Download Auditory: Stimulus Auditory

9/27/2011
Auditory: Stimulus
Auditory: Receptors
• Outer ear Æ Middle ear Æ Inner ear = receptors
440 Hz
1500 Hz
• Sound waves vary in amplitude (intensity) and frequency (Hz or cycles per second – tone)
• Humans can detect 20 Hz to 20,000 Hz frequencies at their best (i.e. no environmental damage)
Auditory: Transduction
Auditory: Afferent Signals
• The tectorial membrane “bounces” on the hair cells in rhythmic fashion to physically open ion channels and produce action potentials in the cochlear nerve
• Receptors in the cochlea are “tuned” to send action potential only for certain frequencies from high (outermost) to low (innermost).
• More receptors/ afferent signals for 500‐5,000 Hz
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Auditory: Pathways
Auditory: Perception (Left Brain)
• Each auditory cortex receives input from both ears BUT
primarily from the contralateral ear.
• There are specialized brain regions for complex sounds and language. (secondary auditory cortex = language & music)
• Neural signal goes to the primary auditory cortex in the temporal lobe.
Brown = language comprehension area
Auditory: Perception (Right Brain)
• The same areas of the right hemisphere appear to be specialized to detect all environmental sounds other than language.
Green = primary auditory cortex
Gold = language production area
Auditory: Summary
• Stimulus: 20‐20,000 Hz sound waves
• Receptors: Hair cells in the cochlea
• Transduction: Physical opening of ion channels in the cochlea by the tectorial membrane
• Afferent Signals: unevenly distributed to allow most signals for range of human speech
• Pathway: contralateral to primary auditory cortex • CNS Areas: Primary in superior temporal lobe; Wernicke’s, Broca’s, and right hemisphere specializations
• Perception: Complex processing of language in the left hemisphere and music in the right hemisphere.
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The chemical senses . . .
Olfactory Receptors
• Taste and olfaction are grouped together as “chemical senses” because chemicals are the stimuli that activate both senses.
PHYSICAL STIMULUS:
• Taste (gustatory system) stimuli are dissolved chemicals (in solution or saliva)
• Smell (olfactory system) stimuli are typically chemicals suspended in the air
• Olfactory receptors are found in the back of the nasal cavity. (regenerate)
• Protected by mucous layer
• Olfactory receptors are modified neurons with cilia
Olfactory
Transduction
Olfactory:
Signals
• Olfactory receptors are similar to the neurotransmitter receptors (both stimuli are chemicals, right?)
• Odorants fit only in certain receptors like a lock & key
LOCK & KEY
LOCK & KEY
• Chemicals bind to
the receptor causing a reaction
in the neuron to send a neural signal to the olfactory bulb
• Each odor has its own pattern of activity
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Olfactory: Pathways & CNS Areas
• What is your perception of odors?
• Olfactory bulb to:
• Amygdala –
emotional value of stimuli
• Frontal Lobe –
memories, moods
Olfactory: Summary
• Stimulus: Chemicals in air from nostrils or mouth
• Receptors: Olfactory receptor neurons with lock & key shape binding
• Transduction: Binding activates sending signal
• Afferent Signals: Odors produce unique patterns
• Pathway: Olfactory bulb Æ Amygdala & Frontal Lobe
• CNS Areas & Perceptions: Amygdala (emotional value), Frontal Lobe (memory association), Orbitofrontal cortex (flavor perception)
Olfactory Perception
• Projections to the limbic system and frontal lobe closely link smell with memories and emotions
• Evolutionary pressure to remember harmful stimuli – strong long‐term memory mechanism
• Olfaction is a powerful component to flavor linked with gustation (taste).
Gustatory: Receptors
• Taste buds are found on the surface of the tongue
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Gustatory: Transduction
Chemicals dissolve in saliva
and enter the taste bud pits
Chemicals are interact
with receptors on the
taste receptor cells
Taste cells release
neurotransmitters
on afferent taste
neurons
Gustatory: CNS Areas
• Many secondary areas
Limbic system & Frontal Lobe
Hypothalamus – hunger & satiety
Gustatory: Pathways
• 3 afferent sensory nerve relay taste sensations to the brain.
• Insular cortex = primary taste quality (between the temporal & parietal lobes)
Flavor Perception
Black bars = normal
Gray bars = pinched nose
% subjects correctly
identify flavored solutions
• Flavor ‐ the overall perception of an oral stimulant
• Includes taste, smell, texture, temperature, even color & appearance
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Gustatory: Summary
Somatosensory: Stimuli & Receptors
Stimulus: Chemicals in saliva or solutions
Receptors: Taste receptor cells in taste buds
Transduction: Different for each taste category
Afferent Pathway: 3 nerves from oral cavity to the NST of the brainstem • CNS Areas & Perceptions: Insular cortex (primary taste categories), Amygdala (cravings and aversions), Frontal Lobe (memory association), Hypothalamus (hunger & satiety) Orbitofrontal cortex (flavor perception)
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Somatosensory: Transduction
• Mechanoreceptors (Meisners, Merkel, Ruffini, Pacinian, and Hair cells): physical movement opens ion channels to depolarize the neuron and send action potentials to the brain
• Bare or Free Nerve endings: detect increases or decreases in temperature and chemicals released in response to tissue damage (like histamines)
Somatosensory: Pathway
• Sensory neurons enter the spinal cord organized by body region from toe to head, form synapses, & then send axons up the dorsal spinal cord
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Somatosensory: Pathway
• Temperature and Pain pathway comes into the spinal cord, crosses over, then goes up through the anterolateral pathway
• Special destination: anterior cingulate gyrus (pain perception)
Temperature & Pain
• Free nerve endings are specialized to detect:
– changes in temperature
• Cold fibers respond only to decreasing temp.
• Warm fibers respond only to increasing temp.
Plasticity in the CNS
• You can not replace CNS neurons but their connections can change with use (experience)
• For example: each finger is represented in the sensory cortex.
Loss of a finger does not
result in loss of cortical
use - cortical area is
taken over by adjacent
areas.
Pain Perception
• Pain following injury is often 2 sensations:
• Sharp immediate pain - myelinated neurons
• Dull throbbing secondary pain - unmyelinated neurons
• Speed difference due to myelin on axons!
Immediate Sensation
Delayed Sensation
Overall Perception
– extreme stimuli
• Pressure (cuts) or temperature (burns)
– chemical signals from tissue damage
• Histamines, capsaicin, etc.
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Controlling Pain
Sensations & Perceptions
• Pain perception can be modified!
• Reduction of the sensation through the :
Somatosensory: Summary
• Stimulus: mechanical, thermal, and chemical
• Receptors: Mechanoreceptors & Free nerve endings
• Transduction: Physical movement, change in temp., or chemicals released by tissue damage
• Afferent Pathway: Dorsal column pathway for touch, anterolateral pathway for temp and pain
• CNS Areas & Perceptions: Postcentral gyrus is the primary somatosensory cortex (touch organized by body part); anterior cingulate gyrus (pain)
• Gate control theory is natural pain management
Natural Pain Management System
• Efferent signal
from the brain
to the spinal
cord at pain
sensation level.
• Localized
release of
endorphins to
block the
sensation at a
precise site
Brain output
to block pain
Pain input
to brain
Psychophysics
• Psychophysics is the study of the our perceptions (behavior) to physical stimuli
• The first field of experimental psychology!
• Two types of thresholds:
– Absolute: can you detect a stimulus or not?
– Difference: can you detect a change in a stimulus?
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Signal Detection Theory
• The sensory system must deal with interference and noise to detect appropriate stimuli
• There are four possible outcomes when trying to detect stimuli: hits, misses, false alarms, and correct rejections
Difference Thresholds
• Usually our perception of stimuli does not exactly match changes in the actual stimulus • Weber’s law provided an equation to quantify (& predict) changes in perceptions based on changes in stimuli
• 1800s – 1st quantification of brain activity!
Sensory Adaptation
• Decrease in perception due to a constant stimulus
• Bottom‐up adaptation at the level of the receptor:
– Olfactory and Gustatory Systems
• Top‐down adaptation at the level of the thalamus:
– Auditory & Somatosensory Systems
• Allows us to focus are relevant stimuli and ignore unimportant stimuli
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