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STUDY NOTES FOR MODULE 7 - physiology notes

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MODULE 7 Sensory System In the last module we talked about how our brain causes muscles to move or actions to occur in our bodies. This module will focus on how we sense our environment and relay that information to our brain. 1) How are all the stimuli around us translated into action potentials, the language our nervous system understands? - Receptors In order for us to sense and detect changes in the environment, we use receptors. These act as sensors of our body and are responsible for initiating what we perceive as one of our many senses (taste, smell, hearing, sight, touch). We have numerous sensory receptors all over our body, each particularly good for sensing one thing (adequate stimuli). However, this doesn’t mean it ONLY senses one type of stimuli. A good example of this is your photoreceptors in your eyes. They are particularly good at sensing Light (adequate stimulus) but if you take your fingers and press on your eyelids, you will notice that you begin to see spots! So while these receptors are most sensitive to light, they can also be stimulated by a certain amount of pressure. 2) How are adequate stimuli translated into action potentials? Surprise, it is all about ions moving! The movement of ions in a sensory receptor is called a receptor potential. Receptor potentials ONLY occur in receptors. However, they share many of the same properties you learned about in the last module for EPSP’s and IPSP’s. Hint-It would be helpful to make a chart of the similarities and differences between these potentials so that you can understand them fully. Receptor potentials behave very much like the post-synaptic potentials: Receptor potentials are graded (size matters), local, and deteriorate in strength over time and distance. Receptor potentials can be depolarizing (mainly Na+ moving in) or hyperpolarizing (mainly by K+ moving out). The most important detail to take away is that just like EPSP’s or IPSP’s, receptor potentials may OR may not lead to an action potential…you need a large enough depolarization i) Sometimes, receptors are directly attached to their sensory neuron…and then the receptor potential spreads to the “firing area”. A good example of these types of receptors are your pain receptors. ii) Some receptors are not attached to their sensory neuron, but there is a cleft or space between them and the dendrites of the sensory neuron. How can the receptor potential spread in this case? These receptors have little vesicles containing neurotransmitters that are released when a depolarizing receptor potential spreads, opening voltage-gated Ca ++ channels and neurotransmitter travels across the cleft and binds to their chemically-gated ion channels. These channels open allowing ions to move and cause a graded potential just as you learned for the generation of EPSP’s and IPSP’s. Examples of these types of receptors (that have to release a neurotransmitter) are rods and cones in the eyes.
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3) Summary of receptors
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