Chordotonal organs are stretch receptor organs found only in insects and crustaceans. They are located at most joints and are made up of clusters of scolopidia that either directly or indirectly connect two joints and sense their movements relative to one another. They can have both extero- and proprioceptive functions, for example sensing auditory stimuli or leg movement. The word was coined by Vitus Graber in 1882, though he interpreted them as being stretched between two points like a string, sensing vibrations through resonance.
Structure
Chordotonal organs can be composed of a single scolopidium with only a single sensory, bipolar neuron (such as the tympanal ear of a notodontid moth), or up to several thousand scolopidia, each equipped with up to four sensory neurons (as in the mosquito Johnston's organ). The bipolar sensory neurons each have an apical dendritic structure with a cilium densely packed with microtubules and surrounded by two specialized cells, the scolopale cell and the attachment (cap) cell, plus a glial cell. Mechanically gated ion channels are located distal to the ciliary dilation, a characteristic part of the upper dendritic cilium. The cavity between the scolopale cell and the sensory neuron is filled with a specialized receptor lymph similar to the endolymph that surrounds the mechanosensory hair bundles of cochlear hair cells (high in potassium and low in sodium). The dendritic cilia can have one of two major forms: in the mononematic form, the major connection between the attachment site and the cilium is a microtubule-rich attachment cell. The electron-dense extracellular material is small and localized mainly to the junction between the cilia and the attachment cell. The femoral chordotonal organ is mononematic. In contrast, in the amphinematic form, the extracellular material of the cap forms a dense, tubular sheath that surrounds the sensory cilium and extends all the way to the cuticle at the attachment site. In this form, the attachment cell contains both microtubules and actin-rich scolopale rods similar to those present in the scolopale cell. The Johnston's organ is an example of an amphinematic chordotonal organ. The functional significance of the morphological differences of the two forms is unknown, but may confer different viscoelastic properties on the sensory units.
Functional diversity
In a chordotonal organ, individual sensory neurons can respond to different types of mechanosensory stimuli (for example, sound vs gravity), and those that respond to a particular stimulus can have different tuning properties (for example, tuned to different position of a joint). One way to generate these functional diversity is by having sensory neurons with different types of mechanosensory channels or intrinsic properties. For example, in Johnston's organ of Drosophila melanogaster, sensory neurons that detect sound may express nompC, an ion channel that belongs to the transient receptor potential (TRP) superfamily, while those that detect gravity may express another member of the TRP channel, painless. Another way to generate functional diversity is by having sensory neurons that are attached to the joint through different types of connections. For example, in the femoral chordotonal organ of the locust, the ligament in which sensory neurons are embedded is divided into several strands that are sequentially pulled as the joint is flexed, providing a mechanism for differential activation of the sensory neurons at different position of the joint.
Major chordotonal organs
Insects
Femoral chordotonal organ
The femoral chordotonal organ is located in the femur of the insect leg and it detects position, speed, acceleration, and vibration of the tibia relative to the femur. In Drosophila melanogaster, where it is possible to systematically analyze neuronal populations using genetic tools, the sensory neurons of the femoral chordotonal organ can be separated into at least 3 functionally and genetically distinct populations: the club, claw, and hook. The club neurons encode bi-directional movements and vibrations of the tibia, the claw neurons encode position of the tibia, and the hook neurons encode directional movements of the tibia. Information encoded by the femoral chordotonal organ is thought to be used during behaviors that require precise control of leg movements, such as walking and target reaching. The femoral chordotonal organ is thought to be functionally homologous to muscle spindles.