6 3 Mechanoreception Mechanoreceptors include those responsible for touch tactile receptors hearing tactile receptors hair cells balance tactile receptors hair cells internal blood pressure baroreceptors and proprioception proprioceptors Unlike chemoreceptors which are often concentrated in sensory organs mechanoreceptors are small and dispersed making them difficult to study Mechanoreceptors express two main types of membrane protein ion channels the epithelial sodium channel ENaC and the transient receptor potential channel TRP Both are attached to the cytoskeleton and extracellular matrix proteins so pressure on the cell pulls the ion channel open Receptor cells can be grouped roughly by location and function baroreceptors internal in vertebrates pressure in blood vessels heart digestive reproductive urinary tract tactile receptors body surface vary greatly between vertebrates and invertebrates detect touch pressure vibration and proprioceptors vary between vertebrates and invertebrates detect body position Many mechanoreceptors detect waves of pressure in the media around them water or air and perceive these as either vibration or sound Physical aspects of the pressure waves affect how the wave is perceived The figure below shows how variation in pressure waves are perceived as differences in sounds Insect mechanoreceptive cells are of either Type I or II Type I are always connected to the cuticle the protective outer layer of the insect s body that includes most of the exoskeleton These mechanoreceptive cells are involved in proprioception touch and hearing are associated with complex mechanosensory organs and express TRP channels Type II are internal and used to detect stretch and proprioception so they are simpler like stretch receptors and express ENaC channels Some Type I cells are in the cuticle of the sensilla where bending of the sensilla opens stretch sensitive TRP ion channels causing depolarization Others are attached by a ligament underneath the cuticle to form tympanal organs that function as insect ears The cuticle covering these is thin and covers an air pocket When waves of pressure in the air disturb the cuticle it vibrates which is detected by the sensory neuron The location of these tympanal organs is highly variable they can be completely absent on some insects or found on the legs abdomen thorax or wing base Vertebrate tactile receptors are widely distributed and function in isolation We looked briefly at these modified epithelial cells on Monday There are tactile receptors with very small receptive fields which are used for fine tactile discrimination due to their high acuity Some of these are tonic receptors that slowly adapt so they make the skin extremely sensitive to light touch and pressure Others are phasic receptors that adapt rapidly making the skin also sensitive to change For example this allows animals to detect a small bug moving across its skin but perhaps not one that is sitting still Still other tactile receptors in the skin detect stretch These are located deeper in the skin as well as in the muscles joints and internal organs where they play an important role in the sense of proprioception or the position of the body One of these sensory cells the Pacinian corpuscle is surrounded ay a layer of viscous gel When pressure is exerted on the skin it physically perturbs the gel and thus the membrane of the cell causing mechanoreceptive channels to open and the cell to depolarize As the pressure continues the gel will slowly regain its original shape so the animal stops detecting and perceiving pressure even though it is still present When the pressure is removed surrounding tissues will be perturbed which again stimulates the cell to initiate the perception that pressure changed Mechanoreceptors are also used to maintain balance or impart a sense of equilibrium This sense is necessary to orient the animal to the its environment and direct its movements In many invertebrates the sensory organ responsible is called a statocyst which is a hollow organ lined with mechanoreceptors and a fluid filled lumen The mechanosensory neurons are highly ciliated and on top of these cilia sit crystals of calcium carbonate called statoliths When the animal changes direction or moves the dense statoliths move within the fluid of the statocyst moving the cilia and opening ion channels in the mechanoreceptor membrane Cephalopods have an entire system of statocysts that are arranged on either side of their heads and composed of multiple chambers that are at different angles Thus movement in different directions will disturb statoliths in different chambers providing the cephalopod with detailed information about its body position and movement In vertebrates equilibrium and hearing are both due to modified epithelial cells called hair cells These are found in the ear as well as along the lateral line in teleost fish Their name is due to their highly ciliated apical membrane side facing away from the underlying tissue These cilia called stereocilia are thin and flexible so they move in response to pressure waves in the fluid that surrounds them These waves can be conducted through air or water water moves 4x faster to the animal and movement of these projections results in TRP channels opening and the hair cell depolarizing As you can see in the figure these projections can vary in length which makes the hair cell more sensitive to pressure waves from one direction than the other Nearby hair cell projections are often grouped together in a gelatinous cupula which connects the projections to the fluid around it and coordinates the activation of multiple hair cells in response to fluid movement In the lateral line of fish many hair cells may be connected via a cupula an organization is referred to as a neuromast The lateral line system in fishes uses hair cells to detect small movements in the water This sensitive system provides important information for survival and effective schooling The basic layout of the lateral line is shown below on this bitterling from Bleckmann and Zelick 2009 with dots illustrating the locations of hair cells In addition to sensing the presence of a predator as you saw in the escape reflex teleost fish may also use lateral line input to infer the size of conspecifics during a fight A larger animal will displace more water as it moves and leave a larger wake behind it as it swims These differences in pressure will cause more
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