User:Tony J. Prescott/Proposed/Invertebrate mechanoreception
Mechanoreceptors are sensory structures that detect mechanical stimuli such as touch, pressure, vibration and sound from the external and internal environments. Invertebrate mechanotransduction describes the processes that occur in mechanoreceptors of invertebrate animals, and usually includes all steps between the physical stimulus and the neural signal that is sent to the central nervous system. The invertebrate animals comprise very large number of species in many phyla, but only a small number of invertebrate preparations have been used to investigate invertebrate mechanotransduction, principally from the arthropods, annelids, nematodes and protozoa.
Mechanoreceptors respond to mechanical signals, such as displacement, acceleration or force. They always contain at least one sensory neuron that responds to mechanical stimuli with a change in membrane potential. This graded receptor potential usually initiates or modulates action potentials that propagate along an axon to the central nervous system (CNS), but not all mechanosensory neurons produce action potentials. Some vertebrate mechanoreceptors, such as vertebrate auditory organs, separate the two functions between two different cells, with the first cell releasing a chemical transmitter to excite action potentials in a second cell. However, no examples of such separated functions have yet been described in invertebrates. In contrast, some invertebrate sensory neurons are close enough to the CNS to transmit graded receptor potentials directly to interneurons within the CNS. Mechanoreceptors detect a wide range of mechanical stimuli from both the external environment (e.g. touch, vibration and hearing) and the internal environment (e.g. muscle length and hollow organ distention).
Mechanotransduction is a three stage process
The mechanical stimulus is usually coupled by a physical structure to stress the sensitive membrane of the sensory neuron. These physical structures are often complex, and their morphologies have been used to identify and classify different types of mechanoreceptors. Invertebrate examples include the chordotonal organs of arthropods that provide hearing and vibration sensitivity, or the touch receptors of nematodes that cause the animal to change direction as it encounters barriers to progress. Transduction is the process by which mechanical stress in the cell membrane produces the receptor current, leading to the receptor potential. This essential and defining property of all mechanosensory neurons is due to the presence of mechanically-activated ion channels in the cell membrane. However, mechanically-activated ion channels also occur in cells that are not mechanosensory neurons. The receptor potential is usually encoded into action potentials by a similar process to action potential encoding in other neurons. Variations in the sensitivity and dynamic properties of encoding often contribute to the functional properties of the mechanoreceptor, such as the high frequency response of some vibration detectors.
Mechanosensory neurons may also be modulated by signals from the CNS. Such signals may arrive specifically, via synaptic inputs from efferent neurons that directly innervate the sensory organ, or via circulating chemicals. Such modulation can change both the overall sensitivity and the dynamic properties of mechanotransduction.
Arthropods have highly specialized cuticular structures that allow external mechanical stimuli to reach the sensory endings through the hard exoskeleton. These can be classified into four basic types: hair, campaniform, chordotonal and slit sensilla. Hair sensilla respond to disturbances of air or fluid outside the animal, campaniform sensilla detect stress in the cuticle, and chordotonal sensilla respond to a variety of stimuli that originate inside or outside the body, including vibration and sound. Slit sensilla have only been found in arachnids and serve similar functions to the campaniform sensilla of insects. Arthropods also contain multipolar receptors. These are found throughout the body, from just below the integument to deep tissues such as muscle, gut and reproductive organs. They have highly branched nerve endings that are usually sheathed by glial cells and connective tissue, except for the extreme tips of the dendrites, where naked nerve endings are assumed to transduce mechanical displacements of the innervated tissues.
Hair sensilla form levers that allow movements of external air or water to compress sensory dendrite endings, while the bell-shaped caps of campaniform sensilla convert stress in the cuticle to compression at the tips of sensory dendrites. Slit organs of spiders perform similar stress-detecting functions, although the coupling mechanisms are not so clear. Chordotonal sensilla of insects and crustaceans contain particularly elaborate structures whose functions are uncertain. While all arthropod cuticular sensilla have dendrites derived from modified ciliary structures, there is evidence that the cilia of chordotonal sensilla may retain the capacity to move actively, leading to the idea of feedback loops between mechanotransduction and ciliary movement. Positive feedback could enhance sensitivity to small stimuli, or negative feedback could serve as a gain control for large stimuli. Possibly, both mechanisms operate. Interestingly, chordotonal sensilla function as auditory receptors in many insects, and similar positive or negative feedback loops have been proposed between the outer and inner hair cells of vertebrate auditory systems. Although the general functions of many arthropod coupling structures are understood, their dynamic properties remain largely unknown; partly because the mechanical properties of the individual components are difficult to measure or estimate. The dynamic properties of spider slit organs have been studied intensively, and there is some evidence for frequency tuning by slit length, but the overall dynamic responses seem dominated by ionic events during action potential encoding. In general, the evolutionary advantages of such varying and complex coupling structures are enigmatic, particularly since arthropod multipolar receptors seem to provide many similar functions without any comparable coupling structures.
Dr. Andrew French accepted the invitation on 16 May 2008 (self-imposed deadline: 16 November 2008).