Problem Set #3 - Neurons & Sensory Systems

Problem Set #3 - Neurons & Sensory Systems - Problem...

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Unformatted text preview: Problem Set #3 – Neurons & The Sensory System 1. How do sensory neurons encode stimulus modality (i.e. type of stimulus), stimulus location, stimulus intensity and stimulus duration? Provide the level of detail necessary to understand these concepts. How sensory neurons encode stimulus modality and stimulus intensity is described by the theory of labeled lines. In general, the brain detects the type of stimulus based on the particular type of receptor that is stimulated (i.e. light is only detected by photoreceptors and so if a photoreceptor is stimulated, it must be light). Additionally, for most sensory systems there is a specific pathway from the sensory cell to the integrating center and therefore the brain knows where the receptor is located. Animals can improve the precision of their stimulus location by having afferent neurons with overlapping receptive fields (i.e. population coding). Stimulus intensity is encoded by changes in action potential frequency. Stronger stimuli typically trigger higher-­‐‑frequency series of action potentials, whereas weaker stimuli trigger lower-­‐‑frequency series of action potentials. Sensory receptors can only detect a stimulus over a range of intensities. The threshold for detection is the weakest stimulus that produces a response in a receptor 50% of the time. At some point the receptor will become saturated and cannot increase its response to increasing stimuli. Stimulus duration is encoded by tonic or phasic receptors. Tonic receptors fire action potentials as long as the stimulus continues and therefore can convey how long the stimulus lasts. Phasic receptors can code changes in the stimulus (i.e. when it starts or when it ends) but not explicitly the stimulus duration. 2. Dynamic Range is the range of stimulus intensities over which a receptor can respond. Receptors that have a smaller (or narrower) dynamic range have much better discrimination of changes in stimulus intensity than receptors that have a large dynamic range (although I can’t test figures here, you should be able to draw a neuron with a wide vs. narrow dynamic range). Please explain the trade-­‐‑off between dynamic range and discrimination using the relationship between number of action potentials fired and changes in stimulus intensity for both receptors with narrow vs. large dynamic ranges (FYI: I should see numbers in this explanation to explain the figure that is on the slide I presented). I am looking for you to think about interpreting differences in slopes of lines. If a slope is 5, then there is a change of 5 units on the y-­‐‑axis for every change in one unit on the x-­‐‑axis (this is the case for a neuron with a narrower dynamic range). If a slope is 2 then there is a change 2 units on the y-­‐‑axis for every change in one unit on the x-­‐‑axis (this is the case for a neuron with a larger dynamic range). If we think about this in terms of number of action potentials being fired on the y-­‐‑axis and stimulus intensity on the x-­‐‑axis, it tells you that a neuron with a greater slope (i.e. 5) will increase the rate of firing of action potentials by one action potential for every 0.2 stimulus intensity units (i.e. 1/5). It can therefore distinguish between 0.2 differences in stimulus intensity. The neuron with the slope of two will increase the rate of firing of action potentials by one action potential for every 0.5 stimulus intensity units (1/2). It can therefore only distinguish between 0.5 differences in stimulus intensity and therefore has a poorer discrimination. 3. Compare and contrast receptor vs. generator potential. Provide enough details that I can clearly understand the similarities and differences between these two mechanisms of receiving sensory stimuli. Both receptor and generator potentials are involved in sensory receptor cells and therefore in detecting incoming stimuli (from the internal and external environments) and transmitting the signal to the integrating centers. The two types of potential differ in where the change in membrane potential occurs (i.e. sensory afferent neuron vs. sensory receptor cell) If the receptor protein that detects the stimulus is on an afferent sensory neuron, the incoming stimulus that activates this receptor protein causes a depolarization called a generator potential. The generator potential triggers action potentials in the axon of the neuron. If the receptor protein that detects the stimulus in on the surface of a receptor cell, the incoming stimulus that activates this receptor protein causes a depolarization called a receptor potential. The receptor potential causes voltage-­‐‑gated Ca2+ channels to open, causing the release of neurotransmitter onto the primary afferent neuron. A graded potential is generated that can in turn generate action potentials that are conducted to the integrating centers along the axon of the afferent neuron. 4. Contrast ionotropic and metabotropic receptors. Specifically describe how they differ in their mechanisms of converting a sensory stimulus to a change in membrane potential. Provide one example of an ionotropic receptor we discussed in class and briefly describe how it works. Provide one example of a metabotropic receptor we discussed in class and briefly describe how it works. Ionotropic receptor: A receptor protein that acts as a gated ion channel. An example is a hair cell with a mechanically-­‐‑gated K+ channel. When the pressure signal causes the stereocilia to pivot toward the kinocilium, the mechanically-­‐‑gated K+ channels open, causing an influx of K+ and a depolarization that opens voltage-­‐‑gated Ca2+ channels. When Ca2+ enters the cell through these voltage-­‐‑gated Ca2+ channels it causes the release of neurotransmitters that bind to the afferent neuron, increasing the release of action potentials. When the pressure signal causes the stereocilia to pivot away from the kinocilium, the the mechanically-­‐‑gated K+ channels close, ultimately decreasing the frequency of action potential release from the afferent sensory neuron. Metabotropic receptor: A receptor that signals via a signal transduction pathway. An example is a photoreceptor that transduces light through the activation of a G-­‐‑protein coupled receptor (i.e. rhodopsin bound to a vitamin A-­‐‑derived chromophore, retinal) that signals a G protein (transducing) to activate a downstream signal transduction pathway that closes a cGMP-­‐‑gated Na+ channel. 5. As a continuation of the Challenge Question from last Friday (cheetah growling and me screaming). The cheetah starts to walk towards me and so I run. As I am running in the forward direction, I look downward to make sure I get my hand on the handle for the exit door. I thankfully get outside but then fall down a huge hole because of all the construction going on around Scrub Oak. As I am falling a twist my head to look up only to see the cheetah stairing down at me. Explain how my brain knows (and able to differentiate between) I am running forward, tilting my head to open the door, falling into a pit and then twisting my head to see the cheetah. Running forward – Hair cells in utricle (increase in APs from rest) Tilting head – hair cells in utricle (decrease in APs from rest) Falling – hair cells in saccule (vertical movement) Twisting head – Hair cells in ampullae that picks up angular acceleration (change in APs depends on direction I twisted my head) ...
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