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Robert K. Josephson
Professor, Neurobiology and Behavior
School of Biological Sciences
Ecology & Evolutionary Biology
School of Biological Sciences
PH.D., University of California, Los Angeles
Phone: (949) 824-5956
Fax: (949) 824-2447
University of California, Irvine
2205 McGaugh Hall
Mail Code: 4550
Irvine, CA 92697
Neural control of muscle contraction; muscle contraction kinetics and power output; relations between muscle ultrastructure and performance; neurotrophic control of muscle properties
Neural events are converted into behavior through effectors, of which muscles are by far the most important. The properties of the muscles-their speed, power efficiency, endurance-impose major limitations on the performance of organisms. Our laboratory work is oriented toward characterizing the mechanical performance of muscle and the underlying reasons for the diversity in performance found in muscles from different sources. We have been particularly interested in very fast muscles and in muscles with the capacity for high mechanical power output.
There is considerable variation in the contraction properties of muscles within an organism and often among different fibers within a single muscle. One of the factors determining the mechanical, structural, and biochemical properties of a muscle fiber is the source of its innervation. Neutrophic control of muscle properties is well documented in vertebrate muscles. We have recently shown that there is neurotrophic control of muscle properties in insects as well. Insect muscles have the same basic physiology as muscles of higher animals, but in insect muscles there is considerably greater cellular homogeneity than in most vertebrate muscles. This homogeneity greatly simplifies physiological and structural analysis of muscle properties and should aid us in our studies of the mechanisms of neurotrophic control of muscle properties.
There are correlations between muscle fiber ultrastructure and contractile performance. For example, muscle fibers that give short twitches tend to have thin myofibrils and an abundant sarcoplasmic reticulum. (Sarcoplasmic reticulum is the internal tubular system within muscle fibers that is involved in calcium regulation.) One of the thrusts of the research is quantification of the relations between muscle ultrastructure and contraction kinetics. We are seeking rules that w ill allow us to accurately predict a muscle's performance from its ultrastructure or the comerse-rules that may also identify rate-limiting steps in muscle control.
A third line of research concerns mechanical work output by muscle. The most imporunt functional capacity of muscle is its ability to shorten against a load and thus to do work. Despite its obvious significance, work output by muscle is poorly characterized, largely for lack of an appropriate procedure for quantifying mechanical work output under conditions that approximate those of normal muscle use. We have developed an experimental approach for determining the work output of muscles during cyclic contraction. We are now combining this technique with data from the measurements of the oxygen consumed by working muscles in order to determine the efficiency of muscle in converting chemical energy to mechanical energy.
Josephson, R.K. and Stokes, D.R. (1999). Work-dependent deactivation of a crustacean muscle. Journal of Experimental Biology 202, 2551-2565.
Josephson, R.K. and Stokes, D.R. (1999). The force-velocity properties of a crustacean muscle during lengthening. Journal of Experimental Biology 202, 593-607.
Josephson, R.K. and Edman, K.A.P. (1998). Changes in the maximum speed of shortening of frog muscle fibres early in a tetanic contraction and during relaxation. Journal of Physiology 507, 511-525.
Josephson, R.K. (1993). Contraction dynamics and power output of skeletal muscle. Annual Reviews of Physiology 55, 527-546.