1 6 3 Sensory Systems Mechanoreception BIO 361T Fall 2014 1 Recall that the chambers of the vestibular system ampullae utriculus and sacculus in vertebrates are lined with hair cells The hair cells in the ampullae the chamber just outside the semicircular canals are arranged in cupulas and those of the utriculus and sacculus have dense mineralized otoliths atop their stereocilia a Where else do we find hair cells connected via cupulas What is their function Lateral line for detecting waves of pressure in the water b What are these otoliths similar to in invertebrates Calcium carbonate containing statoliths c One set of hair cells is responsible for detecting linear acceleration and the other detects angular circular acceleration Which set of hair cells detects which kind of acceleration How do the fluid filled semicircular canals contribute to one or both of these processes Explain your answer Vestibular sacs utriculus sacculus detect linear acceleration due to dense otoliths that drag on the hair cells when the head tilts or starts and stops in space Utricle is oriented horizontally and saccule is oriented vertically so they can detect movement on the horizontal and vertical planes respectively Ampullae detects angular circular acceleration due to fluid movement in and out of the semicircular canals which are each arranged in a different plane Thus fluid movement will differ in each canal depending on what plane the animal s head moves in and the CNS can compare these levels of activation d Mechanoreceptive input from the semicircular canals also allows animals to keep their eyes focused on a single point while the head moves For example try to read this sentence while shaking your head Then try to read this sentence while shaking your paper Explain why you can read in one case but not the other Sensory input from semicircular canals about head movement is being compensated for by motor efferents that move your eyes You can keep your eyes fixed on a single point while moving your head in this way However if you move the object you are not getting this sensory input because it is not your head that is moving so your eyes cannot compensate 2 2 Vertebrates have a variety of tactile receptors with varying electrical properties The three graphs below illustrate how three different tactile receptor sensory neurons fire in response to mechanical stimulation Label each graph with a cell type a firing pattern a rate of adaptation and a location Cell type options Pacinian corpuscles detect when pressure against the body changes Merkel s disks detect light touch on the skin s surface Ruffini corpuscles detect skin stretch and proprioception Firing pattern options tonic or phasic Rate of adaptation options fast or slow Location options surface of skin connective tissue just under the surface deep within the skin Merkel small receptive field fine tactile discrimination sensitive to indentation sense light touch and pressure on surface of skin Tonic slow adapting Ruffini in connective tissue associated with collagen fibers detect skin stretch involved in proprioception Tonic little to no adaptation Pacinian deep in skin in muscles joints internal organs Large dendrite is surrounded by gel and connective tissue Very phasic fast adapting only detects change Described in homework 3 proprioceptors are typically slow adapting constantly informing the CNS about body position Sensory cells that provide the sense of movement kinesthesia are rapidly adapting Why do aquatic animals lack both outer and middle ears but a variety of middle e g tympanic membrane ossicles and outer ear e g pinna structures can be found in terrestrial animals Although sound waves travel faster in water than air four times faster water is denser than air so it provides greater resistance to sound waves Thus when sound waves hit the fluid of the inner ear of terrestrial vertebrates a lot of the sound is reflected instead of detected These structures are adaptations to increase sound detection on land 4 Propose an ultimate explanation for the excitability of the hair cell membrane Why does firing a low frequency of action potentials without sensory input provide a selective advantage Vestibular system can detect direction of movement not just forward movement by increasing OR decreasing AP frequency Cochlea as sound waves bounce around and make their return trip hair cells will be bent in their non preferred direction causing hyperpolarization closing of K channels that are constitutively open causing low AP firing frequency in auditory nerve fibers without any sensory input This prevents perception of sound as sound waves are exiting and only perception as sound waves are entering 5 Why is it more difficult to localize a sound that comes from above your head than one that comes from somewhere beside you When sound comes from beside you your ears receive this sound at slightly different times Your brain interprets this lag as distance and angle relative to your body 3 6 The graph below shows the detectable sound frequencies across some taxa List as many factors as you can think of that would affect what frequencies an animal can detect Would any of these factors change over the lifetime of an individual animal flexibility of tympanic membrane length of cochlea thickness of basilar membrane stiffness of oval window build up of ear wax outside of oval window stiffness of round window presence health of hair cells stereocilia and auditory nerve fibers outer hair cells inner hair cells amplification of outer ear structures and middle ear structures number of cochlear turns The best models for predicting detectable frequency range include basilar membrane length and number of cochlear turns
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