||Motor Patterning Mechanisms||Page 15|
Three different motor patterns of locomotion have been indicated in the figure, each corresponding to a different motivational mechanism, and each differing in the sensory analyzers and synthesizers that appear to be involved. Patrol/marking produces a general increase in locomotion without a particular goal except that the animal tends to remain within its territory and to investigate odors that are encountered. There are projections from neurons of the medial preoptic region (patrol/marking motivational mechanism?) to the midbrain locomotor region (Takeo and Sakuma, 1995) which may be the motor patterning mechanism. Offense produces approach locomotion which then, depending on the behavior of the animal being attacked, may be intensified into chase or shifted into sideways posture, upright posture or full aggressive posture which may culminate in a bite-and-kick attack. In this case the motor pattern requires releasing and directing stimuli of object recognition and orientation, as mentioned in the following section. The neural basis of the motor patterning mechanisms is unknown. Defense produces flight locomotion, if not the competing responses of freezing, full submissive posture, sideways posture, upright posture or lunge-and-bite attack. As discussed in the following section, flight locomotion also requires specific releasing and directing stimuli, in this case indicating an escape route. Again, the neural basis of the motor patterning mechanism is not yet understood.
The best studied motor patterning mechanisms are those of defense vocalizations which have been studied by Luthe et al. (2000) in monkeys. In response to vocalizations elicited by chemical stimulation of the midbrain central gray (the defense motivational mechanism?), they determined by single neuron recording that the principal location of the motor patterning mechanisms appear to be in the parvocellular reticular formation, with feedback control involving as well the nucleus of the solitary tract and the spinal trigeminal nucleus. In another study, Yajima et al. (1982) found that neural activity in the nucleus ambiguus (NA) and adjacent medullary regions in rats was linked to ultrasound vocalization. Since they also found that lesions of this nucleus abolished ultrasound produced by electrical stimulation of the midbrain central gray, it would appear that this is the motor patterning mechanism for defense ultrasound vocalization.
In principle, lesion studies should be able to separate out the pathways from motivational mechanisms to motor patterning mechanisms by leaving some motor patterns intact and eliminating others. This has received little attention in the case of offense motor patterns. However, unpublished studies by students in our laboratory found that different lesions near those in the ventral tegmentum which abolish offense in the rat selectively abolish offensive sideways posture and bite-and-kick attack. This suggests that the output pathway of offense passes through the ventral tegmentum and then diverges toward the two motor patterning mechanisms at this point. Lesions immediately posterior to those which abolish all offense were found to eliminate offensive sideways posture while leaving bite-and-kick intact. On the other hand, lesions located laterally near the pedunculopontine tegmental nucleus abolish the bite-and-kick attack while leaving offensive sideways posture intact. The motor patterns of defense elicited by chemical stimulation of the dorsal midbrain lateral to the central gray have been systematically interrupted by knife cuts in the rat (Shehab et al., 1995b). Escape locomotion, squealing vocalization and biting are all reduced or abolished by ipsilateral knife cuts of descending fibers, but not by contralateral knife cuts or by so-called "rostral" cuts which interrupted fibers ascending toward the forebrain. Different motor patterns of defense (jumping, rearing and squealing) are produced by electrical stimulation at different points in and around the central gray (Sandner et al., 1987), which could, in principle, reflect the differential activation of outputs to motor patterning mechanisms. In another related study (Schmitt et al., 1979), the radial connections from the midbrain central gray, assumed to contain the outputs from the defense motivational mechanism, were severed, but the animals were not tested for the various motor patterns of defense.