MOTOR NEURONAL CONTROL OF TAIL-DIRECTED AND HEAD-DIRECTED SIPHON RESPONSES IN APLYSIA-CALIFORNICA

1. Cutaneous stimulation of opposite ends of the body causes qualitati vely different siphon responses: tail stimulation causes flaring and b ackward bending (the siphon T response), whereas head stimulation caus es constriction and slight anterior bending (the siphon H response). T his paper characterizes the motor neuronal control of siphon T and sip hon H responses. 2. The siphon response to tail nerve (p9) shock in a semi-intact preparation was indistinguishable from the siphon T respon se in intact or parapodectomized animals. Similarly, the siphon respon se to head nerve (c2) shock in this preparation was indistinguishable from the siphon H response in intact or parapodectomized animals. 3. C entral siphon motor neurons (SMNs) were found to cause a wider variety of movements than previously reported. The movements produced by the LF(SB) cells strongly resemble the flaring response of the siphon to t ail or tail nerve stimulation, The movements produced by RD(S) and LD( S1) resemble components of the constricting response of the siphon to head or head nerve stimulation. 4. Among central SMNs, the LF(SB) cell s show the strongest activation by posterior stimulation, whereas RD(S ) and LD(S1) show the strongest activation by anterior stimulation. Th e LF(SA), cells, which produce much weaker siphon constriction, are on ly activated slightly by posterior stimulation and are inhibited by an terior stimulation. Peripheral SMNs are inhibited by stimulation of he ad and tail nerves, and thus their activity does not directly contribu te to siphon T and H responses. 5. Artificially activating central SMN s with the pattern of activity previously exhibited after tail or head nerve stimulation indicated the sufficiency of the LF(SB) cells for t he siphon T response, and of RD(S) and LD(S1) for the siphon H respons e. 6. Dramatic behavioral deficits produced by hyperpolarizing the LF( SB) cells during tail nerve stimulation, or by hyperpolarizing RD(S) a nd LD(S1) during head nerve stimulation, indicated the necessity of th ese cells for the expression of directed siphon responses to tail or h ead stimulation, respectively. 7. Because of their apparent necessity and sufficiency for directional siphon responses to anterior and poste rior stimulation, these few cells provide well-defined vantage points for studying neural mechanisms underlying the motor control and transf ormation of siphon responses. The four LF(SB) cells offer a special ad vantage for cellular analysis because they form a homogeneous function al unit in which any sampled LF(SB) cell can be used as a precise moni tor of the total motor output underlying the siphon T response. 8. Del ayed, long-lasting excitation of motor neurons with actions antagonist ic to the primary reflex response indicates that distributed activatio n of motor neurons at levels subthreshold for muscle contraction contr ibutes to general defensive arousal after threatening stimulation. Thu s a function for the weak activation of numerous neurons by tactile st imulation (observed in the abdominal ganglion with optical recording m ethods) may be to ready many defensive systems for possible attack.