Selection and Initiation of Motor Behavior

Successful locomotion in vertebrates requires not only generation of the appropriate propulsive synergy (flying, swimming, or walking), but also goal-directed steering and control of body orientation. In most vertebrates, a correct positioning of each foot is also required during selected phases of the movement. This review focuses on the role of the forebrain and brainstem control in the initiation of locomotion. It considers the basal ganglia, hypothalamus, the mesencephalic locomotor region, including the cuneiform and pedunculopontine regions, as well as the medullary reticular areas. The various corticospinal systems are also reviewed because they most likely contribute to accurate foot placement during locomotion over a complex terrain. In the absence of their forebrain, mammals like decerebrate cats and rats can be made to walk, trot, and gallop by activation of different loco-motor regions in the brainstem. Corresponding findings have been made in all classes of vertebrates. The movements are well coordinated and accompanied by a largely appropriate equilibrium control. In character, however, the movements are robotlike-neither goal directed nor adapted to the environment. The latter qualities are added by neural structures in the forebrain. However, even in advanced mammals like the cat, goal-directed locomotion is retained after an ablation of the entire cerebral cortex that leaves the rest of the forebrain intact (including the basal ganglia and hypothalamus). Kittens, decorticated during their first few weeks of life (Bjursten et al., 1976), can be kept alive for years. The casual observer finds it difficult to distinguish their movements from those of cats with intact nervous systems. They exhibit periods of rest (and possibly sleep), become active, search for food, and are able to remember the location of food. Complex patterns of behavior, such as searching for and finding water or food, or attacking other individuals , require a sequential recruitment Figure 1.1 Forebrain and brainstem structures important for initiation of the basic locomotor synergy in mammals. The basal ganglia are included within the box The inputs to accumb are indicated to the left, including modulation by dopamine (DA) from the ventral tegmental area (VTA) and also by 5-HT, histamine (Hist.), neurotensin (NT), and enkephalin (Enk.). The descending control from the MLR activates reticulospinal neurons (ret. spin.), which in turn activate the spinal central pattern generator (CPG). The reticulospinal neurons can also be activated by a direct projection from the lateral hypothalamus (l. hyp.). of the motor programs coordinating locomotion in combination with …