Historical Survey

The early morphological description of nerve fibers has been credited to Antoine van Leeuwenhoek (1718), who studied nerve bundles from cow or sheep spinal cord under the microscope. In his report, however, the individual fibers are described as hollow tubes. In cross section, given the suboptimal preservation of the specimens, the dark core of axoplasm would be shrinken, and myelinated axons would therefore appear hollow.

Further characterization of the axon, through the second half of the 19th century, was hindered by technical limitations as well as by the controversies surrounding the basic organization of nerve cells. One ofthe most important contributions during this period was made by O. F. K. Deiters. By microdissection of individual nerve cells from histologically treated specimens of spinal cord, Deiters successfully established the continuity of the soma and its processes and distinguished between the protoplasmic processes ("dendrites") and what he called the more slender axis cylinder. He correctly concluded that the nerve cell is a cell that bears on its soma one axon and several dendrites.

The advent of the Golgi silver stain (1883), which demonstrates a small number of neurons, often in their complete or near complete entirety, stimulated a highly creative period in cellular neuroanatomy, culminating in the Nobel prize awards in 1906 to Camillo Golgi and

Ramon y Cajal. These morphological studies, especially those of Cajal, firmly established the neuron as the basic cellular unit of the brain, as opposed to the "reticularist" position promulgated by Golgi. They also clarified the dynamic polarization of the neuron, as a cell with receiving and transmitting elements (respectively, the multiple dendrites and single axon).

With this work, the investigation of axons rapidly expanded to their functional anatomy and their connectivity and network properties. Axons were defined as two major subtypes, those with local ramifications and those with extensive ramifications (Fig. 2). These became known as Golgi type II and type I, respectively (or local circuit and projection neurons). These two types were later found to correspond generally to inhibitory and excitatory connections, each using different neurotransmitters. In the same period, many of the specific sensory, motor, and associational pathways interconnecting subcortical nuclei and cortical areas were successfully mapped.

Breaking Bulimia

Breaking Bulimia

We have all been there: turning to the refrigerator if feeling lonely or bored or indulging in seconds or thirds if strained. But if you suffer from bulimia, the from time to time urge to overeat is more like an obsession.

Get My Free Ebook


Post a comment