NeuroFIG 9D-J: Mechanosensory neurons that sense gentle touch.
D. At the time of hatching, lateral touch neuron processes are located adjacent to the body wall muscle (left panel). During larval development, processes of lateral touch neurons become embedded in the hypodermis just beneath the cuticle (right panel). An electron-dense ECM, called mantle, surrounds the touch receptor processes and attaches them to the body wall. Mantle ECM is secreted by the touch receptor neurons as well as by the surrounding cells. The amount of mantle varies along the length of the process. Touch receptor processes are filled with 15-protofilament MTs. (ER) Endoplasmic reticulum. (Image source: N501 [Hall] S3 N517 [right panel]; N2L1C [MRC] A1082-19 [left panel].)
E. Touch neuron processes form attachments to the cuticle at intervals through fibrous organelles (FO), structures similar to those seen underlying muscle cells. Although these attachments were found not to be essential for touch sensitivity, they are thought to ensure that a very slight deformation of the process gets transduced by the receptor (Vogel and Hedgecock, 2001; Goodman and Schwarz, 2003; Emtage et al., 2004).
F&G. Mechanotransducing degenerin channel complex, made from the assembly of MEC-4, MEC-6, and MEC-10, a channel-associated protein (MEC-2), and collagen (MEC-5), colocalize in a punctate manner. These puncta of the mechanotransduction apparatus align with the cuticular annuli along the touch neuron process.
H. Schematic representation of mechanotransduction complex. MEC-4 and MEC-10 form the transduction channel, which is attached via MEC-2 to 15-protofilament MTs. MEC-6 interacts with the channel proteins to regulate channel activity. The complex is suggested to be linked to the ECM and cuticle (Cu) through fibrous organelle-like structures. Mechanical deflection of the ECM mantle relative to the MT bundle may gate the transduction channels. (Based on Gu et al., 1996; Goodman and Schwartz, 2003.)
I. A lateral touch cell process with 15-protofilament MTs (inset) is shown. The number of protofilaments in the MTs of touch cells may vary between 12 and 16. These MTs have an average outer diameter of approximately 16 nm. (Reprinted, with permission, from Chalfie and Thomson, 1982.)
J. In C. elegans, all singlet MTs, other than those in touch cells, have 11 protofilaments. Shown is a VNC neuron process with 11 protofilament MTs (inset). These MTs have an average outer diameter of approximately 13 nm. (Reprinted, with permission, from Chalfie and Thomson, 1982. ©Rockefeller University Press.)
See also NeuroFIG 9A-C
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