NEURAL ADAPTATION IN CRAYFISH

Motor neurons of crayfishes can be broadly classed as ''phasic'' and ' 'tonic'' on the basis of their normal activity patterns, synaptic phys iology, 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 thei r axons contain fewer and less metabolically active mitochondria than those of tonic motor neurons. Adaptive changes in all of these feature s take place in phasic motor neurons of the crayfish Procambarus clark ii when they become more active, and the neurons assume a more tonic p henotype. Axonal mitochondria become metabolically more productive, EP SPs are smaller and more fatigue-resistant, and synaptic terminals bec ome more varicose, with larger mitochondria. Since maintained synaptic transmission is strongly dependent upon oxidative metabolism, the obs erved mitochondrial changes are thought to be involved in producing gr eater fatigue resistance. Normally occurring adaptive changes are seen in crayfish neurons during different seasons of the year. Thus, neuro nal adaptation to different activity levels is one of several adaptive responses available in the nervous system which equip a species for c hanges in its environment.