Bristle sensilla

Bristle sensilla (sometimes also called tactile hairs) are a class of mechanoreceptors found in insects and other arthropods that respond to mechanical stimuli generated by the external world.[1] 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.[2][3] 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.[4][5]

Schematic cross-section of an insect bristle sensillum. Each bristle is composed of a hair with its base fixed to the dendrite of a sensory neuron. The hair acts as a lever that exerts force on the dendrite, inducing mechanotransduction channels to open and producing electrical currents.

Bristle sensilla on the edge of a fruit fly wing. Green fluorescent labeling shows where sensory neurons innervate the bristles. In this image, some of the neurons are mechanosensory and some are gustatory.

Each bristle sensillum is composed of a hollow hair with its base fixed to the dendrite of a sensory neuron. The hair acts as a lever. When the hair is deflected, for example by dirt or parasites, force is exerted on the dendrite. This induces mechanotransduction channels to open, producing an electrical signal that is carried along an axon to the central nervous system.[1]

Bristles are directionally selective to mechanical deflection. Fly bristles are typically angled 45° relative to the cuticle, and most bristle neurons are most sensitive to forces that push the bristle toward the cuticle.[6] Movement of even a single bristle is sufficient to trigger an insect to groom.[7]

Bristle neurons generally refers to the mechanosensory neurons that can be found at the base of each bristle sensilla. The cell body is located at the base with a dendrite extending far enough into the hair that deflection of the hair will open mechanotransduction channels and produce electrical currents. While the cell body and dendrite are located peripherally, the axon of bristle neurons project to either the brain or the ventral nerve cord (VNC) of the animal - a correlate to the mammalian spinal cord. Axons from bristle sensilla on the head terminate in the subesophageal zone (SEZ) in the brain, whereas bristle neurons from the legs, thorax and abdomen project to and terminate in the ventral surface of the VNC.[8] It is thought that in both the SEZ and the VNC that the spatial organization of these axons represents a somatotopic map of different body segments.[9][10] Bristle neurons are also found at the base of chemosensory sensilla. These hairs tend to be longer with a distinct curve to their shape, they also contain a pore at the very tip that is used to sense various chemical signals. Instead of just a single mechanosensory neuron at the base of these hairs, they contain the mechanosensory neuron along with 2-4 gustatory receptor neurons. While gustatory and mechanosensory neurons project to different regions in the SEZ and VNC, it is still unclear which neuropil the mechanosensory neurons at the chemosensory sensilla project to.