Functional morphology of insect mechanoreceptors

Abstract

This paper reviews the structure and function of insect mechanoreceptors with respect to their cellular, subcellular, and cuticular organization. Four types are described and their function is discussed: 1, the bristles; 2, the trichobothria; 3, the campaniform sensilla; and 4, the scolopidia. Usually, bristles respond to touch, trichobothria to air currents and sound, campaniform sensilla to deformation of the cuticle, and scolopidia to stretch. Mechanoreceptors are composed of four cells: a bipolar sensory neuron, which is enveloped by the thecogen, the trichogen, and the tormogen cells. Apically, the neuron gives off a ciliary dendrite which is attached to the stimulustransmitting cuticular structures. In types 1-3, the tip of the dendrite contains a highly organized cytoskeletal complex of microtubules, the ''tubular body,'' which is connected to the dendritic membrane via short rods, the ''membrane-integrated cones'' (MICs). The dendritic membrane is attached to the cuticle via fine attachment fibers. The hair-type sensilla (types 1, 2) are constructed as first-order levers, which transmit deflection of the hair directly to the dendrite tip. In campaniform sensilla (type 3), there is a cuticular dome instead of a hair and the dendrite is stimulated by deformation of the cuticle. In these three types, a slight lateral compression of the dendrite tip is most probably the effective stimulus. In scolopidia, the dendritic membrane is most probably stimulated by stretch. On the subcellular level, connectors between the cytoskeleton, the dendritic membrane, and extracellular (cuticular) structures are present in all four types and are thought to be engaged in membrane depolarization.

Drosophila Hook-Related Protein (Girdin) Is Essential for Sensory Dendrite Formation

“…From their distal tip, wild-type sensory neurons contain a sensory cilium (outer segment), followed by a connecting cilium and two basal bodies (centrioles). These basal bodies exist at the distal end of the dendrite (inner segment) (Keil 1997). Within the specialized mechanosensory cilium, a rich array of tightly packed microtubules (tubular body) is surrounded by the plasma membrane and delimited by the dendritic sheath ( Figure 1I).…”

“…The dendrite of many of these neurons is composed of two parts: the inner segment and the sensory cilium (Keil 1997;Swaroop et al 2010) ( Figure 1A). The sensory cilium is composed of a connecting cilium, which is a specialized ciliary transition zone (Malicki and Avidor-Reiss 2014), and an outer segment, which houses the sensory transduction machinery .…”

“…Although the sensory cilium is responsible for the initial detection of a sensory stimulus, the inner segment is essential for the transmission of this signal to the neuronal cell body, as well as for the proper formation and positioning of the sensory ending. Another characteristic of ciliated sensory neurons is that the function and formation of their dendrite depends on their interactions with surrounding support cells (Keil 1997;Bonilha 2014). However, how the dendrites of ciliated sensory neurons form their morphology and how support cells function in the formation of the sensory dendrite is poorly understood in molecular terms.…”

“…They are bipolar, with an axon ending in a synaptic zone on one end and a single dendrite on the other end. In addition, the dendrites of these ciliated sensory neurons are composed of an inner segment that terminates in a sensory cilium composed of a connecting cilium and an outer segment on the other end ( Figure 1A) (Thurm 1965;Kernan et al 1994;Keil 1997). Second, mutations affecting the connecting cilium, such as cep290 (den Hollander et al 2006;Cideciyan et al 2007;Basiri et al 2014), and sensory cilium, such as intraflagellar transport (IFT) proteins, have a similar effect in both human photoreceptor and Drosophila sensory function (Avidor-Reiss et al 2004;Crouse et al 2014).…”

Expression of the rubber‐like protein, resilin, in developing and functional insect cuticle determined using a Drosophila anti‐rec 1 resilin antibody

“…Thurm (1964) showed that the cuticle at the base of each sensillum of the honey bee hair plate is elastic and stains positively with methylene blue, one of Weis-Fogh's (1960) diagnostic characteristics for identification of resilin. Subsequently, transmission electron microscopy has shown that the flexible socket cuticle, termed the joint membrane, is composed of a homogeneous material, possibly resilin (Matsumoto and Farley, 1978;Keil and Steinbrecht, 1984;Keil, 1997), embedded in a matrix of fibrous cuticle. Touch sensilla are likely to undergo many repeated deflections in an insect's lifetime.…”