0306-4522/79/l

Neuroscience Vol. 4. pp. 1717 to 1743 pqamon Press Ltd 1979. hinted in Great Britain

IOl-1717102.00/0

Q IBRO

SUBTHALAMIC

NEURONS IN PRIMATES: A QUANTITATIVE AND COMPARATIVE ANALYSIS J. YELNIK and G. PERCHERON

U3 INSERM, Uniti de recherches neurophysiologiqucs, HBpital de la Salp&i&re, 47 Bd de l’H6pital, 75013 Paris, France Abstrsct-The anatomy of the subthalamic nucleus in primates was studied in the macaque and in man with relation to stereotaxic ventricular coordinates. Its cytoarchitecture was studied on cresyl violet stained sections and the subthalamic neurons were analysed on Golgi impregnated material from macaque, baboon and man. The distribution and morphology of the neurons were described by means of both algebraic (dendritic numbers) and geometric (lengths, surfaces and three-dimensional reconstructions) statistical parameters. The primates’ subthalamic neurons were compared with those of cats. The unimodal distribution of all the statistically studied parameters strongly suggests that there is only one variety of subthalamic Golgi type I neuron which is identical in cat, monkey and man. The cell body is ovoidal in shape, giving it a fusiform appearance when it is observed paralkl to its long axis, whereas it looks round or polygonal in other directions. Gn average, 7 dendritic stems originate from various points on the periphery of the soma; they branch out in successive biiurcations giving rise to a mean of 27 dendritic tips. In the centrally located neurons, the dendritic domains are ellipsoidal. Their mean dimensions are 1200,600 and 300 m. The long axis of the dendritic domains is usually parallel to the main rostrocaudal axis of the nucleus. The marginal neurons that are close to the borders of the nucleus seem to lack the part of their dendritic arborization that could have crossed the borders. Their longest dendrites are parallel to the border of the nucleus. Cell bodies are somewhat smaller and dendrites shorter in the caudal part of the nucleus than in its oral part, but the dendritic domains are still ellipsoidal. In addition to the Golgi type I neurons, small neurons have been looked for in cat, baboon, macaque and human brains. None of the small cells found in baboon and in man had the characteristic morphology of a local circuit neuron. Thus the subthalamic nucleus appears to be a homogeneous closed nucleus whose Golgi type I neurons seem to belong to a single neuronal species that does not change from cat to man. As a&rents and efferents to the nucleus also apparently remain the same, it is probably legitimate to extrapolate from studies on experimental species to man.

NUCLEUS subthalamicus was 6rst discerned by Lurs (1865) and so was named ‘corpus Luysii’ by FOREL(1877). The nucleus is known to play an important role in motor regulation, its lesion causing hemiballism (MARTIN, 1927). Morphological studies have aimed to describe its‘ constituent cells in detail, Numerous studies used Nissl-stained material (VON KCUIKFS, 1896; RAM& Y CAJAL, 19104911; Fotx & NICOLBCO, 1925; KODAMA, 1928; WHITIRZR & METIWR, 1949). Golgi studies have been less numerous (VON KOLLSER, 1896; MIRTO, 1896; RN&N Y CAJAL, 1910-1911), some of them forming only a part of more general studies (RAIUOPI A4oLm 1962; L~N?~VICH&ZHUKOVA, 1963). Apart from the old study of MIRTO (1896) on human foetuses, none dealt with primates until one of us (YELNIK, 1976) initiated a study in the human brain. At the same time a paper on macaque neurons appeared (RAFQLS & Fox, 1976) which was on several points in disagree ment with our observations. In spite of these numerous anatomical studies, several important contradictions remain, concerning the presence or THE

Abbreviations: F, S, L, dendritic tips, stems and length, respectively.

absence of subthalamic Golgi type II neurons, the identification of diRerent groups among the Golgi type I neurons and their position in neuronal classifications, The contradictory descriptions of the types of subthalamic neurons, which make the correlations between physiological and morphological features almost impossible, could be due to difkrent causes. First, the numbcT of neurons studied have often been small. !Secondly, previous morphological descriptions of the subthalamic neurons have been based on mainly qualitative parameters. Thirdly, they have been made on standard Golgi sections, i.e. about 100~ thick sections which are too thin to contain the whole dendritic arborization of a single neuron. Fourthly, the three-dimensional morphology of these neurons has never been studied, though an oriented pattern of their dendrites could make their appearance different according to the plane of observation. In order to avoid these defects, we have studied the neurons on the basis of quantitative parameters recorded in completed neurons, and have made threedimensional rotations of their dendritic arborizations. These measurements have been made in both man and monkey and the results were then compared to those on several cat neurons. A paper on subthalamic

1717

1718

J. YII.WK

neurons in the cat has recently HORI. 197X), which

data. particularly

to light

with regard to axonal

patterns of the subthalamic compared

been published

has brought

and G

PtaciiEao% wIthIn

(IWA-

the nucleus was delcrmmed

plotter.

important

and afTeren

nucleus. They have been

Dendrltlc

arbori7abtms

ol’a modification

of the technique

ilrc’ lnhelled

This field is explored

PROCEDURES

.Ind blood The shape and dimensions

OC the subthalamic

nucleus

were studied In stereotaxic atlases established according

to ventricular stereotaxic coordinates: in man that of SCHALTENBRAND & BAILEY (1959) and in Mocaca PI.RCtitRoN

(1977).

The

three-dimensional

whole nucleus was reconstructed

that

aspect

of

of the

from the serial stereotaxic

The

the longest one was chosen as the main axis of reference for the nucleus.

drawmg. camera

of the subthalamic

on both N&l-stained

neurons was studied

and Go@-impregnated

material

luclda

somata were studied in one brain of Mamcu thick

2 human I20 pm

sections stained

by cresyl violet

brains (one cut sagittally thick

AVERSON

and one frontally)

sections counterstained

by the

in on

method

of

(1954). Several samples, limited by the circular

field of the microscope

with

the 40x

were selected in the oral and caudal in the frontally

cut brain

parts in the sagitally

objective

cut brain.

the section were taken soma was drawn surface realized

through

and

lucida

projections

(Coradi).

the

of their

(Leitz).

The

was measured

The histograms

and the mean values and standard

calculated

lateral

visible in

The outline

the camera

hy means of a planimeter

4).

In these fields only

into account.

by these planar

(Fig.

parts of the nucleus

and in the medial

neurons which had their nucleus and nucleolus

drawn

and

were

deviations

were

for each region

field

in

1’ plotter

with rhc CIII rlcments

of 111~.

The

drawmg

1ttc

of

)A and B; figures

nc:lrl)n

Ihc

I\ then C‘OIW

represcntmp

drawings

;uc

m a group at thr end oi the WXII Such completed WFROS.

neurons

were then

and ‘geometric’

,tud~cJ

paramelcrs

197Yo) has rcccntly

branching

shown

h)

mean\

One of us (PI Hth.11 the

dendrntc

01 neurons may easily he quantitatively A graphical

stud&

representation

of cane

neuron IS shown m FIN. l(C). The dendrltlc

arborlzariorl

transformed

and segments.

inhI ;I set of dendriric

arc linked

various Neuronal

the same field.

which IS observed al the s3mr’ !Imc through

pleted (Fig.

They

on 50pm

!I

corr\-spending

until ;~II It\ CIII lkicmcnts (dcndrllcs

by using graph-analysis.

The morphology

i

(1975). The

by msans (II rhr \’

vessels) comcldc

01‘ ‘algchraic’

sections and the different axes of symmetry were analyscd;

01 M,V.SFX

of other ncuronz

on the drawing.

adJaccnt sectIons is localized EXPERIMENTAL

.\

hy means

cut dendrites 01‘ the neuron. as well as 311 sectioned blood vcsscls and dendrites

with our results.

~1 n cahbrared

wcrc compleled

hy very simple

numbers

point\

mathematical

of poinrs l)r segment\

from two fundamental

dendrlric

ma)

numhcrb

laws

IS The

bc ohlanled

that of dendrtuc

LIPS(FI and that of dendritic stems(S). These numbers were studied

in e\ery

neuron

Their

disrrihlltion

and

mciln

values were studlcd. Plmong the geometric

parameters.

WC measured

ones such as the length and width ol’ the somala length of different (T in Fig. ID).

dendritic

‘linear‘ and the

segmenta. length of the stems

distance from the soma 11.1the most distal

nodes (R In Fig.

ID)

and the distance from the soma 10

rhc most distal dendritlc

tip (L In Fig

tion of all thesr parameters.

11)). The dlstrihu-

whose meal1 ~alucs were com-

puted. was studied hy means of histograms ‘Planar’ different

geometric

parameters

ucre

t~sed to study

\;urfaceq that could bc obser\cd

of the neuron\.

‘The outline

,111;trca whose marlma

the

1.m the drawmg

of the somnta

clrcumscrihes

length and width and whose sur-

I’XC can bc measured. Thlc area is a proJected surface and not the true surface of the cell bodies. The polygon Our Golgi collection comprises to date 18 human brains. 20 macaque baboon

brains

(II

MCIUCU irus. 9 M.

After 6 weeks to several months of fixation brains

were first cut sagittally

of the positions of the anterior

commisures

and orientation

in relation

dinates (PERCHERON, 1975).

From tried

have

been

methods of DAMNFORT

&

the

of 2- 3 mm

stereotaxic 16 Golgi

coorstaining

in the laboratory.

COMES (No.

all

to allow

and posterior

of the sectioning

to these ventricular

procedures

that

in formalin.

on the midline

observation blocks

mulur~o). 7

brains (Papio papio), 8 cat brains and 7 rat brains.

2) (1954)

ANDEHWN (1954) that both use silver nitrate without

the and

osmic

is delineated

by the most distal dendritic

‘the polygon

of dcndritic

projection’.

dritlc

IYS6), is an

field’ of BOK (1936)

individual

planar

entity

and StioLi

which

‘dendritic

domain’

an individual dendrltic

such a way that adjacent

in

sections could be easily found.

several hundred subthalamic neurons have been observed. The material from 3 adult human brains, I adult macaque brain and 1 adult cat brain. settioned frontally. 3 other adult human brains sectioned From

this material,

sagittally

and I adult baboon

were selected to illustrate Neurons

were drawn

brain

sectioned horizontally

a L&z

40x

oil obJectlve

(or a lo0 x oil objective for very fine structures) by means

of a camera lucida. The precise location of each neuron

(YH 31~;. 1976) from entity

(1967) which

domams cannot be made on planer drawings

The principles the following:

of three-dimensional

the ‘completed’

drawing

lucida is a representation

of any point of the dendrirlc which gives the position Z coordinate). characteristic

reconstructions ohtained

are

through

of of neuron

arhorizatlon

in the

Y coordinates may be deter-

grid. The third dimension.

in the depth

of the section (the

of showing in focus. for a given position screw, only elements

the

basis

corresponding

of these

of

that are in a plane

parallel to the plane of section. All the observable have the same Z coordinate On

hut

may he studied by usmg the microscope’s

the micrometre

method

is

reconstructions.

the .Y and

mined by means of a milhmetric

of the

The study of the

necessitates the use of three-dimenslonal

plane.

the present paper. under

lo the plane

of C;I.OHI:S & ScHFInLI

three-dimensional

rhe camera

and numbered

(1951.

is linked

section. I( must he distingulshed

plane of section. In this plane

(ANDERSON. 1954). mounted

planar

to the ‘den-

in celloidin. All sections. from I20 IO 3OOpm thick, were left in their relative position, counterstained

is another

entity. This surface. to some extent comparable

acid gave the best results in fixed adult primate brains. All blocks were embedded

which

lips (see Fig. ID),

principles.

elements

to that of the the

fokwing

was used for the present study. The whole depth

of the sections is examined from top to bottom by ~uc(xssivc IO pm steps. All dendritlc points or Tegments that arc

1719

Subthalamic neurons in primates in focus at each step are marked on the drawing (observed through the camera lucida) with the number of the step. We then obtain a drawing extended in X and Y dimensions, and numbered in Z dimension. To get the X-Z orthogonal rotation, it suffices to substitute, for the original drawing where X was in the abscissa and Y in the ordinates, a graph where X remains in the abscissa and Z replaces Y in the ordinates. Each point of the X-Y drawing is transferred on the X-Z coordinates system, its X position being reported by a simple projection while its Z value determines its position along the Z axis. The whole graph is reconstructed point by point (see Fig. 2). The other orthogonal rotation (Y-Z) is obtained by replacing the X-Y drawing by a Y-Z graph.

RESULTS Shape and dimensions of the subthalamic primates

nucleus in

The frontal and horizontal aspects of the nucleus in macaque (A and B) and in man (C and D) with rela-

tion to ventricular stereotaxic coordinates are shown in Fig. 3. The subthalamic nucleus appears in both species as a biconvex lens-shaped structure. One may observe that the longest axis of the nucleus is in the rostrocaudal direction. This axis, which is considered here as the reference-axis of the nucleus, is determined by the first (oral pole) and last appearance (caudal pole) of the nucleus in frontal sections. The stereotaxic coordinates of these two poles, given in Table 1, were used to compute the orientation of the main axis. This orientation is the same in macaque and in man. The main axis is closer to the horizontal (20”) and sagittal (35”) planes than to the frontal plane (55”). This inchnation of the main axis is of particular interest because it shows that frontal sections (which are more commonly used) cut the nucleus in an oblique plane, neither perpendicular to its main axis nor parallel to it. Conversely, horizontal sections are probably more relevant for studying the subthalamic

nucleus because they are closer to its longest axis. Measurements of the successive frontal sections demonstrate that in both species, the subthalamic nucleus is larger and thicker in its oral half than in its caudal half. The dimensions of the nucleus differ for man and macaque. The length of the main axis (computed from the stereotaxic coordinates of the oral and caudal poles) is 5.5 mm in Macaca and 13.2 mm in man, exhibiting an increase of 2.4 times. The subthalamic material

neurons

11 x 11 m in man. The longest were 33 x 6 pm in Macaca and 40 x 9pm in man. The distribution of

the projected surfaces of the somata (histograms in Fig. 4) shows that there is no part of the nucleus where the distribution is bimodal or plurimodal. The mean surfaces are not very different when they are measured in the same plane of section: they are comparable in the oral and caudal parts of the nucleus in frontal sections (335 and 347 pm’) and in the medial and lateral parts in sagittal sections (257 and 280~~). These surface-differences appear to be linked more to the plane of section than to the location of the neurons inside the nucleus. Total mean POLESOF

Macaca mulatta

THE SUBTHALAMIC

Mall

C&l&l

pole

Oral pole

Distance between each pole and the three stereotaxic planes Horizontal CA-CP plane 2.7 5.8 Mid-sagittal plane :.; 6.8 L6.9 Frontal CP plane 8:4 3.9 14.0 Horizontal Sagittal Frontal

violet stained

The distribution and appearance of cell bodies in the medial and lateral parts of the human nucleus (as observed in different sagittal sections) and in its oral and caudal parts (as observed in different frontal sections) are shown in Fig. 4. Cell bodies look more numerous in the oral and medial parts than in the caudal and lateral parts. They are either round, fusiform or polygonal. These different shapes of somata are intermingled so that no part of the nucleus contains only one of them. However, there are more fusiform somata in sagittal sections and more round and polygonal ones in frontal sections. The same arrangement was observed in Macaca. The smallest somata we found were 10 x 10pm in Macaca and

TABLE ~.STEREOTAXIC COORDINATESOFORAL AND CAUDAL NUCLl?USINMONKEYANDMAN

Oral pole

on cresyl

Caudal pole

3.2 14.1 3.2

Inclination of the orocaudal axis on the three stereotaxic planes 20” 20 34” 35” 56 55” Length of the orocaudal axis 5.5 13.2

Inclination and lengths of the orocaudal axis are calculated by means of simple trigonometric relations. Distances are given in millimeters. CA, anterior commissure; CP, posterior commissure.

1720

J. YEINK and G.

PERCHERON

FIG. 1. Illustration of the method used to build up the complete dendritic arborization. (A) Camera lucida drawing of a subthalamic neuron as it was observed in human brain H7 fin a 120~~1 section (H7C28). A large number of dendrites are cut. The points of section are indicated by small bars, J is the longest dendrite. cd, cut dendrites; F, number of dendritic tips; S, number of dendritic stems. (B) Drawing of the same neuron completed by the projection of the cut portions of the dendrites found in adjacent sections (H7C27, H7C26 and H7C29). Those found in H7C27 are represented by continuous biack, those found in H7C29 by dashed black. On the upper left-hand part of the Figure, opposite triangles show the points of coincidence of the cut ends of one dendrite between H7C28 and H7C27 and between H7C27 and H7C26. In this part of the Figure, the dendrites are cut distally. The prolongation of the dendrites does not increase defidritic branching. Conversely. in the lower right-hand part of the Figure, the cutting is proximal. Two nodes (black stars) and two tips (open circles) are added by further branching. A complete tree with one root (square shown by an arrow) and five tips (open circles) is added. K is the longest dendrite. FE. 1 (continued). Illustration of algebraic and geometric parameters studied for subthala#c neurons. (C) Same neuron as in Fig. 1A and B transformed into a graph made up of dendritic points and dendritic segments. The dendritic points (roots indicated by squares, nodes indicated by black points and dendritic tips indicated by open circles) -have been left in the same position q in Fig. 1B. The point surrounded by a circle is a trifurcation point. The dendritic stems extended between a Toot and the proximal node, are represented by two parallel lines. The internodal and pendant segments are represented by straight lines drawn between two successive points. Some have been drawn with incurvation points. The segment represented by a continuous tine and a parallel dashed. line is a ‘scion’ or pendant stem. It is counted as a stem. S, the number of stems, is thus 6. The dendritic tips have been numbered in the figure. F, the number of dendritie tips, is thus 31 and the dendritic formula S-F is 6-31. (D) Same neuron drawn for geometrical studies. The crossed area is the projected surface of the soma which is measured with the aid of a planimeter. T is the length of the stems (the scion is not taken into account there). R is the distance from the centre of the soma to a most distal node. L is the distance from the centre of the soma to the most distal tip of the neuron. A dashed line is traced between the most distal tips and circumscribes a ‘polygon of dendritic projection’ which is a bi-dimensional projected area. FIG. 2. Three-dimensional reconstruction of a subthalamic neuron. ypper part. Drawing of the same neuron as in Fig. 1. placed in X-Y coordinates and numbered in 2 coordinates. On X and Y axes the distance between two successive bars is 10 pm. The 2 numbers correspond to successive planes, IO pm from each other, where dendritic portions were observed in focus, when the depth of the sections was examined from bottom to top by successive 10 pm steps. The Z positions of all the dendritic tips are indicated. The successive Z positions of the whole dendrites are indicated for only four dendrites on the picture. Lower part. Orthogonal rotation of the same neuron in an X-Z plane. The X coordinate is the same as above. The Z coordinate replaces Y in the ordinate. On the right, the numbers refer to the different sections where the futl picture of the neuron have been completed.

s _,5 F ~24 cd-_1 7

29

x --

FIG. l(A,B).

1721

--

\

o II I /

‘Q

I

I

FIG. I(C,D). 1722.

/

\

\

\

\

\

\

--

--

FIG. 2. 1723

I

--

--

- __-

INF

HOMO

FIG. 3. Shape and dimensions of the subthalamic nucleus in primates. Three-dimensional aspects of macaque (A and B) and human (C and D) subthalamic nuclei reconstructed from serial. sections in ventricular stereotaxic coordinates. Frontal (A and C) and horizontal (B and D) views are shown with the main axis of the nucleus passing through its oral and caudal poles. The inclination of the axis on the different planes is indicated. Ahhretdations: ANT, anterior; CA-CP, inter commissural line or plane passing through the two commissural points; CP, posterior commissure; INF, inferior; LAT, I: ,ral; SN. substantia nigra: THAL, thalamus; V3. third ventricle.

1724

‘L 5

I

k50

Oral

m.257 S.73

5

Medial

HOMO

Lateral -10

-

~46

FIG. 4. Shape, size and distribution of the cell bodies in the subthalamic nucleus of primates. Somata have been drawn by means of a camera lucida with a 40x oil objective. Various samples have been made in the nucleus. Several microscopic fields (shown by circles) were selected in frontal sections for the oral and caudal poles and in sagittal sections for the medial and lateral parts of the nucleus in man. For each field are given n the number of neurons observed in the field, m the mean values of the projected surfaces (see methods) of somata and s the standard deviation from the mean. Histograms show the distribution of the values of the projected surfaces of the somata. The 100 pm scale is the same for the four microscopic fields of the figure. In the centre of the figure are given the total values for the four samples and the total histogram. At bottom left the histogram shows the distribution of 631 somata in Macaca. FIG. 5. Small cells in the subthalamic nucleus. A-G are from one Papio brain. In A and B are shown the somata and the proximal portions of the dendrites, one with two spines (white star in black circle), of large neurons to furnish a comparison of the relative size of the different types of cells (all are represented with the same magnification). C, D and E are oligoglial cells: C with regular spherules (arrow), D with grape-like elements (arrow) and E with thin branches (arrow). F and G have still thinner branches with swellings (arrows in G) that contact the somata and proximal dendrites of large neurons (arrows in F). H is a neuron observed in a human brain. Its soma is smaller, its dendrites are thinner and shorter than in large neurons. Some thin processes with intermittent swellings arise from several dendrites (arrows).

1725

FIG. 6. Large subthalamic neurons in cats and monkeys. The neurons of one cat (Fe/is) and of one macaque (Muctr~ :I) arc observed in frontal sections. Their precise locations in the nucleus are indicated by dots on the X- Y plotter drawings. The polygon of dendritic projection of centrally located neurons (A, C. D and F I) may either be rather ‘round’ IA. G and I) or somehow elongated (D and F) along the mediolateral diameter of the nucleus (shown by arrows on the X Y plotter drawings). Neurons B and E are ‘marginal neurons’. The orientation of some oftheir dendrites is modified in the direction of the border of the nucleus, which they do not cross. This modifies their polygon. The neurons of borh ypecies. cut in the same plane, appear very similar. The neurons of one baboon (P~cpio) are observed in horizontal sections Centrally located neurons K and L are more elongated than centrally located neurons of cat and macaque cut in frontal sccti~ms. They seem to have longer dendrites. The marginal neuron J is extremely elongated. The 100 pm scale is the same for ~111the neurons of the whole Figure. The I mm scale is the same for the three X Y plotter drawings FIG. 7. Large subthalamic neurons in man. In the upper part of the Figure. the neurons are observed in frontal sections: on the left from mediorostral sections (med). on the right from laterocaudal ones (lat). A. B and C are marginal neurons. The dendrites of neuron C. located in the upper corner of the nucleus. are modified either by the superior horder and hy the lateral one. The other neurons are very similar to those observed in frontal sections in cat and macaque m Fig. 6. At the bottom of the Figure. portions of the thalamus (thal). of the zona incerta (zi) and of the substantia nigra (sn) have been shown to point out the different neuronal typology of the neurons of these neighbouring regions. The location of the thalamic neuron is indicated by a square. that of the neuron of the zona incerta by an open circle, that of the two neurons of the substantia nigra by stars and that of the subthalamic neurons by black circles. The neurons arc observed in sagittal sections. Three neurons of the subthalamic nucleus are very elongated marginal neurones (I) for instance). Neuron E, observed in the substantia nigra on the other side of the capsule, may be a heterotopic subthalamic neuron because its morphological features, different from those of nigral neurons, are similar to those of suhthalamic neurons located above. lu, subthalamic nucleus. The 100pm scale is the same for all the neurons of the whole Figure. The I mm scale is the same for the two X- Y plotter

drawings.

FIG. 8. Direction of the axons of subthalamic neurons m Mucc~co. A and B are two neurons observedin the medial corner of the nucleus (section 19GD9). The axon of B is unbifurcated and directed medially. The axon of A goes dorsomedially, crosses the capsule of the nucleus and then bifurcates (asterisk) to give a medial and a lateral branch. Neurons C, D and E, placed more laterally in a 6OOpm more posterior section (19GD7). send their axons laterally. The precise location of the neurons in the nucleus is shown in the middle left-hand part of the Figure where the direction of the axons of two other neurons, not shown in the Figure, is also indicated. All axons stem from an axon-hillock (h in A). haw: a very thin initial portion (i in A) shown between two small arrows on the drawings, and then thicken. inf. inferior: lat. lateral;

med. medial.

FIG. 10. Three-dimensional reconstructions of two human subthalamic neurons. Neuron A was drawn from ;I frontal section (H2A16) and completed from the adjacent sections (H2A14. 15 and 17). Its two orthogonal rotations arc shown on the sagittal and horizontal planes. Neuron B was drawn from a sagittal section (HI 1 DDIO) and completed from the adjacent sections (HllDD9 and 11). Its two orthogonal rotations are shown on the frontal and horizontal planes. The right picture shows the location of these neurons in the three planar projections of the subthalamic nucleus (horizontal, frontal and sagittal). The orientation of the three planes (LATERAL, DORSAL and CAUIIAI.) is identical in the three pictures. Note that neuron A has a ‘round’ polygon of dendritic projection on the frontal plane and an ‘ovoidal’ one on the sagitlal plane. Conversely, neuron B had an ‘ovoidal’ polygon on its sagittal appearance while its projection on the frontal plane is more ‘round’. FIG. 11. Similarity one-dimensional morphology of the simian and human subthalamic neurons, Three subthalamic neurons, one from macaque (Macacu), one from baboon (Papio) and one from human brain (How) are shown with, in each case, their three orthogonal projections on sagittal, horizontal and frontal planes. The planes where the neurons were first drawn are indicated by stars. This picture shows that, despite their different initial appearances and their belonging to different species, all three neurons are very similar. Frc;. 12. Diagrammatic representation of the dendroarchitecture, efferent and afferent connections of the subthalamic nucleus. (A) Statistical model of the Golgi type I subthalamic neurons. Two of the seven dendritic stems are pendant stems or scions. The number of dendritic stems and dendritic tips give the ‘dendritic formula’ 7-27. The ellipsoidat dendritic domain is drawn and its three semi-axes, which measure 600, 300 and 150 pm are represented. (B) The areas of terminal ramifications of four aBerent axons are represented within the cat’s subthaIamic nucleus in a fontal section (dor, dorsal; lat, lateral; med, medial) (from IWAHORI, 1978). Note that each axon does not cover the whote nucleus, making a more or less ventrodorsally oriented column. (C) Schematic drawing of a cat’s subthalamic neuron placed within the nucleus and compared with the areas of axonal ramifications (same orientation as in B). Note that the width of ones area of axonal ramification (L Af) is roughly equal to the length of the longest dendrite (Ld) and that the length of the dendritic domain (2Ld) corresponds to two axonal ramifications. Note also that about two dendritic domains suffice to cover the whole length of the nucleus (L Nu). (D) Schematic drawing of afferent (dotted lines) and efferent (fuil lines) connections of the subthalamic nucleus (Lu). Percentages of subthalamic neurons projecting to the entopeduncular nucleus (Entoped) or medial pallidum (Pal, med), paliidum (Pal) or lateral pallidnm (Pal. lat) and substantia nigra (SN) are computed from the data of DENIAU et al. (1978) in rats. Projection towards the nucleus tegmenti pedunculo pontinus (n. tegmenti Ped. ponl.) was demonstrated by NAUTA & COLE (1978). Afferent connections from the motor cortex and the lateral pallidurn could be due to collaterals. I726

\

G

c

l

/

50ym

J

FE LIS frontal ----

h-

e

med

1 lat

HOMO- _

------

_

_

-

/

I I

\

--

lmm U

1729

Macaca,

Papio,

,A-’

,/’

aned

motor

cof tex

Subthalamic neurons in primates

1735

tion of the border only go as far as that border- which they never cross. Their polygons di@er from those of the oentrally located neurons mainly by the fact that Subthaiamic neurons in Gol&stained material they lack the part that could have crossed the border (Fig. 6B). In frontal sections of human brain (upper When studying Go&i-stained sections, the prepart of Fig. 7), the same observations may be made. dominant impression is one of neurons with rather The neurons of all three species finally appear to be long, in~~~~ dendrites. Small cells have to be very similar. The neuron C, located in the upper deli~tely looked for, as they are much less obvious corner of the nucleus (Fig, 7), is of particular interest and apparently less numerous. These small cells will as it shows the e&t of the proximity of two borders. he described first. In horizontal sections (Papi0 in Fig. 6)1 the polygons Small cells in the ~t~~a~~ nucleus. Cells with of centrally located neurons are rarely ‘round. ~arti~~~~y small somata (greater diameter from 5 to Conversely, the ‘ovoidal’ polygons are usually more 8 grn) and very thin branches arising from 3 to 4 stems were found (Fig. 5). Among them some have elongated than in frontal sections (Fig. 6K and L). regular disseminated spherules (Fig xl), others have The longest dendrites, which appear longer than those seen in frontal sections, are more often than not grape-like elements (Fig. SD) and still others have oriented along the rostrocaudal diameter of the very thin, short branches (Fig. 5E). They were all nucleus. Marginal neurons (Fig. 6J), in the same manidentified as oligoglial cells. Other small cells (Fig. 5F ner, have longer dendrites than in frontal sections and and G) appeared to have fewer branches bearing more elongated polygons, In sag&al sections (bottom longer and even thinner processes with some irregular part of Fig. 7) one may observe that marginal neurons swellings that may bifurcate in secondary processes (for instance neuron D) are comparable to those seen ending in spherules. Such spherules were observed to make contact either with the somata or with the in horizontal sections (Fig. 6J). A comparison, for each neuron, between the shape proximal dendrites of the large neurons (Fig. 5A and B). Such structural differentiations make them some- of its soma and that of its polygon of dendritic projection was made, In the centrally located neurons, what different from the oligoglial ceils described there is no obvious ~Iation~ip between the appearabove. However, the length of their branches and the size of theii somata were very similar. ance of their somata (polygonal or fusiform) and the In one human brain, only one small cell (Fig. SH) shape of their polygon of dendritic projection (‘round’ was found that could indubitably be considered as or ‘ovoidai’). In the marginal neurons, the somata are a neuron. Its dendrites are shorter (16Om for the usually fusiform and the polygons ‘ovoidal’. longest one) and thinner than in large neurons From The pattern of dendritic branching looks simifar in some of these dendrites arise some thin processes with every plane of section. Some dendrites arising from intermittent swellings (arrows). No axon is visible. the soma do not bifurcate. In bifurcated dendritic The signiticance of this neuron will be discussed later. trees, the dendritic stems are usually thick short and straight. There are several (from 1 to 4) nodes of bifurtarge Gdgi type I neurons. The somata of these cation. More distal dendritic segments are always neurons may be polygonal or fu~fo~ as has already thinner, longer and more undulating than the more been seen on cresyl violet sections. The somatic spines proximal ones. Trifurcations also occur in subthalaare very scarce. The somata of the centrally located neurons are fusiform or polygonal and in spite of a mic neurons. Dendritic spines which are not very numerous in each neuron, are thin and pedunculated, careful observation of numerous neurons, no preferential location of each of these shapes of somata has and are more often located on distal dendrites. These been found within the nucleus. The somata of the dendritic features make the subthalamic neurons very marginal neurons are most often fusiform with their different from other neighbouring neuronal species, long axis parallel to the border of the nucleus (Figs 6 already observed and analysed in our material (botand 7). tom part of Fig. 7). Thalamic neurons have shorter, The ‘polygons of dendritic projection’ (see Fig. 1D) and far more numerous dendrites, while dendrites in must be considered according to the plane of section neurons of the substantia nigra (Fll~Nm& PERand to their location inside the nucleus. In frontal C~XON, YELNIK& HEYNER,1979) are less numerous, sections (F&s and Macaca in Fig. 6), some centrally longer, thicker and straighter. The neuron in the zona located neurons have dendrites extending almost incerta (Fig. 7) is less ram&d and has thicker denequally in all directions, thus having rather ‘round’ drites than subtile neurons. These differences polygons (Fig. 6A, G and I). Others have longer denand the sharp limits which constitute the field H of drites extending preferentially along one direction Fore1 make the delimitation of the nucleus quite easy parallel to the mdioiaterial diameter of the nucleus, in Go&i sections. However, we were rather surprised thus having rather ‘ovoidal’ or ‘elongated’ poiygons to find extranuclear neurons such as neuron E in (Fig. 6D and I?). “Marginal neurons’, i.e. neurons Fig. 7, exhibiting all the mor~hoIog~ features of a located along the borders of the nuclcu~ have their subthalamic neuron, though it was located within the longest dendrites extended along that border, On the substantia nigra. Conversely, in one macaque we other hand, those dendrites which extend in the direc- found a large neuron whose den&tic features are

surfaces are larger in man (304 m’) (205 @2).

than in macaque

J. YELNIK

1736

and G. PERCHERON

those of the nigral neurons, although it is located in the medioventral part of the subthalamic nucleus. Its thick and long dendrites are oriented ventratty, cross the capsule and reach the substantia nigra. Several axons were well-impregnated in our macaque material (Fig. 8A-E). All originate from an axon-hillock which emerges from the soma or proximal dendrites and is 34/1m thick at its base and approx. 13 pm long. The axon-hillock is followed by 2 axonat portions: an initial portion (i in Fig. 8) which is 0.2 pm in diameter and 50 pm long and a distal portion, somewhat thicker (0.5 pm) that might correspond to a myetinated portion of the axon. Most of the time, in our material, the axons were unbifurcated. Some are directly medially (Fig. 8B) but most of them laterally (Fig. 8C, D and E). The latter seem to be more laterally located inside the nucleus than the former. We found one bifurcating axon in a medially located neuron (Fig. 8A). This axon reaches and crosses the capsule and bifurcates into a medioventral branch and a laterodorsal branch. The importance of this finding will be further discussed. Quantitatioe

studies of large

Golgi

type I neurons.

Algebraic and geometric parameters (see Fig. 1) were recorded for 38 human neurons. The mean fundamental dendritic numbers are 6 (6.37 &- 1.18) for the dendritic stems and 21 (21.34 + 4.77) for the dendritic tips. The ratio F/S which gives the mean rate of dendritic branching for one neuron is 3.5. 34”/, of the dendritic stems are unbranched (scions). Although bifurcation is the most frequent pattern of branching, trifurcations also occur (5.40;;,) in subthalamic neurons. The mean dendritic lengths of stems, last nodes and longest dendrite (T, R and L in Fig. JD) are 22 (22 + 11.48), 70 (69.67 + 26.24) and 343 (342.6 + 123.1) pm respectively. The statistical distribution of these different parameters is shown in the histograms (Fig. 9). The algebraic parameters have a narrow unimodal distribution with a Gaussian appearance and a small standard deviation (S and F in Fig. 9) whereas the geometric parameters (T, R and L in Fig, 9) are more randomly distributed. with a larger standard deviation. However, in spite of the large sample which was used, none of the parameters exhibits a bi- or plurimodal distribution which could indicate separate groups among the subthatamic large neurons. One may further analyse the reasons why the distributions are different in algebraic and geometric parameters. Our Golgi material is the more often cut in I20 pm thick sections. Whilst the longest dendrites we observed are 750 pm long (the average length is 343 pm), they are unavoidably shortened by the KCtion. In the 38 human neurons, 54% (ranging from 10 to 80%) of the dendrites were sectioned (cd in Figs IA and 9). The point where they are cut, which determines the length which will be measured, is randomly located along the whole dendrite. Conversely, the computation of the dendritic numbers is not influenced by the location of the section along the den-

Fro. 9. Histograms of some dendritic quantitative parameters of thirty-eight human subthalamic neurons. The number of neurons in each class is indicated in ordinates. S is the number per neuron of dendritic stems. F that 01 dendritic tips, cd the percentage of cut dendrites (see Fig. 1A and C). T is the mean length in pm for each neuron of the denritic stems, R that of the distance between the centre of the soma and the most distal nodes. L is the length in Mm of the distance between the centre of the soma and the tip of the longest dendrite of each neuron (see Fig. ID).

drite, unless the dendrite is cut before a bifurcation point (see Fig. 1B). As the most distal bifurcations are 70 pm (25-125) from the soma, some of them were probably sectioned in our 120 pm thick material. These remarks show that the dendritic numbers are less influenced by the sections than are the dendritic lengths. and that they remain very useful quantitative parameters in as much as all neurons are studied under the same conditions (i.e. same thickness of section). On the contrary. the geometrical parameters are highly distorted by the sections and their description can be correct only for complete neurons, We completed the analysis of seven neurons following the procedure described above. Figure 1 shows one human subthalamic neuron as it was drawn from a 120 pm thick section (A) and as it appears after completion (B). The two pictures look so different that only a careful examination convinces one that they represent the same neuron. What distinguishes (A) from (B) must be further analysed. The dendritic arborization looks delicately thin and wavy in (B) whereas it is straight in (A) because the section cuts the distal dendrites in particular, which have already been noticed to be thinner and more wavy than the proximal ones. The dendrites look more numerous in (B) than in (A). The fundamental dendritic numbers (S-F) actually increase from 524 to 6-31 (1 stem and 7 tips were removed by the section). On the seven completed neurons, the mean dendritic numbers increase almost equally (from 6-22 to 7--27) but their distribution is still narrow and unimodal. This demonstrates that the dendritic numbers, as computed for 120pm thick sections, are not useless or wrong parameters, simply that they are underesti-

Subthalamic ncurw

in primates

TABLE 2. DENDRITIC PARAMETERS OF SUBTHALAMK SPECIES

1737 NEURONS IN FOUR

6-21

Macaca 6-17

Papio

S-F F/S L LNu LNuj2L

3.5 349 1.5 2.15

2.8 324 3.5 5.40

3.7

Felis

6-22

Homo 6-21 3.5 342 6.2 9.06

A comparison in four spccics of the mean numbers of dendritic stems (S) and tips (F), longest dendrites (L in pm) and mediolateral diameter of the subthalamic nucleus (LNu in mm).

mated equally in all neurons. The dendritic lengths are more or less shortened by the section. The longest dendrite (I) in Fig. l(A) is no longer the longest in Fig. l(B) (K). The dendrites which are sectioned are usually those which are perpendicular to the section, i.e. those whose projection on the plane of the drawing make them appear to be the shortest. For the same reason, their completion in the same projected plane does not greatly increase their length. The mean length of all the dendrites, however, is 97~ in (A) and 147 pm in (8).

describe a single statistical three-dimensional model. This model is ellipsoidal in shape (Fig. 12A). The longest axis is on average 1200pm long and is oriented parallel to the long axis of the nucleus. The second axis is on average 600~ long and is perpendicular to the first and parallel to the mediolateral diameter of the nucleus. The third axis (mean length 300 pm) is perpendicular to the first and second axis and parallel to the dorsoventral diameter of the nucleus.

Three-dimensional reconstructions of subthalamic Go& type I neurons. Orthogonal rotations of several

cat, monkey and man inasmuch as they are observed under the same conditions (frontal sections in Figs 6 and 7). Quantitative parameters demonstrate this similarity. One can see that the fundamental dendritic numbers are almost the same (S and F in Table 2) and that, consequently, the rate of dendritic branching (F/S in Table 2) does not change at all. Their dendritic lengths are also identical (L in Table 2) though they vary according to the plane of section (Table 3). This suggests that subthalamic neurons of cat, baboon, macaque and man all have the same morphological features. Consequently, the neurons of the subthalamic nucleus in cat, in monkey and in man may all be statistically represented by the same model (Fig. 12A). As these neurons do not change from cat to man, the only modification that occurs during evolution is an increase in the dimensions of the nucleus and of the total number of its neurons. Table 2, in which the mediolateral diameter of the nucleus is compared to the mean length of the longest dendrites on frontal sections, demonstrates that a single neuron can fill half of the nucleus in cat, a fifth in monkey and a ninth in man.

neurons in man, macaque and baboon have been carried out following the procedure already described. Two human neurons (Figs 2 and 1OA) were drawn in frontal sections, in which their polygon of dendritic projection had a ‘round’ appearance. Their sagittal rotations are clearly elongated. On the other hand, another human neuron (Fig. 10B) was elongated in a sagittal section; its frontal appearance is more radiated. The same features were observed in baboon and macaque neurons (Fig. 11). This figure shows three neurons, observed in three different planes, whose polygons of dendritic projection look different. They are in fact very similar when they are compared to each other in the same plane after rotation. All three neurons are rather ‘round’ in frontal sections and rather ‘elongated’ in horizontal sections (their sagittal appearance is intermediate). These polygons of dendritic projection thus appear to be different planar aspects of rather constant three-dimensional ‘dendritic domains’. The fact that these individual dendritic domains are all similar makes it possible to

TABLE 3. LBNG~

Planes of section Frontal SIXgittal Horizontal

Compari~n of subthalamic neurons in cat, monkey and man. Subthalamic neurons look quite similar in

OF LONGJ?STDENDRITES OF SUBTHALWC FOUR sPecIEs

Felis

Macaca

349

324

Papio

NEURONS IN

Homo

342 474 501

A comparison of the mean lengths of the longest dendrites as measured in frontal, sagittal and horizontal Goigi sections in four diierent species. Lengths are given in j4m.

J.

113x

YELN~K

and Ci. PERCHERON

DISCUSSION ‘Neuronal

types’ and their quuntitatiw

determination

In our opinion the distinction of neuronal types is important as it does not only have morphological significance. Following LENHOSSEK (1895) and RAM~N Y CAJAL (191@1911). what is important is to define ‘neuronal species’ (neuronal sets with the same morphological features, with the same aRerents and efferents) which are postulated to be the fundamental elements of a given nervous region. Qualitative criteria are often used for that purpose but we think that ncuronal types can. and should, be quantitatively determined by means of both algebraic and geometric parameters. In the case of the subthalamic nucleus, two varieties of Golgi type I neurons were described by RAFOLS& FOX (1976) ‘on the basis of cell body shape, dendritic configuration, presence of somatic spines and quantity of dendritic spines’, while three types were isolated by IWAHOR~(1978) ‘on the basis of size of the cell bodies’. We have seen that, the variations in the shape of the soma are mainly linked to the plane of section. This is an example of a qualitative parameter which may vary according to the conditions of observation. Quantitative parameters studied from large samples of neurons are much more reliable. The fundamental dendritic numbers have already been shown to be good algebraic parameters for the isolation of neuronal groups, comparison between different groups and comparison between different species (PERCHERON, 1979b). The present study confirms this. In the subthalamic nucleus, dendritic numbers (S F) are identical or very similar in cat, monkey and man: 6-21 in our material and 6-20 in that of other authors (RAMONMOLINER, 1962; IWAHORI, 1978). We have shown that when dendritic arborizations were completed, the mean dendritic numbers increased from 6-21 to 7-27 but that the increase was about the same in all neurons and their statistical distribution remained identical. Comparisons between neuronal groups in different species require large samples of neurons of each group and it would be a long and laborious process to carry out a complete study on each of these neurons. Thus, the dendritic numbers are the simplest and probably the best parameters for that purpose. On the contrary, the precise description of a single neuronal group must be made for those neurons that have been completely studied. Study of the geometric parameters demonstrated that the linear ones (dimensions of the somata, lengths of the dendrites) and the planar ones (projected surfaces of the somata and polygons of dendritic projection) must be interpreted in relation to the thickness of the sections and to their orientation (frontal, sagittal or horizontal). The spatial parameters (three-dimensional position of dendritic points and orientation of the dendrites) are the onty ones that give an undistorted representation of the neurons. They also lead to the determination of the ‘dendritic domain’. This

space, first described by &K (1936) for rhe pyramidai cells of the cortex was called by him %tcal dendritic field’. SHOLI. (1953. 1956) has showu its functional importance as it is the only space whcrc the neuron may receive its afferents. GLOBSs Br SCHI:IIEI ( I967; studied the dendritic domain ot’ the pyramldai neurons of the cerebral cortex. SCHELNB.I KLSC~IEIHEI (1966) studied those of the thalamlc ncuronb and 01 the reticular neurons of the brain stem (Sr~t~nw. & SCHEIBEL.1968). The peculiar ellipsoid:11 domain:, of the subthalamic neurons are different from rhe spherical domains of the thalamic neuron>. .md from the cylindrical ones of the pyramidal neurims. It should be pointed out that the ellipsoidal domain has the same shape as the biconvex lens-shape of the nucleus. This could suggest that the closure of the nucleus ‘influences’ the extension of the dendrites. just as it has been said that the laminar organization of the cortex ‘influences’ the orientation of the dendrites of the pyramidal neurons.

Many papers dealing with the typology of the subthalamic neurons, including the earl&t ones (VOX KOLLXER 118961 in mice and rats; MIKTO [t896J in human fetuses: RAM~NY CAJAL[1910- 1911] in mice and cats) but also more recent ones (RAUONMOLINER 119621 in cats; LEONTOVICH & ZHC’KOVA[ 1963J in dogs) do not mention the existence t)!”different subthalamic neuronal types. FOIX & N~c~~.esco (19X) were the first to indicate, on Nissl material in man. that subthalamic neurons look smaller in the ventral and external part of the nucleus than in its dorsal and internal part. Also on Nissl material, the reverse was observed by KODAMA(1928) in man and by WHITTEK & METTLER (1949) in monkeys. The iatter authors placed the smaller cells in the medial part and the larger ones in the lateral part. RGOLS & Fox (19761 localized the smaller cells of the Macal. in ‘a narrow strip along the ventral border at the medial end of the nucleus’. On Golgi material, two recent papers distinguished several neuronal types. 1H’4HURI t 19781 separated three Golgi I neuronal types in the cat; type I with medium sized oval or polygonal somata. typ II with large multipolar or polygonal somata~ and type III with small polygonal somdta. R+&-S & I%\ (1976) distinguished two Golgi I neuronal types 111 Mucacu (‘radiating’ and ‘elongated fusiform’ neurons1 but also described Golgi II neuron< :~b ‘local inter. neurons’. The existence of such neuron\ will be d!scussed first. Do internarrons occur in the subthukmic IIU&U.~ ’ RAFOLS & Fox (1976) have beautifully illustrated two ‘local interneurons’ or ‘intrinsic neurons’ in thcil Figs 3, 4, II, 15 and 16. These ~~11s have small somata, few, thin and wavy dendrites which have ‘dendritic appendages’ and ‘axon-like processes’. These authors assume that these interneurons are similar to other local circuit neurons found elsewhere in the nervous system and correspond to inhibitory

Subthalamic neurons in primates interneurons found in the su@thalamic nucleus by physiological methods (TSVBOKOWA t SUTM, 1972). In our laboratory material, we already found such local circuit neurons in the centralis-parafascicularis complex (PEWHEJ~ON, YELNIK, FRAN& Hfxmw 19783,in the substantia nigra of baboon and macaque (mcus et al., 1979), in the cat thalamus and in the striatum of the macaque (unpublished observations). We have been looking for similar neurons in the subthalamic nucleus for 3 yr. TWO types of small cells have been found which could be considered to be possible l&al interneurons. Small cells of the baboon (Fig. 5F and G) have somata which are as small as those of glial cells (Fig. SC, D and E). Their very thin branches with some structural differentiations which contact the somata and proximal dendrites of the large neurons, make them resemble the ‘axonless cells’ described in the cat thalamus by ~CHEIBEL,DAMES & ~CHEIBEL (1972). However, the morphology of all these small cells are not sufficiently different from that of the glial cells to be sure of their neuronal nature. The second type of small cell which has beem found in our material is represented by neuron H in Fig. 5. Its cell body is clearly larger than those of the glial cells and somewhat smaller than those of large neurons. It has shorter and thinner dendrites than all other large neurons and above all, has some thin processes. The presence of such thin processes has been shown to characterize many local circuit neurons (FRANCOISet al., 1979) but those of the present neuron are somewhat thicker and less numerous than in the intrinsic neurons of R~LS & Fox (1976) and even than those found in our material in the substantia nigra (FRANCOISet al., 1979). The small length and diameter of the dendrites and the presence of some thin processes could permit us to classify this neuron among the local circuit neurons. However, the large number of its dendrites makes it resemble more closely the Golgi type I neurons. The absence of convincing subthalamic Golgi type II neurons in our material could be due to an unfortunate lack of impregnation, as the Golgi method is notoriously capricious However, we studied a large amount of material in different planes of section, in different species and with various staining procedures, one of which is very close to that used by RMOU & FOX (1976). Among the other authors who have studied the subthalamic nucleus, neither RAM~N Y CIVAL(1930-1911) nor IWAHORI(1978), who studied 250 cats, observed any Golgi type II neurons. The Nissl stained material that we analysed did not bring us more convincing arguments than did the Golgi material. Histograms of the projected surfaces of the s~mata have been shown to be bimodal in the cerebra1 regions, such as the thalamus, where local circuit neurons certainly exist (DE WULF, 1971). We have made such measurements in various parts of the subthalamic nucleus of both man and macaque, in different planes of section, and each time the histograms

1739

were unimodal. These arguments led one of us (YELNIK, 1976), just before the paper by RAM)Ls & Fox (1976) was published, to conclude that local circuit neurons might be absent in the subthalamic nucleus (at least in macaque and man). We are less sure now, but we do not yet have morphological evidence to corroborate the observation of RAFTS & Fox (1976). Is there more than one kind of Golgi type I neuron? Among the Golgi type I neurons, different varieties have been described in previous publications. These were suggested by the different morphological aspects of both somata and dendrites. We also observed all these different aspects in the present material. Neurons with ‘round’ polygons of dendritic projection such as neurons A, G and I in Fig. 6 are similar to neuron A in Fig. 286 of RA&N Y CAJAL(1910-1911), to the ‘radiated’ neurons C and D in Fig. 1 of RAF~LS & Fox (1976) and to neurons A, C and D in Fig. 2 of IWAHORI (1978). Neurons with ‘ovoidal’ polygons such as neurons D, F, K and L in Fig. 6 are similar to neuron E in Fig. 2 of IWAHORI (1978) and probably to the ‘elongated’ neuron C in Fig. 2 of RAFOLS& FOX (1976). Such patterns are still modified by the proximity of the borders of the nucleus. The bending of the dendrites on the ‘wall’ of the nucleus gives pictures such as the ‘cellules marginales’ in Fig. 286 of RAM~N Y Cu_u (1910-1911X neurons J in Fig. 4 of IWAHORI (1978) and B in Fig. 2 of RMOLS & Fox (1976), and our neurons B, E and J in Fig. 6, and A and B in Fig. 7. When individual neurons are considered alone, one thus has an impression of highly variable pictures. Conversely, if numerous neurons are analysed according to the planes of section and to their exact location in the whole nucleus, the ellipsoidal dendritic domain of the subthalamic neurons (Fig. 12A), the orientation of their dendrites in relation to the main axis of the nucleus and their modification by the proximity of the borders explain the individual pictures of neurons which, in fact, belong to a single neuronal species. Furthermore, the existence of a single variety of Golgi type I neurons in the subthalamic nucleus is in better accordance with the physiological results. DENIAU, Hwom, CfIEvtiw & FEGER (1978) concluded ‘that the cells of the subthalamic nucleus behave as a homogeneous neuronal population’. The lesions of different parts of the nucleus in man do not provoke different clinical motor disturbances but always the same ballistic movements (MYRON, 1927; CARPENTER & CARPENTER,1951; MARTINET,1953). This suggests that all parts of the nucleus are devoted to the same motor function. SUbthalamiC neurons in relation to neuronal, classijcations

The first classif%zations using the dendritic pattern of neurons classified the subthalamic neurons as ‘isodendritic’ (RAM~NMOLINER,1967) along with those of the globus pallidus, substantia nigra and reticular formation. RAFOLSBr FOX (1976) have already corrected that point of view, showing that the ‘delicate dendrites

1740

J. YELNIKand G. PERCHERON

of the subthalamic neurons’ are different from the thicker dendrites of neurons in the globus pallidus and substantia nigra. In addition, the dendritic numbers have been shown (PERCHERON,1979b) to be different. Comparison with other neuronal populations shows that subthalamic neurons are moderately branched neurons, like the neurons of the centre median and the dentate nucleus. However, they cannot be classified with those groups, as, unlike them, they do not change in phylogenesis. Conversely. they exhibit a relatively high number of ‘scions’ and ‘bushes’ (PERCHERON.1979b) as do the motoneurons of the spinal cord and the neurons of the oculomotor nucleus. Although subthalamic neurons are less branched. we think now that they can be considered as special motor neurons.

horn of the spinal cord [respectively m Figs 292. 295 and 296 of RAT&N Y CAJAL(1910-191 I,], seems to be a characteristic of motor nuctei. As there are no other dendrites in the cerebral peduncle except those 4 the subthalamic neurons, one may conclude that the dendritic organization of the subthalamic nucleus is certainly that of a ‘closed nucleus’ whose: dendrites do not overlap with any dendrites of ttrher neuronal groups. Exceptions have been found to this general law. and will be discussed now. Heterotopic

subthulamic neurons

While the boundaries of the subthalamic nucleus are sharply outlined several neurons have been observed outside their original nucleus. In !Mecaca, a typical substantia nigra neuron (large soma. thick, straight and long dendrites) was located in the medial Dmdrourcllitectonic organization of‘ the subthulumic end of the subthalamic nucleus. Its dendrites reached mlcleus the substantia nigra after having crossed the capsule. MANNEN(1960) has shown that the Golgi study of RAFOLS& Fox (1976) showed that a subthalamic denthe brain stem led to a distinction between ‘open nudrite could cross over a dendrite of a substantia nigra clei’ and ‘closed nuclei’. In the latter, the cytoarchitecneuron (their Fig. 7) or even over thar of a tegmcntal tonic limits represent a boundary which the dendrites neuron (their Fig. 8). In man, a typical subthalamic do not cross. Consequently. only the a&rents which neuron (E in Fig. 7) was located- in the most dorsal end inside these limits can in any way directly make part of the substantia nigra. In macaque. subthalamic contact with the neurons of the nucleus. These ‘closed neurons were found at the medial tip of the globus nuclei’ are assumed to support a ‘specific function’. In pallidus, on the other side of the internal capsule. the ‘open nuclei’, the dendrites pass beyond their Such heterotopic neurons have been observed in P&l cytoarchitectonic limits, becoming what has been sections of both monkey and man (SCHNEIDER,L968). called ‘extra-focal’ dendrites by RAT&N Y CAJAL This author suggested that these heterotopies could (1910-1911). These dendrites are then intermingled be due to individual variations of the place where the with dendrites of other structures, making a ‘dendritic internal capsule passes between the globus pallidus, continuum’ (RAMONMOLINR~& NAUTA, 1966). This the substantia nigra and the subthalamic nucleus. arrangement may only support unspecific functions because a single afferent fibre, arriving in that interAfirents and somatotopy mingled territory. probably forms contacts with several different neuronal groups. Finally, such a ‘retiThe main afferents to the subthalamic nucleus cular organization’ is opposed to the ‘nuclear organicome from the external pallidurn (NAIJTA & MEHLER, zation’ of the ‘closed nuclei’. 1966; CARPENTER& STROMINGFX 1Yh7 ;CARPENTER. Comparison between the cytoarchiteetonic limits of FRASER& SHIUVER,1968; KIM, NAKANO,JAYARAMAN the nucleus and the extension of the dendrites showed & CARPENER, 1976) or from its equivalent in rats and that the subthalamic nucleus is a ‘closed nucleus’. cats, the pallidum (CARTER & FIRIGER, 1978). The First, no subthalamic dendrite crosses the cytoarchiexistence of sparse afferents coming from the motor tectonic limits of the nucleus. This is also clearly cortex has also been demonstrated (PENAL. 1965: shown in Fig. 1 of IWAHORI (1978). Secondly. the den1969). Following VON KOLLIKER(1X%) and Mntn, (1896), RAM~N Y CAJAL (191@-19111 showed that in drites of the dorsally located neurons always bend cats (Ram& y Cajal’s Fig. 286) and in mice. subthalaalong the nucleus border, which they never cross. The mic afferents were mainly collaterals from rather thick bending of these dendrites demonstrates the ‘material’ axons running into the cerebral peduncle. This has character of the dorsal border of the nucleus. In fact, been confirmed by Figs 5, 7 and 9 of IWAHORI(1978). Forel’s field H2, which is made up of myelinated fibrcs passing between the zona incerta and the sub- We have no data of our own concerning the afferents to the subthalamic nucleus but we want to raise a thalamic nucleus, acts as a capsule. What happens along the ventrolateral border is different. Some den- problem: since the main afferents of the nucleus are drites (or even some cell bodies) may extend into the collaterals, what are the fibres from which they cerebral peduncle [neuron A in Fig. 286 of RAM& Y branch? Some collaterals could @z b%inCb of cortiCAJAL (1910.,191l); Fig. 1 of IWAHORI(197811.The cal axons descending in the peduncle (Fig. 12D) but same thing was observed in macaques (R.@oLS 8~ the quantity of collaterals does not fit in with the sparse character of the cortical projection. ConFox. 1976), and in one baboon brain. This disposiversely, lateral pallidal a&rents, which are proFuse, tion. also found in the nucleus of the hypoglossal are usually said to end in the subthalamic nucleus. nerve. the nucleus of the spinal nerve and the anterior

Subtbalamicneurons in primates

1741

This would mean that they are terminals and not The thicker portion, which is perhaps the myelinated part of the axon, begins within the nucleus, in cats collaterals. Terminal afferents exist in the subthala& nucleus b in Fig. 288 of RAM6N Y CAJAL [Fig 4 of Iwlworu: (1978)) and in macaques (Fig. 8). (1910-1911); J and K in Fig. 7 of IWAHORI(197811 Intranuclear collaterals terminating within the nucleus are probably exceptional since Figs 286 and 295 but they are not as numerous as pallidal a&rents should be. HATTORI, F~BIGER & MCGEER (1975), of R&N Y CAJAL(1910-1911) do not show any one CAR’IER& FIBIGER(1978) and KANAZAWA,MARSHALL &them in cats, and we did not find any in monkeys. & KELLY(1976) in rats, GROFOV~(1975) in cats have Only one of the twelve neurons shown in Fig. 4 of IWAH~RI (1978) has such a short axonal collateral. shown that a moderate number of pallidal neurons Some axons do not bifurcate either inside or in the (the lateral pallidum) also send axons to the substantia nigra, mainly to the dopaminergic neurons of its vicinity of the nucleus. Some of them, starting from pars compacta. Thus, the observed collaterals could medially located neurons, run medially and caudally also be branches of pallidonigral fibres (Fig. 12D). in cats [Figs 286 and 295 of RAI&N Y CAJAL. NAUTA & MEHLER(1966), CARP~TER & STROM- (1910-191111 and in macaques (B ia Fig. 8). ApparMGER(1967), CARPENTERet al. (1968) found that the ently more of them, which start from more laterally located neurons, run laterally in cats [A-G in Fig. 4 projections from the lateral pallidum are spatially of IWAHORI(197811as well as in macaques (C, D and organized. The existence of a somatotopical distribution within the subthalamic nucleus has been also E in Fig. 8). Bifurcated axons also exist in cats [H, I, J suggested by clinical cases of hemiballismus (WHIT- and L in Fig. 4 of IWAHORI(1978)] and in macaques (A in Fig. 8). One branch runs medially, the other one TIER, 1947; CARPENTERt CARPENTER,1951) though laterally. The main efferent pathway of the subthalathe motor disturbances were not always clearly located The topographical organization of the sub mic nucleus is lateral (and rostral) towards the medial thahunic nucleus may explain that rather crude somapallidum (CARP& STROMINGER, 1967) but also towards the lateral pallidum (NAUTA & COLE, 1978). totopy on the basis of the following hypothesis. No The unbifurcated axons or the axonal branches that somatotopy would be possible in a closed nucleus if all the dendritic domains of its neurons filled enrun laterally could then be Luyso-pallidal efferents tirely the whole space of that nucleus. Indeed, in this that join the ansa lenticularis (NAUTA& COLE, 1978). case, any afferent could contact the dendrites of any KANAZAWAet al. (1976) in rats and NAUTA & COLE neurons of the nucleus, and thus no spatial organiza(1978) in cats and macaques have shown that subthation could be possible. The ratio between the size lamic neurons also projected to the substantia nigra. of the nucleus and that of the dendritic domains is, The uabifurcated axons and the axonal branches that then, a first determining factor, and may allow the go medially could be Luyso-nigral e&rents (Fig. determination of the ‘grain’ of the nucleus. Similarly, 12D). NAUTA& COLE (1978) also showed that subthano somatotopy could exist in one nucleus if the arborlamic neurons projected to the nucleus tegmenti ization of any afferent axon also fill the whole space pedunculo pontinus. Axons running inferiorly and of the nucleus. In this case also, any afferent could caudally such as those described by RAYONY CAJAL make contact with the dendrites of any neuron. The (191+1911) could be Luyso-pedunculo pontine efferamount of possible somatotopy would then be linked ents. DENUU et al. (1978) gave corroborative physioto the relative size of the nucleus and of the dendritic logical data for the rat. They furnished quantitative domains, and to the spaces of axonal tierent arboridata for the different e&rents; these are summarized zation. In cats, the axons that end in the subthalamic in Fig. 12D. Unfortunately, such a study is not availnucleus ramify in spaces that do not cover the whole able in primates. Only NAUTA & COLE (1978) noticed nucleus [Fig 288 of &I&N Y CAJAL (1910-1911); that the e&rents to the substantia nigra are not ‘as Figs 6, 7 and 8 of IWAHORI(19781-J.These spaces, massive’ as are those to the pallidurn. When interwhich make more or less ventrodorsally oriented preting subthalamic lesions or simulations, one must remember that some nigrostriatal axons have been columns, are five times smaller than the nucleus in this species (Fig. 12B), while the dendritic domains shown to pass through the subthalamic nucleus (CARof the subthalamic neurons are only half as big a~ PEN'IER 8c MCMASTERS,1964; CARPEN’IW& PETER, 1972). Such an axon, apparently thin and giving no the nucleus in cats (Fig. 12C). This organization probably only supports a very crude somatotopy. In mon- branch to the subthalamic nucleus could correspond to axon N in Fig. 7 of IWAHORI(1978). key and in man where the subthalamic nuclei we larger, there is room for five and nine non-overlap Ping neurons respectively, probably allowing a rather Conclusions and possible functional implications more precise somatotopy. The subthalamic nucleus appears to be a homoThe axons of the Go@ type I subtbalumic neurons geneous closed nucleus w&h receives its own a&The axonal cource of the subthalamic neurons is r&s from the lateral pallidum and, to a less extent, the morphological basis of the efferent pathway of the from the motor cortex. Lesions or stimulations inside nucleus. These axons originate in the axon-hillock the nucleus would only influence subthalamic neurons and are very thin for a short distance (about sopm). (with the exception of some nigrostriatal axons). me

1742

J. YELNIKand G.

belong to a single homogeneous neuronal species with ‘motor features’

Golgi type I neurons apparently

that may be quantita&ly characterized by stable dendritic numbers and by dimensions of their ellipsoidal domain. Their axons (either unbifurcated or bifurcated) project on the two parts of the pallidum, the substantia nigra and the nucleus tegmenti pedunculo pontinus. Thus, the suhthalamic nucleus is not only a part of a Luyso-pallidal loop, but also a part of a more complex Luyso-nigrostriatopallidal loop and even sends descending fibres. Comparative studies show that the subthaIamic neurons do not change from cat to man. As afferents and efferents also apparently remain the same, this legitimizes homologies

PERCHERON

taken from experimental studies on non-primates species. The main change consists of an increase in the dimensions of the nucleus and in the number of

neurons. What is expected to change is the amount of somatotopical organization. In all probability absent in cat, it appears, to some extent, possible in man. Acknowledgements-It is a pleasure to acknowledge Mrs S. HEYNERfor her excellent histological work. Mr 1. R. TEILHAC for his photographic work and Miss C. FRANCOI~ for her competent advice. We are grateful to Dr R. NAQU~T who gave a babocm’s brain. Mr K. Hnro~ kindly reviseti the English. This work was supported by Grant ATP 29 76 61 or INSERM (France).

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(Accepted 18 June 1979)