Subtypes of Connections 1 Morphological Evidence

The current evidence for subdivisions within the main connectional groups derives mainly from the characteristics of the cells of origin. Neurons belonging to one projection class (e.g., corticoclaustral vs corticogen-iculate projection neurons in layer 6 of primary visual cortex) frequently have a distinctive laminar pattern of their local axon collaterals. Other experiments suggest that projection systems can be distinguished by the distribution of neurofilament protein in the cell bodies. That is, some feedback projections in the visual pathway are reported to exhibit a higher proportion of neurofilament protein-containing neurons that the feedforward projections (70-80 vs 25-36% for projections to area V4). Additional criteria, such as specificity of surface molecules on the cell body, have been more difficult to establish, although it seems likely that this is due to technical limitations rather than any basic uniformity.

There are a few known examples of strikingly distinct structural specialization of axons. For example, corticothalamic projections to association thala-mic nuclei (subdivisions of the pulvinar and the mediodorsal thalamus) originate from two groups of neurons: large pyramidal neurons in cortical layer 5 and smaller neurons in layers 5 and/or 6. This has been known since the late 1970s from the results of retrograde tracers injected into the thalamus and transported to cortical areas. Recently, studies using highresolution anterograde tracers combined with serial section reconstruction have shown that the axons of these two groups differ dramatically (Fig. 9). Axons differ in caliber, being respectively greater than and less than 1.0 mm, and in the configurations of their terminal arbors. The larger axons terminate in small, round arbors with a small number of large terminal specializations. The smaller axons have elongated fields bearing a larger number of slender specializations. The physiological characteristics of these two types are not known, although it seems likely that the larger axons would be faster because of their larger diameter and larger terminations.

Another recognizable axon type is the connections originating from the giant Meynert cells in the deeper layers of primary visual cortex (Fig. 10). The axons of these cells are thick (2 or 3 mm in diameter) and have distinctively large terminal specializations. Both features are suggestive of a fast conduction velocity, although the functional properties of Meynert cells are not known. Some but not all of these axons can branch to multiple targets, including the pulvinar, cortical area MT/V5, and superior colliculus. The latter two structures are known to have a role in motion processing.

2. Physiological Evidence

Other evidence of axonal subtypes comes from physiological investigations of conduction velocity. Conduction velocity depends on several factors: axon caliber (larger axons are faster), myelination (myeli-nated axons are faster), the existence of branching, synaptic dynamics, and integration delay at the postsynaptic target. Axon length is less important. Conduction velocity is analyzed by measuring the latencies of spikes evoked by antidromic activation of axons by electrical stimulation or by calculating the temporal relationships between the firing times of two impaled neurons in cross-correlograms.

Long-distance projecting axons typically exhibit a spectrum of conduction velocities and caliber (Fig. 11). Projections from cortical motor areas to the spinal cord (corticospinal tract) are subdivided into slow and fast components. Slow fibers, comprising the majority, have an antidromic latency of about 2.6 msec from the brainstem pyramid, whereas for fast fibers the equivalent latency is 0.9 msec. These latencies respectively correspond to conduction velocities greater than or less than 30 msec 1, and axon calibers <4 mm or >6 mm (in man). Curiously, in this system there does not seem to be any correlation of fiber size with either phylogenetic status or digital dexterity. From comparative studies, the largest fibers (25 mm) have been reported in the seal.

Figure 9 Photomicrographs of two morphologically distinct types of corticopulvinar axons. (A) A field of thin axons with slender, spine-like terminations. Portions of two thicker axons pass in the vicinity (asterisk). (B) A single, round arbor bearing a small number of larger boutons. (C) Higher magnification of a field of large boutons, mixed with a small group (asterisk) of the thinner terminations.

Figure 9 Photomicrographs of two morphologically distinct types of corticopulvinar axons. (A) A field of thin axons with slender, spine-like terminations. Portions of two thicker axons pass in the vicinity (asterisk). (B) A single, round arbor bearing a small number of larger boutons. (C) Higher magnification of a field of large boutons, mixed with a small group (asterisk) of the thinner terminations.

Several studies have investigated the distribution of latencies to visual stimulation in different areas of the visual system of macaque monkeys. These suggest an average latency difference of 10 msec between neurons in successive areas. All areas, however, exhibit a range in latencies, and it has been noted that short-latency neurons in higher order areas in inferotemporal cortex can respond to a given stimulus before long-latency neurons in area V1. The range in conduction velocity may correlate with axon caliber, which for most interareal cortical connections ranges from about 0.5 to 2.0 mm in diameter. How these timing differences contribute to cortical processing is the subject of ongoing investigations.

The temporal-computational characteristics of individual axons have been analyzed by simulating the propagation of an action potential in arbors traced with histological techniques. Simulated results on latencies and conduction velocities are in good agreement with those found electrophysiologically, and this technique may provide an effective tool for probing issues such as how the velocity of spike propagation is affected by changes in axon diameter at branching points or at boutons, the delay in activation latencies between boutons, and how conduction velocity might be modified by the previous occurrence of an action potential. An interesting general result from this work is that different axon geometries can lead to similar patterns of spatiotemporal activation. This raises the issue of how close a correspondence there is between the morphology and computational properties of an axon. A larger sample of different axons and more

Figure 10 Photomicrographs of axons originating from large Meynert cells of primary visual cortex. These are large caliber and terminate in arborizations with large boutons. [A and (higher magnification) B] Cell body, dendrites, and axon (arrowheads) filled with BDA. [C and (higher magnification) D] One of several terminal arbors. Open arrows point to corresponding features in A, C, and D. (Portions in the deeper planes of the histological section are out of focus.)

Figure 10 Photomicrographs of axons originating from large Meynert cells of primary visual cortex. These are large caliber and terminate in arborizations with large boutons. [A and (higher magnification) B] Cell body, dendrites, and axon (arrowheads) filled with BDA. [C and (higher magnification) D] One of several terminal arbors. Open arrows point to corresponding features in A, C, and D. (Portions in the deeper planes of the histological section are out of focus.)

precise data on the biophysical properties of the terminal arbor are necessary to extend this line of research.

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