ABSTRACT
Neocortical axonal systems convey the results of cortical processing to cortical target structures. This chapter is concerned with the timing of this information flow and with delays introduced by different types of cortical axon. Neocortical white matter is composed of axons that range from <0.1 µm to >10 µm in diameter. Such axons generate a similarly wide range of conduction velocities, and conduction times between soma and axon terminals. Work from the author’s laboratory is reviewed in which conduction properties of corticocortical and descending corticofugal efferent systems of rabbit neocortex were examined in each of four sensory and motor cortical regions. These data are compared with corresponding axonal systems of primates, cats and rodents. Whereas some cortical efferent systems are composed of exclusively fast-conducting axons, others have a large number of very slowly conducting elements, and impulse conduction times along these latter elements may take several tens of milliseconds. Properties of specific thalamocortical axonal systems are also reviewed, and the role of axonal conduction time in the generation of sharply and broadly synchronous cortical activity is explored. KEYWORDS: Axonal conduction; Cerebral cortex; Cortico-cortical axons; Conduction velocity; Conduction delay; Synchrony
1. INTRODUCTION
Neocortical axonal systems convey the results of cortical processing to a host of destinations. This chapter is concerned with the timing of this information flow, and with delays introduced by different types of cortical axons. Considerable information is available concerning the laminar and regional origins of neocortical efferent pathways and the locus of terminal synapses. Less attention has been paid to the axon lying between soma and synaptic terminals. General surveys of cortical white matter (Bishop and Smith, 1964; Waxman and Swadlow, 1977) have shown that neocortical axons exhibit a wide spectrum of diameters. Unmyelinated axons range from <0.1-0.6 µm, and often have a median diameter of ~0.2 µm (e.g. Waxman and Swadlow, 1976b; Swadlow et al., 1980). Most such axons cannot be visualized without the aid of electron
microscopy. Myelinated axons may be as fine as 0.2 µm in diameter (e.g. Harding and Towe, 1985; Waxman and Bennett, 1972). The diameter of the largest myelinated axons found in a given cortical system is species dependent (see below), and may be >10 µm (e.g. Biedenbach et al., 1986). Thus, neocortical axons span a range of >1:10,000 in cross-sectional area, and a range in expected conduction velocities of >1:100 (Waxman and Bennett, 1972, see below).
