The problem of ‘competition’ between neurones innervating the same target can be studied in simple neural systems such as the central nervous system of the leech and the lobster neuromuscular junction. Intracellular injection of pronase to kill selectively a single neurone shows that, in the leech, removal of one neurone is a sufficient signal to produce compensatory changes. After removal of a given neurone, only neurones of the same function respond to innervate the ‘vacant territory’. This was shown both for a motor neurone (annulus erector) and sensory neurones (T or N). Thus the response is very specific. The lobster neuromuscular junction, with its multiple excitatory and inhibitory innervation, has advantages for the study of changes in synaptic efficacy of the remaining neurones after removal of a defined neurone releasing the same or a different transmitter. Killing the inhibitory neurone produced prolongation of the excitatory synaptic current because of a prolonged channel open time. When an excitatory axon is killed the remaining excitatory axon releases more transmitter. Over a period of 10 days, there is first a strengthening of existing synapses, then the appearance of new release sites and sprouting. Only those terminals of a neurone that innervate a territory with reduced innervation become stronger, while other terminals of the same axon remain normal. Cutting of axons produces different responses from those seen after killing single neurones.

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