Neural Adaptation in Crayfish

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SYNOPSIS. Motor neurons of crayfishes can be broadly classed as “phasic” and “tonic” on the basis of their normal activity patterns, synaptic physiology, and morphology. Phasic motor neurons produce large excitatory postsynaptic potentials (EPSPs) which fatigue rapidly with repetition; their synaptic terminals are slender, with few mitochondria, and their axons contain fewer and less metabolically active mitochondria than those of tonic motor neurons. Adaptive changes in all of these features take place in phasic motor neurons of the crayfish Procambarus clarkii when they become more active, and the neurons assume a more tonic phenotype. Axonal mitochondria become metabolically more productive, EPSPs are smaller and more fatigue—resistant, and synaptic terminals become more varicose, with larger mitochondria. Since maintained synaptic transmission is strongly dependent upon oxidative metabolism, the observed mitochondrial changes are thought to be involved in producing greater fatigue resistance. Normally occurring adaptive changes are seen in crayfish neurons during different seasons of the year. Thus, neuronal adaptation to different activity levels is one of several adaptive responses available in the nervous system which equip a species for changes in its environment.