Swimming CPGs Tadpole Lamprey

Swimming is an undulatory rhythmic movement that requires activation of muscles in alternation on either side of the body. The cycle frequency of swimming ranges from 0.2-10 Hz in the lamprey and from 10-20 Hz in the tadpole. The basic structure of the swimming CPG network is illustrated by Figures 5.1 and 5.2, which depict the CPG networks in the tadpole and lamprey (Grillner 2003; Roberts et al. 1998). The similarities between the two CPGs are striking. In the lamprey, it has been shown that each one of the spinal segments contains a CPG unit, with CPG neurons on both the left and right side of the cord. The overall structure of the CPG unit is composed of two types of interneurons: excitatory glutamatergic neurons and inhibitory glycinergic commissural interneurons. Excitatory glutamatergic neurons provide monosynaptic excitation of motor neurons. Commissural interneurons inhibit contralateral neurons by axons crossing in the midline to ensure segmental alternation: when one side is active, all neurons on the other side are silenced. Finally, CPG activity is initiated by descending fibers originating in reticulospinal neurons in the brain. For further details, see Sillar and Grillner (this volume).

Figure 5.1 Basic schematic representation of the microcircuits in the spinal locomotor networks of the tadpole. Note that on each side of the spinal cord, as shown on the right top side, an excitatory kernel is located that interacts with the contralateral side via mutual inhibition (for details, see Sillar and Grillner, this volume). The details of the central pattern generating network are shown at the bottom. E: excitatory interneuron; I: inhibitory interneuron; M: motor neuron; circles denote groups of neuron types; ▲ denote excitatory synapses; • denote inhibitory synapses; zigzag lines denote coupling via electrical synapses.

Tadpole

Tadpole

Figure 5.1 Basic schematic representation of the microcircuits in the spinal locomotor networks of the tadpole. Note that on each side of the spinal cord, as shown on the right top side, an excitatory kernel is located that interacts with the contralateral side via mutual inhibition (for details, see Sillar and Grillner, this volume). The details of the central pattern generating network are shown at the bottom. E: excitatory interneuron; I: inhibitory interneuron; M: motor neuron; circles denote groups of neuron types; ▲ denote excitatory synapses; • denote inhibitory synapses; zigzag lines denote coupling via electrical synapses.

Glutamate

Glutamate

Figure 5.2 Basic schematic representation of the microcircuits in the spinal locomotor network of the lamprey. Within each hemi-segment of the spinal cord, an excitatory kernel is located that interacts with the contralateral hemi-segment via mutual inhibition, as shown on the right top side (for details, see Sillar and Grillner, this volume). Detailed topology of the segmental CPG is shown at the bottom. E: excitatory interneuron; I: inhibitory interneuron; M: motor neuron; circles denote groups of neuron types; ▲ denote excitatory synapses; • denote inhibitory synapses; arrows denote influences.

Figure 5.2 Basic schematic representation of the microcircuits in the spinal locomotor network of the lamprey. Within each hemi-segment of the spinal cord, an excitatory kernel is located that interacts with the contralateral hemi-segment via mutual inhibition, as shown on the right top side (for details, see Sillar and Grillner, this volume). Detailed topology of the segmental CPG is shown at the bottom. E: excitatory interneuron; I: inhibitory interneuron; M: motor neuron; circles denote groups of neuron types; ▲ denote excitatory synapses; • denote inhibitory synapses; arrows denote influences.

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