Campaniform sensilla


Campaniform sensilla are a class of mechanoreceptors found in insects, which respond to stress and strain within the animal's cuticle. Campaniform sensilla function as proprioceptors that detect mechanical load as resistance to muscle contraction, similar to mammalian Golgi tendon organs. Sensory feedback from campaniform sensilla is integrated in the control of posture and locomotion.

Structure

Each campaniform sensillum consists of a flexible dome, which is embedded in a socket within the cuticle and innervated by the dendrites of a single bipolar sensory neuron. Campaniform sensilla are often oval-shaped with long axes of about 5-10 µm.
Campaniform sensilla are distributed across the body surface of many insects. Sensilla with similar orientations are often grouped together in regions where stress is likely to be high, including on the legs, antennae, and wings. For example, stick insects have groups of campaniform sensilla on the trochanter, a group on the proximal femur, a group on the proximal tibia, and a small number of sensilla on the distal end of each tarsomere. In Diptera like blow flies, the highest density of campaniform sensilla is found at the base of the modified hind-wings, or halteres, which function as gyroscopic sensors of self-motion during flight.

Function

The activity of campaniform sensilla was first recorded by John William Sutton Pringle in the late 1930s, who also determined that the oval shape of many sensilla makes them directionally selective. When cuticular deformations compress a campaniform sensillum along its short axis, the socket edges indent the cuticular cap. This squeezes the dendritic tip of the sensory neuron and opens its mechanotransduction channels, which leads to the firing of action potentials that are transmitted to the central nervous system. Campaniform sensilla signal the magnitude and the rate of cuticular deformation.
In walking control, sensory feedback from leg campaniform sensilla is thought to reinforce muscle activity during the stance phase and to contribute to inter-leg coordination, much like sensory feedback from mammalian Golgi tendon organs.
In flight control, sensory feedback from haltere and wing campaniform sensilla is thought to mediate compensatory reflexes to maintain equilibrium.