ing forward when the tail is touched. Laser ablation experiments have revealed that six
touch receptor neurons are responsible for sensing these mechanical stimuli (Chalfie et
al. 1985). These six neurons have processes that lie embedded in the hypodermis near the
cuticle of the worm and seem to be attached to the hypodermis via extensive connections
to the ECM (Garcia-Anoveros and Corey 1997).
By patch-clamping these neurons in vivowhile gently touching the nematode cuticle,
100 PLANT TROPISMS
Figure 5.4 (also see Color Section). Structures of mechanosensitive channels gated by membrane tension
(MscL) and tethering to the cytoskeleton and extracellular matrix (Mec). In the bacterial MscL channel (a),
five subunits form the channel, each contributing two transmembrane domains (green). In the closed state,
the S1 domain (red) sits close to the inner face of the pore. When the membrane experiences tension, the
transmembrane helices tilt and the pore expands in an iris-like fashion. S1 is drawn closer to the membrane,
partially occluding the pore. As the channel fully opens, the S1 domains are dispersed, leaving the pore un-
obstructed. In the Mec channel of C. elegans(b), the Mec-4 and Mec-6 subunits form the conducting pore,
which is gated through a complex of proteins that interact with the extracellular matrix and the cytoskeleton.
Structures in (a) were rendered using MMDB from NCBI (Chen et al. 2003), and models of MscL open-
ing and Mec channel structure (b) were redrawn from http://www.life.umd.edu/biology/faculty/sukharev and
O’Hagan et al. (2005) respectively.