Brain Mechanisms

Motor patterning mechanisms for defense. There are a large number of motor patterning mechanisms activated by defense motivational mechanisms. In muroid rodents these mechanisms organize sideways or quadrupedal defense postures, defensive upright posture, freezing crouch, escape leaps and fleeing locomotion, lunge-and-bite attack, hissing, squeal or chit vocalization, urination, defecation, release of defense pheromones, activation of the adrenal medulla and the pituitary-adrenal axis and various warning or threat signals, including piloerection, teeth-chattering, tail-rattling, tail-raising, hind-foot thumping, and forefoot pattering (Adams; submitted for publication). In the cat many analogous motor patterns are activated during defense, including sideways and upright postures, escape leaps and fleeing locomotion, lunge-and-bite attack, hissing, screaming, urination, defecation, endocrine activation, and piloerection (Leyhausen 1956). In addition, striking is shown by cats during defense.

The defense patterns do not all occur at the same time but are organized in a graded hierarchical series corresponding to threat at low intensities and attack or escape at high intensities. This is apparently organized within the brain in terms of the strength of neural connections from the defense motivational mechanism to the motor patterning mechanisms. As shown in the classical work on brain stimulation of defense in cats (Hess & Brugger 1943), low-intensity brain stimulation of appropriate loci produces piloerection, pupil dilation, and low-intensity vocalization. Intermediate intensities of stimulation produce higher intensities of the foregoing motor patterns along with postural effects such as sideways postures and arching of the back. High-intensity stimulation produces attack or escape, provided that the appropriate releasing stimuli are present. Apparently the motor patterning mechanisms of piloerection, vocalization, and other threat patterns have low thresholds for activation by the defense motivational mechanism, while motor patterning mechanisms of attack and fleeing have higher thresholds. Threshold differences are also indicated by the fact that the threat patterns have shorter latencies for activation by brain stimulation than do the attack and fleeing patterns.

Since most of the motor patterns for defense have been obtained after removal of the forebrain in rats and cats, it would appear, as noted earlier, that the motor patterning mechanisms lie at midbrain or hindbrain levels. The one exception, it may be assumed, is pituitary-adrenal activation, which accompanies defense; this would be expected to involve ascending pathways from the midbrain to the hypothalamus and thence to the anterior pituitary. Some evidence for such a pathway may be found in the work of Giuliani et al. (1961), who found that midbrain sections abolish the pituitary-adrenal response to ether anesthesia, abdominal surgery, electric shock, anoxia, and certain neurotransmitters. As one would expect, electrical stimulation of the region just lateral to the central gray activates ACTH secretion in the chronic cat (Slusher & Hyde 1966).

The efferent fibers from the midbrain central gray projecting to various caudal neural structures that presumably function as motor patterning mechanisms may correspond to those fibers that leave the central gray in a radial stream pattern throughout the lateral extent of the midbrain. These fibers have been called Weisschedel's radiations (Nauta 1958). One set of such fibers has been traced and related to the function of hissing and screaming vocalization in the cat. Fibers of this pathway leave the midbrain central gray laterally and swing posteriorly and ventrally to travel in the ventral pons beneath the medial lemniscus (Magoun et al. 1937; Kanai & Wang 1962; Berntson 1972); their ultimate destination, and the location of the motor patterning mechanisms for hissing and screaming, have not been determined, however.

The motor patterning mechanisms for freezing and fleeing may include the so-called "locomotor region" of the midbrain. This is a region in the midbrain tegmentum beneath the inferior colliculus, where electrical stimulation produces running in the cat, and where single-pulse stimulation produces monosynaptic excitation on reticulospinal neurons thought to provide the "throttle" for locomotion in the cat (Orlovsky 1970). This region, often called the cuneiform nucleus, receives a major input from the central gray (Hamilton & Skultety 1970). Excitation of these neurons would presumably cause fleeing, and inhibition would cause freezing.

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