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a. Disposition of chemosensory neurons in the animal. Each of the two amphids contains 12 associated chemosensory or thermosensory neurons. Each of the two phasmids contains 2 chemosensory neurons, PHA and PHB. There are six inner labial organs, each of which contains one IL2 chemosensory and one IL1 mechanosensory neuron. There are two URX neurons, one AQR neuron, and one PQR neuron; the endings of these neurons are within the animal, and not exposed. b. Detailed structure of the amphid sensory opening showing the socket (so), sheath (sh), and ciliated nerve endings. AWA, AWB, AWC, and AFD endings are buried in the sheath and not exposed through the amphid pore. c. Detailed structure of the cilia in the 12 classes of amphid neurons. Panels b and c are adapted from Perkins et al. (1986).

(A) Wild-type worms wave their heads in a narrow arc while foraging. (B) Mutants that lack GABA curve their heads in exaggerated flexures when foraging. (C) RMED motor neuron connectivity (White et al., 1986). The long undifferentiated processes of SAA and SMB may contain stretch receptors that are activated by positive curvature. These could stimulate the RME neuron which releases GABA onto the contralateral side to relax the flexed muscles. The RMED and RMEV neurons also have processes extending into the dorsal and ventral nerve cords, respectively. In addition, the dorsal and ventral RMEs receive input from the mechanosensory neurons IL1 and IL2, and the lateral RMEs receive input from the mechanosensory neurons OLQ and OLL, which could cause the head to move away from a stimulus by causing the muscles to relax on the touched side. Open arrows, stretch input; horizontal lines, gap junctions; solid arrows, chemical synapses. Excitatory inputs are indicated by dark shading in the muscle, inhibitory inputs are shown as white areas.

Mechanosensory circuits. (Ovals) Sensory neurons; (rectangles) interneurons; (inverted triangles) motor neurons. Cells that express glr-1 glutamate receptors are black. Synaptic connections are indicated by lines with triangles; gap junctions by lines with bars. Connectivities are as described by White et al. (1986). To simplify the illustration, gap junctions between ASH and AVD and between PVC and AVA are not shown. (A) ASH neurons respond to three stimuli, all of which stimulate backward locomotion (Bargmann et al. 1990; Kaplan and Horvitz 1993; Troemel et al. 1995). ASH provides synaptic input to interneurons that control backward (AVA and AVD) and forward (AVB) movment. Animals lacking glr-1 glutamate receptors are defective for ASH-mediated touch sensitivity but are normal for ASH-mediated osmotic and volatile-repellent sensitivity (Hart and Kaplan 1995; Maricq et al. 1995). (B) Mechanosensory neurons IL1 and OLQ, and the RMD motor neurons regulate the rate of spontaneous foraging and mediate an aversive response to touch to the ventral or dorsal tip of the nose (J. Kaplan and H.R. Horvitz, unpubl.). glr-1 mutants forage abnormally slowly and are defective for the head withdrawal reflex (Hart and Kaplan 1995). (C) The mechanosensory neurons ALM, AVM, and PLM sense touch to the body and provide input to the command neurons via both synaptic connections and gap junctions (Chalfie et al. 1985). Anterior and posterior body touch stimulates backward and forward locomotion, respectively. The anterior touch cells ALM and AVM are connected by a gap junction, which creates an anterior touch cell network. AVM neurons also function independently of the ALM neurons, presumably via the gap junctions with AVD and the synaptic connections with AVB. (D) Harsh touch sensitivity is mediated by the PVD sensory neurons, which provide synaptic input to AVA and PVC (White et al. 1986; Way and Chalfie 1989). These synapses appear to require GLR-1 glutamate receptors, since glr-1 mutants are insensitive to harsh touch (Hart and Kaplan 1995). (E) A diffuse mechanical stimulus (tap) simultaneously activates both the anterior and posterior touch circuits. Two behavioral responses to tap are observed, accelerations of forward movement and reversals. Which of these responses occurs appears to be determined by integration of the antagonistic activities of the anterior and posterior touch circuits (Wicks and Rankin 1995). Not shown are PVD (but see panel D) and DVA which provide input to both the anterior and posterior circuits and appear to enhance both reversals and accelerations.