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I. SensationA. Coding of signals into action potentials
1. receptor cells
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I. SensationA. Coding of signals into action potentials
1. receptor cells2. afferent neurons
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I. SensationA. Coding of signals into action potentials
1. receptor cells2. afferent neurons3. sensory cortex
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I. SensationB. Modalities (types of input)
1. touch: mechanoreceptors2. hearing: mechanoreceptors3. vision: photoreceptors4. taste: chemoreceptors
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I. SensationB. Modalities (types of input)
5. smell: chemoreceptors6. unconscious
- interoceptors (include proprioceptors)7. thermoreceptors
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I. SensationC. Perception (awareness) of stimulus
1. transduction - conversion of one form of energy to another2. action potentials reach brain from sensors
- sensory (afferent) pathways3. interpretation (meaning)
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I. SensationD. Specificity
1. most neurons will produce only one type of stimulus2. response specific no matter what the stimulus
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I. SensationE. Mechanisms
- promote conformational change (of protein)- activate second messenger cascade- open an ion channel
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I. SensationE. Mechanisms
1. detectiona. via receptorsb. commonality of receptor structural motifs
- vision, smell, sweet/bitter taste- similarity to muscarinic
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I. SensationE. Mechanisms
2. amplificationa. single photon activates transducin (G protein)b. leads to activation of multiple cGMPsc. each cGMP modifies an ion channel
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I. SensationE. Mechanisms
3. encodinga. due to a change in gm (conductance through ion channels)b. depolarization action potential?c. can impart information about intensity of stimulus
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I. SensationE. Mechanisms
4. adaptation - allows detection of new stimulus in the presence of ongoing inputa. tonic (continuous action potentials)
- provide input about duration of stimulus
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I. SensationE. Mechanisms
4. adaptation b. phasic (rapidly adapting)
- action potentials at onset of stimulus- amplitude may eventually drop below threshold- not much info about duration
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II. Receptor Potentials and Impulse Propagationgenerator potentials generated on neurons having the sensory
receptorsA. Generator potentials analogous to EPSPs (pictured in (a) below)1. can vary in amplitude (graded)
- receptor current2. generate action potentials at threshold
3. stimulus of sensor generator potential current AP?
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II. Receptor Potentials and Impulse PropagationB. Intensity of stimulus determines:
1. amplitude of generator potentials2. frequency of action potentials3. brain receives action potentials
- only variation is frequency- an AP is an AP
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II. Receptor Potentials and Impulse PropagationC. Initial stimulus can be on sensory epithelial cells (as in (b) below)
- does not generate an action potential- graded receptor potentials- graded release of neurotransmitter onto primary sensory neuron
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II. Receptor Potentials and Impulse PropagationD. Primary (first-order) sensory neuron
1. may also be the receptor2. axon may travel to CNS as a sensory (afferent) fiber3. will synapse with second-order (2˚) neuron
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II. Receptor Potentials and Impulse PropagationE. Adaptation
sometimes sensation is just a matter of perceptionis the intensity less, or is our brain just adapting?
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II. Receptor Potentials and Impulse PropagationE. Adaptation (several mechanisms)
1. transducer molecules can be “used-up”2. sustained stimulation may cause electrical changes
Ca++ in cell3. enzyme cascade inhibited by accumulation
4. sensory adaptation at higher levels
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II. Receptor Potentials and Impulse PropagationF. Sensitivity
1. many receptors always on (just modify up or down)- greater sensitivity
2. or in frequency can imply direction of stimulus (hair cells)
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II. Receptor Potentials and Impulse PropagationG. Sensitivity
3. lateral inhibitiona. interneurons inhibit neurons receiving less stimulusb. sharpens cutaneous sensation
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II. Receptor Potentials and Impulse PropagationG. Sensitivity
4. feature detectiona. selective detection of given features of a sensory stimulusb. examples: shape, angle, or movement by the visual cortex
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Cerebral CortexMap of cerebral hemispheres
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Cerebral CortexMap of cerebral hemispheres (functional organization)
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Cerebral CortexMap of cerebral hemispheres (Brodmann’s cytoarchitectural map)
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Cerebral CortexLaminar organization
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Cerebral CortexColumnar hypothesis: views the cortex as being organized vertically
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Cerebral CortexCortex that is predominantly sensory has a prominent layer IV
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Motor areas have a prominent layer V
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Cerebral CortexAfferent impulses will project project first to lamina IV.They will then project vertically to layers II, III, and V.These will then project to other cortical and subcortical regions
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Cerebral CortexCerebral cortex forms in a vertical fashion from cells arising from the areas immediately surrounding the ventricles (neural tube)
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Ideas on Perception“Grandmother cells”
The cell at the top of the column does the “perceiving”
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Ideas on PerceptionParallel pathways
Each pathway analyzes one specific aspect of the stimulus
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Ideas on PerceptionDistributed system theory
one single column may be a member of a number of different pathways
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Ideas on PerceptionBoth ideas are basically correct
vertical hierarchy used in the different cortical regions used to perceive sensory input
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