Defense Date


Document Type


Degree Name

Doctor of Philosophy


Anatomy & Neurobiology

First Advisor

J. Ross McClung


Nine lateral rectus motoneurons of the principal abducens nucleus, intracellularly stained with HRP, were morphometrically analyzed by light microscopy using a new method for determining motoneuron size. Particular emphasis was placed on devising a method of estimating total dendrite size from the proximal dendritic diameter alone.

The dendrites of these cells were divided into three types. One type, the microdendrites, had a consistent diameter of l micrometer, variable but short lengths, and added very little to the overall cell size. The majority of the dendrites on these cells (83) were standard in appearance but they could be separated into two further types. Six dendrites differed from the other 77 in that they were tapering processes which branched minimally, had both a rostrally and a caudally directed secondary dendrite and showed a larger ratio for the sum of the secondary dendrite diameters to the proximal dendrite diameter. The remaining 77 branched extensively and traveled either rostral or caudal in the brainstem. However, the most significant difference was quantitative. The tapering dendrites were approximately 2X the size of the prevalent branching dendrites based on proximal diameter measurements. Correlation coefficients of the relation between proximal diameter and surface area or volume of the entire dendrite increased when the correlations were separated into two types. Therefore, to insure the most accurate total size calculations, the regression lines used for estimating dendrite size were of the separate correlations. Total neuron size was calculated by adding the soma and dendrite surface areas. An intraneuronal comparison of size indicated that the size of the soma was not indicative of the size of the cell and it constituted between 2% to 7% of the total cell size. Comparison of the motoneuron size to the mechanical properties of their muscle units was inconclusive. However, a general tendency for small motoneurons to innervate muscle units of lower force output was observed. The smaller motoneurons were generally more dorsally located in the nucleus.


Scanned, with permission from the author, from the original print version, which resides in University Archives.


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