THE JOURNAL OF COMPARATIVE NEUROLOGY 306:24-38 (1991)

Somatosensory Cortex of the Neonatal Pig: I. Topographic Organization of the Primary Somatosensory Cortex (SI) SANDRA L. CRANER AND RICHARD H. RAY Department of Physiology, East Carolina University, Greenville, North Carolina 27858-4354

ABSTRACT The cerebral cortex of adult mammals contains several somatotopic representations of the body surface. Although the organization of the various somatosensory cortices of numerous species of adult animals has been elucidated, data on the somatosensory representations of fetal and neonatal animals are limited. As part of an investigation into the perinatal development of the somatosensory cortices, it was necessary to delineate the organization of the somatosensory cortices of the perinatal pig. This study presents the topographical organization of the primary somatosensory cortex (SI)of the perinatal pig. Multiunit microelectrode mapping methods were used to produce topographic maps of SI from barbiturate anesthetized pigs ranging in age from 7 days preterm to 2 months postpartum. It was demonstrated that the overall organization of this region of cortex was similar to that of other mammals: a somatotopic projection of predominantly the contralateral body surface was delineated in which the hindlimb is represented medially and the face laterally across the cortex. A disproportionately enlarged rostrum representation was mapped in detail, and multiple representations of the rostrum, face, and mouth were found. Several of these representations exhibited bilateral and ipsilateral input. The SI trunk and hindlimb representations were located on the medial wall of the hemisphere; these representations were small but their presence refutes speculation that ungulates do not have a complete body representation in SI. Key words: cortical mapping, somatic sensory, localization of function, multiunit recording, ungulate

The mammalian brain has been shown to contain multiple somatosensory representations. For all species examined, these areas include at least the primary somatosensory area (SI) and the secondary somatosensory area (SII). Additional somatosensory representations have been described in some; the cat reportedly has at least four somatosensory areas (Marshall et al., ’41; Tasker, ’60; Clemo and Stein, ’82, ’83) and a “supplementary sensory area” has been described in primates (Penfield and Jasper, ’54). “SI” has been traditionally described in primates as a single and continuous representation of the body surface spanning four cytoarchitecturally distinct regions, areas 3a, 3b, 1, and 2 (see Merzenich et al., ’81).More recent studies, however, have determined that these cytoarchitectural areas in primates contain two complete (or almost complete) cutaneous representations of the body surface (areas 3b and 1)and two representations of deep body structures (areas 3a and 2). In contrast to the primate SI, only one cutaneous body representation is contained in areas 3b, 1,and 2 of the cat, whereas a “deep body” representation is present in area 3a 0 1991

WILEY-LISS, INC.

(Felleman et al., ’83).The SI region of rodents (see Sur et al., ’78 for review), opossum (Pubols et al., ’761, and tree shrews (Weller et al., ’79;Sur et al., ’80)also contains only a single cutaneous representation, and a 3a “deep body” field may be present in both galagos and tree shrews (see Kaas et al., ’81 for review). The evidence now indicates that the single cutaneous representation in cats, rodents, tree shrews, and opossum is homologous to the area 3b representation of monkeys, whereas the 3a “deep body” representation of these animals is homologous to the 3a representation of monkeys (Kaas et al., ’81). The present study and its companion work (Craner and Ray, ’91) are part of an investigation into the perinatal development of the SI and SII areas. With the notable exception of the whisker “barrel” cortex of rodents, developmental studies of somatic receiving areas have been conspicAccepted December 19,1990, Address reprint requests to Dr. Sandra L. Craner, who is now at the Dept. of Neurohiology, Anatomy and Cell Science, School of Medicine, University of Pittsburgh, Pittsburgh, PA 15261.

PRIMARY SOMATOSENSORY CORTEX OF THE PIGLET uously few and little information is available concerning the development of the peripheral representation as a whole. No study to date has compared the perinatal development of multiple somatic representations. The domestic pig has been widely used in cardiovascular and nutritional studies because of its remarkable physiological similarities to humans (see Dodds, '82). Compared with the young of the dog, cat, goat, and sheep, the neonatal pig appears in many aspects to be most like the human newborn infant. The development of the fetal pig shows similarities to that of the human fetus (Patten, '64): the two are basically alike at corresponding developmental stages ( k e y , '66). The pig, therefore, has become the standard experimental model for mammalian embryology. Despite its suitability for such developmental studies, the pig has not been extensively used in neurological studies. In early mapping studies of the pig using evoked potentials, the loci of the SI representation for the snout and forelimb areas (Adrian, '43; Woolsey and Fairman, '46) and the loci of the SII representation for the face, forelimb, and hindlimb areas (Woolsey and Fairman, '46) were determined. However, a detailed map of the somatosensory cortices of the pig was never produced. This study presents the topographical organization of the primary somatosensory cortex (SI) of the piglet, and the companion work presents the map of the secondary somatosensory cortex (SII).

25

leakproof chamber. The chamber was then filled with warm mineral oil (38"C), keeping the cortical surface warm and preventing dehydration. After reflecting the dura, the exposed cortex was photographed so that the locations of all recording sites could be marked upon photographic enlargements of the cortical surface. In order to record from the medial wall of the left cerebral hemisphere, the superior sagittal sinus was ligated anteriorly, centrally, and posteriorly in a manner that isolated blood vessels entering the sinus. The median wall was then exposed by removing a portion of the right hemisphere by aspiration. The sinus was cauterized between the ligatures and carefully moved to allow complete examination of the superior and mesial portions of SI. To record from cortex within sulci, the electrode was either driven in at an angle or a portion of the adjacent gyrus was removed by suction, permitting direct access to the desired area. All experiments were terminated by the delivery of an overdose of sodium pentobarbital followed by intracardiac perfusion, first with a 0.9%saline solution, followed by a 10% formal-saline solution. The brain was removed and stored in 10%formal-saline for later histological examination.

Microelectrode mapping

Electronics. Tungsten microelectrodes with characteristics favoring extracellular recording from small clusters of neurons and single neurons were employed (12 megOhm). METHODS These electrodes were either obtained from A-M Systems, General procedures Inc. or were prepared in our laboratory from electrolytically A total of 46 domestic Yorkshire pigs, ranging in age from sharpened tungsten stock insulated with epoxy resin. The microelectrode was held in a micromanipulator 7 days preterm (normal gestational age = 114 days) to 2 months postpartum, were obtained locally.' Animals were driven by a hydraulic microdrive. The amplified neural used experimentally the day they were obtained and were activity (Grass P511) was fed through a window discriminaplaced into the following age groups: (1) 7 days premature tor (WPI) and displayed on a Tektronix oscilloscope. The (-7 days), (2) birth to 1day, (3) 3 to 4 days, (4) 6 to 7 days, neural activity was made audible through a Grass AM5 audio monitor. (5) 14 to 15 days, (6) 18 to 21 days, and (7) 2 months. Mapping procedure and analysis. Under microscopic Preterm animals were obtained by inducing premature labor (at gestational age of 106 days) in sows with an observation, the microelectrode was lowered normal to the intramuscular injection of the prostaglandin LutalyseTM pial surface and the location of the electrode was marked on an enlarged photograph of the exposed brain. The electrode (3-7 ml, depending on the weight of the sow). Acute, anesthetized preparations were used for all neuro- was slowly driven through the cortex with the hydraulic physiological studies. General anesthesia was induced by an microdrive while the body was continuously stimulated by intraperitoneal injection of sodium pentobarbital (Nembu- hand. When a driveable cortical response was detected, the tal) (30-35 mg/kg). Atropine (0.2 m a g ) was injected depth of the electrode tip in the cortex was recorded and, using a small wooden probe or Semmes-Weinstein filaintramuscularly to prevent excessive salivation. The trachea was cannulated and respiration maintained ments, the receptive field of the cell or cluster of cells was with a positive-pressure pump. The left femoral artery was delineated. The receptive field was then drawn on a sketch cannulated to allow direct recording of arterial pressure of the pig's body and the neural response characteristics, and the left femoral vein was cannulated for supplementa- including threshold of response (low, medium, high), submotion of anesthesia as necessary to maintain areflexia and for dality (hair, cutaneous, joint, deep, claw), and laterality of the administration of fluids (5% dextrose in saline). Body response (bilateral, contralateral, or ipsilateral), were retemperature was monitored rectally throughout the experi- corded. ment and maintained at 38-40"C with a water circulating As the electrode was driven through the cortex, any noticeable change in the location of the receptive field was heating pad. The head was immobilized in a stereotaxic unit (David regarded as a new responding locus, and the process of Kopf Instruments) and a craniotomy performed, exposing receptive field delineation was repeated. the somatosensory cortices (SI and SII) of the left hemiSuccessive electrode penetrations were made perpendicusphere. The exposed edges of bone were sealed with bone lar to the surface 0.5 mm to 1 mm apart, or as close as the wax. Small strips of gauze soaked in a 3% agar-saline vascular pattern would permit. In portions of cortex where solution were placed around the skull opening, forming a much detail was desired, as many as 10 penetrations were made per mm2 of surface. Boundaries of SI were determined by penetrations that were unresponsive to light cutaneous stimuli. Two or more consecutive unresponsive 'G and P Pork Producers, Blackjack, NC, Charles Gaskins, proprietor

S.L. CRANER AND R.H. RAY

26 sites were determined at the boundaries of SI for each row of recording sites. Each body region was mapped a number of times in detail in different animals. The map of the body parts on the cortical surface was determined by relating receptive fields to recording sites and depths of electrode penetations. The body subdivisions of SI were drawn on enlarged photographs of the brain. Composite body maps were created by overlaying the results of all the experiments and outlining the corresponding body subdivisions.

The portion of the cruciate gyrus extending down into the longitudinal fissure and along the mesial aspect of the hemisphere is the median wall region (MW). This area of the cortex receives somatosensory information from the contralateral hindlimb, trunk, and part of the forelimb. The cortical areas surrounding SI were carefully examined and, other than the SIT cortical region, found to be unresponsive to cutaneous stimulation. The auditory area of the pig was not delineated.

RESULTS

Some body subdivisions of SI occupy a disproportionately large area of the cortex, presumably reflecting behavioral adaptations. Accordingly, the rostrum (rostral snout) of the pig occupies a disproportionately large cortical area. Representative penetration points and receptive fields of a typical experimental animal are shown in Figure 2. The neuronal receptive fields illustrated in Figure 2B were found at the penetration sites similarly numbered in Figure 2A. As shown, an orderly progression of receptive fields around the snout of the pig corresponds to an orderly somatotopic progression of recording sites around the cortical rostrum region. The anterior portion of the gyrus (pts. 9, 10, 1) receives contralateral afferent information from the lower rostrum, and the dorsomedial aspect of the gyrus (pts. 2, 3, 4)contains the representation for the lateral region of the rostrum. The posterior end of the rostrum gyrus (pts. 5, 6) receives sensory information from the upper rostrum, and the ventrolateral portion of the gyrus (pts. 7, 8 ) demonstrates receptive fields along the midline of the rostrum. These data reflect the same general pattern of representation found in the evoked potential studies of mature animals conducted by Woolsey and Fairman (’46). The neuronal receptive fields in the rostrum region have several characteristics: they are well delineated, mostly of a cutaneous nature (only 7% hair), and primarily are contralateral (92%).As can be seen, large areas of the cortex receive afferent information from the same surface area of the rostrum. Of 356 receptive fields (362 penetrations in 18 animals) localized in the rostrum region, the only consistent bilateral receptive fields were those that extended across either the top or the bottom of the nose. Few bilateral or ipsilateral RFs were found, primarily on the lateral edges of the nose. However, a small number of receptive fields of the lips and teeth were also found in this cortical region, and they were often bilateral. A second complete representation of the rostrum (R,) was found in the cruciate gyrus (Fig. 3). The lower snout (LS) and upper snout (US) boundaries, as shown in Figure 3A, were determined by making a composite of this region from all animals studied. A lengthwise region in the center of the gyrus receives sensory information from the upper rostrum. Anterior, medial, and lateral to this region is an area of cortex devoted to the lower nose. Figure 3 also illustrates representative penetration points and receptive fields from one typical experimental animal. The neuronal receptive fields illustrated in Figure 3B, C, D were found at the corresponding penetration sites numbered in Figure 3A. Of 49 receptive fields delineated on the rostrum, 72% were contralateral, 26%were bilateral, and 2%were ipsilateral. The receptive fields in the cruciate gyrus exhibited a much greater degree of bilaterality than those found in the rostrum gyms (see Table 1). Due to the presence of a complete rostrum representation and a higher degree of

A total of 2,071 electrode penetrations were made in the SI region of 46 domestic pigs. From these penetrations, 1,593receptive fields (either of single units or small clusters of neurons) were isolated. As a result, details of the SI representation of the body surface of the neonatal domestic pig were revealed. This SI representation has both specialized characteristics and characteristics common to the primary somatosensory cortices of other mammals. The overall organization of SI is similar to that of other mammals in that it contains a complete and somatotopically organized projection from the body surface. The SI representation of the body is distorted with a disproportionately enlarged representation of the rostrum (rostral snout). The receptive fields are primarily contralateral, although ipsilateral and bilateral receptive fields are found within the face representations. SI of the domestic pig, as mapped in this study, also exhibits several specialized features. Two separate representations of the rostrum and two mirror-image representations of the face and mouth exist. Unlike the lip representation in most other ungulates (Adrian, ’43; Johnson et al., ’74),the SI lip representation of the pig receives primarily contralateral sensory afferents.

Location and topographical organization of SI The overall features of the representation of the body parts in SI of the pig are shown schematically in Figure 1. SI of the neonatal pig brain occupies a large proportion of the hemisphere. To simplify the description of the various areas of SI, this somatosensory representation is divided into four cortical regions: (1) “rostrum” region (RR), (2) ipsilateral face region (FJ, (3) the cruciate gyrus (Cru), and, (4) median wall (MW). The oblong gyrus located lateral to the coronal sulcus, medial to the middle and rostral suprasylvian sulci, and including the coronal gyrus receives information from the contralateral rostrum and is referred to as the “rostrum” region (RR) (Fig. 1). Surrounding the anterior pole of the rostrum region and lateral to the cruciate sulcus is the ipsilateral face region (F,) as first described by Woolsey and Fairman (’46). This is a large area of cortex that receives contralateral, ipsilateral and bilateral sensory information from the face. The cruciate gyrus (Cru) extends rostrally and medially between the coronal and cruciate sulci (Fig. 1). The lateral crest of the cruciate gyrus includes the forelimb representation of SI. This area receives afferent input from the shoulder, forelimb, and front hoof. An area of cortex along the rostral and medial edges of the coronal sulcus receives input from the chin and face of the animal. In addition, the anterior portion of the cruciate gyrus receives afferent information from the mouth.

SI representation of the rostrum

PRIMARY SOMATOSENSORY CORTEX OF THE PIGLET

27

Fig. 1. Anatomy of the left cerebral hemisphere of the pig brain. The mesial aspect is illustrated as if it were reflected in a mirror. 1 = sigmoid gyrus, 2 = cruciate sulcus, 3 = coronal sulcus, 4 = longitudinal fissure, 5 = coronal gyrus, 6 = rostral suprasylvian sulcus, 7 = middle suprasylvian sulcus, 8 = caudal suprasylvian sulcus, Cru = cruciate gyrus, RR = rostrum region, F, = ipsilateral face region, and MW = median wall. TABLE 1. Summary of Lateralities of Various Regions of the SI Cortex of

the Neonatal Pig Cortical representation

%

%

%

Con tralateral

Bilateral

Ipsiiateral

RR Forelimb Hindllmb F, MI

92 94 98 65 26

6 6 32

2 2

61

13

R2 F2 M*

72 -

38 96

Upper lip Lower lip Tongue

80 46 12

26 28 4 14

34

2

27 16

2

7 21 72

bilaterality than that seen in the rostrum region, the cruciate gyrus rostrum representation is believed to be a duplicate representation, separate from the rostrum region.

SI representation of the forelimb The SI forelimb representation of the domestic pig was found in the lateral cruciate g y m s and the median wall of

the cruciate gyrus, as illustrated in Figure 4A. As shown, the forehoof, foreleg, and shoulder have separate areas of representation, although there is considerable overlap of these areas. Extending onto the median wall, the forehoof region is located along the caudal bank of the cruciate sulcus. The forelimb representation is located caudal to the forehoof region and rostral to the coronal sulcus. This representation also extends onto the median wall. In contrast, the representation for the shoulder is located primarily on the lateral surface of the hemisphere. Representative penetration points and receptive fields of one typical experimental animal are shown in Figure 4.The neuronal receptive fields illustrated in Figure 4B, C, D were found at the corresponding penetration sites numbered in Figure 4A. As can be seen by comparing the penetration sites with the corresponding receptive fields, receptive fields on the more distal portions of the forehoof and forelimb are represented “deeper” on the median wall. Receptive fields located more proximally on the front limb are found on the lateral surface of the hemisphere.

28

Fig. 2. Somatotopic representation of the SI rostrum region (RR). A. Location of representative penetration sites in an experimental animal. B. Receptive fields on the rostrum of the pig. These receptive fields were found at the correspondingpenetration sites numbered in A. Note that an orderly progression of receptive fields around the snout of

S.L. CRANER AND R.H. RAY

the pig corresponds to an orderly somatotopic progression of recording sites around the cortical rostrum region. Penetration site 10 was located in a second animal; however, receptive field 10 is representative of receptive fields found in all animals at this site.

29 .....

Fig. 3. Somatotopic representation of the cruciate gyrus rostrum region (RJ. A. The large box contains an enlargement of the cortical area outlined in the inset. Numbers indicate the locations of representative penetration sites in an experimental animal. The dark boundary outlines the location of the R, region. LS = lower snout representation, US = upper snout representation. B. Receptive fields found on the

snout and upper and lower mouth of the animal. C. Receptive fields found on the contralateral and ipsilateral face and neck. D. Receptive field delineated on the tongue. Receptive fields in B, C , and D were found at the corresponding penetration sites in A. Receptive fields alphabetically labeled (i.e. 9a, 9b, 9c) were found at progressively greater depths in the penetration site.

S.L. CRANER AND R.H. RAY

30

D

C D4

Fig. 4. Somatotopic representation of the SI forelimb region. A. chin. C. Receptive fields found on ventral forehoof. D1 = digit 1,D2 = Boxed area of cortex is enlarged in adjacent drawing. The enlargement digit 2, D3 = digit 3, D4 = digit 4. D. Receptive field on ventral illustrates composite representations of forehoof (FH), forelimb (FL), shoulder. Receptive fields in B, C, and D were found at the correspondinpnpnotmtinn e i t e c in A and shoulder (S), as well as representative penetration sites from an ~~. “---_I.l-yl.-I._ experimental animal. B. Receptive fields foGnd on the forelimb and ~~

...D~---l_--

PRIMARY SOMATOSENSORY CORTEX OF THE PIGLET Unlike the digit representation in some animals, a distinct progression of digit representations is not apparent in the pig. However, there is a tendency for the D1 representation to be located more rostrally and laterally on the cruciate gyrus, whereas D4 is located more caudally and medially. The D2 and D3 representation is generally located in the middle of the forehoof representation. The dorsal aspect of the forehoof is represented more laterally on the cruciate gyrus, whereas the ventral forehoof is represented more medially, especially down the median wall of the gyrus. Most digit receptive fields are located on both D2 and D3, whereas few receptive fields are confined to either D1 or D4. Receptive fields on the ventral pads of the forelimb often extend entirely over one pad and sometimes over two or more pads. Most receptive fields on pads are on D2 or D3; few are confined to the pad of either D1 or D4. Fewer receptive fields were delineated on the ventral aspects of the shoulder and forelimb, making it difficult to determine this representation. However, there appears to be a tendency for the ventral aspects to be located more caudally on the cmciate gyrus along the coronal sulcus. Of 583 receptive fields recorded in the forelimb representation (550 penetrations in 27 animals), 88%were cutaneous, 6% were claw receptors, and 4% were hair receptors. Less than 2% of the receptive fields characterized were deep or joint receptors; 94% of the responses were contralateral and 6% were bilateral.

31

fields on the chin were found along the medial bank of the coronal sulcus connecting the two cortical face representations, and the greatest density of receptive fields in the two face representations was located on the chin. The characteristics of these receptive fields suggest that afferent information to the cortex changes caudally to rostrally in the cruciate gyrus. Caudally (i.e. the F, face representation) afferent information is represented contralaterally. Progressing rostrally (ie. the cruciate gyrus MI mouth representation), sensory information becomes more bilateral in nature, and at the most rostral portion of SI (ie. the F, ipsilateral face region) information is received primarily from the ipsilateral side of the body. A large area of cortex is devoted to the upper and lower lip representation and the tongue representation (Fig. 6). In contrast to the mirror-representations of the mouth and face, the lips and tongue have only a single representation located in the rostral end of the cruciate gyms and in the ipsilateral face region. A great deal of overlap occurs among the lower lip and upper lip representations. However, the upper lip representation tends to be located slightly more caudolateral than that of the lower lip. A very few receptive fields on the lower and upper lips were scattered more posteriorly in the cruciate gyrus. In contrast to the lip representations of sheep and goats (Adrian, '43; Johnson et al., '74), both the upper and lower lip representations of pigs were predominantly contralatera1 in nature. In the upper lip representation, 80% of the somatosensory responses were contralateral, 7% were ipsilateral, and 14%were bilateral. In the lower lip representaSI representation of the face and mouth tion, 46% of the receptive fields were contralateral, 27% A large area of cortex anterior and medial to the rostrum were ipsilateral, and 27% were bilateral. Even though these region (RR), including the cruciate gyrus, receives both representations primarily received contralateral sensory contralateral and ipsilateral afferent information from the information, a high percentage of ipsilateral and bilateral face and mouth. Figure 5 is a composite of receptive fields input was received. delineated in 40 animals. As seen in Figure 5 , distinct The cortical representation of the tongue is located representations of the face and mouth exist on either side of anterolaterally in the ipsilateral face region (Fig. 6). As the cruciate gyms representation of the rostrum (RJ. The determined in other ungulates, the cortical tongue represenface subregions receive projections from the chin, cheek, tation receives afferent information primarily from the jaw, lateral snout, and the area around the eye. Included in ipsilateral side of the body (72%); 16% of the tongue the mouth representations are receptive fields from the receptive fields were bilateral and 12%were contralateral. hard palate, gums, incisors and canines, and the floor of the The representations of the neck and chest of the animal mouth. No projections from the molar teeth to the cortex are located in approximately the same cortical area as the were found; however, this may reflect the difficulty in representations for the shoulder and face (Figs. 4, 5 ) . In stimulating the posterior oral cavity. many instances receptive fields on the neck extended either Both anterior and posterior to R, are two distinct cortical onto the shoulder, the chest, or the face. representations of both the mouth and face. The two When comparing Figures 3,4,5, and 6, it can be seen that mirror-representations of the mouth have distinct charac- a great deal of overlap of cortical representations occurs in teristics. The representation located caudally in the cruci- the cruciate gyrus and the ipsilateral face regions of SI. ate gyrus (MI) is predominantly bilateral in nature (61% It should be noted that the scalp of the animal could not bilateral, 26%contralateral, and 13%ipsilateral). In compar- be stimulated due to the craniotomy. Thus no scalp repreison, the mouth representation located anteriorly (M,) in sentation was determined. the ipsilateral face region receives predominantly ipsilateral(96%ipsilateral, 4% bilateral) sensory information. SI representation of the trunk and hindlimb One of the mirror-image face representations is located As hypothesized by Woolsey and Fairman ('46), the SI on the crest of the posterior cruciate gyrus along the banks of the coronal sulcus. The coronal sulcus was carefully hindlimb representation of the domestic pig is located deep explored, and it was determined that the chin, cheek, and within the longitudinal fissure on the median wall. Due to face representations dip into the lateral branch of the its small area and its location, the hindlimb representation sulcus, but not into the caudal branch. This face representa- was very difficult to locate. The median walls of many tion (F,) receives primarily contralateral input (65% con- animals were mapped before this representation was found. tralateral, 2% ipsilateral, 32% bilateral). The second face However, once the existence and location of this representarepresentation (F,) is located anteriorly in the ipsilateral tion was determined, the area was delineated in all further face region F,. In contrast to F,, F, is primarily ipsilateral in experiments. In two animals the location of the hindlimb nature (38%ipsilateral, 34% contralateral, and 28% bilat- region was confirmed by penetrating the electrode deeply eral). Two observations should be noted: a few receptive through the caudal branch of the coronal sulcus, thus

S.L. CRANER AND R.H. RAY

32

B 11

C

Fig. 5. Somatotopic representation of the SI face and mouth regions (F, and M, and F, and MJ. A. Illustration of rostral hemisphere. The anterior boundary encircles the ipsilateral face area (FJ. The unshaded area within F, is the ipsilateral mouth region (MJ. The posterior boundary encircles the F, face area. The unshaded area within F, is the M, mouth region. The numbers indicate locations of representative penetration sites in two animals. The F, and M, regions were mapped in one animal and the F, and M, regions were mapped in another. B.

Receptive fields found on the snout and upper and lower mouth. C . Receptive fields delineated on the tongue. D and E. Receptive fields delineated on the contralateral and ipsilateral sides of the body. F. Receptive fields found on the snout and upper mouth. G. Receptive field delineated on the ventral forehoof. The receptive fields were found at the corresponding penetration sites in A. Receptive fields alphabetically labeled (e.g. 16a, 16b) were found at progressively greater depths in the penetration site.

D3

D2

G D4

Figure 5D-G

D1

S.L. CRANER AND R.H. RAY

34

A

D

Fig. 6. Somatotopic representation of the lips and tongue. A. The large box contains an enlargement of the cortical area outlined in the inset. Numbers indicate the locations of representative penetration sites in an experimental animal. Solid line = lower lip, dashed line = upper lip, dotted line = tongue. These outlined areas are composites

from 18 animals. B. Receptive fields delineated on snout, lips and mouth. C.Receptive fields found on the contralateral chin and ipsilateral jowls. D. Receptive fields delineated on tongue. Receptive fields in B, C, and D were found at the corresponding penetration sites in A.

PRIMARY SOMATOSENSORY CORTEX OF THE PIGLET

35

B

Fig. 7. Somatotopic representation of the SI hindlimb. A. Boxed area of cortex on mesial aspect is enlarged in subsequent box. Numbers indicate the locations of representative penetration sites in an experi-

mental animal. Dashed line = hindhoof, solid line = hindlimb, dotted line = hip. B. Receptive fields delineated on hindlimb. The receptive fields were found at the corresponding penetration sites in A.

positioning the electrode tip at the median surface of the cruciate gyrus. Ninety-one receptive fields were found on the hindlimbs of 10 animals. The size of the representation was highly variable: one animal exhibited 24 receptive fields; in the others as few as one and as many as 16 receptive fields were delineated. Only two receptive fields were bilateral, the remaining contralateral. Most receptive fields were cutaneous in nature, although some deep receptor input was found, especially in the thigh and hip of the pig. A few claw and hair receptive fields were also present. Receptive fields on the distal portions of the hindlimb (i.e. the digits, hoof, sole, and ankle) tended to have much lower thresholds than did receptive fields on the more proximal thigh and hip.

Representative penetration points and receptive fields of one typical experimental animal are shown in Figure 7. The neuronal receptive fields illustrated in Figure 7B were found at the corresponding penetration sites numbered in Figure 7A. The encircled areas of Figure 7 represent a composite of receptive fields found in 10 animals. As shown, the hindhoof representation is generally located rostrally, whereas the more proximal hindlimb and hip are represented more caudally. However, the distal to proximal progression of receptive fields is not as well delineated in the hindlimb as in the forelimb. The trunk representation of SI is very small or possibly nonexistent in neonatal piglets. Of 16 animals examined, only nine receptive fields were found in four animals. These

36

S.L. CRANER AND R.H. RAY

Another prominent feature of the pig cortex is the receptive fields were on the animal’s side or abdomen and responded to either cutaneous stimuli or movement of hair. apparent presence of duplicate representations of body Two of the receptive fields were bilateral, the remaining parts in SI. A very large area of the cortex is devoted to two contralateral. In general, the cortical trunk representation cortical representations of the rostrum-one representawas located on the caudal edge of the hindlimb representa- tion in the rostrum region (RR) and one in the cruciate gyrus (RJ. Mirror-image representations of the face and tion. Occasionally receptive fields on the shoulder were found mouth (F, and M,, and F, and M,) are also present. Double representations in SI are not unique to the pig. to extend onto the pig’s back, and receptive fields on the forelimb extended onto the sides or chest of the animal. Complete double representations of the body are present in These receptive fields were not tabulated as part of the SI of several species of Old World and New World monkeys trunk representation since the major portion of each of (Merzenich et al., ’78; Kaas et al., ’79). Double representathese receptive fields was on an extremity. Also, the ventral tions of body parts have been reported for the squirrel trunk of the animal could not be easily stimulated due to monkey (Zimmerman, ’68), the macaque monkey (Paul et the positioning of the animal, therefore receptive fields on al., ’72), and the slow loris (Krishnamurti et al., ’76). Sur this region of the body surface may not have been detected. and coworkers (’78)have documented a dual representation The hindlimb and trunk representations vary greatly of the hand and forearm in the gray squirrel, with a single from one animal to another. Some piglets exhibited compar- representation of the rest of the body. Mirror-image repreatively large representations of the trunk and hindlimb sentations of the face and dual representations of the directly bordered by the forelimb area, and the neurons forelimb have also been delineated in the ferret (Leclerc et responded to light cutaneous and hair stimulation. How- al., ’87). Even though the SI cortex of the cat has been ever, other piglets had very small hindlimb and trunk studied for over 40 years, its organization is still argued. regions, occasionally surrounded by an inactive cortical Some investigators have reported double representations of region, and the neurons had relatively high thresholds. It body parts (Iwamura and Tanaka, ’78a,b), and complete should be noted that the great deal of variation detected in double body representations in area 3b (Dykes et al., ’80). the hindlimb and trunk representations may have partially Others insist that only a single representation of the body been a result of damage done to the cortex by removal of the surface exists (Felleman et al., ’83) and that the dual superior sagittal sinus and tissue of the right cerebral representations reported may be part of SIII, another hemisphere. However, based on direct microscope observa- somatosensory region (Iwamura and Tanaka, ’78; Felleman tions, the cortical blood flow and color appeared normal in et al., ’83). all such experiments. This confusion is a result of the ambiguous “definitions” of the various somatosensory cortices, the experimental DISCUSSION techniques employed, and individual interpretations of In early mapping studies using evoked potentials, Adrian data. The various somatosensory regions are inconsistently (’43) roughly outlined the somatic, auditory, and visual defined. Electrophysiological mapping, cytoarchitectural cortices of the goat and pig. He localized the SI representa- delineations, and thalamic connectivity have all been used tion for the snout of the pig, but other body regions were as primary criteria for determination of field boundaries. No two methods delineate identical areas, thereby leading not mapped. Woolsey and Fairman (’461, using evoked potentials, to confusion when interpreting data. The present study delineated the loci for the SI snout and forelimb areas, and relies solely upon electrophysiological responses to periphthe loci of the face, forelimb, and hindlimb regions in SII. eral stimulation for a definition of primary somatosensory An ipsilateral face area was also tentatively outlined ros- cortex. It is conceivable that future cytoarchitectural studtrally. No other body regions were delineated; SI hindlimb ies with older animals could result in the division of the and trunk subdivisions were not found. It was suggested representation into separate fields. The duplicate representations of the rostrum and the that the leg area of SI might be found on the mesial aspect face and mouth of the pig show specializations that may be of the hemisphere. The present study provides a detailed micromap of SI of indicative of their individual functions. Large amounts of the pig (see Fig. 8). The map reveals both specialized and cortex in RR receive afferent information from essentially generalized features of the primary somatosensory area. the same contralateral receptive fields and the topographic The overall organization of SI is similar to that of other progression of receptive fields is highly organized. R,, in mammals with a complete and somatotopically organized contrast, is not as well organized topographically and projection from the body surface progressing from the receives a significant proportion of bilateral afferent inforhindlimb to the head with medial to lateral locations in the mation. The duplicate representations of the face and mouth are very similar in organization, varying primarily cortex. A common feature of cortical organization of the mamma- in the laterality of their input. F, and MI receive primarily lian SI area is that the representation of body surfaces most contralateral input, whereas F, and M, receive primarily important for the survival of the species have the largest ipsilateral information. Within the cruciate gyrus of the piglet, the laterality of cortical representations. Thus the SI representation of the body is distorted, due to the behavioral adaptation of the receptive fields changes considerably. Progressing caudally species. For example, raccoons have a very large forepaw to rostrally, receptive fields generally change from contralatrepresentation (Welker and Seidenstein, ’59),sheep, goats, eral to bilateral to ipsilateral in nature. Similarly, ipsilatand llamas have enlarged representations of the lips (Adrian, eral connections are located rostrally in the hemisphere of ’43; Johnson et al.,’74; Welker et al., ’761, and rats exhibit various other species. In llamas the ipsilateral perioral large representations of the mystacial vibrissae (Welker, representation is situated rostra1 to the contralateral lip ’71). In agreement with this concept, the domestic pig, a region (Welker et al., ’76) and ipsilateral face regions of “rooting” animal, has a very large cortical representation sheep, goats, cats, and dogs (Adrian, ’43; Woolsey and of the rostrum. Fairman, ’46) are located rostrally in the hemisphere.

PRIMARY SOMATOSENSORY CORTEX OF THE PIGLET

37

/ Fig. 8. Summary map of the SI somatotopic representation of the neonatal pig. FB = forelimb region, HB = hindlimb region, F, and M, = contralateral face and mouth area, R, = cruciate g y r u s rostrum region,

F,and M, = ipsilateral face and mouth area, LR = lower rostrum, TR =

One exception to this progression of connections in the cruciate gyrus of the pig is the lip representation. This single, rostrally located lip representation is primarily contralateral in nature. In contrast, receptive fields from other body surfaces delineated in this same cortical region are more bilateral or ipsilateral in nature. The contralateral lip representation of the pig is also exceptional in that it differs from the lip representation found in some other ungulates. In goats (Adrian, '43) and sheep (Johnson et al., '741, the lip representations are primarily ipsilateral. However, similar to the pig, the lip representation of the llama is primarily contralateral (Welker et al., '76). Although the presence of rostral ipsilateral face areas have been noted in some species, the differences in the lateralities of receptive fields in different regions of SI and in duplicate SI body representations have not been extensively studied in other mammals. Perhaps lateralities of connections in SI have not been systematically studied because SI has classically been thought to receive primarily contralateral input. However, a significantly large area of the pig neonatal S1 cortex, as we define it, receives ipsilatera1 input. This observation extends across all age groups

included in this study, but the possibility remains that the activity evoked ipsilaterally could become less apparent in mature animals. Upon examination of the sheep cortex in 1974, Johnson et al., determined that cortical representations of the body and limbs in the sheep SI were apparently absent. It was speculated (Johnson et al., '74) that the apparent absence of SI trunk and hindlimb representations in sheep (Johnson et al., '741, llamas (Welker et al., '761, andpigs (Woolsey and Fairman, '46) might be typical of ungulates and might suggest that true functional differences between SI and SII exist. The present study, however, clearly delineates a SI hindlimb and trunk representation in pigs. In the study of sheep, Johnson et al., ('74) apparently made limited numbers of electrode penetrations in four animals and found responses at only three loci, The responses were to deep pressure upon the joints or muscles of the hindlimb. Similarly, only a few penetrations were made in the probable hindlimb region of llamas (Welker et al., '74). If a more concentrated mapping had been made, it is anticipated that the SI trunk representation of sheep and llamas would have been better delineated.

lateral rostrum, UR = upper rostrum, and MR = midline of rostrum. LR, TR, UR, and MR are subdivisions of the rostrum region (RR).

S.L. CRANER AND R.H. RAY

38 Since cytoarchitectural studies have not yet been conducted in mature pigs, there was some concern that the hindlimb representation delineated on the median wall in our study may actually have been a portion of a supplementary sensory area, similar to that of monkeys, rather than a part of SI. However, unlike neurons in the primate supplementary sensory area, no neurons in this region responded to stimulation of the head or neck. Neurons were exclusively responsive to the hindlimb and or trunk, thus supporting our belief that this region was part of SI proper. In a study of the motor cortex of the domestic pig by Breazile and coworkers (’66), the motor cortex of the pig was found to be quite similar to that of the sheep and goat. In addition, their findings indicate that the motor cortex of the pig is adjacent to the electrophysiologically defined somatosensory representation of SI. The pig, similar to other nonprimate mammals, does not have a sulcal delineation between the sensory and motor representations. Although Breazile and coworkers used different anatomical terminology to describe the motor cortex, their text and illustrations indicate that the rear limb and forelimb motor representations are adjacent to the corresponding somatosensory representations. The motor forelimb region is located on the lateral wall and base of the cruciate sulcus without extending onto the crest of the cruciate gyrus. In contrast, the somatosensory forelimb region is located primarily on the crest of the gyrus and does not extend medially down the cruciate sulcus. From Breazile’s description, the hindlimb motor region also appears to be located on the median wall anterior to the somatosensory region. The rostral to caudal sequence of body regions in the motor cortex also corresponds to the sequence in SI. The motor lip representation is rostral to the motor forelimb representation, as it is in the somatosensory cortex. However, the motor lip representations do not extend as far anteriorly as the somatosensory lip representations and are located in the same cortical area as the somatosensory mouth area. An interesting aspect needing further investigation is the possible presence in the pig SI of a “deep body” field similar to that found in cats and primates. Receptive fields of the R, rostrum region and F, and M, areas along the medial crest of the cruciate gyrus often had a high threshold, This area of the somatosensory cortex may be a transition zone between the motor and somatosensory cortices. Further physiological and histological studies are needed to determine whether this region of the pig cortex may be a homologous 3a region.

ACKNOWLEDGMENTS We would like to thank Alan Branigan and the East Carolina University Center for Health Sciences Communications for their help with the illustrations. Partial support of this work was provided by Sigma Xi, the Scientific Research Society.

LITERATURE CITED Adrian, E.D. (1943) Afferent areas in the brain of ungulates. Brain 66t89103. h e y , L.B. (1966) Developmental Anatomy. Philadelphia: W.B. Saunders Co. Breazile, J.E., B.C. Swafford, and W.D. Thompson (1966) Study of the motor cortex of the domestic pig. Am. J. Vet. Res. 27:1369-1373. Clemo, H.R., and B.E. Stein (1982) Somatosensory cortex: a ‘new’ somatotopic representation. Brain Res. 235: 162-168. Clemo, H.R., and B.E. Stein (1983) Organization of a fourth somatosensory area ofcortex in cat. d. Ncurophys. 50:910-925.

Craner, S.L., and R.H. Ray (1991) Somatosensory cortex of the neonatal pig: 11. Topographic organization of the secondary somatosensory cortex (SII). J. Comp. Neurol. 306:39-48. Dodds, W.J. (1982) The pig model for biomedicai research. Fed. Proc. 41:247-256. Dykes, R.W., D.D. Rasmusson, and P.B. Hoeltzell (1980) Organization of primary somatosensory cortex in the cat. J. Neurophysiol. 43:15271546. Felleman, D.J., J.T. Wall, C.G. Cusick, and J.H. Kaas. (1983) The representation of the body surface in SI of cats. J. Neuroscience 3:1648-1669. Iwamura, Y., and M. Tanaka (1978a) Functional organization of receptive fields in the cat somatosensory cortex. I: Integration within the coronal region. Brain Res. 151:49-60. Iwamura, Y., and M. Tanaka (1978b) Functional organization of receptive fields in the cat somatosensory cortex. 11. Second representation of the forepaw in the ansate region. Brain Res. 151:61-72. Johnson, J.I., E.W. Rubel, and G.I. Hatton (1974) Mechanosensory projections to cerebral cortex of sheep. J. Comp. Neurol. 158.3-106. b a s , J.H., M. Sur, R.J. Nelson, and M.M. Merzenich (1981) The postcentral somatosensory cortex: Multiple representations of the body in primates. In C.N. Woolsey (ed.): Cortical Sensory Organization, Vol 1: Multiple Somatic Areas. Clifton, NJ: Humana Press, pp. 2 9 4 5 . Kaas, J.H., R.J. Nelson, M. Sur, C . 3 Lin, and M.M. Merzenich (1979) Multiple representations of the body within ‘SI’ of primates: a redefinition of ‘primary somatosensory cortex’. Science 204r521-523. Krishnamurti, A., F. Sanides, and W.I. Welker (1976) Microelectrode mapping of modality-specific somatic sensory cerebral neocortex in slow loris. Brain Behav. Evol. 13.267-283. Leclerc, S.S., F.L. Rice, and R.W. Dykes (1987) Multiple representations within the ferret first somatic cortex (Mustela putorius furo). SOC Neurosci. Abstr. I3:249. Marshall, W.H., C.N. Woolsey, and P. Bard (1941) Observations on cortical somatic sensory mechanisms of cat and monkey. J. Neurophysiol. 4:l-24. Merzenich, M.M., J.H. Kaas, M. Sur, and C.-S. Lin (1978) Doublerepresentation of the body surface within cytoarchitectonic areas 3b and 1 in “SI” in the owl monkey (Aotus triuirgatus).J. Comp. Neurol. 181:41-74. Merzenich, M.M., M. Sur, R.J. Nelson, and J.H. Kaas (1981) Organization of the SI cortex: Multiple cutaneous representations in areas 3b and 1 of the owl monkey. In C.N. Woolsey (ed): Cortical Sensory Organization Vol 1:Multiple Somatic Areas. Clifton, NJ: Humana Press, pp. 47-66. Patten, B.M. (1964) Foundations of Embryology, 2nd ed. New York: McGraw-Hill. Penfield, W., and H. Jasper (1954) Epilepsy and the Functional Anatomy of the Human Brain. Boston: Little, Brown. Paul, R.L., H. Goodman, and M.M. Merzenich (1972) Alterations in mechanoreceptor input to Brodmann’s areas 1 and 3 of the postcentral hand area Of Macaca mulatta after nerve section and regeneration. Brain Res. 39:l-19. Pubols, B.H., L.M. Pubols, D.J. Dipette, and J.C. Shaw (1976) Opossum somatic sensory cortex: A microelectrode mapping study. J. Comp. Neurol. 165:229-245. Sur, M., R.J. Nelson, and J.H. Kaas (1978) The representation of the body surface in somatosensory area I of the grey squirrel. J. Comp. Neurol. 19269-92. Sur, M., R.E. Weller, and J.H. Kaas (1980) Representation of the body surface in somatosensory area I of tree shrews, Tupaia glis. J. Comp. Neurol. 194:71-95. Tasker, R.R. (1960) A third somatic tactile sensory area. Physiologist 3:162. Welker, C. (1971) Microelectrode delineation of fine grain somatotopic organization of SmI cerebral neocortex in albino rat. Brain Res. 26r259275. Welker, W.I., and S. Seidenstein (1959) Somatic sensory representation in the cerebral cortex of raccoon (Procyon lotor). J. Comp. Neurol. I I1:469501. Welker, W.I., H.O. Adrian, W. Lifschitz, R. Kaulen, E. Caviedes, and W. Gutman (1976) Somatic sensory cortex of llama (Lama glama). Brain Behav. Evol. 13t284-295. Weller, R.E., M. Sur, and J.H. Kaas (1979) Representation of the body surface in SI of the tree shrew. Anat. Rec. I33:716. Woolsey, C.N., and D. Fairman (1946) Contralateral, ipsilateral and bilateral representation of cutaneous receptors in somatic areas I and I1 of the cerebral cortex of pig, sheep and other mammals. Surgery 19:684-702. Zimmerman, I.D. (1968) A triple representation of the body surface in the sensorimotor cortex of the squirrel monkey. Exp. Neurol. 20:415431.