The primate visual system is composed of multiple, functionally specialized cortical areas. The functional diversity among areas is thought to reflect different contributions from early parallel visual pathways to the area V1 neurons providing input to "higher" cortical areas. The M pathway is believed to provide information about motion and contrast, via layer 4B of V1, to dorsal visual areas. The P pathway is believed to provide information about shape and color, via layer 2/3 of V1, to ventral visual areas, with specialized contributions from cytochromeoxidase (CO) blob versus interblob neurons. However, the detailed anatomical relationships between the M and P pathways and the neurons in V1 that provide input to higher extrastriate cortical areas are poorly understood. To study these relationships, spiny stellate neurons in the M-and P-recipient layers of V1, 4C␣ and 4C␤, respectively, were intracellularly labeled, and their axonal and dendritic arbors were reconstructed. We find that neurons with dendrites in upper layer 4C␣ project axons to layer 4B and CO blobs in layer 2/3, thus relaying M input to these regions. Other neurons in lower layer 4C␣ provide M input to interblobs. These cells have either (1) dendrites restricted to lower layer 4C␣ and axons specifically targeting layer 2/3 interblobs, or (2) dendrites in lower 4C␣ and 4C␤ and axons targeting blobs and interblobs. P-recipient layer 4C␤ neurons have dense axonal arbors in both blobs and interblobs but not layer 4B. Quantitative analyses reveal that 4C␣ cells provide approximately five times more synapses than 4C␤ cells to layer 4B, whereas 4C␤ cells provide five times more synapses than 4C␣ cells to layer 2/3. These observations imply that M input is dominant in layer 4B. In layer 2/3, both blobs and interblobs receive M and P input, but the P input is dominant, and M input to interblobs derives exclusively from a subpopulation of M afferents that targets lower 4C␣, not from afferents targeting only upper 4C␣ (cf. Blasdel and Lund, 1983). We speculate that the M and P pathways to interblobs are "X-like" linear systems, whereas blobs also receive nonlinear "Y-like" M input. Key words: functional streams; local circuits; macaque; primary visual cortex; primate; V1The relative contributions of the f unctionally distinct M and P pathways are believed to be important determinants of the functional differences between extrastriate cortical areas (Livingstone and Hubel, 1988; for review, see Merigan and Maunsell, 1993). Nevertheless, the actual relationships between the M and P streams and V1 neurons projecting to extrastriate cortex are poorly understood. We have addressed these relationships by studying spiny stellate neurons in layer 4C of V1, because they lie at the heart of the problem; they provide a direct link from LGN afferents to extrastriate projection neurons.In the retino -geniculo -cortical system, parallel M and P pathways converge on V1, where they segregate their inputs into layers 4C␣ and 4C␤, respectively (Hubel and Wiesel, 1972;Hen...

Relationships between the M and P retino-geniculo-cortical visual pathways and “dorsal” visual areas were investigated by measuring the sources of local excitatory input to individual neurons in layer 4B of primary visual cortex. We found that contributions of the M and P pathways to layer 4B neurons are dependent on cell type. Spiny stellate neurons receive strong M input through layer 4Cα and no significant P input through layer 4Cβ. In contrast, pyramidal neurons in layer 4B receive strong input from both layers 4Cα and 4Cβ. These observations, along with evidence that direct input from layer 4B to area MT arises predominantly from spiny stellates, suggest that these different cell types constitute two functionally specialized subsystems.

Pyramidal neurons in superficial layers of cerebral cortex have extensive horizontal axons that provide a substrate for lateral interactions across cortical columns. These connections are believed to link functionally similar regions, as suggested by the observation that cytochrome-oxidase blobs in the monkey primary visual cortex (V1) are preferentially connected to blobs and interblobs to interblobs. To better understand the precise relationship between horizontal connections and blobs, we intracellularly labeled 20 layer 2/3 pyramidal neurons in tangential living brain slices from V1 of macaque monkeys. The locations of each cell body and the cell's synaptic boutons relative to blobs were quantitatively analyzed. We found evidence for two cell types located at characteristic distances from blob centers: (1) neurons lacking long-distance, clustered axons (somata 130-200 microm from blob centers) and (2) cells with clustered, long-distance axon collaterals (somata < 130 microm or >200 microm from blob centers). For all cells, synaptic boutons close to the cell body were located at similar distances from blob centers as the cell body. The majority of boutons from cells lacking distal axon clusters were close to their cell bodies. Cells located more than 200 microm from blob centers were in interblobs and had long-distance clustered axon collaterals selectively targeting distant interblob regions. Cells located less than 130 microm from blob centers were found within both blobs and interblobs, but many were close to traditionally defined borders. The distant synaptic boutons from these cells were generally located relatively near to blob centers, but the neurons closest to blob centers had synaptic boutons closer to blob centers than those farther away. There was not a sharp transition that would suggest specificity for blobs and interblobs as discrete, binary entities. Instead they appear to be extremes along a continuum. These observations have important implications for the function of lateral interactions within V1.

Previous studies indicate that cat jaw-muscle spindle afferents can be divided into two types (type I and II) on the basis of their axonal trajectories. The present study examined the relationship between spindle afferent fibers and their target masseter alpha-motoneurons in the cat by using the intracellular horseradish peroxidase (HRP) injection technique, and provided several new findings on the synaptic organization generated between the two. Five type I afferent fiber-motoneuron pairs and nine type II afferent-motoneuron pairs were well stained with HRP. The following conclusions were drawn: 1) A motoneuron received contacts from only one collateral of any given spindle afferent. 2) The number of contacts made between an afferent and a motoneuron ranged from one to three. 3) The contacts made by a spindle afferent were on the same dendrite or dendrites branching from the same primary dendrite. 4) The vast majority of the contacts made by an afferent on a motoneuron were distributed in the dendritic tree within 600 microns from the soma, i.e., in the proximal three fourths of the dendritic tree. The differences observed between the two afferent types were as follows. First, type II afferent terminals made contacts on more distal dendrites of the motoneurons than did type I afferent terminals. Second, the contacts made between a type I afferent and a motoneuron were clustered together, but those made between a type II afferent and a motoneuron were widely dispersed. The present results provided the general rules of synaptic contacts between the spindle afferents and masseter alpha-motoneurons, and demonstrated that the spatial distribution of synaptic contacts on the dendritic tree was different between type I and type II afferents.

A previous study indicated that in adult rat, a distinctive neuronal group in the dorsomedial division of the subnucleus oralis of the spinal trigeminal nucleus (SpVo) and the rostrolateral part of the nucleus of the solitary tract (Sn) is stained for nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d), and suggested that the labeled structures are involved with sensorimotor reflexive functions. This study aimed to characterize the developmental expression of NADPH-d in SpVo and Sn, including other areas of the lower brainstem and cervical spinal cord, by means of the enzyme histochemical staining technique, from the prenatal through the postnatal period. On embryonic day 12 (E12), no neurons in the brain were stained for NADPH-d, whereas blood vessels were stained. Labeling in the vessels was consistently present throughout pre- and postnatal periods but decreased with development. On E15, labeled neurons appeared in the dorsomedial part of SpVo and the rostrolateral part of Sn, but not in the other nuclei. The labeled neurons in both nuclei increased in numbers drastically to E17. Postnatally, they tended to increase gradually in Sn, but to decrease slightly in SpVo. The cell size of labeled neurons reached a plateau at E17 in SpVo, but at postnatal day 4 (P4) in Sn. In other nuclei on E17, labeling appeared in the lateral paragigantocellular reticular, intermediate reticular, medullary reticular, pedunculopontine tegmental, and spinal vestibular nuclei, and laminae V, VI, and X of the cervical spinal cord. On E20 and P0, labeling appeared in the dorsal column, laterodorsal tegmental, raphe obscurus, parvocellular reticular, ventral gigantocellular reticular, and parahypoglossal nuclei, and laminae IX of the cervical spinal cord. On P4 labeling appeared in the parabrachial and median raphe nuclei, medial and caudolateral Sn, the magnocellular zone of subnucleus caudalis of the spinal trigeminal nucleus (SpVc), and laminae III/IV of the cervical spinal cord. On P10, labeling appeared in the paratrigeminal and dorsal raphe nuclei, the superficial zone of SpVc, and laminae I/II of the cervical spinal cord. No newly labeled neurons appeared in any nuclei after P14. The very early appearance of NADPH-d staining in SpVo and Sn, which precedes the appearance of NADPH-d elsewhere in the brainstem, suggests that the nitric oxide (NO)/cyclic guanosine monophosphate (cGMP) system has an important role for primitive orofacial sensorimotor reflexive functions. Furthermore, the pattern of developmental expression of NADPH-d in SpVo and Sn suggests that the NO/cGMP system is organized in a distinct manner in different nuclei.