The Pyramidal System of the Woodchuck

Abstract

The pyramidal system of the woodchuck was examined anatomically and electrophysiologically. The pyramidal tract was found to originate entirely within the anterior half of the cerebral hemispheres and to follow a course typical of most rodents, decussating almost entirely and descending the length of the spinal cord in the ventral part of the dorsal funiculi. It decreased in size uniformly with distance along the spinal cord; most of its fibers terminated in the medial half of the dorsal horn, though they scattered widely and even appeared to terminate on motoneurons. Each tract contained 140,000±20,000 fibers, with 60–80% of the fibers being about 1 µm and 90% being less than 3 µm in diameter. Stimulation of the medullary pyramid evoked a minute antidromic potential (a wave) which was generally obscured by a large surface-positive response that reversed polarity deep in the cortex and that appeared to be synaptic in origin (r wave). It is proposed that the r wave results from intracortical pyramidal cell collateral activity. Though largest in the apparent region of origin of the pyramidal tract, the r wave also showed local maxima in the forepaw and hindpaw foci of somatosensory cortex. The somatosensory cortex was organized in a manner similar to other rodents, but an ''association'' area lacking topographical organization was found near the anterior pole of the hemispheres. In an allometric sense, the woodchuck was found to be a ''normal'' rodent and a ''normal'' mammal.

Cerebral Response to Medullary Pyramid Stimulation in the Rabbit

“…In the second paper [Chang, 1955b] he argued that the subse quent two components represented (4) in terneuronal activity evoked by PT axon collaterals and (5) reexcitation of PT cells by those interneurons. This interpretation differs significantly from that developed for the cat [Jabbur and Towe, 1961;Ken nedy and Towe, 1962;Humprey, 1968a, b] on the one hand , and for the opossum [Towe and Biedenbach, 1969] and wood chuck [Doetsch and Towe, 1981a] on the other. The properties of the response in the rabbit, as described by Woolsey and Chang [1948], by Chang [1955a,b] and by Conway et al [1969], are similar to those in the opossum and woodchuck, suggesting that a similar process may be involved.…”

“…The earlier of these was largest caudally and the later one was largest rostrally on the cortex. By analogy with the opos sum [Towe and Biedenbach, 1969] and woodchuck [Doetsch and Towe. 1981a], this large surface-positive response will be termed the r wave, and the two compo nents will be distinguished as rl and r2 (Chang [1955a] termed these components 1 and 2; in the opossum [ Towe and Bieden bach, 1969], r2 was referred to as r').…”

“…Yet, the r wave has been found on ly in animals that have few PT axons grea ter than 4-5 jum in diameter. Doetsch and Towe [1981a] calculated the shape and maximum amplitude to be expected of a purely antidromic response in the wood chuck, and found that it should begin 0.7 ms after the medullary pyramid stimu lus and should attain a peak amplitude of 0.08 mV about 1.4 ms after the stimulus. Such an event was identified in the wood chuck, though it was difficult to analyze with clarity.…”

Antidromic Response to Medullary Pyramid Stimulation inRats and Its Relation to That in Cats (Part 2 of 2)

“…Fast PT neurons make up about 10% of the pyramidal sys tem in domestic cats, whereas by the same criterion [Takahashi, 1965], they make up less than 1 % in rats. By the structural crite ria of Deschenes et al [ 1979], only slow PT neurons are present in rats [Donoghue and Kitai, 1981;Landry et al, 1984], The ex pected configuration of the antidromic re sponse in rats, calculated from the 0.1 umbin fiber spectrum measured by Harding and Towe [1985], is a surface-positive event nearly identical to that calculated by Doetsch and Towe [1981a] for wood chucks. However, because of the short conduction distances involved in rodents, the response bears a greater similarity to the a wave (fast PT) than to the beta com ponent (slow PT) of the antidromic re sponse seen in cats.…”

“…The organization and structure of the pyramidal system in rodents are differ ent from those in carnivores, and one im portant difference relates to the cerebral circuitry. Both fast and slow PT neurons are present in domestic cats [Deschenes et al, 1979] and macaque monkeys [Hamada et al, 1981], whereas there is doubt con cerning rodents [Doetsch and Towe, 1981a;Donoghue and Kitai, 1981;Hard ing and Towe, 1985]. Fast PT neurons make up about 10% of the pyramidal sys tem in domestic cats, whereas by the same criterion [Takahashi, 1965], they make up less than 1 % in rats.…”