest observed in the nucleus (Fig. 3Aa). At a depth of 1.0 m m from the ventral surtace and 2.1 mm caudal, the amplitude of the superior buccolabialis field was maximal at 2.3 mm from the midline and decreased abruptly further laterally, Fig. 3C. This result was characteristic, for the fields were always largest in the lateral aspect of the nucleus, tn a lateral rostrocaudal plane the field extended from 1.5 to 3.4 mm caudal to the earbars with a maximum at 2.1 m m (Fig. 3D). The superxor buccolabialis field was generally located ventral and somewhat lateral to the second, caudolateral, maximum of the temporal-zygomatic field (TZ ll). The field also extended more rostral as well as more caudal than the temporal-zygomatic field. (4) Inferior bueeolabialis (IBL) nerve. Like the superior buccotabialis field the inferior buccolabialis field (5 animals) was pronounced throughout a large part of the nucleus (Fig. 4Aa, B and D) but was maximal in the lateral aspect (Fig. 4C) from 1.95 to greater than 2.5 mm from the midline. The superior and inferior buccolabiales fields were compared in two animals which showed that the maximum from the inferior branch was medial and ventral as well as somewhat caudal (Figs. 3D and 4D) to that from the superior branch. Pontamine dye marks indicated that the superior buccolabialis is in the dorsal and the inferior buccolabialis is in the ventral portion of the lateral subdivision (L in Fig. 1). (5) Cervical (CERV) nerve. The cervical antidromic field was studied in 4 animals and was always found to be small, discrete and predominantly in the ventral two-thirds of the nucleus, Fig. 4E. In the electrode track illustrated (Fig. 4Ab and B) the field extended from 0.75 to 1.5 mm from the ventral surface. In the same coronal plane a ventral position for the field was generally confirmed and reached a peak at 2.15 m m from the midline (Fig. 4C) where it sharply decreased. In the rostrocaudal plane the maximum amplitude was found in the ventral portion of the median aspect of the nucleus with a peak at about 2.3 mm (Fig. 4E), diminishing caudally and no
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