ACTIVITY-DEPENDENT MODULATION OF ADAPTATION PRODUCES A CONSTANT BURSTPROPORTION IN A MODEL OF THE LAMPREY SPINAL LOCOMOTOR GENERATOR

The neuronal network underlying lamprey swimming has stimulated extens ive modelling on different levels of abstraction. The lamprey swims wi th a burst frequency ranging from 0.3 to 8-10 Hz with a rostrocaudal l ag between bursts in each segment along the spinal cord. The swimming motor pattern is characterized by a burst proportion that is independe nt of burst frequency and lasts around 30%-40% of the cycle duration. This also applies in preparations in which the reciprocal inhibition i n the spinal cord between the left and right side is blocked. A networ k of coupled excitatory neurons producing hemisegmental oscillations m ay form the basis of the lamprey central pattern generator (CPG). Here we explored how such networks, in principle, could produce a large fr equency range with a constant burst proportion. The computer simulatio ns of the lamprey CPG use simplified, graded output units that could r epresent populations of neurons and;that exhibit adaptation. We invest igated the effect of an active modulation of the degree of adaptation of the CPG units to accomplish a constant burst proportion over the wh ole frequency range when, in addition, each hemisegment is assumed to be self-oscillatory, The degree of adaptation is increased with the de gree of stimulation of the network. This will make the bursts terminat e earlier at higher burst rates, allowing for a constant burst proport ion. Without modulated adaptation the network operates in a limited ra nge of swimming frequencies due to a progressive increase of burst dur ation with increasing background stimulation. By introducing a modulat ion of the adaptation, a broad burst frequency range can be produced. The reciprocal inhibition is thus not the primary burst terminating fa ctor, as in many CPG models, and it is mainly responsible for producin g alternation between the left and right sides. The results are compar ed with the Morris-Lecar oscillator model with parameters set to produ ce a type A and type B oscillator, in which the burst durations stay c onstant or increase, respectively, when the background stimulation is increased. Here as well, burst duration can be controlled by modulatio n of the slow variable in a similar way as above. When oscillatory hem isegmental networks are coupled together in a chain a phase lag is pro duced. The production of a phase lag in chains of such oscillators is compared with chains of Morris-Lecar relaxation oscillators. Models re lating to the intact versus isolated spinal cord preparation are discu ssed, as well as the role of descending inhibition.