Chordotonal organ

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Chordotonal organs are stretch receptor organs found only in insects and crustaceans. [1] [2] [3] They are located at most joints [2] 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. [4] 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. [5]

Contents

Structure

Diagram of the primary components of a chordotonal organ scolopidium Schematic of chordotonal scolopidium.png
Diagram of the primary components of a chordotonal organ scolopidium

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). [6] 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. [2] 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). [6] 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. [7]

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). [2] 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. [8] 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. [9]

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. [10] [11] [12] [13] [14] 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. [14] 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. [14] Information encoded by the femoral chordotonal organ is thought to be used during behaviors that require precise control of leg movements, such as walking [15] and target reaching. [16] The femoral chordotonal organ is thought to be functionally homologous to muscle spindles. [17]

In the femoral chordotonal organ, the scolopidia are organized into groups called scoloparia. Scoloparia may be functionally distinct from one another, with separate scoloparia containing vibration-sensitive or position-sensitive sensory neurons. [18] [19] Drosophila melanogaster has three scoloparia. [20]

Johnston's organ

The Johnston's organ is located in the pedicel (the second segment) of the insect antennae, and it detects the position and the movement of the flagellum (the third segment of the antennae) relative to the pedicel. [21] Johnston's organ exists in nearly all orders of insects. [2] In Drosophila melanogaster , in most mosquito species and some midge species, different subsets of Johnston's organ neurons are tuned to different amplitude and frequency of the movements allowing them to detect various stimuli including, sound, wind, gravity, wing beats, and touch. [22] [23] [24] [25] [26]

In several species of Diptera, the Johnston's organ is sexually dimorphic. Males possess both greater numbers, greater diversity, and a more highly organized distribution of scolopidia. [4] Some species of mosquitoes may possess as many as several thousand scolopidia. [6] Males of these species likely use the Johnston's organ to identify potential mates.

Janet's organ

In addition to the Johnston's organ, antennae of Hymenoptera possess a second chordotonal organ, the Janet's organ, which detects flexion of the antennal joints, somewhat like the femoral chordotonal organ. [4]

Subgenual organ

The subgenual organ is found in all insects except Diptera and Coleoptera. It is located in the proximal part of the tibia and detects high-frequency acoustic vibrations transmitted through the substrate as well as sound through air. [27]

Tympanal organ

Tympanal organs are specialized hearing organs that have evolved in at least seven different orders of insects. They consist of a tympanal membrane backed by an air-filled space and are innervated by a chordotonal organ. Tympanal organs detect air-borne vibrations and are used to detect predators, prey, and potential mates and rivals. They can be found in a variety of locations on the body, including the abdomen, wing base, metathorax, and ventral prosternum. [28]

in Drosophila melanogaster , the Wheeler's organ [29] [30] is a type of tympanal organs in the first two abdominal sternites. It is named after the American entomologist William Morton Wheeler, who first described it in 1917. [31]

Wheeler's organ is composed of about 20 scolopidia, which are sensory structures that are sensitive to movement and vibration. The scolopidia are innervated by a single neuron, which sends signals to the fly's brain.

The function of Wheeler's organ is not fully understood, but it is thought to be involved in sensing the position of the abdomen and the distension of the abdomen. It may also play a role in the fly's courtship behavior.

Wing and halteres

There is a chordotonal organ located at the base of the wings in many insect orders, and, in Dipterans, there are also two chordotonal organs found at the base of the haltere. Their function is currently not well understood. In lacewings, a tympanal organ is located in the radius vein of the forewing and is thought to monitor ultrasound. [2]

Crustaceans

Myochordotonal Organ

In the order Decapoda, there are chordotonal organs located in the legs, antennules, antenna, chelipeds, and mandibles. [32] [33] Each leg joint also contains a chordotonal organ. [34] Similar to the antennal and leg chordotonal organs in insects, the leg chordotonal organs in crustaceans are sensitive to both proprioceptive and auditory information, including airborne and substrate-borne vibrations. [35] [36] [37] Myochordotonal organs are also called Barth's Myochordotonal Organs and were first studied by Barth in 1934. [32]

See also

Related Research Articles

<span class="mw-page-title-main">Halteres</span> Pair of small club-shaped insect organs

Halteres are a pair of small club-shaped organs on the body of two orders of flying insects that provide information about body rotations during flight. Insects of the large order Diptera (flies) have halteres which evolved from a pair of ancestral hindwings, while males of the much smaller order Strepsiptera (stylops) have halteres which evolved from a pair of ancestral forewings.

<i>Drosophila melanogaster</i> Species of fruit fly

Drosophila melanogaster is a species of fly in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly", "pomace fly", or "banana fly". Starting with Charles W. Woodworth's 1901 proposal of the use of this species as a model organism, D. melanogaster continues to be widely used for biological research in genetics, physiology, microbial pathogenesis, and life history evolution. As of 2017, six Nobel Prizes have been awarded to drosophilists for their work using the insect.

A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are innervated by sensory neurons that convert mechanical pressure into electrical signals that, in animals, are sent to the central nervous system.

<span class="mw-page-title-main">Range fractionation</span>

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<span class="mw-page-title-main">Octopamine</span> Group of stereoisomers

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Johnston's organ is a collection of sensory cells found in the pedicel of the antennae in the class Insecta. Johnston's organ detects motion in the flagellum. It consists of scolopidia arrayed in a bowl shape, each of which contains a mechanosensory chordotonal neuron. The number of scolopidia varies between species. In homopterans, the Johnston's organs contain 25 - 79 scolopidia. The presence of Johnston's organ is a defining characteristic which separates the class Insecta from the other hexapods belonging to the group Entognatha. Johnston's organ was named after the physician Christopher Johnston, father of the physician and Assyriologist Christopher Johnston.

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<span class="mw-page-title-main">Campaniform sensilla</span> Class of mechanoreceptors found in insects

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<span class="mw-page-title-main">Tympanal organ</span> Hearing organ in insects

A tympanal organ is a hearing organ in insects, consisting of a membrane (tympanum) stretched across a frame backed by an air sac and associated sensory neurons. Sounds vibrate the membrane, and the vibrations are sensed by a chordotonal organ. Hymenoptera do not have a tympanal organ, but they do have a Johnston's organ.

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<span class="mw-page-title-main">Proprioception</span> Sense of self-movement, force, and body position

Proprioception, also called kinaesthesia, is the sense of self-movement, force, and body position.

<span class="mw-page-title-main">Scolopidia</span>

A scolopidium is the fundamental unit of a mechanoreceptor organ in insects. It is a composition of three cells: a scolopale cap cell which caps the scolopale cell, and a bipolar sensory nerve cell.

Crista acustica is a part of the hearing organ in some insects. It is a collection of sensory cells that form a crest on top of the hollow tube behind the hearing membrane (tympanum) on the legs of the insect.

TRPN is a member of the transient receptor potential channel family of ion channels, which is a diverse group of proteins thought to be involved in mechanoreception. The TRPN gene was given the name no mechanoreceptor potential C (nompC) when it was first discovered in fruit flies, hence the N in TRPN. Since its discovery in fruit flies, TRPN homologs have been discovered and characterized in worms, frogs, and zebrafish.

<span class="mw-page-title-main">Subgenual organ</span>

The subgenual organ is an organ in insects that is involved in the perception of sound. The name refers to the location of the organ just below the knee in the tibia of all legs in most insects.

Hair-plates are a type of proprioceptor found in the folds of insect joints. They consist of a cluster of hairs, in which each hair is innervated by a single mechanosensory neuron. Functionally, hair-plates operate as "limit-detectors" by signaling the extreme ranges of motion of a joint.

<span class="mw-page-title-main">Reinhard F. Stocker</span> Swiss biologist

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<span class="mw-page-title-main">Femoral chordotonal organ</span> Sensory organ in insect legs

The femoral chordotonal organ is a group of mechanosensory neurons found in an insect leg that detects the movements and the position of the femur/tibia joint. It is thought to function as a proprioceptor that is critical for precise control of leg position by sending the information regarding the femur/tibia joint to the motor circuits in the ventral nerve cord and the brain

<span class="mw-page-title-main">Bristle sensilla</span> Class of sensory hairs

Bristle sensilla are a class of mechanoreceptors found in insects and other arthropods that respond to mechanical stimuli generated by the external world. As a result, they are considered exteroceptors. Bristle sensilla can be divided into two main types, macrochaete and microchaete, based on their size and physiology. The larger macrochaete are thicker and stouter than the smaller microchaete. Macrochaete are also more consistent in their number and distribution across individuals of the same species. Between species, the organization of macrochaete is more conserved among closely related species, whereas the organization of microchaete is more variable and less correlated with phylogenetic relatedness.

References

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