Neuroscience Letters, 135 (1992) 75 79 © 1992 Elsevier Scientific Publishers Ireland Ltd. All rights reserved 0304-3940/92/$ 05.00
75
NSL 08343
The organisation of fibres within the rat basis pedunculi Mitchell Glickstein, Ines Kralj-Hans, Charles Legg, Barbara Mercier, M a r k R a m n a - R a y a n and Elisabetta Vaudano Department of Anatomy and Developmental Biology. University College London, London ( U.K. ) Kev words.
Peduncle; Corticofugal; Pons; Efferent; Subcortical; Cerebellum; Pyramidal tract; Basis pedunculi
The organisation of corticofugal fibres within the basis pedunculi of rats was studied using wheat germ agglutinin-horseradish peroxidase as an orthograde tracer. Following cortical injections, labelled fibres were distributed within the cerebral peduncle in an orderly way. Fibres which originate from cells in the frontal cortex maintain a position in the ventromedial part of the basis pedunculi. Fibres from the occipital and temporal cortex travel in the most dorsolateral part. Somatosensory fibres travel between these two. The extent of labelled fibres within the peduncles is correlated with the relative density of corticopontine cells arising from different areas of the cerebral cortex.
In all mammals, fibres in the basis pedunculi originate from cells in the cerebral cortex. One of the major targets of peduncle fibres is the pontine nuclei. These fibres project either directly to the pontine nuclei or by way of a collateral of cortico-spinal and cortico-bulbar [5, 20] or cortico-tectal [1, 10] fibres. The pons in turn relays information to the cerebellar cortex. The concentration of corticopontine fibres within the peduncle offers the opportunity to study the behavioural effects of selectively interrupting the input to the cerebellum from one or another cortical area by a small fibre lesion or a reversible blocking agent. The spatial organisation of peduncle fibres has been studied for over a century. However, there was confusion and disagreement in the early literature on the origin and termination of different components of the peduncle. The picture was clarified by D6j6rine [6] who charted systematically the location of degenerating fibres in the basis pedunculi in post-mortem studies of the brains of patients who had sustained injury to the cerebral cortex or its efferent fibres. In contrast to earlier claims, he reported that all of the fibres within the basis pedunculi have their cells of origin in the cerebral cortex. The largest number of peduncle fibres originate from the cortex within or adjacent to the central sulcus. Fibres which originate from precentral cortex travel in a medial region of the peduncles. Subsequent evidence extended these conclusions. Fibres from pre-frontal cortical areas travel Correspondence." M. Glickstein, Department of Anatomy and Developmental Biology, University College London, Gower Street, London WC1E 6BT, U.K.
in the most medial part of the human peduncle [3]. Fibres from the parietal and temporal lobes travel in the most lateral portions [15]. The same basic organisation of peduncular fibres is true for primates [2, 14, 19, 22] and other mammals although the data are less systematic. There are studies which identify the location within the peduncles of efferent fibres from different cortical areas in ferrets [23], rabbits [7], and goats [9]. Krieg [11], used the Marchi stain to follow degenerating fibre tracts in the rat. He illustrated the fact that efferent fibres from different cortical areas are segregated within definite regions of the internal capsule and the peduncle. Nauta and Bucher [17], using silver degeneration staining techniques, noted that the efferent fibres from visual cortex travel in the dorsolateral region of the peduncle. The present study was directed at extending these descriptions using a more sensitive method for analysing the course and the degree of segregation of cortico-peduncular fibres in the rat at rostro-caudal levels from the junction with the diencephalon to the pons. A total of 11 adult rats were studied. Pressure injections (50-100 nl) of 4% wheatgerm agglutinin-horseradish peroxidase (WGA-HRP) were made under hypnorm/hypnoval anaesthesia, supplemented as necessary by halothane. The cortex was exposed over the injection site and the tip of a Hamilton microsyringe or micropipette was lowered to a maximum depth of 1 mm below the cortical surface. Following a 48 h post-operative survival period, the animal was anaesthetised with an overdose of sodium pentobarbitone and perfused via the heart. The brain was removed from the skull and frozen
76 TABLE I S U M M A R Y OF THE N U M B E R OF SEGMENTS WHICH CONTAINED LABELLED FIBRES WITHIN THE CEREBRAL P E D U N C L E Cortical area
Frl Fr3 Cing Barrels Par I OC I OC21 RSA Prh OCr Te2
Case No.
89/3 87/26 R18 SMCI 89/73 89/3 90/1 89/73 89/29 R/12 88/20
Rostro-caudal section level I
II
I11
IV
V
VI
7,8 5,6,7 5,6,7 2,3 1,2,3,4 1,2 1,2 1,2,3 1,2 1,2,3 1,2,3
8 5,6,7,8 6,7~8 2,3 1,2,3,4 1~2 1,2 1,2 1,2 1,2,3 1,2,3,4
8 6,7,8 6,7.8 1,2~3 1,2,3 1.2 1,2 1,2,3 1,2 1,2 12.3
8 6,7,8 8 3,4,5 3,4~5 1,2 1 1,2,3 1,1 1 1,2
8 5,6,7,8 7.8 2,3~4,5,6 3.4,5,6 1.2~3,4 1.2 1,2,3 1,2 1,2,3 1,2
7,8 5,6,7 6,7 1,2,3 2,3,4 1,2,3 1,2,3 1,2,3 1 1,2,3,4 l
sections cut transversely at 50 ~ m in a flat skull plane [18]. Every third section was processed according to a modified tetramethylbenzidine method [8, 16]. The sections were mounted onto gelatin-coated slides, counterstained with Cresyl violet, dehydrated, cleared and coverslipped using D P X mountant. In no case did the injected W G A - H R P extend below the cortex to include underlying white matter. The location and extent of the primary label was reconstructed onto a standard map of the cortical surface [24]. In order to supplement the identification of the cortical injection sites, in all cases the orthogradely and retrogradely labelled terminals and cells within the thalamus were also studied. These observations were used only to confirm the locus and extent of the cortical injection site, hence they are not reported in detail here. All sections which contained the basis pedunculi were examined for the presence of W G A - H R P - I a b e l l e d fibres and these were plotted using a camera lucida system under bright-field cross-polarized illumination. The data are presented in two ways. The location of labelled fibres for each individual case is shown at 6 rostro-caudal levels through the basis pendunculi on a standard atlas of the rat brain [18]. In addition, at each of the 6 levels sampled, the peduncles were divided into 8 equal segments, measured along an axis running from the dorsolateral to the ventromedial pole. The presence of labelled fibres within each of these 8 segments was recorded and is summarised in Table I. This table reflects the degree of segregation of fibres arising from different cortical areas and the extent to which the labelled fibres maintain this pattern throughout their course. Note that the table shows the presence of labelled fibres within a given segment but it does not reflect their relative density.
Fibres arising from cells in primary m o t o r cortex (Frl and Fr3) travel in the most ventromedial part of the peduncle. The most concentrated grouping of fibres arise from the most rostral cortical area studied, area Frl. Fibres from Fr3 and cingulate cortex are distributed more diffusely and occupy a somewhat more lateral position in the peduncle. Fibres from the primary somatosensory cortex (Parl and Barrels) are located dorsolateral to those from the frontal cortex. Occipital Ocl and Oc2, temporal Te2, perirhinal cortex (Prh), and retrosplenial area (RSA) give rise to fibres which travel in the most dorsolateral region of the peduncle. There was little or no overlap between fibres from frontal and parietal cortex, but some overlap of fibres from the parietal and occipital cortex and considerable overlap between the fibres from occipital and temporal cortex. Segregation of fibres was less sharp at more caudal levels of the peduncles and the fibres formed multiple terminal patches within the pontine nuclei. The extent and density of labelled fibres contained within the peduncles correlates well with the relative density of cells arising from different cortical areas [13]. In each of the 11 cases we measured the total area within the peduncle containing label at 6 different levels. We rated the density of labelled fibres as light, medium, or heavy, scoring them 1, 2, or 3, respectively. We then multiplied the area times the scored density and calculated an average over the 6 different levels at which the measurements were made. In an earlier study [13] we had calculated the density of filled cells in different regions of the rat cerebral cortex after a W G A H R P injection which filled the pontine nuclei. The correlation between these two measures of the relative density of corticopontine cells was 0.76. These observations confirm that fibres within the basis
77 OcZRM
89/,3
89/,.3
87/26
R18
90/1
89/73
89/29
SMCl
R12
89/7,3
88/20
Fig. 1. cortical injection sites and position of labelled fibres within the basis pedunculi. The relative density of labelled fibres is indicated by the density of the stippling. The diagram in the upper left of the figure is from Zilles [24] and presents in outline the major cyto-architectonic regions of the rat brain. Nine of the injections are represented on the dorsolateral surface of the cerebral cortex; the other two on the medial surface of the hemispheres. To the left of the figure is an outline drawing of 6 sections through the rat brainstem to illustrate the level within the peduncle from rostral to caudal. The caudalmost sections are at the level of the rostral pons.
78
pedunculi of rats are spatially organized. Fibres from the most rostral areas of the frontal lobe travel in the most ventromedial portions of the peduncles. Fibres from the occipital and temporal lobes travel in the most dorsolateral portion. Parietal fibres travel between these two. The overall pattern of this pathway in the rat is similar to that in man and the higher primates. Although on a gross level the pattern is orderly, at a finer level, it may not be. A large number of cases with very small injections would be needed in order to establish whether efferent fibres from closely adjacent cortical regions remain segregated throughout their entire course. For example efferent fibres from occipital and temporal cortex overlap in the most dorsolateral region of the peduncle, although the temporal lobe fibres on average are located more dorsolaterally. Moreover, although the fibres maintain an orderly course within the peduncles they eventually lose that organisation, terminating in small and diffusely distributed patches within the pontine nuclei. Although these data reveal a general plan of organisation we recognise that any method of orthograde tracing at the light microscopic level may favour the largest calibre fibres. Verhaart [23] attempted to avoid the problem by using a stain for surviving normal fibres after he made cortical lesions in monkeys. In every case he found a region of the peduncle in which the largest calibre fibres were absent following restricted cortical lesions, but small fibres often appeared to survive within that region. He suggested that smaller fibres may have a less orderly course through the peduncles. However, subsequent studies by Van Crevel in Verhaart's laboratory [21] showed that central nervous system fibres degenerate at differing rates depending on their diameter, with the largest calibre fibres degenerating most rapidly. The survival time may have been too short to allow all of the fibres which would eventually die to do so. The WGAHRP method is probably more sensitive than either of these earlier methods for determining the course of efferent fibres, although the possibility remains that the very smallest calibre fibres may not have been revealed. There is a good correlation between the percentage of the peduncle which is occupied by fibres from a given cortical area and the relative density of cortico-pontine cells which arise from that area in the rat. This correlation suggests that pattern of the pathway as revealed by WGA-HRP label is consistent with the density of its cortical origins. Visual and somatosensory efferent fibres are segregated within the peduncle, and hence either could be selectively ablated or temporarily blocked. Rats can perform a number of skilled tasks such as accurate jumping [12] and reaching [4]. The contribution of sensory circuits to
such behaviour can be studied by selectively interrupting this major system which links sensory to motor areas of the brain.
I0
11 12
13
14 15
16
17
18 19
20
Baker, J., Gibson, A.. Mower, G., Robinson, F. and Glickstein, M., Cat visual corticopontine cells project to the superior colliculus, Brain Res., 265 (1983) 222 232. Barnard, J.W. and Woolsey, C.N., A study of localization in the cortico-spinal tracts of monkey and rat, J. Comp. Neurol., 105 (1956) 25 50. Beck, E., The origin, course, and termination of the prefrontal to pontine tract in the human brain, Brain, 73 (1950) 368 391, Bracha, V., Zhuravin, I.A. and Bureg, J., The reaching reaction in the rat: a part of the digging pattern?, Behav. Brain Res., 36 (1990) 13-64. Cajal, S., Ramon, Y, Histologie du Syst6me Nerveux de l'Homme et des V6rtebr6s C.S.I.C., Madrid, 1909 1911. D6j6rine, J., Anatomic des Centres Nerveux, Vol. II, Rueff, Paris, 1901. Droogleever Fortuijn, J., The systematic arrangement of fibres in capsula interna, thalamus opticus, and pes pedunculi, Proc. Kon. Nederl. Acad. Wet., 41 (1938) 3 12. Gibson, A.R., Hansma, D.I., Houk, J.C. and Robinson, F.R., A sensitive, low artifact TMB procedure for the demonstration of WGA-HRP in the CNS, Brain Res., 298 (1984) 235 24l. Haarsten, A.B. and Verhaart, W.J.C., Cortical projections to brain stem and spinal cord in the goat by way of the pyramidal tract and the bundle of Bagley, J. Comp. Neurol., 129 (1967) 189 201. Keizer, K., Kuypers, H.G.J.M. and Ronday, H.K., Branching cortical neurons in cat which project to the colliculi and to the ports: a retrograde fluorescent double-labeling study, Exp. Brain Res., 67 (1987) I 15. Krieg, W.J.S., Connections of the cerebral cortex. I. The albino rat c. extrinsic connections, J. Comp. Neurol., 86 (1947) 267 394. Legg, C.R. and Lambert, S., Distance estimation in the hooded rat: experimental evidence for the role of motion cues, Behav. Brain Res., 41 (1990) 11 20. Legg, C., Mercier, B. and Glickstein, M., Corticopontine projection in the rat: the distribution of labelled cortical cells alter large injections of horseradish peroxidase in the pontine nuclei, J. Comp. Neurol.. 286 (1989) 427 436. Levin, P.M., The efferent fibers of the t¥ontal lobe of the monkey, Macaca mulatta, J. Comp. Neurol.. 63 (1936) 369 419. Marin, O. and Angevine, J., Topographical organisation of the lateral segment of the basis pedunculi in man, J. Comp. Neurol., 118 (1962) 165-183. Mesulam, M.M., Principles of horseradish peroxidase neurohistochemistry and their applications for tracing neural pathways - axonal transport, enzyme histochemistry, and light microscopic analysis, In M.M. Mesulam (Ed.), Tracing Neural Connections with Horseradish Peroxidase, Wiley, Chichester, 1982, pp. 1 151. Nauta, W.J.H. and Bucher, V.M.. Efferent connections of the striate cortex in the albino rat. J. Comp. Neurol., 100 (1954) 257 285. Paxinos, G. and Watson, C., The Rat Brain in Stereotaxic Coordinates, Academic Press, Sydney, 1982. Sunderland, S., The projection of the cerebral cortex in the pons and cerebellum in the macaque monkey, J. Anat., 74 (1940) 201 226. Ugolini, G. and Kuypers, H.J.M., Collaterals of corticospinal and pyramidal fibres to the pontine grey demonstrated by a new appli-
79 cation of the fluorescent fibre labelling technique, Brain Res., 365 (1986) 211-227. 21 Van Crevel, H. and Verhaart, W.J.C., The rate of secondary degeneration in the central nervous system. I. The pyramidal tract of the cat, J. Anat., 97 (1963) 419~t49. 22 Verhaart, W.J.C. and Mechelse, K., The pedunculus cerebri and the capsula interna, Monatschr. Psychiat. Neurol., 127 (1954) 65-88.
23 Verhaart, W.J.C. and Noorduyn, N.J.A., The cerebral peduncle and the pyramid, Acta. Anat., 45 (1961) 315-343. 24 Zilles, K., Anatomy of the neocortex: Cytoarchitecture and myeloarchitecture. In B. Kolb and R. Tees (Eds.), The Cerebral Cortex of the Rat, MIT, Cambridge, MA, pp. 77-112.
75
NSL 08343
The organisation of fibres within the rat basis pedunculi Mitchell Glickstein, Ines Kralj-Hans, Charles Legg, Barbara Mercier, M a r k R a m n a - R a y a n and Elisabetta Vaudano Department of Anatomy and Developmental Biology. University College London, London ( U.K. ) Kev words.
Peduncle; Corticofugal; Pons; Efferent; Subcortical; Cerebellum; Pyramidal tract; Basis pedunculi
The organisation of corticofugal fibres within the basis pedunculi of rats was studied using wheat germ agglutinin-horseradish peroxidase as an orthograde tracer. Following cortical injections, labelled fibres were distributed within the cerebral peduncle in an orderly way. Fibres which originate from cells in the frontal cortex maintain a position in the ventromedial part of the basis pedunculi. Fibres from the occipital and temporal cortex travel in the most dorsolateral part. Somatosensory fibres travel between these two. The extent of labelled fibres within the peduncles is correlated with the relative density of corticopontine cells arising from different areas of the cerebral cortex.
In all mammals, fibres in the basis pedunculi originate from cells in the cerebral cortex. One of the major targets of peduncle fibres is the pontine nuclei. These fibres project either directly to the pontine nuclei or by way of a collateral of cortico-spinal and cortico-bulbar [5, 20] or cortico-tectal [1, 10] fibres. The pons in turn relays information to the cerebellar cortex. The concentration of corticopontine fibres within the peduncle offers the opportunity to study the behavioural effects of selectively interrupting the input to the cerebellum from one or another cortical area by a small fibre lesion or a reversible blocking agent. The spatial organisation of peduncle fibres has been studied for over a century. However, there was confusion and disagreement in the early literature on the origin and termination of different components of the peduncle. The picture was clarified by D6j6rine [6] who charted systematically the location of degenerating fibres in the basis pedunculi in post-mortem studies of the brains of patients who had sustained injury to the cerebral cortex or its efferent fibres. In contrast to earlier claims, he reported that all of the fibres within the basis pedunculi have their cells of origin in the cerebral cortex. The largest number of peduncle fibres originate from the cortex within or adjacent to the central sulcus. Fibres which originate from precentral cortex travel in a medial region of the peduncles. Subsequent evidence extended these conclusions. Fibres from pre-frontal cortical areas travel Correspondence." M. Glickstein, Department of Anatomy and Developmental Biology, University College London, Gower Street, London WC1E 6BT, U.K.
in the most medial part of the human peduncle [3]. Fibres from the parietal and temporal lobes travel in the most lateral portions [15]. The same basic organisation of peduncular fibres is true for primates [2, 14, 19, 22] and other mammals although the data are less systematic. There are studies which identify the location within the peduncles of efferent fibres from different cortical areas in ferrets [23], rabbits [7], and goats [9]. Krieg [11], used the Marchi stain to follow degenerating fibre tracts in the rat. He illustrated the fact that efferent fibres from different cortical areas are segregated within definite regions of the internal capsule and the peduncle. Nauta and Bucher [17], using silver degeneration staining techniques, noted that the efferent fibres from visual cortex travel in the dorsolateral region of the peduncle. The present study was directed at extending these descriptions using a more sensitive method for analysing the course and the degree of segregation of cortico-peduncular fibres in the rat at rostro-caudal levels from the junction with the diencephalon to the pons. A total of 11 adult rats were studied. Pressure injections (50-100 nl) of 4% wheatgerm agglutinin-horseradish peroxidase (WGA-HRP) were made under hypnorm/hypnoval anaesthesia, supplemented as necessary by halothane. The cortex was exposed over the injection site and the tip of a Hamilton microsyringe or micropipette was lowered to a maximum depth of 1 mm below the cortical surface. Following a 48 h post-operative survival period, the animal was anaesthetised with an overdose of sodium pentobarbitone and perfused via the heart. The brain was removed from the skull and frozen
76 TABLE I S U M M A R Y OF THE N U M B E R OF SEGMENTS WHICH CONTAINED LABELLED FIBRES WITHIN THE CEREBRAL P E D U N C L E Cortical area
Frl Fr3 Cing Barrels Par I OC I OC21 RSA Prh OCr Te2
Case No.
89/3 87/26 R18 SMCI 89/73 89/3 90/1 89/73 89/29 R/12 88/20
Rostro-caudal section level I
II
I11
IV
V
VI
7,8 5,6,7 5,6,7 2,3 1,2,3,4 1,2 1,2 1,2,3 1,2 1,2,3 1,2,3
8 5,6,7,8 6,7~8 2,3 1,2,3,4 1~2 1,2 1,2 1,2 1,2,3 1,2,3,4
8 6,7,8 6,7.8 1,2~3 1,2,3 1.2 1,2 1,2,3 1,2 1,2 12.3
8 6,7,8 8 3,4,5 3,4~5 1,2 1 1,2,3 1,1 1 1,2
8 5,6,7,8 7.8 2,3~4,5,6 3.4,5,6 1.2~3,4 1.2 1,2,3 1,2 1,2,3 1,2
7,8 5,6,7 6,7 1,2,3 2,3,4 1,2,3 1,2,3 1,2,3 1 1,2,3,4 l
sections cut transversely at 50 ~ m in a flat skull plane [18]. Every third section was processed according to a modified tetramethylbenzidine method [8, 16]. The sections were mounted onto gelatin-coated slides, counterstained with Cresyl violet, dehydrated, cleared and coverslipped using D P X mountant. In no case did the injected W G A - H R P extend below the cortex to include underlying white matter. The location and extent of the primary label was reconstructed onto a standard map of the cortical surface [24]. In order to supplement the identification of the cortical injection sites, in all cases the orthogradely and retrogradely labelled terminals and cells within the thalamus were also studied. These observations were used only to confirm the locus and extent of the cortical injection site, hence they are not reported in detail here. All sections which contained the basis pedunculi were examined for the presence of W G A - H R P - I a b e l l e d fibres and these were plotted using a camera lucida system under bright-field cross-polarized illumination. The data are presented in two ways. The location of labelled fibres for each individual case is shown at 6 rostro-caudal levels through the basis pendunculi on a standard atlas of the rat brain [18]. In addition, at each of the 6 levels sampled, the peduncles were divided into 8 equal segments, measured along an axis running from the dorsolateral to the ventromedial pole. The presence of labelled fibres within each of these 8 segments was recorded and is summarised in Table I. This table reflects the degree of segregation of fibres arising from different cortical areas and the extent to which the labelled fibres maintain this pattern throughout their course. Note that the table shows the presence of labelled fibres within a given segment but it does not reflect their relative density.
Fibres arising from cells in primary m o t o r cortex (Frl and Fr3) travel in the most ventromedial part of the peduncle. The most concentrated grouping of fibres arise from the most rostral cortical area studied, area Frl. Fibres from Fr3 and cingulate cortex are distributed more diffusely and occupy a somewhat more lateral position in the peduncle. Fibres from the primary somatosensory cortex (Parl and Barrels) are located dorsolateral to those from the frontal cortex. Occipital Ocl and Oc2, temporal Te2, perirhinal cortex (Prh), and retrosplenial area (RSA) give rise to fibres which travel in the most dorsolateral region of the peduncle. There was little or no overlap between fibres from frontal and parietal cortex, but some overlap of fibres from the parietal and occipital cortex and considerable overlap between the fibres from occipital and temporal cortex. Segregation of fibres was less sharp at more caudal levels of the peduncles and the fibres formed multiple terminal patches within the pontine nuclei. The extent and density of labelled fibres contained within the peduncles correlates well with the relative density of cells arising from different cortical areas [13]. In each of the 11 cases we measured the total area within the peduncle containing label at 6 different levels. We rated the density of labelled fibres as light, medium, or heavy, scoring them 1, 2, or 3, respectively. We then multiplied the area times the scored density and calculated an average over the 6 different levels at which the measurements were made. In an earlier study [13] we had calculated the density of filled cells in different regions of the rat cerebral cortex after a W G A H R P injection which filled the pontine nuclei. The correlation between these two measures of the relative density of corticopontine cells was 0.76. These observations confirm that fibres within the basis
77 OcZRM
89/,3
89/,.3
87/26
R18
90/1
89/73
89/29
SMCl
R12
89/7,3
88/20
Fig. 1. cortical injection sites and position of labelled fibres within the basis pedunculi. The relative density of labelled fibres is indicated by the density of the stippling. The diagram in the upper left of the figure is from Zilles [24] and presents in outline the major cyto-architectonic regions of the rat brain. Nine of the injections are represented on the dorsolateral surface of the cerebral cortex; the other two on the medial surface of the hemispheres. To the left of the figure is an outline drawing of 6 sections through the rat brainstem to illustrate the level within the peduncle from rostral to caudal. The caudalmost sections are at the level of the rostral pons.
78
pedunculi of rats are spatially organized. Fibres from the most rostral areas of the frontal lobe travel in the most ventromedial portions of the peduncles. Fibres from the occipital and temporal lobes travel in the most dorsolateral portion. Parietal fibres travel between these two. The overall pattern of this pathway in the rat is similar to that in man and the higher primates. Although on a gross level the pattern is orderly, at a finer level, it may not be. A large number of cases with very small injections would be needed in order to establish whether efferent fibres from closely adjacent cortical regions remain segregated throughout their entire course. For example efferent fibres from occipital and temporal cortex overlap in the most dorsolateral region of the peduncle, although the temporal lobe fibres on average are located more dorsolaterally. Moreover, although the fibres maintain an orderly course within the peduncles they eventually lose that organisation, terminating in small and diffusely distributed patches within the pontine nuclei. Although these data reveal a general plan of organisation we recognise that any method of orthograde tracing at the light microscopic level may favour the largest calibre fibres. Verhaart [23] attempted to avoid the problem by using a stain for surviving normal fibres after he made cortical lesions in monkeys. In every case he found a region of the peduncle in which the largest calibre fibres were absent following restricted cortical lesions, but small fibres often appeared to survive within that region. He suggested that smaller fibres may have a less orderly course through the peduncles. However, subsequent studies by Van Crevel in Verhaart's laboratory [21] showed that central nervous system fibres degenerate at differing rates depending on their diameter, with the largest calibre fibres degenerating most rapidly. The survival time may have been too short to allow all of the fibres which would eventually die to do so. The WGAHRP method is probably more sensitive than either of these earlier methods for determining the course of efferent fibres, although the possibility remains that the very smallest calibre fibres may not have been revealed. There is a good correlation between the percentage of the peduncle which is occupied by fibres from a given cortical area and the relative density of cortico-pontine cells which arise from that area in the rat. This correlation suggests that pattern of the pathway as revealed by WGA-HRP label is consistent with the density of its cortical origins. Visual and somatosensory efferent fibres are segregated within the peduncle, and hence either could be selectively ablated or temporarily blocked. Rats can perform a number of skilled tasks such as accurate jumping [12] and reaching [4]. The contribution of sensory circuits to
such behaviour can be studied by selectively interrupting this major system which links sensory to motor areas of the brain.
I0
11 12
13
14 15
16
17
18 19
20
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