Figure 2.

Subsystems involved in the control of goal-directed locomotion. A, Selection of a motor program is performed in the basal ganglia, which receive inputs from the cortex (pallium) and the thalamus. The basal ganglia output stage (pallidum) inhibits command centers in the DLR and MLR during resting conditions. Through a well controlled inhibition of pallidal regions, the spinal CPG for locomotion can be activated via the reticulospinal (RS) neurons. In the brainstem, information is further integrated based on visual, sensory, and vestibular inputs to control both steering and posture. In all vertebrates, the spinal cord CPG neurons are modulated by local sensory feedback. Generation of locomotor activity in the spinal cord. B, The CPG network is activated from the locomotor regions via reticulospinal neurons (Ret.sp.) in the brainstem. Excitatory interneurons (E) excite all types of interneurons, including other excitatory interneurons. The inhibitory glycinergic interneurons (I) provide inhibition to the contralateral side and are also responsible for the left–right alternation seen during normal swimming. M indicates motoneurons. Some phasic feedback modulation (both glutamatergic and glycinergic) from the spinal cord network back to the brainstem also occurs. C, Activation of the spinal CPG by reticulospinal neurons (R). Neurons can drive the local network activity and cause a left–right alternating pattern over the experimentally observed frequency range in the simulations. L, Lateral interneurons.