Quantal nature of neuromuscular transmission in crayfish phasic abdominal flexors

Abstract

1. The electrophysiology and structure of the crayfish (Procambarus clarkii) deep abdominal flexor neuromuscular system were studied at several stages of development for the purpose of establishing and analyzing the quantal nature of synaptic transmission in these muscles. 2. In adult crayfish we were unable to obtain evidence for quantized neurotransmitter release in this system; spontaneous miniature synaptic potentials were never observed and evoked synaptic potential amplitude fluctuations suggested a mean quantal size of no more than a few microvolts. By recording from very young crayfish, so as to select fibers of higher input resistance, we were able to reveal the existence of a population of miniature potentials with a mean amplitude of less than 100 μV. Their amplitude distributions were always positively skewed, with the mode buried in the background noise. The time intervals between successive events were approximately exponentially distributed. 3. The extremely small quantal size was accounted for in terms of the input resistance (RN) of the muscle fibers. The exceptionally lowRN resulted from a low specific membrane resistivity combined with an unusual organization of the muscle fibers into a system of electrically coupled subunits. Thus the total membrane area of each fiber is disproportionately large for its diameter. The developmental origin and functional significance of this form of neuromuscular organization are discussed. 4. The mean quantal content per muscle fiber was estimated to be several thousand in adult animals, an order of magnitude higher than has been reported for other crayfish muscles. The large quantal content appears to result from a greater than usual total number of synaptic contacts. 5. Synapses formed by the motor giant (MoG) and non-giant (NG) motor neurons displayed the same use-dependent labilities in young crayfish as previously reported in adults. With repetitive stimulation at low to moderate rates the NG synapses facilitated mildly due to an increase in mean quantal content, while the MoG synapses depressed even at rates of once per minute or less.