Peter Bräunig, Michael Schmäh and Harald Wolf are fascinated by the neurophysiology of locusts, but each from a slightly different perspective: Wolf is intrigued by the role of inhibitory neurones in neuromuscular control, Bräunig by the neuroanatomy of the insect's head and thorax, and Schmäh by the insect's segmental organisation. Wolf explains that the three came together as a team when Bräunig noticed that a region of the insect's thorax was supplied by more neurons than expected. Their curiosity aroused, the team began characterising the insect's neurophysiology and were surprised to find that unlike all other arthropod inhibitory neurons, which function on several muscles simultaneously, one of the thorax's inhibitory neurones was `specific', functioning on one muscle alone (p. 1827).
Wolf explains that unlike large animals, which selectively activate fast and slow fibres through a myriad individual motoneurons, muscle function in smaller arthropods is controlled by as few as two or three motoneurons activating both slow and fast muscle fibres. According to Wolf, slow muscle fibres tend to be activated first, while fast muscle fibres are activated only at higher neurone discharge rates during an excitatory signal. So how do arthropods move fast when the muscle fibres that are initially activated are designed for slow endurance performance? Wolf explains that arthropod muscles are innervated with inhibitory neurones that act only on slow muscle fibres,effectively switching off the slow muscle fibres when the creatures need to get a move on. Wolf adds that almost all inhibitory neurones act on several muscles simultaneously, which he explains makes sense; if a crab wants to walk fast, it might as well inhibit all the slow muscle fibres in its leg muscles with a single neuron. However, only two inhibitory neurones, found in decapod crustacean legs, are known to function `specifically' on single muscles,allowing the crustaceans to independently activate two muscles that are excited by a single neuron. Curious to know which neurons in the locust's thorax were inhibitory, the team traced the delicate neural tissue through a segment of the insect's thorax.
The team washed thorax nerves with antisera that stained neurones producing the inhibitory neurotransmitter, GABA. Surprisingly, three of the neurons were inhibitory, even though most body segments are inhibited by only two. Curious to know which muscles were innervated by the inhibitory neurones, the team infused the insect's nervous tissue with cobalt and nickel ions, to track the neuron's course. Sure enough, two of the neurons served several muscles, but when the team traced the third, it only inhibited one muscle, known as M60. The team confirmed their unexpected finding by measuring electrical activity on the three neurons, and correlated their activity with signals in the muscle tissue.
So why is the inhibitory neuron that innervates M60 `specific'? Wolf suspects that there are several possible explanations. Either the neuron was previously connected to another muscle and is in the process of losing that connection, or the segment with three inhibitory neurones is an evolutionary snapshot, linking the locust to early dragonflies. Wolf explains that each modern dragonfly segment is innervated by three inhibitory neurons, and the locust could prove to be the missing link between the neurophysiology of early dragonflies and modern insects.