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Chapter 7
ascending pathways, as well as collaterals from the specifi c
ascending pathways, end in the brainstem reticular formation
and regions of the thalamus and cerebral cortex that are not
highly discriminative, but are important in controlling alert-
ness and arousal.
Association Cortex
and Perceptual Processing
The
cortical association areas
presented in Figure 7–14 are
brain areas that lie outside the primary cortical sensory or
motor areas but are adjacent to them. The association areas
are not considered part of the sensory pathways, but they play
a role in the progressively more complex analysis of incoming
information.
Although neurons in the earlier stages of the sensory
pathways are necessary for perception, information from the
primary sensory cortical areas undergoes further processing
after it is relayed to a cortical association area. The region of
association cortex closest to the primary sensory cortical area
processes the information in fairly simple ways and serves basic
sensory-related functions. Regions farther from the primary
sensory areas process the information in more complicated
ways. These include, for example, greater contributions from
areas of the brain serving arousal, attention, memory, and lan-
guage. Some of the neurons in these latter regions also inte-
grate input concerning two or more types of sensory stimuli.
Thus, an association area neuron receiving input from both
the visual cortex and the “neck” region of the somatosensory
cortex might integrate visual information with sensory infor-
mation about head position. In this way, for example, a viewer
understands a tree is vertical even if his or her head is tipped
sideways.
Auditory
cortex
Central sulcus
Frontal lobe
association
area
Parietal lobe
association
area
Somatosensory
cortex
Visual
cortex
Taste
cortex
Occipital lobe
association
area
Temporal lobe
association
area
Figure 7–14
Primary sensory areas and areas of association cortex. The olfactory
cortex is located toward the midline on the undersurface of the
frontal lobes (not visible in this picture).
The specifi c ascending pathways that transmit informa-
tion from
somatic receptors
—that is, the receptors in the
framework or outer walls of the body, including skin, skeletal
muscle, tendons, and joints—go to the
somatosensory cor-
tex.
This is a strip of cortex that lies in the parietal lobe of
the brain just posterior to the
central sulcus,
which separates
the parietal and frontal lobes (see Figure 7–14). The specifi c
ascending pathways from the eyes go to a different primary
cortical receiving area, the
visual cortex,
which is in the
occipital lobe. The specifi c ascending pathways from the ears
go to the
auditory cortex,
which is in the temporal lobe.
Specifi c ascending pathways from the taste buds pass to a cor-
tical area adjacent to the region of the somatosensory cortex
where information from the face is processed. The pathways
serving olfaction project to portions of the limbic system and
the
olfactory cortex,
which is located on the undersurface of
the frontal lobes.
Finally, the processing of afferent information does not
end in the primary cortical receiving areas, but continues from
these areas to association areas in the cerebral cortex where
complex integration occurs.
In contrast to the specifi c ascending pathways, neurons
in the
nonspecifi
c ascending pathways
are activated by sen-
sory units of several different types (
Figure 7–15
) and there-
fore signal general information. In other words, they indicate
that
something
is happening, without specifying just what or
where. A given ascending neuron in a nonspecifi c ascend-
ing pathway may respond, for example, to input from several
afferent neurons, each activated by a different stimulus, such
as maintained skin pressure, heating, and cooling. Such path-
way neurons are called
polymodal neurons.
The nonspecifi c
Spinal cord
Touch
Temperature
Thalamus and
brainstem
Cerebral cortex
Temperature
Touch
Specific ascending
pathways
Nonspecific ascending
pathway
Figure 7–15
Diagrammatic representation of two specifi c ascending sensory
pathways and a nonspecifi c ascending sensory pathway.
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