Somatotopic organization of corticospinal and corticotrigeminal neurons in the rat

SOMATOTOPIC ORGANIZATION OF CORTICOSPINAL AND CORTICOTRIGEMINAL NEURONS IN THE RAT S. P.

WISE, E. A. MURRAY and J. D. COULTER

Marine

Biomedical Institute, Departments of Physiology and Biophysics and of Psychiatry and Behavioral Sciences. University of Texas Medical Branch, Galveston. Texas, Ii S.A.

Abstracts-The method of retrograde axonal transport of horseradish peroxidase was employed to examine the topographic organization of corticospinal and corticotrigeminal neurons in the rat. In both the first somatic sensory (9) area and the motor (MI) area of the cortex these labeled corticofugal neurons, all of which are found in layer V. are grouped in ;1 well organized. somatotopic pattern. Corticospinal projections which extend to lumbar levels of the spinal cord origmate only from neuronal somata located in the hindlimb representation of SI and MI. Those neurons projecting to the cervical enlargement have 5omata mainly in the forelimb representation of SI and MI and the ventrolateral part of the trunk representation within SI. Cortical projections to the rostra1 cervical spinal segments appear to originate mainly from the neck and posterior head representations of SI and MI, though this conclusion is clearest for SI. Finally, neurons located exclusively within the head, muzzle, and vibrissal representation of SI project to the spinal trigeminal complex. Corticofugal neurons near the frontal pole and in an area of cortex ventrolateral to Sl also project to the spinal cord. The areas involved are probably homologous to the supplementary motor (MII) and second somatic sensor) (%I) areas respectively. The corticospinal and corticotrigeminal projections from these areas also appear to be organized in a somatotopic manner. It is concluded that in the rat, as in other species. the corticospinal and corticotrigeminal neurons in the sensorimotor cortex are arranged somatotopically. The somatotopic pattern found correlates remarkably techniques.

well hith

that

determined

by single

unit.

evoked

potential

and

cortical

stimulation

RECENTinvestigations

of the organization of the cortiIn contrast to the findings in primates and carnisystem in cats (COULTER,EWING & CARTER, vores it has recently been reported (D’AMATO & HICKS, 1975; HICKS & D’AMATO, 1977) that the corti1976; GROOS, EWING, CARTER & COULTER, 1978) and monkeys (BIBER, KNEISLEY, LAVAIL & RAKIC, 1976; cospinal neurons of the rat show virtually no somatotopic organization. In view of previous studies of the COULTER et al., 1976; MURRAY & COULTER. 1976; corticospinal system in cats and monkeys and the evi1977; JONES & WISE, 1977), employing the retrograde dence in rats (WISE & JONES, 1977) that the cortical tracing technique, have demonstrated that within output from the first somatic sensory cortex to the each cortical field which sends axons to the spinal cord, the corticospinal neuronal somata are arranged region of the spinal trigeminal complex originates exclusively from the head representation, this reported in a somatotopic pattern. Further, in each cortical lack of somatotopic organization of the corticospinal field for which detailed electrophysiological data are available, the distribution pattern of the corticospinal system of rats is most unexpected. Accordingly, the somatotopic organization of the rat corticospinal sysneurons as determined anatomically correlates well tem has been reinvestigated by the retrograde horsewith the somatotopic map obtained by examining either the peripheral receptive fields of cortical radish peroxidase fiber tracing method. neurons or the movements elicited by stimulating small parts of the cortical field. Hence, in cats and monkeys, corticospinal axons projecting to lumboEXPERIMENTAL PROCEDURES sacral levels of the spinal cord arise from layer V Sixteen albino rats, male and female (ZS(r35Og). were pyramidal cells situated mainly in the hindlimb repreobtained from Texas Inbred Strains, Inc. The animals were sentations of sensorimotor cortical fields. those proanesthetized with (40mg,ikg) sodium pentobarbitai and a jecting to cervical levels arise from cells in the forelaminectomy performed to expose either the lumbar or cerlimb representations, and those projecting to thoracic vical enlargements or the rostra1 cervical segments of the levels arise from cells in intermediate locations. cospinal

Abbrraiurions: MI, first motor area of the cerebral tex; MII, supplementary motor area; SI, first somatic sory area; SIT, second somatic sensory area.

spinal cord. A small portion of the dura was then removed from the dorsolateral aspect of the spinal cord and a glass micropipette (25-50 pm tip diameter) which was cemented over a 10pm Hamilton syringe, was inserted into the cord. Care was taken to avoid passing the pipette through the dorsal columns since the corticospinal axons run in the

corsen-

65

ventral

aspect of the dorsnl

BROWN.

uere mxic num.

of the spmal trigeminal

in ;I similar mimner through

Between

Mdea (Miles oxid;lse

columns ln the rat (KI~c,.

injections

1971).

0.3 and

l.Oltl

Laboratories

15 min ;It either

fused

period.

through

;L cooled

trigernmld

the

heart

(5 C’) solution

?.5”,, glutaraldehyde brains the

nnd spimd

complex.

;it

were

.4ftcr

with

buffer

\pmal

;I 2.0 2.5 day

saline

bull’er,

then

5 C for

sucrose in phosphate

per-

approk.

;md per-

and

then

bq

I .O”,,paraformaldrh>dc

of 0.7

cords

o\cr

site\ 111:I \mgle

in phosphate

s&me fixative

horseradish

were reanesthetized

briell!,

of

smglc site (0.05 /Al) wils

itt iI

the animals

mag-

wlutton

w~ls injected

one or two intection

m:~de in the spinal

twb)

England]

saline

segment. A cniallcr injection hur\i\,31

the foramen

of ;i W,,

Ltd.

in physiological

191(l.

complex

and

pH 7.1 7.4. The

removed.

post-fixed

4 6 h. placed

in

in cold

30”,,

I.(- 7.0 d;l><. and scct~oned

for

on a freezing microtome at 50/m Two types of procedure were employed. In the lirst. the brain was cut in the frontal plane and e:tch section wils collected:

alternate

section3

were

phate buffer or cold acetate m phosphate

buRer were then

c;11 localiration WiNvoh

placed

buffer.

Those

treated

of peroxidase

in either sections

for

of

LAVAIL. tetr;t-

and O.Ol”,, H,02.

method

The hcctions

collected

acetate buffer were treated with o-dianisidine oxybenzidine

dihydrochloride)

modification

by

of the

DI

similar

to that

Also. for

method.

on

ice rather

than

in

of the reaction

communication).

All

cresSI

and dark-field

were

hemisphere

glass slides, 1976: WN sectioning

was

of a hemisphere upon

the

;I slight

prior

varlatlon

Results).

Serial

dianisidinc

method

with with

between (W~LK~K,

procedure

layers.

The

enables

orientation

wits varied

depending

21 procedure

in the appearance

producing

of the ‘flattened’

of the SI area within

sections

were

of procedure.

pressure.

1977). This

location:

and the orientation

gelatin

szctlons

pre\lousl)

to Hattening

site

on

examined

type

under

described

were collected

or bj the method

them

and reacted of HAN)\

b!

\ec(see the

& HCIM~R

(1977). using ?,3’.5.5’-tctramethyIbenr.idine.

The location

retrogradelq

and the densit)

of labeling of labeled

labeled

was determined neurons

were found with

cresyl

tion

violet in layer

to

plane

and and

segments

trigeminal

containing

after

patch.

the

pattern

treatment

spinal treated

cord

of the The

of

number

labeled

neuron5

cover-glass

counterstained of

All

granule

cell

of the injec-

were

sectloned

:is described

of the injection

here as those spinal product

ratio

and the sections

reveal

The extent

complex

the

in which

IV of SI (see Results).

Gtes In the medulla sections.

plotted,

continuous

removed.

in the horizontal

frontal

by

to the area

in a single

was subsequently aggregates

cells was then

for the

sites :lre defined

or that part of the brainstem readily

observable

by the dianisidine

granul;lr

Lyer

cells

;irc

14 not

continuous

grouped

through-

in 2ggregatcs

c;ich

receibcs its input from a $pecifc part L>tthe contralatcral hod! surface in a \omatotoplc;ill! organi7ed manner (WI.l.lit3. 1976). (‘ertam of 1hehc dense granule cell aggregates. particularI! the one containing the head and SIIOLI~ representation. ;trc diarupted by the cortical ‘barrels described iti rat5 I~\ WLLKFR & W(H)LSI,\I (1973). Phc peripheral input\ to these aggregates are arranged m ;I manner c‘on\lstertt of which

the 1971.

the rat

homatotopic SI

1976:

cortex 13GI.l

pattern

described

(WOOPLY &

LII\I~tIOL.M.

previousI!

in

iY52; 19%: WI LKI.K. 1974).

both

\\lthiii

an individual aggregate and with respect to their qtial rclatlon to each other. The SI I reprejcntarioil I, \entrola1cral to SI. The motor arca. including XII, lies rostra1 and medial to SI (Fig. I(‘) (Wi:.hf-H Kc SINHA. i972: HAI I+ 62 I_I~I)IIOLM. 19731. ‘The pattern of retrograde labeling of the c:orticotrigernln~II and corticospinal projectionh ~lill he described with rcspect to the granule cell ;qgregate> of SI.

found

and were examincd

to the cortical

injection

not

The dianisldine-treated

flattened

& .IONI,S, 1976: parallel

:md

In the second

in u manner

were

treated

microscopy.

microscopy.

agent

dense

SI. Granule

solutions

mounted

violet.

were not counterstained

bright-lield

tions

product

sections

with

the re;iction temperatures.

(R. B. L~OKAKI). personal

slides. The diaminobenLidine

counterstained

each

and allows

IS

con-

aa li penetration

and were omitted

sections

the

method

nitroprussidc-methanol

stabilization

bright-

thic

at sub-zero

to be necessary coated

(3,3’-dlmethobser\,utiona)

Briefly.

the use of dimethylaulfoxide post-incubation

in

following

of D+ OLMOS (1977). but uses ;I lower

(0.03”,,) of o-dianisidine

to proceed

HzOz.

LIAINARI) (unpublished

OLMOS (1977)

centration

and

R. B.

and

This out

with

the hiatochemi-

& TISH (1973). using 3,3’-diaminobenzldme

hydrochloride

by the

phos-

collected

The first somatic \ensor! (SI) cortex 01 the ri:t receives peripheral input from mechanorccep(oi 5 located on the entire cnntralatcral c~dc of [he botl? Most of SI cortex contain\ an extremcl\. ctcn~cl~ packed granular layer, laker II’. which can hc c;i\~l> distinguished from the less dcnselq p:lcked lacr\ \\itlr larger cell bodies supcrticial :InJ Jeep to la!121 IV.

method.

reaction

Injections of horseradish peroxidasc were made into the lumbar spinal segments in three rats. The injection sites were confined mainl) to one side of the spinal cord, but a significant amount of reaction product could be observed on the side contralateral to the main injection site. All of the injections of the lumbar cord were confined to the lumbar enlargement. One of these appeared to fill the enlargement. while the other two were mainly in the caudal half of the lumbar cord. Each hemisphere contralateral 10 the focus of the spinal injection has a similar pattern of retrograde labeling with respect to distribution within the cortex and laminar organization. As seen in Fig. 2. the retrogradelq labeled somata arc conlined to lalcr V. arc mainl! concentrated in the deepest part of that layer, and are among the largest pyramidal cells. This pattern is obberved following all spinal cord or trigeminal injections (see also West & Jovr:s, 1977: PORTEK & SANIYSRSON. 1964). With regard to topographic distribution, the cortical neurons labeled from the lumbar spinal cord are not found scattered homogeneously throughout the sensorimotor cortical areas. but are located instead in a single. relatively compact group in the dorsomedial aspect of the hemisphere (Fig. I). No labeled cells are found in the cortex ventrolateral to SI or near the frontal pole of the cortex. The area that con-

Rat corticospina)

r!btem

LUMBAR

ENLARGEMENT

DORSOMEDIAL

A

5

mm

B

Ftc;. I. Pattern of retrograde labeling in the sensorimotor cortex after peroxidase injections into the lumbar enlargement. (A) Layer IV granule cell aggregates in SI reconstructed from Nissl-stained sections of a ‘flattened’ hemisphere. The location of each aggregate (shaded area) and its peripheral input is indicated. This hemisphere was flattened from the dorsal aspect, which gives it an overall shape somewhat different than usual (cf. Fig. 3). (B) Area of cortex containing labeled neuronal somata in layer V. The shaded area represents a reconstruction of labeled areas from several sections. (C) Somatotopic map of SI (from WELKER, 1976) and the relattve locations of the SII cortex and the MI cortex. (D) The location of the granule cell aggregates (as seen in Al and the area containing labeled cells (as seen in B) is compared by reference to blood vessels (dots). The area of retrograde11 labeled cells is within the hindlimb representation and the dorsomedial part of !he trunk representation in SI and extends rostrally and dorsomedially into the agranular MI cortex. .Ahhviurion.s: FL. granule cell aggregate representing the contralateral forelimb; H, head and vibrissal aggregate: HL. hindlimb aggregate: LJ, lower lip and jaw aggregate; MI. first motor area: SI, the tirst somatic sensory area: SII. the second somatic sensory area: T. trunk aggregate.

tams the corticospinal

neurons

which

project

to lum-

in rostrocaudal extent. and l-2 mm in mediolateral extent. This group of labeled neurons completely underlies the granule cell aggregate which WELKER (1976) identified as receiving its peripheral input from mechanoreceptors on the contralateral hindlimb. In addition, the cortical zone of labeled cell bodies extends up to 500pm more medially than the medial border of this dense granule cell aggregate into what is probably a part of the motor cortex (Figs 1, 5). Labeled neurons also extend caudally to lie beneath the dorsomedial part of the granule cell aggregate containing the trunk representation (Figs 1,5; see WELKER, 1976). Figure l(B) shows an example from one animal of the pattern of corticospinal neurons retrogradely labeled from the lumbar enlargement. Because the density of labeled cells is so high (4OO/mm’), the cortical zone containbar

segments

is approximately

4mm

the labeled neurons as seen in serial sections through layer V. is shown as an enclosed shaded area plotted on the flattened hemisphere. Counterstaining these same sections for Nissl substance allowed identification of the granule cell aggregates of layer IV (Fig. 1A). which are identified with reference to the SI map of Welker (Fig. 1C). Using the blood vessels to align the sections. the zone of retrogradely labeled neurons can be seen to underlie the SI granule cell aggregate corresponding to the hindlimb and the dorsomedial part of the aggregate representing the trunk (Fig. 1D). No retrogradely labeled cells are seen. however, in any other part of the SI cortex. The pattern of retrogradely labeled neurons is identical in sections stained by the diaminobenzidine method and those stained by the dianisidine or tetramethylbenzidine methods. However, two to six times as many retrogradely labeled neurons could be ing

observed m the diamsidine or ~ztrametl~ylhenrid~ne treated maternal as in the dl~tminc~henritiille material (cl: Fig. 6( and DL In all C;ISC‘S.111~‘ dianisitiine reac-. tion product demonstrating the ictrograclely Iransported pcroxidase is denser and inorc i-eadil! ohsensecable than that seen in adjacent tllalnlnobentidine tions. The labcling (Fig. 2B) appears to he of three levels of intensity. These three intensities 01’labeling are similar to the ‘solid’. ‘granular-’ and ‘dense-granular’ labeling described recently by Krt+r~ (197X). Many of the most dcnscly labeled ccllh have reaction product in the apical dendrite which can he followed from the cell soma into layer II. The basilar dendritic system is occasionally tilled several q into fourth or fifth order dcndritcs. However, most of the labeled neurons are less densely lab&d and the observable reaction product in these cells is mainly confined to the neuronal somata.

In tive animals peroxidase injections were made into the cervical enlargement. Two of these injections essentially tilled the enlargement. The others had injections confined to its caudal half. In all cases, three patches of retrograde labeling are seen in the cortex. The largest patch of labeled cells both in size and number of labelled cells (90”,, of the labeled cells in the hemisphere) is seen in SI, beneath the granule cell aggregates. and in the rostromedially adjacent agranular cortex in MI (Figs 3, 5). The extent of labeling in the agranular cortex is variable (cf. Figs 3B and 5) and probably dependent on the precise location of the effective in.jection site. This labeled area is approx. 4 5 mm in rostrocaudal and 1 2 mm in mediolateral extent. The labeled cells in SI are beneath the forelimb granule cell aggregate. beneath the ventrolateral aspect of the trunk aggregate, and in the intervening less granular SI cortex (Figs 3. 5. 6). Labeling beneath the granule cell aggregate representing the hindlimb and caudal trunk is virtually absent, in contrast to the pattern seen following pcroxidase injections in the lumbar enlargement. In addition to the large patch of neurons in the SI and MI cortical areas. two other small cortical zones contained labeled neurons. One group of labeled cc!ls is seen immediately ventrolateral to the SI head representation (Fig, 3A, B). Labeled cells may occur in a single group or as 2 3 distinct. small clusters, These cells appear to be within the second somatic sensory (31) representation (WLLKER & SINHA, 1972). The other group of neurons labeled from the cervical enlargement lies rostra1 and medial to the MI cortex as traditionally described (Wtxn.src\~. 1958; HAL.I, & LINI)HOLM. 1974). This group of labeled cells is best seen in frontal sections. since distortions of the rostra1 and medial cortex inevitably occur in flattened hemispheres. Labeled neurons are observed at the frontal pole and posteriorly along the dorsal aspect of the hemisphere for approx. 1.5 mm.

Four rats had injection\ which wcrc contincd between the second ccrv ical \egmcnt iro\trall\ ;IIII.I tllc fourth cervical segment caudally. The general paltcrn of retrograde labeling in the cortex wax similar tn all cases. Scvcral patches of retrograde labeling
into the .spinu/ trigettlinul

covrplrs

Small injections of peroxidase were confined to the dorsolateral aspect of the caudal medulla (unilaterally) in four animals. These injections involved mainly the caudal nucleus of the spinal trigeminal complex. but extended beyond its borders ventrally and medially into the dorsolateral reticular formation. The pattern of labeling in SI (which contains 79”,, of the labeled neurons) is similar to that described previously in a study employing the diaminobenzidine method (WISE & JONES, 1977). The labeled neurons are densely packed (500 labeled neurons/mm’) in layer V and are found beneath the head. vibrissal and muzzle related granule cell aggregates including the posteromedial barrel subfield of WEL.KER& W~LSEY (1974) (Fig. 3E, F). In no case does the labeling fill

69

FIG. 2. Labeled corticospinal neurons. (A) N&l-stained frontal section through the SI cortex (x 70). (B) retrogradely labeled corticospinal neurons from the adjacent, dianisidine-stained section after an injection of peroxidase into the rostra1 cervical spinal cord. AH retrogradely labeled cells are within layer V and concentrated in the deep part of that layer. Labeled apical dendrites extend into layer 111 (x 75). The arrow in A and B marks the same blood vessel in adjacent sections. Ahbreviarions: I&VI, layers of the cerebral cortex.

CERVICAL

ENLARGEMENT

5 ROSTRAL

CERVICAL

TRIGEMINAL

E VENTROLATERAL

FIG. 3. Pattern of retrograde labeling in the sensorimotor cortex after peroxidase injections into the cervical enlargement (A and B), the rostra1 cervical spinal cord (C and D), or the spinal trigeminal complex (E and F). The pattern of granule cell aggregates in the hemispheres for which the labeling is shown is illustrated in A, C and E as in Fig. IA. Areas containing retrograde labeling are compared to the location of the aggregates in B, D and F as in Fig. 1D. Dots indicate blood vessels used for aligning the sections. Ahhre~~iations as in Fig. I : VIB. vibrissal representation.

mm

71

FIG. 4. Tetramethylbenzidine stained corticospinal neurons in layer V of a flattened hemisphere. (A) The pattern of labeling is similar to that described in Fig. 3 (C and D), but from a different rat. The peroxidase injection was in the rostra1 cervical spinal cord. Labeled cell clusters are seen as dark patches. Open arrows surround the SII region where labeling can be more readily discerned at higher magnification. Rostra1 is to the left. dorsomedial is up ( x 10). (B) Granule cell aggregates in a Nissl-stained section passing through layer IV of the same hemisphere (x 7). Parts of the forelimb. hindlimb and head aggregates can be seen. The head aggregate can be seen to extend more caudally in other sections through layer IV. Solid arrows mark the same blood vessels in A and B. By reference to the observable aggregates, blood vessels, and the known orientation of the SI representation within the cortex, the topographic pattern of corticospinal neurons can be determined. (C) Shows part of A at higher magnification (X 25). Solid long arrow marks the same blood vessel in A, B and C.

LUMBAR

ENLARGEMENT

CERVICAL

ENLARGEMENT

FIG. 5. Comparison of the pattern of labeled corticospinal neurons in comparable frontal sections after peroxidase injections at lumbar or cervical levels. Each dot represents one labeled cell (dianisidine preparation) and its location within the cortex. The dotted lines indicate the location of dense, granule cell aggregates as observed in the adjacent Nissl-stained sections. The number to the right of each pair of sections is the distance from the frontal pole (mm x IO) and corresponds to rostrocaudal levels indicated in the flattened hemisphere illustrated m Fig. 7A. Thcrc is httle or no overlap of the regions of origin of the cortical projections to these two spinal levels and the somatotoptc pattern of labcling agrees with that seen in flattened preparations (cf. Figs ID and 38). Ahhr.~,l.irr/io,,.\: C‘C‘. corpus callosum: dg. dentate gyrus; Fx. fornix; Hp. hippocampus. LV. lateral ventricle: S, striatum: WM. subcortical white matter.

FIG. 6. Association of corticospinal neurons to granule cell aggregates. (A) Peroxidase injection site (diaminobenzidine method) in the caudal part of the cervical enlargement of the rat from which Figs B. C and D are taken. Rostra1 is up in this figure only. The constriction of the cord caudally indicates ( x 6). (B-D) Bright-field (B) and dark-field (D) photomicrothe caudal limit of the cervical enlargement graphs of a diaminobenzidine treated frontal section. counterstained by the Nissl method (medial. left; dorsal. up). Hollow arrows in B mark the borders of the hindlimb (HL) and forelimb (FL) aggregates as \een in layer IV at a level approximated by section 59 (right) in Fig. 5 (see also Fig. 7A). The retrogradely labeled cells are shown in D from the same section photographed in dark-field and in C‘ from the adjacent dianisidine treated section. Arrows mark the same blood vessels. The corticospinal cells projecting to the cervical enlargement are found beneath the forelimb granule cell aggregate and do not extend medially (left) toward the hindlimb aggregate. No labeled neurons are found beneath

Rat corticospinal

the entire complement of cortex related to the head and face (Fig. 3E, F). This observation undoubtedly arises from the rather small injections made in the spinal trigeminal complex which would involve only a portion of the caudal nucleus. Two rostrally situated patches are seen. One of these extends from about 2mm ventrolateral to the frontal pole to the level of the frontal pole. The other rostra1 patch is slightly caudal to the frontal pole and extends caudally for about 1 mm on the lateral aspect of the hemisphere. Together, these rostrally situated groups of corticotrigeminal neurons comprise about 15”; of the labeled cells in the hemisphere. Another labeled area of cortex, about 1 mm in diameter, is found ventrolateral to the SI representation. It contains about 6”, of the labeled neurons in the hemisphere and is rostra1 to the homologous labeled zone (in the SII cortex) observed after injections into either the cervical enlargement or the rostra1 part of the cervical spinal cord (Fig. 3B, D, F). DISCUSSION Sotnutotopic

organization

Two aspects of corticospinal organization arise from the present study. First, the corticospinal system, together with the corticotrigeminal system. is definitely organized in a somatotopic manner in the sensorimotor cortex of the rat. Second, the somatotopic pattern of these corticofugal neurons correlates remarkably well with that determined by single unit, evoked potential and cortical stimulation techniques. The latter point is clearest for SI where the pattern of labeling can be directly compared to the somatotopic representation as observed histologically. First somutic sensory cortex. In the case of SI. the somatotopic arrangement of corticospinal and corticotrigeminal neurons is quite precise. There appears to be little, if any, overlap between the parts of SI which project to one region of the spinal cord or the trigeminal complex and those which project to other spinal regions (Fig. 7). In view of this strict somatotopic organization in SI, extensive branching of corticospinal neurons (SHINODA, ARNOLD & ASANUMA, 1976) over widely separate spinal cord levels (e.g. to both lumber and cervical enlargements) would appear to be extremely limited. There is a striking correlation of the afferent and efferent projection systems in the rat SI cortex. SI corticospinal neurons project to those spinal segments which receive input from the same region of the periphery as the cortical zone from which the corticospinal projections themselves originate. A similar relationship of afferent and efferent projection systems has been noted for the motor cortex of cats and monkeys (FETZ & BAKER, 1969; ASANUMA& ROSEN, 1972; ASANUMA, 1975; LEMON, HANBY & PORTER, 1976) and for the somatic sensory cortex of cats (DUBNER & SESSLE,1971; S. P. WISE, J. D. COULTER & E. A. MURRAY, unpublished observations; see also GORDON & MILLER, 1969).

systenl

71

First motor area. Since no histological correlate of the somatotopic map can be found in the agranular MI cortex, precise statements concerning corticofugal somatotopy are more difficult than for SI. However. a clear somatotopic pattern can be discerned b! examining the patterns of retrograde labeling in relation to the SI map as revealed by the granule cell aggregates and in relation to established maps showing the spatial relationship of the MI and SI representation in the rat (WOOLSEY, 1958; HALL & LINI)HOLM. 1974). This approach is facilitated by the reported overlap of MI and SI in the hindlimb representation and. to a lesser extent. in the forelimb representation (HALI. &

LINDHOIM. 1974;

WISE &

JOKES. 19771.

Our current results are consistent with ELICIT :III overlap. As might be expected. the corticospinal neurons project to those spinal segments which contain motoneurons innervating muscles which are influenced by cortical stimulation. Thus, neurons in the hindlimb representation of MI, where stimulation produces hindlimb movements (WOOLSEY.1958: HALI. & LINI~HOLM, 1974) and excitatory postsynaptic potentials in motoneurons innervating the hindlimh musculature (JANZEN. SPECKMANN,CASPERS& EL.GI:K, 1977) project to the lumbar enlargement. Corticospinal neurons in the forelimb representation of MI project to the cervical enlargement (see ELGER. SPECKMANN,CASPERS& JANEN. 1977). The projection from MI to the rostra1 cervical segments arises from 31~ area rostra1 to the zone projecting to more caudal spinal levels. This part of MI is likely to be the neck representation. but previous studies (W(X)LSEY. 1952. 1958; HALL. & LINUHOLM, 1974) are not conclusive in this regard, so these interpretations must hc approached cautiously. As in SI, there would appear to be relatively little overlap in the cortical zones in MI which project to different levels of the spinal cord or the region of the trigeminal complex. Secorai somatic sensory area. Although no corticospinal neurons projecting to lumbar levels could be identified in the SII cortex, this area, ventrolateral to the SI head representation (WOOLSEY& FAIRMAN, 1946; WELKER & SINHA, 1972), does appear to project to the trigeminal complex, to the rostra1 part of the cervical spinal cord, and to the cervical enlargement. It is apparent from our results that the SII projection to the spinal trigeminal complex arises from the rostral part of SII and that to the rostra1 cervical cord arises from a more caudal part of SII. The projection to the cervical enlargement arises from the most caudal part of SII in which corticospinal cells have heen observed. The neck representation (related to the rostral cervical spinal cord) would be expected to be rostral to the forelimb representation and caudal to that of the head. Thus the somatotopic pattern, though only crudely determined in this small area of cortex. is consistent with the physiologically defined somatntopy (WELKER & SINHA, 1972).

EIG. 7. Summar>

of cortlcoapmal

aggregates in the hemisphere

and corticotrigeminal

illustrated

somatotop).

levels from which the frontal

sectIons in Fig. 4 are taken.

cervical enlargement

shading). rostra1 cervxal

(vertical

injections are superimposed

(A) The

pattern

of granule

m FYg. 3A. The numbers refer to the approximate

(B) Zones of

labeling

after lumber

(dots). and spinal trigeminal

on the granule cell aggregate pattern

cell

rostra-caudal (circles).

(diagonal shading)

of a Hattened hemisphere.

The regions

of SI which project to each spmal cord lc\el overlap with each other \cry little. 4 clear correapondcnce exists between the projection input to the same \ubdiwsion in A.

~hhr~~r~rrior~r mdicatc

RC.

rowal

Lone of the corticospinal of SI as revealed the mjectlon

cervical spinal \egmentb.

site: LE. Sp

V.

or corticotrigeminal

by the somatotopic lumbar

caudal

.A corticospincrl Llrvo rostrtrl ro Ill. A projection from the extreme rostra1 part of the rat cortex to the spinal cord has been previously described (HK’KS & D’AMATO. 1977). After peroxidase injections into the spinal trigeminal complex (and adjoining medullary regions), the rostra1 cervical cord. and the cervical enlargement. labeling was seen in an area near the rostra1 pole of the hemisphere. Most of this area would appear to be rostra1 at&or medial to the MI representation, but some of it seems to bc ventrolateral to Ml (see HALL & LINIIHOLM. 1974: WGOLSH\. 1958). This rostra1 corticospinal group appears to have a spatial relation to MI that is similar to the position of the supplementary, motor cortex (Mil) in other species (WCXXSEY. MARSHAI.L & BARI). 1942: WOIJLSE\., 1958). Furthermore. the rostra1 area appears to be somatotopicnlly organized and the pattern agrees well with that expected ot an MI1 representation The projections to the region of the spinal trigeminal complex are found ventrolaterally m the most rostra1 2 3 mm of the rat cortex. The projection to the cervical enlargement extends from the frontal pole caudally along the dorsal aspect of the cortex for a distance of about I 2 mm. A projectton to the rostra1 cervical spinal cord also arises from an area near the frontal pole. No distinct zone which projects to the lumbar enlargement could bc seen in the rostral area. It remains conceivable that labeling in the rostra1 part of the cortex represents a non-somatotopic corticofugal projection. if one includes this area within Ml. Although inclusion within MI of this zone is not ruled out by physiological reports (Woo~st.~, 1958; HALL & LINIIHOLM. 1974). the presence of four distinct patches of labeling (SI. SII. MI and in this

granule

enlargement:

neuron& and the peripheral cell aggregate

CE.

cervical

nucleus of the spinal trigeminul

map shown enlargement

1

complex.

rostra1 zone of cortex) seen after both cervical enlargement and rostra1 cervical spinal injections supports our interpretation that this rostra1 cortical area forms a fourth, somatotopicallv organized sensorimotor area.

The failure of a previous study (HICKS & D’AMATO, 1977) to demonstrate the somatotopic patterns of corticospinal labeling in the sensortmotor cortex of the rat appears to have resulted from an inability to label neurons retrogradely within the forelimb and neck representations of SI and MI. coupled with the retrograde labeling of cells as a result of their axons being severed at the injection site. In the study of HICKS & D’AMATO (1977). peroxidase was applied to the cut end of the spinal cord. which was sectioned in the cervical or lumbar regions. The reason for the negative tindings is not clear. though the involvement of fibers of passage readily accounts for the appearance of label in what appears to be the hindtimb representation of MI and SI. Corticospinal axons passing to the lumbar spinal cord are present, of course, at all more rostra1 levels and so might reasonably be expected to take up and retrogradely tranaport peroxidase when their axons are sectioned and peroxidase applied to the wound. The main conclusion of the present study. namely that the corticospinal and corticotrigeminal neurons in the rat sensorimotor cortex are somatotopically arranged, depends to a large degree on ‘negative’ evidence. The absence of retrogradely labeled neurons is as crucial to our conclusions as the observed locations of labeled neuronal somata. Therefore. the sensi-

Rat corticospinal tivit) of the histochemical demonstration of peroxidase must be sufficient to warrant confidence in negative observations. This methodological difficult) has been discussed at length elsewhere (LAVAIL, 1975; MESC:LAM & ROSENE. 1977; JONES & HARTMAN. 1978). In accord with other investigators (MESULAM & Rostu~. 1977; HARIIY & HEIMER. 1977: Dr OLMOS. 1977) we found that the diaminobenzidine reaction method was less sensitive than those using other substrates. Fewer neuronal somata could be identified and labeling was considerably less intense than in dianisidine or tetramethylbenzidine material. However, the high sensitivity of the latter methods supports our mtcrpretations. Other observations also

,!stcm

7’

lend support to our conclusions. The topographic pattern of labeling demonstrated by each of the histochemical methods is identical. Furthermore, the high densitj of the retrograde labeling in all areas where it is observed supports the view that unlabeled parts of a given cortical area (e.g. specific parts of SI) do not project into the particular. injected segtnents of the spinal cord. 4[,kno~/riiyu,t1t,,7i~ The authors thank Dr R. B. LFXYVAKI) for assistance alth our hlstochemical methods and Dr E. G. JONES for his critical review of an earlier version of this report. Alqo thank\ dre given to K. WtSTLU\t) for photographic assistance and to (‘. HoLt and P. WALI)KOPfor typing. Supported b! grant\ NS 12481 and NS 05736

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of the columnar

arrangement

of neurons

wtthin

the motor

ASA?It%lA H. & R&N 1. (1972) Topographical organization of cortical efi’erent Lanes prqjecting to distal forelimb muscles in the monkey. E.upl &air! Rex 14. 243-256. Bun K M. P.. KUFISI.EY L. W.. LAVAIL J. H. & RAKIC P. (1976) C‘ortlcal proJection to cervical and lumbar segment\ of spinal cord in the rhesus monkey using the retrograde transport method. N~~rosci. .‘lhr. 2, 23X BROWN L. T. (1971) Projections and terminations of the corticospinal tract in rodents. Evpl Brrrlrt Rc.s. 13, 432 450. Cot 1.1FK J. D.. Ewrxti L. & CARTER C. (1976) Origin of primar! \onsorimotor cortical projections to lumbar f the developing motor-sensor! cortex (MS( i. Vcuro.wi. 41~. I, 7X4. Dr. OLMOS J. S. (1977) An improved HRP method for the stud! of central ncr\l>us connections. Erpl Brcrijl Kc,\. 29, 541 -551. Dt:tmtx R. & S~SSLE B. J. (1971) Presynaptic excitability changes of primar) atl’erent and corticofugal libers projecting tc) trigeminal hrain stem nuclei. Espl Neural. 30. 223.~238. EI.GIK (‘. E., SPECKMANN E. J., CASPEKS H. & JANZF~NR. W. C. (1977) Corticospinal connections In the rat ~1. Monosynaptic and polysynaptic responses of cervical motoneurons to eplcortical stimulation. Evpl Brtrin Rrs. 28. 385 404. F~rz E. E. & BAKER M. M. (1969) Response properties of precentral neurons in awake monkeys. Ph~~,sio/o~qr.ri 12. 71.3. GOKIIOY G. & MII.LF:K R. (1969) Identification of cortical cells prolectmg to the dorsal column nuclei of the cat. Q. JI <‘up. Ph+o/. 54, 85 -9X. GKOOS W P.. EWING L. K., CAKTFK C. M. & COLLTEK J. D. (197X) Organiratton of corticospinal neurons !n the car. Bruin He. 143. 393~419. HALI R. D. & LINIIHOLM E. P. (1974) Organization of motor and \omatoaensor> neocortex in the albino rat. Brrk Rr\. 66. 23 38. H~KI)\I H. & HEIMEK L. (1977) A safer and more sensitive substitute for diammohenridine in the light microscopic demonstration of retrograde and anterograde axonal transport of HRP. Yruro\ci. Lrrl. 5, 235-240. HlChs S. P. & D’AMATO C. J. (1977) Locating corticospinal neurons bv retrograde transport of horseradish peroxldasc. E.~pl. Yrurr~l. 56, 410 420. JANZEN R. W. c’. SPECKMANN E. J., CASPEKS H. & ELGEK C. E. (19771 Corticospinal connections in the rat+ll. Oligosynaptic and polysynapttc responses of lumbar motoneurons to epicortical stimulation. Evpl Brain Res. 28, 405~420. JONES E. G. & HARTMAN B. K. (1978) Recent advances in neuroanatotnical methodology. -1. Rev. Nrurosci. 1, 215-296. JONES E. G. & WISE S. P. (1977) Size, laminar and columnar distribution of etTerent cells in the sensory-motor cortex of monkeys. .I. camp. IYrurol. 175, 391 438. Kt-EFFK D. A. (197X) Horseradish peroxidase as a retrogradely-transported detailed dendritic marker. Hruirl Rex. 140. 15 32. KIN<; J. L. (1910) The cortico-spinal tract of the rat. hat. Rec. 4, 235 252. LAVAIL J. H. i 1975) The retrograde transport method. fedn Proc. Fdn 4~. Sots cup. Biol. 34, 1618 1624. LAVAtL J. H.. WINSTON K. R. & TISH A. (1973) A method based on retrograde intra-axonal transport of protein for identification of cell bodies of origin of axons terminating within the CNS. Bruin Rex 58. 47&477. LF MON R. N., HENRY J. A. & PORTER R. (1976) Relationship between the activity of precentral neurones during active and passive movements rn conscious monkeys. Proc. R. Sm. B. 194. 341-373. MtrSnLAM M. M. & ROSEN~:D. L. (1977) Differential sensitivity between blue and brown reaction procedures for HRP neurohistochemistry. Nwrovci. Lett. 5, 7714. b'!tJKKAY E. 4 & (‘OIILTEH J. D. (1976) Origms of cortical projection\ to cervical and lumbar spinal cord in monke!. Ncwmc~i. .A~Y.2, 9 17.

MIIKRAI

E-. A

&

Cor~r.rrK

J. D. 11Y771 (‘oriico\plnal

proJections

lrom

the

mcdu~l

ccrchral

hcml\pherc

111 m~,ni\c\

.Vctr,o.scr. ./lb>. 3. 275. POHILK I.o!lti. SIIINOIIA

R. Kr SANIXRSON

J.

(19641Antidromic

H

corr~11

response

IO p>ramldal

tract

\t~mula~~on

111the ~-at. .I

/%~;I~II.

170. 355 370. Y.. ARUOLII

A.

I’. &

AsA~~~~IA

H. 11976) Spinal

branching

ol cortico\pinal

axon\

In

the

cat.

I\/:/

Hr~rr,~

Rr.v. 26. 2 I5 234. WI,LKI.K

(‘. (1971

) Microelectrode

rat. Br~lir~ Re.

(I Y76)

WI.I.LI.K

C‘.

WtI.LFK

C‘. &

delineation

of lint

grain

somatotoplc

orgamzatlon

of Sml cerehr;tl

neocortea

III aib~no

26. 259- 275. Receptive

SIKHA

M.

fields of barrels M.

in the somatosensory

(19721 Somatotupic

organization

neocortex of Smll

of the rat. .I. ~‘,,r,~p. :Vrurol.

cerebral

neocortcx

166.

173 190

in albino

rat.

Br~rr,t RC,\.

of the

ra(:

description

37, 132 136 WFLUR

C‘. 6t WOOLS~:\ T. A. (1974)

and comparison WEE

with

the mouse.

Structure

S. P. & JONES E. G. (1976) The

somatic

sensory

cortex

of layer

J. c’o,~p. h’eurol. organization



cortex.

J. cov~p. R’curol.

C‘. N. (1952) Patterns

7/1(, R;~/cu~L, o( Mrntui WOOI.SFV

C.

~iochcn~~c~tr/ WG+)LSI,I

WCN)LS~,~ (‘.

f.AlRhlAY

I

(eds.

MAKSHAU

W.

neocortes

postnatal

development

of the commissural

H. &

projection

of the

163. 3 13- 343.

and terminal in aenaury

Chap.

of somatic

D. (1946)

as indicated

somatosensor!

distribution

of descending

and motor

14. Hoeber.

sensory

and

area\

New

proJections

of the rat \omatic

motor

of the cerebral

Contralateral, neocortex

BARI)

by evoked

ipsilateral

and

areas

of the

bilateral

of pig, sheep and other

P. (1942)

Representation

potentials.

Johrls

cortex.

In :Mi/h~nk

S!‘~C~CGICI~I.

York. cerebral

HARLOW H. 6t Woo~.sn C. N.). pp. 63 Xl. University

and II of the cerebral

of the monkey

m the

129 15X.

trtrri Diwutr.

(195X) Organization

areas N..

175.

of localization

Hrtrlrl~

Brrsc~,~ of Behior

c‘. N. &

in somatic cortex

N.

and

of the rat. .I. camp. IV~rol.

Wlsl, S. P. & JONES E. G. (1977) Cells of origin

IV

158, 437 -454.

representation mammals.

of cutaneous

Hopkin\

cortex.

of Wisconsin

Bioloyicd

receptor\

19, 6X4- 702.

sensibilit!

tlosp. Bull. 70, 799-441.

urd

Press, Madison.

of cutaneous

Surgery

tactile

In

in the

cerebral