Stephen B. Edwards

The ascending and descending projections of the red nucleus in the cat: An experimental study using an autoradiographic tracing method ☆

Injections of [3H]leucine were stereotaxically placed in the red nucleus of cats, and the ascending and descending rubral projections were traced by following the transport of isotopically labeled protein from red nucleus cell bodies to their terminals. The chief advantage of this technique over fiber degeneration methods is that it is possible to selectively trace a fiber system which is originating in a region containing fibers of passage. Slow axopasmic flow was utilized to demonstrate the course of the efferent fibers as well as their terminal fields. The descending projections to the facial and lateral reticular nuclei, the inferior olivary complex, and the spinal cord, previously established by fiber degeneration methods, were confirmed. In addition, a number of projections were found which have not previously been reported in the cat. These included projections to the main sensory trigeminal nucleus, and nucleus oralis and nucleus caudalis of the spinal trigeminal nucleus, the descending vestibular nucleus, the cuneate and gracile nuclei, and cell groups x and z. No evidence was found of a projection to thalamic nuclei which have been thought to receive rubral efferents in the cat on the basis of degeneration studies. Transport of label by surrounding reticular cells did not account for any of these results.

Projections of the pulvinar-lateral posterior complex to visual cortical areas in the cat

Abstract The projections of the pulvinar-lateral posterior complex of the cat were studied using the autoradiographic tracing method and related to 15 previously defined cortical areas. The results indicate that each of three separate zones within the pulvinar-lateral posterior complex has a different pattern of projection. The most lateral zone, the pulvinar, sends fibers to at least seven cortical areas, most of which are known to have input from other visual areas within the brain: the splenial visual area, the cingulate gyrus, and areas 5, 7, 19, 20a and 21a. A zone located just medial to the pulvinar, the lateral division of the lateral posterior complex, projects to at least eight visual areas in the cortex: areas 17, 18, 19, 20a, 21a, 21b, the posteromedial lateral suprasylvian area and the ventral lateral suprasylvian area. The most medial zone, the intermediate division of the lateral posterior complex, projects to at least four cortical areas: 20a, the posterior suprasylvian area, the posterolateral lateral suprasylvian area and the dorsal lateral suprasylvian area. Of the 15 cortical areas that receive fibers from the pulvinar-lateral posterior complex, only three (areas 19, 20a and 21a) receive projections from more than one of these thalamic zones, and only one of the cortical areas (20a) receives fibers from all three zones. Thus, the data support the division of the pulvinar lateral posterior complex into three zones on the basis of their unique and largely non-overlapping projections to the visual cortex.

The Autoradiographic Tracing of Axonal Connections in the Central Nervous System

Since the conception of the autoradiographic tracing method in the late 1960s (Weiss and Holland, 1967; Goldberg and Kotani, 1967; Lasek et al., 1968) and its popularization and first applications in the early 1970s (Cowan et al., 1972, Edwards, 1972), it has rapidly become one of the most widely used techniques for tracing neuroanatomical connections. Based on the process of axoplasmic transport and the technique of autoradiography, the autoradiographic tracing method represents a radical departure from past tracing methods that relied primarily on the visualization of degenerating axons. As such, it offers several new and significant advantages, the most important being the selective demonstration of pathways arising from neurons surrounded by passing fibers. This advantage alone has meant that the connections of whole new brain territories could be established for the first time and the connections of many previously examined areas clarified. But the technique also has definite limitations and certain disadvantages relative to other tracing methods, and, if naively used, it can produce misleading results.

Efferent projections of the neonatal superior colliculus: Extraoculomotor-related brain stem structures

The development of eye movements is a prolonged process which presumably involves the efferents of the superior colliculus. In the present study we sought to determine which, if any, of the colliculus efferents that influence eye movements in adult cats were present in neonatal kittens. The autoradiographic and orthograde horseradish peroxidase tracing methods were employed in kittens ranging from 6 h to 5 weeks of age and in adult cats. Surprisingly, most of the known projections from the superior colliculus which are believed to be involved in eye movements were already present in the youngest animals studied. These included projections to (a) the ventral central gray matter overlying the oculomotor nucleus, and (b) those portions of the pontine and medullary reticular formation which provide excitatory and inhibitory inputs to abducens neurons. Apparently, the pathways over which the superior colliculus influences eye movements are elaborated quite early in life. However, in the predorsal bundle and pontomedullary reticular areas the density of transported label was less in 1-day-old kittens than in older animals. Thus, anatomical as well as functional development of portions of this circuitry appear to require a significant period of postnatal maturation.

Microinjector for use in the autoradiographic neuroanatomical tracing method.

Abstract A simple microinjector is described for use in making stereotaxic placements of minute quantities of labeled precursor in the CNS. The instrument is used in conjunction with a Hamilton microsyringe and standard stereotaxic electrode carriers. It is capable of delivering volumes as small as 0.05 μl with a high degree of accuracy and repeatability.

Methods for Delivering Tracers

The delivery of chemical tracers into the central nervous system is often an essential, although sometimes problematic aspect of modern tract-tracing experiments. In this chapter we shall concentrate on the most common methods of tracer delivery: small injections of a solution, either using a microsyringe or through glass micropipettes, or extracellular of intracellular iontophoresis. It should be kept in mind, however, that other methods of delivering tracers may be more suitable for certain types of problems. Tracers have been injected into the periphery—for instance, into muscle in order to label motoneurons in the spinal cord or brain stem (Kristenson, 1970; Kristenson et al., 1971; Kristenson and Olsson, 1973)—and they have been applied to the cut or crushed ends of peripheral nerves (Kristenson, 1975; lies and Mulloney, 1971; Tweedle, 1978) and deposited in naturally closed structures such as the eye (LaVail and LaVail, 1972, 1974). In cases where diffusion from the site of deposition is not a serious limitation, tracers may be applied directly to the surface of the brain, either as a liquid or by means of absorbant pellets or wicks that have been soaked in the tracer solution (Arbuthnott, 1969; Descarries and Lapierre, 1973; Held and Young, 1969; Weiss and Holland, 1967).