Femoral chordotonal organ

The femoral chordotonal organ is a group of mechanosensory neurons found in an insect leg (Figure 1) that detects the movements and the position of the femur/tibia joint.

[1] 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[2][3][4][5] The cell bodies of the femoral chordotonal neurons are generally located in the proximal femur,[1] and their dendrites are mechanically coupled to the tibia through different types of tendons.

[11] The sensory neurons of the femoral chordotonal organ encode various kinematic features of the femur/tibia joint including, position, speed, acceleration, and vibration.

[11] The femoral chordotonal organ is thought to be involved in precise control of leg movements, and the experimental manipulations of the femoral chordotonal organ in stick insects and locusts have shown that they play a critical role during walking.

[2] and target reaching[3] One way in which the femoral chordotonal organ contributes to the control of leg movement is through a resistance reflex, in which a sub-group of femoral chordotonal neurons sense the extension of the tibia and activates the motor neurons that flexes tibia to counteract the movement (Figure 3).

Figure 1. The femoral chordotonal organ in the femur of the fruit fly . Left: A picture of the cells of femoral chordotonal organ (labeled with green fluorescent protein ) in the femur of the fruit fly. Right: A drawing of a fruit fly showing the location of the femur and the tibia. The femoral chordotonal organ senses the movements and the position of the femur-tibia joint.
Figure 2. Functional subtypes of femoral chordotonal neurons in the fruit fly. Left: A drawing of a fruit fly showing the location of the ventral nerve cord , where the axons of the femoral chordotonal neurons project to. Center: A drawing of the axon projection pattern of the different subtypes of femoral chordotonal neurons in the fruit fly. Right: Kinematic features of the femur-tibia joint encoded by different subtypes of femoral chordotonal neurons.
Figure 3. A reflex involving the femoral chordotonal organ changes based on the behavioral state of the animal. Left-Top: During standing, flexion of the tibia detected by the femoral chordotonal organ activates a reflex that resists the movement and stabilize the posture. Left-Bottom: During walking, the same flexion of the tibia leads to an assistive reflex that facilitates the flexion of the joint and enables the animal to walk. Right: A drawing of a stick insect showing the location of the femur and the tibia.