Neuroscience Letters, 49 (1984) 57-61 57 Elsevier Scientific Publishers Ireland Ltd.

NSL 02843

COMMISSURAL AFFERENTS TO THE CORTEX SURROUNDING THE POSTERIOR PART OF THE SUPERIOR TEMPORAL SULCUS IN THE M O N K E Y

T. BAN, J. NAITO and K. KAWAMURA

Department of Anatomy, School of Medicine, Iwate Medical University, Morioka 020 (Japan)

(Received April 14th, 1984; Revised version received May 9th, 1984; Accepted May 14th, 1984)

Key words: commissural connections - horseradish peroxidase - monkey

Commissural afferents to the cortex surrounding the posterior part of the superior temporal sulcus were studied in Japanese monkeys by the horseradish peroxidase method. After injection of the enzyme, many caUosal neurons were labeled contralaterally in the cortical area corresponding to the injection site (homotopical area) and in other regions (heterotopical area). Most of the callosal neurons were triangular in shape, occurring for the most part in layer III of both homotopical and heterotopical areas (about 75-90°7o of the total number of labeled cells). Mean diameters of the cell bodies were about 11-13/~m.

The posterior part of the superior temporal sulcus in the monkey has been shown anatomically to be the site of convergence of association fibers from the somatic sensory, auditory and visual cortical areas [7]. The present study is an attempt to examine the distribution of callosal neurons projecting to this particular part of the cortex. Under ketamine and Nembutal anesthesia, 6 Japanese monkeys (Macaca fuscata) received a single injection (Fig. IB) of a small amount (0.15-0.2 #1) of 50% (w/v) horseradish peroxidase (HRP, Toyobo I-c) in cortical areas surrounding the superior temporal sulcus with a glass micropipette connected to the needle of a 1.0-/d Hamilton syringe. In 3 monkeys, the underlying white matter was stained to some degree with the enzyme. Therefore, the remaining 3 animals were used for the analysis of the commissural connection.

Technical procedures of the experiment were the same as described previously [8]. Serial sections from the cerebrum were cut coronally at 50 /zm on the freezing microtome and every 10th section was incubated according to the methods described by Mesulam et al. [9] and Graham and Karnovsky [3].

In monkey Com 38 (Figs. 1 and 2), HRP was injected in the dorsal wall of the superior temporal sulcus (Fig. 1B). Many labeled cells were found in groups in areas corresponding to the injection site of the opposite hemisphere: the most caudal part of the superior temporal gyrus and its adjacent cortical wails (Figs. 2A, C). Labeled cells were also found in the middle part of the lateral sulcal cortex and the dorsal

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wall of the middle part of the superior temporal sulcus as well as in the caudal half of the cingulate cortex (area LC of ref. 1). In addition, some labeled cells occurred in the inferior parietal lobule (areas PG and PF) and the caudal part of the superior

temporal sulcal cortex. In the corpus callosum, HRP-labeled fibers occupied the

splenial region, being free of the most caudal part. In monkey Corn 37, where the enzyme was injected in a slightly more rostral part of the dorsal wall of the superior

temporal sulcus than in monkey Com 38 (Fig. 1A), callosal cells were labeled in the

superior temporal gyrus and the caudal part of the cingulate gyrus. No labeled cells were found in the inferior parietal lobule. In the third monkey (Corn 55, Fig. 1A),

in which HRP was injected in the deeper part of the dorsal wall of the superior tem- poral sulcus, labeled cells occurred contralaterally in the inferior parietal lobule,

particularly in the ventral wall of the intraparietal sulcus, and the caudal part of the cingulate gyrus. Some labeled cells were also observed in the inferior temporal gyrus

(area TE), prestriate cortex (areas OA and OB) and a few in the presubiculum (area 27 of ref. 2). In addition, 20 labeled cells were observed in the prefrontal cortex.

They were located in area 8 as well as in the cortical walls of the principal and ar- cuate sulci. Labeled commissural fibers as observed in cross-sections of monkeys

Corn 37 and Com 55 were likewise found in the splenium corporis callosi.

The distribution of cells of origin of the association fibers in monkey Com 38 is illustrated in Fig. 2B. The distribution pattern of the labeled cells was similar to that

of the commissural connection (Fig. 2A). Although no labeled cells were found in the prefrontal cortex of monkey Corn 38, we obtained clear evidence of the com-

missural, prefrontal projection in monkey Corn 55 (see above). Furthermore, in the other three monkeys (Com 35, Corn 36 and Corn 61) whose white matter was also

involved with the enzyme injected in the posterior association area, some labeled

A B

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Fig. 1. A: diagram represents the maximal extent of HRP injected into different parts of the cortical wall of the superior temporal sulcus in 3 monkeys. Black denotes heavy staining of HRP, hatching varying degrees of lighter staining. Arrows indicate the superior temporal sulcus. B: photomicrograph of a co- ronal section through the injection site in monkey Corn 38 shown in A. Cresyl violet stain, x 8.

CONTRA

59

Q C

IPS I

C

(

Fig. 2. A and B: diagrams to show the distribution of labeled cells of callosal (A) and association (B) fibers on the lateral and medial surfaces of the hemisphere of monkey Com 38. C: the distribution of labeled callosal neurons observed in 3 coronal sections at levels indicated in A. One dot represents one labeled cell. Abbreviations: Cals, calcarine sulcus; Cc, corpus callosum; Cd, caudate nucleus; C1, claustrum; Cings, cingulate sulcus; Lats, lateral sulcus; Put, putamen; STems, superior temporal sulcus.

cells were observed in the p re f ron ta l cortex. The commissura l p ro jec t ion f rom the

p re f ron ta l cor tex to the cor tex o f the super ior t empora l sulcus has been repor ted

with the degenera t ion me thod [11]. The callosal p ro jec t ion f rom the pos ter ior

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association area to the prefrontal cortex has also been shown with the degeneration [10] and HRP [6] methods. The latter projection is apparently heavier than the former.

The number of labeled cells was counted in monkey Com 38. The total number of the cells was 4123:2097 in the area corresponding to the injection site (homotopical area) and 2026 in other cortical fields (heterotopical area). On the other hand, the total of the heterotopical area, where labeled cells were found, was estimated about 480 mm 2, and that of the homotopical area about 75 mm 2. Thus, the density of labeled cells in the homotopical area was about 6 times higher than that of the heterotopical area.

One-tenth of the labeled cells (413) were sampled to measure the diameter (mean and S.D.) and frequency (°7o) in each layer. Most of the labeled callosal or com- missural neurons were triangular or polygonal. Shorter diameters were measured for the ceils of fusiform type. The results are shown in Fig. 3. Large commissural neurons of more than 13 #m were found in 50°7o in the homotopical area, whereas they occurred in 30°7o in the heterotopical area. Most of the labeled commissural neurons were located in layer III in agreement with previous studies [4-6]. In this layer, we observed 90.0% of the labeled cells in the homotopical area and 75.3°7o in the heterotopical area. Frequency ratio (indicated in parentheses in Fig. 3) of the commissural neurons was higher in the heterotopical area in each of the deep cor- tical layers.

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( V " l ' l . l ~ ~ / / J , i J / i l ' S / J Vo " ~ - ~ l " J s I,'

Fig. 3. Frequency histograms of different sizes of callosal neurons in different layers of the heterotopical (left) and homotopical (right) areas. The frequency (°70) of labeled cells in each cortical layer is indicated in parentheses, and somal diameters (p.m) of labeled cells are represented along a line extending from the circle. The total number of the labeled cells (N), the mean diameter and the s tandard deviation (S.D.) are shown inside the circles. Abbreviations: o and i, outer and inner parts of cortical layers II1 and V.

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