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2.26.10 - Sensory systems and sense organs Sense...

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Sensory systems and sense organs Sense organs (sensory cells) monitor internal and external environment. Animals have several dozen types of sensors; we’re consciously aware of 5 of them. Sense organs are arrays of receptor cells , either modified neurons or in close association with sensory neurons. All receptor cells respond to some form of energy (light, heat, pressure, osmotic potential, chemical energy, etc.) Receptors can detect only a very small part of the available stimuli in the environment. They are often extremely sensitive to those stimuli they can detect. Detecting a stimulus and responding Sense organs detect and respond to stimuli in several stages: 1. Stimulus arriving at sense organ is often filtered or otherwise modified (usually, data of interest are amplified and extra data are ‘deleted’) by accessory structures: -- external ears, eyeball and lens, etc. 2. Modified stimulus is detected by a receptor cell, changing that cell’s electrical properties -- usually membrane permeability ( generator potential , etc.) sensory transduction 3. Change in receptor cell’s electrical properties causes a change in the frequency or number of action potentials on a sensory neuron Sensory systems and sense organs Time Time A change in stimulus intensity causes a change in action potential frequency on a sensory neuron: Response is usually non-linear (logarithmic): lets a neuron code for large range of stimulus intensities with small range of AP frequencies. Receptors usually show sensory adaptation or accommodation: declining response over time to a constant stimulus: especially sensitive to changes in stimulus intensity! stimulus stimulus response Sensory systems and sense organs: mechanoreceptors Detect mechanical stimuli: pressure, touch, vibration, acceleration, stretch, or fluid movement. Examples: proprioceptors -- muscle and tendon stretch receptors, let the brain keep track of limb position. Muscle receptor is the spindle cell , a modified muscle fiber; sensitivity under neural control. Golgi tendon sensors also detect stretch sensory neuron sensory neuron bone bone muscle
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Sensory systems and sense organs: mechanoreceptors Detect mechanical stimuli: pressure, touch, vibration, acceleration, stretch, or fluid movement. Examples: gravity detectors -- detect direction of gravity -- especially important in aquatic animals in dark habitats. Receptor is the hair cell ; typical arrangement is the statocyst found in many invertebrates: Hollow fluid-filled ball lined with hair cells and containing a dense, heavy statolith (sand grain, etc.).
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