Fig. 3. Locomotor network of the lamprey.

Diagrams show schematic representation of the brainstem and spinal components of the neural circuitry that generates rhythmic locomotor activity.

A. All neuron symbols denote populations rather than single cells. Descending commands exerted through glutamatergic neurons excite all classes of spinal interneurons and motoneurons. The excitatory interneurons (E) excite all types of spinal neurons, i.e. the inhibitory glycinergic interneurons (I) that cross the midline to inhibit all neuron types on the contralateral side and motoneurons (M). Stretch receptor neurons are of two types; one excitatory (SR-E), which excites ipsilateral neurons and one inhibitory (SR-I), which crosses the midline to inhibit contralateral neurons and provide sensory feedback during actual swimming. In addition, metabotropic receptors are activated during locomotion and are an integral part of the network as indicated in the box below the network (5-HT, dopamine (DA), GABA, tachykinins (TK) and mGluRs).

B. Cellular mechanisms contributing to burst termination. During a burst, Ca²⁺ ions entering via NMDA and voltage-dependent channels activate Ca-activated K+ channels (K_Ca) that will have a hyperpolarising effect with different time courses (fast (f) and slow (s)). Similarly the entry of Na⁺ ions (Na) during the burst will activate sodium-dependent K+ channels (K_Na). Voltage-dependent K+ channels activated during the action potential are grouped and indicated as (K_D).