296 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 37 



Rearward extension of the powerful metathoracic legs is an im- 

 portant component of the startle response pattern. Accordingly, an 

 estimate was made of the minimum number of discrete neural events 

 occurring between deflection of hair sensilla on the cerci and contrac- 

 tion of the extensor tibiae muscle. These are: (i) Excitation of the 

 hair sensilla, (2) impulse conduction in the cereal nerve fibers, (3) 

 transmission across the relatively stable cereal nerve-giant fiber syn- 

 apses in the last abdominal ganglion, (4) impulse conduction in the 

 giant fibers up the abdominal nerve cord, (5) transmission across 

 unstable synapses between the giants and motor system in the meta- 

 thoracic ganglion, (6) conduction in the fast motor fibers to the ex- 

 tensor muscles, e.g., in nerve 3B to the extensor tibiae, (7) neuro- 

 muscular excitation and muscle membrane depolarization (muscle 

 potential), and (8) development of tension in the muscles. 



The times occupied by some of these neural events were measured 

 in the following way. A cockroach was pinned to a cork plate and 

 appropriate regions of the nervous system were exposed by dissection. 

 An abrupt mechanical stimulus was applied to one cercus in the form 

 of an electrically timed blow delivered by a small stylus attached to a 

 loud-speaker element. The resulting movement of the cercus was 

 detected by a transducer, and had an abrupt onset followed by a vibra- 

 tion lasting for several milliseconds (pi. 5, upper trace in A, B, and 

 C). Electrodes were placed at various points on the nerve pathway 

 to determine the time of arrival of nerve impulses ehcited by the 

 mechanical stimulus (pi. 5, lower trace in A, B, and C). Measure- 

 ment of arrival times at certain points were made from a number of 

 records similar to that illustrated. Average values were as follows: 

 The afferent volley arrives at a point midway on the cereal nerve in 

 1.2 msec, the initial giant fiber volley leaves the last abdominal gan- 

 glion 3.4 msec, and enters the metathoracic ganglion 6.2 msec, after 

 the onset of the cereal stimulus. The synaptic delay in the last ab- 

 dominal ganglion occupies i.i to 1.5 msec. (Roeder, 1948, 1953)- 

 Subtraction of this delay from the last measurement leaves 4.7 to 5.1 

 msec, occupied by events i, 2, and 4 — that is, by impulse conduction 

 in the cereal nerves and giant fibers. 



The synapses in the metathoracic ganglion (event 5) could not be 

 made to transmit with any reliability under the conditions of this ex- 

 periment. The reasons for this are discussed below. Therefore, this 

 event was bypassed as unmeasurable, and events 6, 7, and 8 (motor 

 nerve and muscle response) were examined. This was done by apply- 

 ing an electric stimulus to nerve 3B at the point where it leaves the 

 metathoracic ganglion. The arrival at the extensor tibiae of impulses 



