202
Chapter 7
SECTION A SUMMARY
I. Sensory processing begins with the transformation of stimulus
energy into graded potentials and then into action potentials in
nerve fi bers.
II. Information carried in a sensory system may or may not lead to
a conscious awareness of the stimulus.
Sensory Receptors
I. Receptors translate information from the external and internal
environments into graded potentials, which then generate
action potentials.
a. Receptors may be either specialized endings of afferent
neurons or separate cells at the ends of the neurons.
b. Receptors respond best to one form of stimulus energy,
but they may respond to other energy forms if the stimulus
intensity is abnormally high.
c. Regardless of how a specifi c receptor is stimulated,
activation of that receptor can only lead to perception of
one type of sensation. Not all receptor activations lead,
however, to conscious sensations.
II. The transduction process in all sensory receptors involves—
either directly or indirectly—the opening or closing of ion
channels in the receptor. Ions then fl ow across the membrane,
causing a receptor potential.
a. Receptor potential magnitude and action potential
frequency increase as stimulus strength increases.
b. Receptor potential magnitude varies with stimulus strength,
rate of change of stimulus application, temporal summation
of successive receptor potentials, and adaptation.
Primary Sensory Coding
I. The type of stimulus perceived is determined in part by the
type of receptor activated. All receptors of a given sensory unit
respond to the same stimulus modality.
II. Stimulus intensity is coded by the rate of fi ring of individual
sensory units and by the number of sensory units activated.
III. Perception of the stimulus location depends on the size of
the receptive fi eld covered by a single sensory unit and on the
overlap of nearby receptive fi elds. Lateral inhibition is a means
by which ascending pathways increase sensory acuity.
IV. Stimulus duration is coded by slowly adapting receptors.
V. Information coming into the nervous system is subject to
control by both ascending and descending pathways.
Neural Pathways in Sensory Systems
I. A single afferent neuron with all its receptor endings is a
sensory unit.
a. Afferent neurons, which usually have more than one
receptor of the same type, are the fi rst neurons in sensory
pathways.
b. The area of the body that, when stimulated, causes
activity in a sensory unit or other neuron in the ascending
pathway of that unit is called the receptive fi eld for that
neuron.
II. Neurons in the specifi c ascending pathways convey information
about only a single type of stimulus to specifi c primary
receiving areas of the cerebral cortex.
III. Nonspecifi c ascending pathways convey information from
more than one type of sensory unit to the brainstem reticular
formation and regions of the thalamus that are not part of the
specifi c ascending pathways.
Association Cortex and Perceptual Processing
I. Information from the primary sensory cortical areas is
elaborated after it is relayed to a cortical association area.
a. The primary sensory cortical area and the region of
association cortex closest to it process the information in
fairly simple ways and serve basic sensory-related functions.
b. Regions of association cortex farther from the primary
sensory areas process the sensory information in more
complicated ways.
c. Processing in the association cortex includes input from
areas of the brain serving other sensory modalities, arousal,
attention, memory, language, and emotions.
SECTION A KEY TERMS
acuity
195
adaptation
194
adequate stimulus
192
ascending pathway
199
auditory cortex
200
central sulcus
200
chemoreceptor
193
coding
194
cortical association area
200
labeled lines
195
lateral inhibition
197
mechanoreceptor
193
modality
194
nociceptor
193
nonspecifi c ascending
pathway
200
olfactory cortex
200
perception
192
photoreceptor
193
polymodal neuron
200
rapidly adapting receptor
198
receptive fi eld
194
receptor potential
193
recruitment
195
sensation
192
sensory information
192
sensory pathway
198
sensory receptor
192
sensory system
192
sensory transduction
192
sensory unit
194
slowly adapting receptor
198
somatic receptor
200
somatosensory cortex
200
specifi c ascending pathway
199
stimulus
192
thermoreceptor
193
visual cortex
200
SECTION A CLINICAL TERMS
phantom limb
201
SECTION A REVIEW QUESTIONS
1. Distinguish between a sensation and a perception.
2. Defi ne the term
adequate stimulus.
3. Describe the general process of transduction in a receptor that is a
cell separate from the afferent neuron. Include in your description
the following terms: specifi city, stimulus, receptor potential,
neurotransmitter, graded potential, and action potential.
4. List several ways in which the magnitude of a receptor
potential can vary.
5. Differentiate between the function of rapidly adapting and
slowly adapting receptors.
6. Describe the relationship between sensory information
processing in the primary cortical sensory areas and in the
cortical association areas.
7. List several ways in which sensory information can be distorted.
8. How does the nervous system distinguish between stimuli of
different types?
9. How does the nervous system code information about stimulus
intensity?
10. Describe the general mechanism of lateral inhibition and
explain its importance in sensory processing.
11. Make a diagram showing how a specifi c ascending pathway relays
information from peripheral receptors to the cerebral cortex.
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