Download Lecture 6

I. Sensation
A. Coding of signals into action potentials
1. receptor cells
I. Sensation
A. Coding of signals into action potentials
1. receptor cells
2. afferent neurons
I. Sensation
A. Coding of signals into action potentials
1. receptor cells
2. afferent neurons
3. sensory cortex
I. Sensation
B. Modalities (types of input)
1. touch: mechanoreceptors
2. hearing: mechanoreceptors
3. vision: photoreceptors
4. taste: chemoreceptors
I. Sensation
B. Modalities (types of input)
5. smell: chemoreceptors
6. unconscious
- interoceptors (include proprioceptors)
7. thermoreceptors
I. Sensation
C. Perception (awareness) of stimulus
1. transduction - conversion of one form of energy to another
2. action potentials reach brain from sensors
- sensory (afferent) pathways
3. interpretation (meaning)
I. Sensation
D. Specificity
1. most neurons will produce only one type of stimulus
2. response specific no matter what the stimulus
I. Sensation
E. Mechanisms
- promote conformational change (of protein)
- activate second messenger  cascade
- open an ion channel
I. Sensation
E. Mechanisms
1. detection
a. via receptors
b. commonality of receptor structural motifs
- vision, smell, sweet/bitter taste
- similarity to muscarinic
I. Sensation
E. Mechanisms
2. amplification
a. single photon activates transducin (G protein)
b. leads to activation of multiple cGMPs
c. each cGMP modifies an ion channel
I. Sensation
E. Mechanisms
3. encoding
a. due to a change in gm (conductance through ion channels)
b. depolarization  action potential?
c. can impart information about intensity of stimulus
I. Sensation
E. Mechanisms
4. adaptation
- allows detection of new stimulus in the presence of ongoing
input
a. tonic (continuous action potentials)
- provide input about duration of stimulus
I. Sensation
E. Mechanisms
4. adaptation
b. phasic (rapidly adapting)
- action potentials at onset of stimulus
- amplitude may eventually drop below threshold
- not much info about duration
II. Receptor Potentials and Impulse Propagation
generator potentials generated on neurons having the sensory receptors
A. Generator potentials analogous to EPSPs (pictured in (a) below)
1. can vary in amplitude (graded)
- receptor current
2. generate action potentials at threshold
3. stimulus of sensor  generator
potential  current  AP?
II. Receptor Potentials and Impulse Propagation
B. Intensity of stimulus determines:
1. amplitude of generator potentials
2. frequency of action potentials
3. brain receives action potentials
- only variation is frequency
- an AP is an AP
II. Receptor Potentials and Impulse Propagation
C. 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
II. Receptor Potentials and Impulse Propagation
D. Primary (first-order) sensory neuron
1. may also be the receptor
2. axon may travel to CNS as a sensory (afferent) fiber
3. will synapse with second-order (2˚) neuron
II. Receptor Potentials and Impulse Propagation
E. Adaptation
sometimes sensation is just a matter of perception
is the intensity less, or is our brain just adapting?
II. Receptor Potentials and Impulse Propagation
E. Adaptation (several mechanisms)
1. transducer molecules can be “used-up”
2. sustained stimulation may cause electrical changes
 Ca++ in cell
3. enzyme cascade inhibited by accumulation
4. sensory adaptation at higher
levels
II. Receptor Potentials and Impulse Propagation
F. Sensitivity
1. many receptors always on (just modify up or down)
- greater sensitivity
2.  or  in frequency can imply direction of stimulus (hair cells)
II. Receptor Potentials and Impulse Propagation
G. Sensitivity
3. lateral inhibition
a. interneurons inhibit neurons receiving less stimulus
b. sharpens cutaneous sensation
II. Receptor Potentials and Impulse Propagation
G. Sensitivity
4. feature detection
a. selective detection of given features of a sensory stimulus
b. examples: shape, angle, or movement by the visual cortex
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Cerebral Cortex
Map of cerebral hemispheres
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Cerebral Cortex
Map of cerebral hemispheres (functional organization)
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Cerebral Cortex
Map of cerebral hemispheres (Brodmann’s cytoarchitectural map)
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Cerebral Cortex
Laminar organization
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Cerebral Cortex
Columnar hypothesis: views the cortex as being organized vertically
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Cerebral Cortex
Cortex that is predominantly sensory has a prominent layer IV
Motor areas have a prominent layer V
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Cerebral Cortex
Afferent 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 Cortex
Cerebral cortex forms in a vertical fashion from cells arising from
the areas immediately surrounding the ventricles (neural tube)
Ideas on Perception
“Grandmother cells”
The cell at the top of the column does the “perceiving”
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Ideas on Perception
Parallel pathways
Each pathway analyzes one specific aspect of the stimulus
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Ideas on Perception
Distributed system theory
one single column may be a member of a number of different
pathways
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Ideas on Perception
Both ideas are basically correct
vertical hierarchy used in the different cortical regions used to
perceive sensory input
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