Crayfish escape behavior and central synapses. 3. Electrical junctions and dendrite spikes in fast flexor motoneurons.

THE LATERAL giant fiber is the decision fiber for a behavioral response in crayfish involving rapid tail flexion in response to abdominal tactile stimulation (27). Each lateral giant impulse is followed by a rapid tail flexion or tail flip, and when the giant fiber does not fire in response to such stimuli, the movement does not occur. The afferent limb of the reflex exciting the lateral giant has been described (28), and the habituation of the behavior has been explained in terms of the properties of this part of the neural circuit (29). The properties of the neuromuscular junctions between the fast flexor motoneurons and the phasic flexor muscles have also been described (13). They are facilitating and transmit reliably at repetition rates above 10 Hz, leading to repeated twitches in the fast flexor musculature. Onlv one neuromuscular junction, that of the motor giant, fatigues on repeated activation at about 1 Hz (3). After only a few motoneuron spikes, the junction fails and its small contribution to muscle tension is lost. The present paper considers the last remaining point in the neural circuit mediating this behaviorthe activation of the fast flexor motoneurons by the lateral giant. The only such junction studied in detail is the synapse between the lateral giant and the motor giant neurons (7). This junction . occurs between the crossing axons near the point at which the motoneuron exits from the ventral nerve cord via the third root; it is electrical, and exhibits rectification. The other junctions . between the giant fibers